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Reduced high intensity training distance had no effect on VLa4 but attenuated heart rate response in 2-3-year-old Standardbred horses

Reduced high intensity training distance had no effect on VLa4 but attenuated heart rate response... Background: Training of Standardbred race horses aims to improve cardiovascular and metabolic functions but studies on the effects of different training strategies from breaking till racing are lacking. Sixteen horses with the goal to race as 3-year-olds were studied from breaking (1-year-olds) to December as 3-year-olds. Horses were allocated to either a control (C) or reduced (R) training program from 2 years of age. The aim was to evaluate the effect of reducing the distance of high intensity exercise by 30% with respect to velocity at lactate concentration 4 mmol/l (V ), blood La4 lactate and cardiovascular response. All training sessions were documented and heart rate (HR) was recorded. A standardized exercise test of 1,600 m was performed 10 times and a V test was performed five times. La4 Results: C horses initially exercised for a longer time with a HR >180 beats per minute compared to R horses (P <0.05) but after 6–9 months, time with HR >180 bpm decreased in C and were similar in the two groups (P >0.05). Over the 2-year period, recovery HR after the 1,600 m-test decreased in both groups but was within 2 months lower in C than in R(P < 0.05). C horses also had lower resting HR as 3-year-olds (P < 0.01) than R horses. In C, post exercise hematocrit was higher than in R (P < 0.05). There was a tendency (P < 0.1) towards a larger aortic diameter in C as 3-year-olds (C: 1.75 ± 0.05, R: 1.70 ± 0.05 cm/100 kg BW). Left ventricle diameter and blood volume (in December as 2-year-olds) did not differ between groups. There were no differences between groups in post exercise blood lactate concentration or in V . Both groups were equally successful in reaching the goal of participation in races. La4 Conclusions: Horses subjected to a reduced distance of high intensity training from the age of 2 showed an attenuated heart rate response, but were able to maintain the same V and race participation as horses La4 subjected to longer training distances. Keywords: Anaerobic threshold, Cardiovascular response, Hematocrit, Lactate, Heart rate Background and glycogen [5]. While all physiological systems are Standardbred horse race performance is positively corre- stressed in order to improve exercise performance and lated to V [1-3] and, in general, European Standardbred quality of musculoskeletal tissues, changes within each La4 horses are subjected to high intensity training for at least may also contribute to fatigue during acute exercise and 1 year before the competition career can begin. Training training [5,6] and (chronic) overload of musculoskeletal is associated with improvements in cardiovascular re- tissues. Amongst sport horses, lameness is a common sponses [4], thermoregulation, aerobic and glycolytic me- cause of a defaulted race career [7,8]. Interestingly, there tabolism, as well as reductions in the accumulation of have been no studies comparing the effects of different extracellular potassium, reduced depletion of muscle ATP training strategies applied from breaking to the start of the racing career on performance related variables in race * Correspondence: [email protected] horses. However, the volume of high intensity training is Deceased positively correlated with increased risk of musculoskeletal Department of Animal Nutrition and Management, Swedish University of problems in race horses [9-11]. Accordingly, one strategy Agricultural Sciences, SE-75007 Uppsala, Sweden Full list of author information is available at the end of the article © 2015 Ringmark et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 2 of 13 to reduce the risk of musculoskeletal overload injuries Training could be a reduction of the volume (distance, duration) of The training programs were designed together with a high intensity training. group of experienced professional trainers and included The aim of this study was to evaluate the effect on heat training, flat interval training and uphill interval V blood lactate and cardiovascular response of two training, which is common practice in Sweden. The La4, training distances (control and reduced by 30%) of high training track mainly used was a 1,000 m oval banked intensity training over a 21-month period in 2 year old sand track, 3 m incline that during wintertime was fro- Standardbred horses with the goal to race as 3-year-olds. zen and covered with crushed snow and ice. Throughout The control training program was designed together the study, the speed was set by the trainer and was the with group of professional trainers and estimated to be same in both groups (see below). Based on the correl- sufficient to get horses in to a condition to race by the ation between post-exercise lactate response and 10-min age of 3 years. recovery HR (y = 4,1513x + 55,944, R = 0.54) observed on these horses in March 2011, a maximal 10 min post- Methods exercise HR of 80 bpm was aimed for (corresponding to The study was performed at the Swedish National a blood lactate concentration less than 6 mmol/L; Trotting School, Wången, Sweden, and the horse use plasma lactate ≤ 8 mmol/L; [23]) and speed was adjusted and testing protocols approved by the Umeå Local according to this in all training sessions as 2-year-olds Ethics Committee according to the Swedish law of but not as 3-year-olds. This meant that if HR10 min post animal welfare. exercise were > 80 bpm in an individual the speed during the next session was decreased for that individual. As Horses there were individual variations of HR10 within training th Sixteen Standardbred geldings born between 19 of groups, two sub-training groups were created with th March and 15 of July in 2009 were used. The horses equally many horses from each training group. These were from four Swedish breeders, were progeny from groups were exercised at a speed adjusted so that all eight sires and were chosen with the criteria to be of horses should have a HR10 of max 80 bpm. The speed mainly American ancestors. The horses were taken was also adjusted due to track conditions (muddy track, care of and trained by high school students with super- lower speed). As 3-year-olds, speed was adjusted accord- vision from professional trainers. Prior to the study, all ing to the trainer’s judgement of the horses’ capability horses had received the same training which started but was always the same for group C and group R. with breaking in September 2010 as 1-year-olds [12]. In January 2011 as 2-year-olds, horses had recently All horses had been at the national training facility for been castrated and therefore training for 2–3 weeks was 2–4 months before the study started, and kept on the mostly walk and slow trot ~3 times/week. In February same diet and management system [12]. The study 2011, exercise in heats of 1,600 m on the oval banked started in January 2011 as 2-year-olds and from January to race track was introduced mixed with jogging 1–2 th the middle of March 2011 all horses were subjected to times/week. Beginning on the 18 of March 2011, all ex- thesametrainingprogram.Inthe middle of March ercise sessions that were expected to cause a heart rate 2011 horses were divided in two groups and one group (HR) > 180 beats per minute (bpm) had the exercise dis- was allocated to a control (C) training program and the tance reduced by approximately 30% for horses allocated other to a reduced (R) training program until December to the R-group. The HR limit of 180 bpm was chosen 2012 as 3-year-olds. The groups were balanced with since workloads producing a HR above that will stress respect to breeder and factors known to affect race both aerobic and anaerobic systems [24]. performance in Swedish Standardbred trotters. These High intensity training was performed ~2 times/week. include sire [13] estimated genetic potential (breeding Heat exercise (C: 1–3 times 1,600 m/training session, R: index calculated from the average of the parents [14,15], 1–3 times 1,100 m/training session, 10 min walk/jog be- percentage of French ancestors [16], inbreeding coeffi- tween bouts until October 2011 as 2-year-olds and cient [17], age [18,19], abnormal radiographic findings therefrom 4 min walk between bouts) was performed in metacarpal metatarsal and tarsal joints, conformation throughout the study (Table 2). Before heat exercise (subjectively determined through visual examinations: horses in both groups were warmed up by jogging x-shaped legs, asymmetric pelvis, tied in below the knee, 3,000 m at an approximate speed of 6 m/s. The mean i.e. leg circumference smaller below knee than above speed during heat exercise when horses had a fetlock), height at withers [20,21] and proportion of type HR > 180 bpm was 8.7 ± 0.1 m/s as 2-year-olds and 9.5 ± IIA / type IIB muscle fibers in musculus gluteus medius 0.1 m/s as 3-year-olds (P< 0.0001). After heat exercise [22] from a muscle biopsy taken in December 2010 as horses were jogged 1,200 m at an approximate speed of 1-year-olds (Table 1). 6 m/s before return to the stable. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 3 of 13 Table 1 Parameters used to balance training groups Group Horse Breeder Breeding Sire French, % Inbreeding Age, Height, cm X-ray Conformation IIA/IIB index coefficient, % days findings remark C 1 A 106.5 1 20.6 7.2 580 153 c 0 0.73 C 2 A 101 1 0 15.8 653 152 d 2 1.34 C 3 A 107.5 1 0 13.5 535 155 0 1.58 C 4 A 109.5 3 26.5 7 630 157.5 1 1.27 C 5 B 115.5 2 0 13.5 611 156 0 1.86 C 6 B 114 2 10.3 8.5 600 157 b 1 1.58 C 7 B 106.5 4 11.0 7.7 567 153 d 2 1.50 C 8 C - 5 10.2 11.1 587 154 a 1 0.83 Mean C-group 108.6 9.8 10.5 595 154.7 7* 1.34 R 9 A 107 1 0 1.8 585 155.8 c 1 1.23 R 10 A - 1 15.3 9.6 612 160 0 1.29 R 11 A - 1 26.5 5.7 623 150.5 a 1 1.31 R 12 D 113.5 6 0 12.6 569 152 a 1 0.95 R 13 B 113.5 2 0 9.2 631 152.5 d 0 2.35 R 14 B 105 2 10.3 8.4 605 161 1 1.08 R 15 B 115.5 7 0 14.9 565 152 0 0.98 R 16 C 103.5 8 0 11.5 604 158 b 1 1.56 Mean R-group 109.7 6.5 10.7 599 155.2 5* 1.34 X-ray performed in November as 1-year-olds on tarsal, metacarpal and metatarsal joints, horses with defects validated to similar severity received the same letter and were separated between the groups. * = Total number of remarks. Information used to allocate horses in to 2 groups (control: C and reduced: R). Groups were balanced to be as equal as possible with respect to breeder (A-D), breeding index (average of breeding index of parents, − if no breeding index for parents available), sire, part of French ancestors in pedigree, inbreeding st 1 coefficient, age (days in the 1 of January as 2-year-olds), height at withers (cm), x-ray findings conformation remarks (0 = no serious remark, 1–2 = remarks) and quote of percentage of type IIA / type IIB muscle fibers in musculus gluteus medius. From August 2011 as 2-year-olds, interval exercise and Saltin [6] (interval exercise: 5.3 ± 3.3 mmol/L after the th th (performed at straight parts of the oval track, 4 interval and 7.2 ± 5.6 mmol/L after the 6 interval, up- th 500 m/interval in August and September and there- hill interval exercise: 10.0 ± 1.4 mmol/L after the 4 inter- th after 700 m/interval, 1 min walk between each inter- val and 8.8 ± 1.2 mmol/L after the 6 interval). val [C: 6 times 500–700 m, R: 4 times 500–700 m]) On a few occasions, as a preparation for participation was also performed. The mean speed during interval in official races, heat exercise of 2,000 m was performed exercise on flat ground for both groups when horses by all horses, since most official races in Sweden are had a HR > 180 bpm were 8.3 ± 0.2 m/s as 2-year-olds 2,140 m. and 9.3 ± 0.2 m/s as 3-year-olds (P< 0.001). The warm Between training sessions both groups were also up before and jog after the intervals were the same as jogged on a mostly flat jog track (2-year-olds: 47 ± 8 ses- for heat exercise. Also, from February 2012 as 3-year- sions, 169 ± 175 s/session with HR > 180 bpm [n = 20], olds, uphill intervals (600 m, 5% incline, 30 m rise per 3-year-olds: 35 ± 12 sessions 38 ± 34 s/session with interval, 4 min walk/jog downhill between each interval HR > 180 bpm [n = 13] [mean ± SD per horse]) and at a [C: 6 intervals, 180 m rise, R: 4 intervals, 120 m rise]) cross country jog track (2-year-olds: 16 ± 4 sessions, were used. Before uphill intervals horses were jogged in 289 ± 122 s with HR > 180 bpm [n = 28], 3-year-olds: a hilly terrain at an approximate speed of 5 m/s for 3 ± 2 sessions, 524 ± 132 s with HR > 180 bpm [n = 26] 5,500 – 6,000 m. After the last interval horses were [mean ± SD]) depending on track conditions. walked 500 m back to the stable. During all interval ex- Equally many training sessions were planned for horses ercise, horses from both groups were exercised together in both groups and all training sessions (type and distance) for the first 4 intervals and then C horses continued for were documented in a protocol and summarized in eight 2 more intervals after the R horses had finished. For the 3-month periods with the exception of the first period th uphill interval exercise when HR > 180 bpm the speed was of 2.5 months that ended on the 17 of March 2011 as 7.3 ± 0.9 m/s. Lactate concentrations were used to confirm 2-year-olds (Table 2). Horses that according to the workloads recommended for interval training by Lindholm head trainer’s opinion were unhealthy were left out of Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 4 of 13 Table 2 Number of training sessions 2-year-olds 3-year-olds Group January-March April-June July-September October-December January-March April-June July-September October-December Heat training C 7.0 ± 0.8 15.9 ± 2.5 11.8 ± 1.3 6.6 ± 0.6 3.6 ± 0.5 5.1 ± 1.0 10.0 ± 1.3 9.3 ± 1.3 R 6.1 ± 0.8 15.6 ± 2.5 16.1 ± 1.3 8.8 ± 0.6 4.5 ± 0.5 7.0 ± 1.0 10.5 ± 1.3 6.3 ± 1.3 Intervals, flat C 4.0 ± 0.5 6.6 ± 1.1 1.8 ± 0.1 1.8 ± 0.3 1.1 ± 0.1 0.4 ± 0.2 R 5.0 ± 0.5 8.1 ± 1.1 1.9 ± 0.1 2.6 ± 0.3 1.0 ± 0.1 0.3 ± 0.2 Uphill intervals C 6.1 ± 1.5 6.6 ± 1.3 6.3 ± 0.7 1.9 ± 0.4 R 9.4 ± 1.5 8.9 ± 1.3 7.8 ± 0.7 2.1 ± 0.4 Cross country jog C 5.5 ± 0.3 6.9 ± 0.6 3.9 ± 1.4 2.3 ± 0.5 0.3 ± 0.2 0.5 ± 0.2 R 6.0 ± 0.3 5.5 ± 0.6 4.6 ± 1.4 2.8 ± 0.4 0.25 ± 0.2 0.0 ± 0.2 Total training C 7.0 ± 0.8 21.4 ± 2.4 22.6 ± 1.0 17.1 ± 2.1 13.8 ± 1.7 13.8 ± 2.3 17.9 ± 1.8 11.5 ± 1.6 R 6.1 ± 0.8 21.6 ± 2.4 26.6 ± 1.0 21.5 ± 2.1 18.5 ± 1.7 18.8 ± 2.3 19.3 ± 1.8 8.6 ± 1.6 Total training All 6.6 ± 0.6 21.5 ± 1.7 24.6 ± 0.7 19.3 ± 1.4 16.1 ± 1.2 16.3 ± 1.6 18.6 ± 1.3 10.1 ± 1.1 Number of sessions performed per horse as heat training (including 1–3 times 1,600 m [C-group] or 1–3 times 1,100 m [R-group], 2,000 m tests, V –tests [4 times 1,000 m] and races [bonus race, qualification race La4 and official races]), intervals on track (500–700 m [C: 6 times, R: 4 times]), uphill intervals (5% incline, 600 m [C: 6 times, R: 4 times]), cross country jogs and total number of training sessions including heat training, intervals on track, uphill intervals and cross country jogs for 8 horses subjected to a control training program (C) and 8 horses subjected to a reduced (by 30% distance, R) training program from March as 2-year-olds to December as 3-year-olds (lsmeans ± SE). High intensity training was scheduled ~2 times/week but horses could miss training due to health problems, lack of staff or poor weather and track conditions. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 5 of 13 training for as long as he thought was needed. Training October and December 2012). Before the test started, could also be cancelled due to poor weather and track horses warmed up with 3,000 m of slow trot (5.6 m/s) conditions or lack of staff. According to a routine that finished with a ~ 200 m fast trot, followed by slow trot check list including drivers’ opiniononfitness and for another 1,000 m. The V -test comprised 4 intervals La4 general condition, horses were sent for a veterinary of 1,000 m with increasing speed (bout 1: 10.3 ± 0.4 m/s, check. After longer pauses (>3 weeks) training then bout 2: 10.7 ± 0.4 m/s, bout 3: 11.1 ± 0.4 and bout 4: started with light exercise until the horse was consid- 11.4 ± 0.4 m/s [mean ± SD]) with the aim of determining ered ready to join the training program of the group V between bouts 2 and 3. Within 1 min after each La4 where it belonged. Planned periods of rest for all bout a blood sample was collected from the jugular vein horses due to practical reasons for the trainer and staff in 7 mL lithium heparin-tubes . The mean ± SD blood occurred for one week in April and 1.5 weeks in July and lactate concentrations were for bout 1: 2.8 ± 1.2 mmol/l, October 2011 as 2-year-olds and one week in January, bout 2: 3.6 ± 1.5 mmol/l, bout 3: 5.6 ± 1.9 mmol/l and March, June and December 2012 as 3-year-olds. for bout 4: 8.7 ± 2.0 mmol/l. Immediately after the blood To reflect the Swedish Standardbred racing industry, sampling, the horses were jogged for 750 m, ~3 min, an overall goal for horses from both groups was to start and then started the next bout. The tests were per- competing in races at latest in the last 6 months of the formed on a 1,000 m race track and the horses were 2012 year’s season as 3-year-olds. When horses were tested in groups of 2–7 horses, balanced between C and considered to be adequately trained (trainer’s opinion) Rhorses. they participated in a preparation race (2,140 m, velocity standardized to 10.5 - 11.4 m/s) as 2-year-olds, a qualifi- Blood samples and analyses cation race (required for permission to participate in Blood samples were immediately stored cooled and official races 2,140 m, > 11.8 m/s) as 3-year-olds and analyzed within 20 min for lactate with a Lactate Pro also in official races (1,600 - 2,140 m) as 3-year-olds. For analyzer , validated for use in horses at lactate concen- the purposes of this study, these races were classified as trations ≤ 12 mmol/L [23]. Hematocrit was determined heat exercise when calculating the amount of different in the last blood sample in the V -test and all samples La4 types of training. from the 1,600 m-test by centrifugation of capillary tubes for 2 minutes at 14,000 X g at ~15°C. Standardized exercise tests Two types of standardized exercise tests were performed. Heart rate recording The first was a single bout of 1,600 m (1,600 m-test), since Heart rate and track speeds during all training sessions the shortest race distance in Sweden is 1,640 m, with the and exercise tests were measured using heart rate purpose to monitor training response [25]. The test was monitors set to record at 1 Hz and synchronized with a performed 5 times when horses were 2 years old (March, GPS (Polar CS600X and G5 GPS sensor, Polar Electro, May, July, August and December 2011) and 5 times when Finland). Data was downloaded and an analysis provided they were 3 years old (April, June, August, October and by Polar ProTrainer 5 Equine Edition Software (Polar December 2012). Before each test the horses did a warm Electro, Finland) was used. From HR recordings, only sec- up consisting of 3,000 m of slow trot (5.6 m/s), ~ 200 m tions where HR was > 180 bpm were used and total time fast trot (free speed, to make sure the horse was alert), with HR > 180 bpm, total distance with HR > 180 bpm and then slow trot for 1,000 m before the test started. The and average speed when HR > 180 bpm were calculated. test aimed for a steady speed of 10.8 m/s, with the excep- Also, HR10 min after the intensive exercise and the tion of the first test which was performed in March 2011 highest recorded HR for each horse and period was prior to the reduced training intensity for R horses which determined from the HR curves. Curves with missing was run at a speed of 10.0 m/s (near maximal speed that values of more than 20 s in the span > 180 bpm were could be maintained at trot for many individuals at that discarded. After removal of records of poor quality, 1,093 time). There were no differences between groups for speed curves remained. in the 1,600 m-test (10.6 ± 0.0 m/s for C and 10.7 ± 0.0 m/s Data were categorized by the type of exercise; heat for R). Within 1 min after the test was finished, a blood training, interval training on flat ground and uphill sample was collected in 7 mL lithium heparin-tubes and interval training. After the 1,600 m-test an average HR th thehorsesjoggedfor 1,000mandthenreturnedtothe for the 10 min after passing the finish line was calcu- stable. lated from the HR-meter output (HR10). Heart rate at The second exercise test was performed to determine 10 min was chosen because of the very rapid decline in the velocity at blood lactate concentration of 4 mmol/L HR within 10 min after exercise and for practical rea- (V ) for each individual [26]. The V -test was per- sons, i.e. horses had returned to the stable and HR mea- La4 La4 formed 5 times as 3-year-olds (May, July, August, surements could be made indoors, while standing still in Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 6 of 13 a calm environment. HR at V was estimated from the used mainly consisted of Meadow fescue, Timothy and La4 last 30 s of each 1,000 m bout in the V -tests and cal- Ryegrass. In total 5 batches were used during the study La4 culated using exponential regression. (47–71% DM content, 10.3-11.7 MJ ME and 93–164 g CP st nd Resting HR was recorded by 2 different methods in per kg of DM), both 1 and 2 cuts. The diet was com- January as 2- and 3-year-olds and in December as 3- plemented with 0.25 - 1 kg (depending on haylage nutri- year-olds. In method 1, HR was determined by ausculta- ent content) of a pelleted Lucerne product to enhance tion with a phonendoscope in the evening with no other mineral supplement intake and meet the CP requirements activities in the stable, thus securing a calm and well [32]. A commercial mineral supplement was used to meet known environment for the horses. In method 2, horses vitamin and mineral requirements except for between were fitted with HR meters (Polar CS600X) and HR re- September 2011 as 2-year-olds and May 2012 as 3-year- corded during night time when quiet. From the HR- olds when the only supplement used was a selenium and meter output the lowest mean of at least 10 min of a Vitamin E product due to sufficient content of Ca, P and consecutive recording were used as resting HR. Mg in haylage during this period. Sodium chloride was al- ways provided from salt blocks placed in a feed box within Cardiovascular assessments and plasma aldosterone each stall and from September year as 2-year-olds an add- When horses were at rest, measurement of blood vol- itional 15 g of NaCl was fed together with lucerne and ume were performed in December 2011as 2-year-olds other minerals due to expected increased sweat losses. using Evans blue dye dilution as described previously Water was offered from two 20 L buckets in the stall that [27] and a hematocrit value obtained from the last V were refilled twice a day, and also provided from a big tub La4 sample (V test performed the same day). Echocardio- in the outdoor paddock. La4 d 2 grams were recorded using ultrasound and a 1.7-3 mHz Horses were stabled individually in 9 m boxes for ap- probe in January as 2-year-olds, December as 2-year-olds proximately 8 hours / day and spent the rest of the time and December as 3-year-olds. Left Ventricular Internal in an outdoor paddock (approximately 20 000 m ) with Diameter during diastole (LVIDd) and aorta diameter access to shelters. Wood shavings were used as litter in were recorded and calculated as recommended by boxes and shelters. Hoof trimming and shoeing was th Pattesson [28]. performed every 5 - 6 week, and during wintertime Respiratory rate (RR) was recorded at the same time (October/November-March) ice chalks were mounted as resting HR by auscultation with a phonendoscope and to the shoes permanently. visual inspection. Blood pressure (BP) was measured at rest with high definition oscillometric (HDO) technique Calculations and statistical analyses (S + B med VET GmbH, Germany) with a cuff placed at All statistical analyses were performed using Statistical the root of the tail. Analysis Systems package 9.3 (SAS Institute Inc., Cary, Plasma aldosterone concentration was analysed to NC, USA). From HR-recordings of training sessions, time, assess sodium status which could affect cardiovascular distance and speed where HR > 180 bpm, HR10 min post responses. The concentration was determined after ex- exercise and maximum HR were analyzed for differences traction of fat and proteins (acetone and petroleum between groups (control or reduced), periods (1–8asde- ether extraction) by use of a radioimmunoassay kit scribed in the training section) and interaction of group (Coat-a-Count, DPC, Los Angeles, CA), earlier used in and period using a mixed model (proc mixed). We also horses [29,30]. The samples for the standard curve modelled correlations between repeated measurements of were extracted in the same way. The quality control individuals using the model: was run using MultiCalc software version 2.0 (Wallac, Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ e ij k i ij k j k jk Turku, Finland). The within and between assay variation was < 10%. Recovery in diluted samples was 95.3%. There where Y is the observation, μ the mean value, a the ij k i was no difference in plasma aldosterone concentration be- effect of individual, β the effect of group, γ the effect of j k tween groups (C-group: 244 ± 37 pmol/L and R-group: period, and e the residuals with a spatial power cor- ij k 264 ± 37 pmol/L, P> 0.05). relation structure between periods (SP-POW). The number of training sessions performed by each Feeding and management horse was also analyzed in a mixed model for the same Horses were fed grass haylage ad libitum and had been effects using an unstructured covariance structure (UN): adapted to ad lib feeding for 4 months prior to the study. Average body condition score according to Henneke et al. Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ e ij k i ij k j k jk [31] (scale 1–9) were maintained around 5 for the whole study period. From the end of April until October some where Y is the observation, μ the mean value, a the ij k i grass pasture was available in the paddock. The haylage effect of individual, β the effect of group, γ the effect of j k Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 7 of 13 period, and e the residuals where all covariance’s be- errors, P values in text are from ANOVA tables if nothing ij k tween periods are estimated separately. else is declared. To assess the effect of training status on exercise test variables, the number of training sessions that each indi- Results vidual performed during the last 3 weeks before each Training with HR > 180 bpm exercise test was summarized and classified as 1: 0–2 As expected, C horses initially exercised for a longer dis- training sessions, 2: 3–5 training sessions and 3: ≥ 6 tance with a HR > 180 bpm in heat exercise (P< 0.05), training sessions. Training status was then included as a flat ground interval exercise (P< 0.0001) and uphill fixed effect in the models for V and 1,600 m-test. interval exercise (P< 0.0001) but after 6–9 months, time La4 From the V -test, blood lactate concentration (mmol/L) with HR > 180 bpm decreased in C and was no longer La4 was plotted against speed (m/s) and exponential regres- significantly different from R (P > 0.05). Overall, the C sion analyses were used to estimate individual V (MS horses exercised for a longer time (Figure 1) with La4 Office Excel 2010). V and hematocrit from the last HR > 180 bpm in the interval training sessions (flat; La4 blood sample in the V -test, and blood lactate, HR10 P< 0.001 and uphill; P< 0.01) but there were no overall La4 and hematocrit from the 1,600 m-tests were analyzed for differences between groups in the heat exercise or cross differences between groups and occasions when the test country jogs (P > 0.05). However, for heat exercise there was performed by the following model: was an interaction of group and period where C horses exercised with a HR > 180 bpm for a longer time in period Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ δ th ij klm i l j k jk 2and 3 (19 of March-end of September 2011as 2-year- þ η þ e olds) than R. The speed when HR >180 bpm was the same ijk l m for both groups during interval exercise, but during heat where Y is the observation, μ the mean value, a ijk l m i exercise C horses had a higher speed (9.2 ± 0.1 m/s vs. the effect of individual, β the effect of group, γ the j k 8.9 ± 0.1 m/s, P< 0.001). effect of occasion, δ the effect of speed (1,600 m-test The highest recorded HR did not differ between only) η the effect of training status and e the m ijklm groups (225 ± 2 bpm vs. 227 ± 2 bpm for C and R, respect- residuals following an ARH(1), i.e. an autoregressive ively, P> 0.05), and peak HRs were lower in the first structure with heterogeneous variances, for the different 3 months than in the last 18 months (216 ± 3 and occasions. 227–231 ± 3 bpm, respectively, P< 0.01). In heat exer- Resting HR, RR and BP were analyzed for differences cise, highest recorded HR were in C 213 ± 1 bpm as between groups using the first value as in January 2011 2-year-olds and 223 ± 1 bpm as 3-year-olds and in R as 2-year-olds as a covariate in the following model: 216 ± 1 bpm as 2-year-olds and 223 ± 1 bpm as 3-year- olds. In the interval exercise highest recorded HR were Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ δ þ e ij kl i l ij kl j k jk 216 ± 2 bpm in C and 219 ± 1 bpm in R and in the uphill where Y is the observation, μ the mean value, a the interval exercise it was 221 ± 1 bpm in C and 224 ± 1 bpm ijkl i effect of individual, β the effect of group, γ the effect of in R, respectively. j k age, δ the effect of starting value as 1-year-olds and e l ijkl the residuals following an ARH(1). Lactate response and hematocrit For all the above analyses, if the overall effect of There were no effects of group, interaction of group and interaction between group and occasion was P < 0.1, a occasion or training status (number of training sessions separate analysis with this excluded was run including 3 weeks prior to test) on blood lactate concentration fol- the effects of age and group alone. If no overall effect of lowing the 1,600 m test (Figure 2). However, there was a interaction was present, P values for effect of group is significant effect of occasion and lactate was lower in reported from this analysis. For differences between October 2012 as 3-year-olds for both groups and also in periods for all horses, data were also analyzed with the December 2012 for C but not R compared with the first effect of group excluded. test in March 2011 as 2-year-olds. There was no differ- Differences between groups in plasma volume, aldos- ence in V between groups and the highest V was La4 La4 terone and mean HR at V for 13 horses (due to miss- observed in July to August 2012 (Table 3). La4 ing values in 3) were analysed using GLM procedure. The hematocrit increased during the study (Figure 2 Also, a Pearson’s correlations test was performed where and Table 3) and was higher overall in C than in R all horses were treated as one group to study possible both after the V -test (56 ± 0 and 55 ± 1, respectively, La4 correlations between lactate, HR10 and hematocrit for P< 0.05) and after the 1,600 m-test (Figure 2, P< 0.05). In the 1,600 m-tests. the 1,600 m-test there was also an effect of occasion Differences were considered as significant if P< 0.05. where haematocrit in all horses increased from 50 ± 1% in Values are presented as least squared means ± standard March 2011 as 2-year-olds to 59 ± 1% in December 2012 Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 8 of 13 A B C Figure 1 High intensity training time and distance. Mean time and distance per training session with heart rate (HR) > 180 bpm in 16 horses subjected either to a control training program (squares) or a reduced (by 30% distance) training program (triangles) from the middle of March as 2-year-olds until December as 3-year-olds. Training was performed as heat training (A), interval training on flat track (B) or uphill interval training (C). * Indicates a difference (P < 0.05) between the control and reduced group. Unfilled dots indicate a significant difference within group from (A) January-June as 2-year-olds, (B) from July-September as 2-year-olds and (C) from January-March as 3-year-olds. as 3-year-olds but there was no interaction between group There was no difference in HR at V between groups La4 and occasion and no effect of training status in either of (C: 211 ± 3 bpm; R: 217 ± 3, P >0.05). the tests (Figure 2 and Table 3). Blood volume, echocardiogram and blood pressure Recovery heart rate, resting HR and HR at V Blood volume was similar in both groups (C: 55.9 ± 1.5 L La4 The HR10 following the 1,600 m-test was lower in C and R: 54.8 ± 1.5 L, P> 0.05). than in R (P< 0.0001) and in August, October and There was a tendency (P< 0.1) for an interaction of December 2012 as 3-year-olds compared with the group and age for diameter of aorta /100 kg BW but no starting value in March 2011 as 2-year-olds (Figure 3). overall effect of group or age (Table 4). The left For both groups, HR10 following the 1,600 m-test, ventricular internal diameter during diastole (LVIDd) compared to values in March as 2-year-olds, was lower was not affected by group or age. There was no differ- in October as 3-year-olds and in December as 3-year- ence in RR between groups, but in R the RR was lower olds forConly. The HR10 afteruphillinterval exercise as 2-year-olds than as 1-year-olds (Table 4). There were was higher for R (P< 0.05). The HR10 after heat exercise no differences between groups in systolic BP, diastolic was higher in R (P < 0.0001) with significant differences BP or mean arterial pressure (Table 4). When all horses for all periods except 1, 2 and 5. In the 1,600 m-test, were analyzed as one group, the RR and diastolic BP HR10 was positively correlated with blood lactate concen- decreased from the age of 1 year until the age of tration (r =0.28, P< 0.05) and negatively correlated with 2 years (11 ± 1 and 9 ± 0 breaths/min and 72 ± 2 and hematocrit (r = − 0.41, P< 0.0001). There were no effects 66 ± 2 mmHg, respectively). of training status on HR10 following any of the exercise regimens. Discussion The resting HR was higher in R than in C, both over- General all and as 3-year-olds (P< 0.05) when both method 1 This study shows that reducing the training distance by (auscultation) and method 2 (HR-meter) were used. In 30% did not affect muscle aerobic capacity, assessed as both groups, resting HR decreased from 1-year-olds to V , by the time horses were expected to race as 3- La4 2- and 3-year-olds when measured with method 1 but year-olds. The training programs used were also efficient only in C when measured with method 2 (Table 4). in terms of getting horses fit for race as horses from Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 9 of 13 Figure 2 Lactate and hematocrit response. Lactate (mmol/L blood) and hematocrit (%) within 1 minute after a 1,600 m-test at an oval racetrack in 16 horses divided in a control (squares) and a reduced (by 30% distance, triangles) training program since March as 2-year-olds. * Indicate difference (P < 0.05) between the control and reduced group. Unfilled dots indicate a significant difference from the starting point in March as 2-year-olds within group. Number of participating horses in each test: 12, 8, 15, 11, 15, 12, 11, 13, 13, 12. both groups were able to pass a national qualification race and preparation race to a higher extent than the rest of the cohort of 2009 in training. Experimental horses also participated in true races as 3-year-olds to the same extent as the rest of the cohort [15]. However, the C horses showed, already within the first 2–8 months of training, more pronounced cardiovascular training responses indicated by the increased hematocrit Table 3 Velocity at blood lactate concentration 4 mmol/l (V ) and haematocrit in V -tests La4 La4 Month as 3-year-olds N V (m/s) Hematocrit (%) La4 a a May 14 10.7 ± 0.1 54 ± 1 b a July 10 11.0 ± 0.1 55 ± 1 Figure 3 Recovery heart rate. Heart rate 10 min after finishing b a August 11 11.0 ± 0.1 56 ± 0 1,600 m-tests (A), number of participating horses in each test: 12, 8, 15, 11, ab b October 10 10.8 ± 0.1 57 ± 1 15, 12, 11, 13, 13, 12,), after training sessions performed as heat training a a (B), interval training on flat track (C) and uphill interval training (D) in 16 December 9 10.6 ± 0.1 55 ± 1 horses divided in a control (squares) and a reduced (by 30% distance, P-value 0.0215 0.2504 triangles) training group since March as 2-year-olds * Indicate difference a, b Means within a column with different superscript letters differ (P < 0.05). (P < 0.05) between the control and reduced group. Unfilled dots indicate V (m/s) and hematocrit (%) after four intervals of 1000 m in 16 3-year-old La4 a significant difference within group from (A) the starting point in March horses trained in either a control (C-group) or reduced (by 30% distance, R-group) as 2-year-olds, (B) from January-June as 2-year-olds, (C) from July- training program since March as 2-year-olds (lsmeans ± SE). N = number of horses September as 2-year-olds and (D) from January-March as 3-year-olds. participating in each test. There were no differences between groups within occasions. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 10 of 13 Table 4 Left ventricle- and aortic diameter, blood pressure and resting heart rate Age January 2 year December 2 year December 3 year Group C R C R C R N8 8 8 8 8 7 LVIDd 2.34 ± 0.09 2.33 ± 0.09 2.40 ± 0.09 2.28 ± 0.09 Aortic diameter 1.70 ± 0.05 1.75 ± 0.05 1.75 ± 0.05 1.70 ± 0.05 2 a b b RR 11 ± 1 12 ± 1 9± 0 9 ±0 10 ± 1 11 ± 1 Systolic BP 116 ± 4 117 ± 4 116 ± 5 114 ± 5 120 ± 4 109 ± 5 Diastolic BP 72 ± 3 73 ± 3 64 ± 3 67 ± 3 66 ± 4 67 ± 4 Mean arterial BP 87 ± 3 89 ± 3 83 ± 4 84 ± 4 86 ± 4 82 ± 4 4 A a B b B b Resting HR1 47 ± 2 50 ± 2 41 ± 2 41 ± 2 35 ± 1 41 ± 1 * 4 A a B a B a Resting HR2 39 ± 1 38 ± 1 36 ± 1 39 ± 1 36 ± 1 40 ± 1 * Cm/100 kg BW. Breaths per minute. mmHg. beats per minute. *indicates a difference between groups within age (P< 0.05). a,A Different superscript letters indicates a difference (P< 0.05) within group between occasions, upper case letters = difference in C-group, lower case letters = difference in R-group. Left ventricle inner diameter diastole (LVIDd) and aortic diameter in 16 1- and 3-year-old Standardbred horses allocated to either a control (C) or a reduced (R) (by 30% distance) training program and resting heart rate determined with phonendoscope (Resting HR1) and HR-meters (Resting HR2), respiratory rate (RR), systolic blood pressure (BP), diastolic BP and mean arterial pressure at 1, 2 and 3 years of age in the same horses (lsmeans ± SE). and lower HR during and after exercise. These cardio- the size of the left ventricle in the present study is unclear vascular differences appear however, not to have any but may be due to a low number of observations. How- major impact on the goal to be fit for racing which is in ever, the aortic diameter showed a tendency to be larger accordance with earlier observations that V has a in C compared to R indicating increased central circula- La4 stronger correlation to race performance than HR dur- tory conductance. Reports of aortic adaptations to training ing exercise [3]. in horses are scarce but a larger aortic dimension has earl- ier been observed in human endurance athletes compared Effects on circulation to strength training athletes [41]. Amongst the tested cardiovascular variables, recovery There was also a more rapid adaptation of cardiovas- HR responded most rapidly to training distance and was cular response in C than R with respect to HR during within 2 months lower in C than in R. However, a sig- exercise. Within 6–9 months of training the time spent nificant decrease within groups was not observed until at HR > 180 bpm did not differ between groups, despite June 2012 as 3-year-olds and by the end of the study, the longer exercise distance in C. With heat training, both groups showed an improvement (reduction) of the C horses trotted at a higher speed than the R when 17–19 beats (~20%) following the 1,600 m-tests. The HR > 180 bpm, which shows the greater aerobic capacity rapid cardiovascular responses observed, especially in in C, than R, horses. This agrees with previous studies C, were consistent with earlier studies. Sixteen weeks showing that HR during exercise may be reduced by of training may increase maximal oxygen consumption 10–20 bpm in horses subjected to 4–10 weeks of train- by as much as 19% [33]. Part of the improvement ing with HR 150 bpm for 20–30 min [42] or 4–10 min might be due to the increased hematocrit where a 10% of interval training [43]. The comparatively fast and increase was associated with a 5% higher circulating marked effects on HR during exercise observed in these O volume [34]. Possible reasons for a decreased HR studies might be due to a low initial level of fitness, include increased stroke volume [35] and cardiac output since the rate of improvement seems to decline the [36], increased plasma volume [37], increased capillary more fit the horse gets [26,44]. Evans and Rose [36] density of skeletal muscles [38] and increased muscle oxi- showed no change in exercise HR response during dative capacity with increased fiber type IIA/IIB ratio [39]. 7 weeks of training at mean velocities of 4–8.3 m/s and In the present study there was no support for an increased Lindner et al.[45]did notfindany improvementinthe cardiac volume based on the size of the left ventricle, nor velocity at HR 180 bpm in horses trained at a blood of blood volume. This is in contrast to Buhl and Ersbøll lactate concentration of 10 mmol/L for 6 weeks, in [40] where an increase in the size of the left ventricle with detrained and untrained horses. In the present study, training and age was reported for Standardbred horses at horses had been in training for 6 months prior to the the age of 2–3.5 years. The reason for lack of response in first test and a more marked HR reduction could Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 11 of 13 probably not be expected. The present study also to follow the development of a horse. Measuring recov- showed a clear effect of training on HR at rest. A reduc- ery HR may also provide a more reliable measurement tion in HR at rest with increased fitness is a well-known than submaximal heart rate as HR-meters may lose con- effect in human athletes [46] but the same response has tact for periods during exercise. In contrast, measure- not until very recently [47] been documented in horses ment of lactate in response to our 1,600 m-test appears and never in horses at the same age submitted to differ- to be of limited value to assess short-term training ent levels of training. For measurement of resting HR effects of young Standardbred horses since the concen- the use of HR meters during night time can be recom- tration did not decrease significantly until October 2012 mended since excitement, especially when horses were as 3-year-olds. This is similar to observations by Ronéus 1 year old, probably affected the manual measurements et al. [25] who measured lactate responses to a submaxi- with a stethoscope. mal work in Standardbred trotters at 24, 26, 29 and There was no effect of training on BP and RR which 40 months of age where a significant decrease was ob- agrees with earlier observations on horses subjected to served only at 40 months. It shall also be noted that the shorter training periods of 10–16 weeks [48,49]. post-exercise sample may not represent the maximal blood lactate response to the test since peak plasma lac- Effects on lactate response tate concentration may occur 0–10 min post exercise The ability to achieve a similar V with both training [53]. To test V as in the present study appears to be La4 La4 volumes demonstrates that the rapidity of skeletal muscle too imprecise to indicate training response. submaximal metabolic adaptation (i.e. oxidative capacity) Interestingly, lost training three weeks prior to exercise was not impaired by reduced distance of high intensity tests did not affect metabolic and cardiovascular re- training. This implies that the volume (duration) of the sponses significantly since there were no effects of train- training sessions may not be a crucial point in improving ing status (quantity of training three weeks prior to the V once volume is above an as yet undetermined thresh- tests) on the HR and lactate responses. Earlier studies La4 old. Once above the volume threshold, intensity (speed or on the effect of reduced activity on oxygen uptake, HR load; i.e. total muscle fibre recruitment) or repetitions and lactate responses to exercise are contradictory (number of training sessions [50]) are likely of greater [43,54-57]. The variation in responses between studies importance. might be due to the level of fitness in the horses, to the When training as 3-year-olds, there was an increase in volume of reduction and to a low number of observa- V in all horses during the summer and autumn period tions in each study. La4 compared to the first test performed in the spring but V then decreased in the last test performed in Conclusions La4 December. In the same month, higher lactate concentra- Horses subjected to a reduced distance of high intensity tion, hematocrit and recovery HR was observed with the training from the age of two showed an attenuated train- 1,600 m-test. These effects are probably attributed to add- ing effect on the cardiovascular system, but were able to itional stress associated with poorer training conditions maintain the same muscle metabolic system responses to during winter (altered track surface, heavier shoes on submaximal exercise and race participation as horses sub- horses and clothing of drivers increasing weight and air jected to a longer training distance. This implies that the friction for example), and possibly other reasons such as duration of training sessions, at least within the interval impaired health status. In general, field tests have been used in the present study, may not be a crucial point for shown to have a good reproducibility in French Standard- achieving race fitness and future studies need to investi- bred trotters when performed on the same track [51], such gate the importance of exercise intensity and number and as occurred in the present study. However, differences in frequency of training sessions. As is now common prac- the physiological responses to different tracks have been tice, measurement of recovery HR after a standardized test reported [52] which may be comparable to seasonal is recommended to monitor training progress. changes in track conditions in the present study. By test- ing equally many horses from each group at the same time Endnotes (all horses exercised in group), the risk of unbalanced con- Venosafe, Terumo Europe, Leuven, Belgium. ditions were minimized. Arkray Factory Inc., Koji Konan-cho, Koka, Shiga, Japan. Assessment of training progress Hemokrit 4, Lic Instruments, Stockholm, Sweden. This study confirms that both working and recovery HR Vivid 3, GE Healthcare, General Electrics, UK. are valuable measurements to detect training progress. 95% Lucerne, 5% molasses, Krafft AB, Sweden. Recovery HR is easy to measure with a stethoscope and 50-150 g of Miner Röd, content/kg: Ca, 110 g; P, 17 g; can therefore be used by trainers as part of their routine Mg, 60 g; NaCl, 125 g; Cu, 1 200 mg; Se, 15 mg; vitamin Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 12 of 13 A, 200,000 IU; vitamin D , 10,000 IU; and vitamin E, 7. Dyson PK, Jackson BF, Pfeiffer DU, Price JS. Days lost from training by two- and three-year-old Thoroughbred horses: A survey of seven UK 15,000 mg or 150 g of Miner Vit content/kg: Ca 55 g; P training yards. Equine Vet J. 2008;40:650–7. 65 g; Mg 60 g; NaCl 125 g; Cu 900 mg; Se 15 mg; vita- 8. Vigre H, Chriel M, Hesselholt M, Falk-Ronne J, Ersboll AK. Risk factors for the min A 100,000 IU; vitamin D 10,000 IU; and vitamin E hazard of lameness in Danish Standardbred trotters. Prev Vet Med. 2002;56:105–17. 5,000 mg, Krafft AB, Sweden. 9. Cogger N, Perkins N, Hodgson DR, Reid SWJ, Evans DL. Risk factors for Protect E-Selen, Lantmännen Lantbruk, Sweden. musculoskeletal injuries in 2-year-old Thoroughbred racehorses. Prev Vet Med. 2006;74:36–43. Abbreviations 10. Estberg L, Gardner IA, Stover SM, Johnson BJ, Case JT, Ardans A. Cumulative BP: Blood pressure; bpm: Beats per minute; C: Group of horses submitted to racing-speed exercise distance clusters as risk factor for fatal musculoskeletal a control training program; HR: Heart rate; LVIDd: Left ventricle internal injury in thoroughbred racehorses in California. Prev Vet Med. diameter at diastole; R: Group of horses submitted to a reduced distance 1995;24:253–63. training program; Resting HR1: HR determined by phonendoscope; Resting 11. Hamlin MJ, Hopkins WG. Retrospective trainer-reported incidence and HR2: HR determined from HR-meters; RR: Respiratory rate; V : Velocity at La4 predictors of health and training-related problems in standardbred blood lactate concentration of 4 mmol/litre. racehorses. J Equine Vet Sci. 2003;23:443–52. 12. Ringmark S, Roepstorff L, Essen-Gustavsson B, Revold T, Lindholm A, Competing interests Hedenstrom U, et al. Growth, training response and health in Standardbred All authors declare that they have no competing interests. yearlings fed a forage-only diet. Animal. 2013;7:746–53. 13. Gaustad G, Kjaersgaard P, Dolvik NI. Lameness in 3-year-old standard-bred Authors’ contributions trotters influence of parameters determined during the first year of life. SR participated in study design, collection and analysis of HR, lactate, and J Equine Vet Sci. 1995;15:233–9. hematocrit data, performed statistical analyses and drafted the manuscript. 14. Arnason T. Genetic evaluation of Swedish standard-bred trotters for racing AL participated in study design, collection of data and drafted the performance traits and racing status. J Anim Breed Gen. 1999;116:387–98. manuscript. UH participated in study design and data collection. ML 15. Swedish Trotting Association Database. [www.travsport.se] participated in the measurements of blood volume and has critically revised 16. Blodbanken Database. [www.blodbanken.nu] the manuscript. CK carried out the measurements of blood pressure, resting 17. Arnason T. Genetic evaluations, genetic trends and inbreeding in HR, left ventricle and aorta and the interpretation of these results. KD Scandinavian trotter populations. In: Annual meeting of EAAP; 29 August - 2 participated in the blood volume and aldosterone measurements. AJ September Stavanger, Norway. The Netherlands: Wageningen Academic conceived and designed the study, participated in data collection, data Publishers; 2011. p. 132. analysis and drafted the manuscript. All authors read and approved the final 18. Collinder E. Födelsetidens inverkan på travhästars avelsvärde. Sv Vet Tidning. manuscript. AL approved the manuscript in August 2014 before he passed 1999;15:703–5. away. 19. Saastamoinen MT, Ojala MJ. Influence of birth-month on age at 1st start and racing performance in young trotters. Acta Agr Scand. 1991;41:437–45. Acknowledgements 20. Dalin G, Magnusson LE, Thafvelin BC. Retrospective study of hindquarter The authors thank National Centre for Trotting Education, Wången, Sweden, asymmetry in standardbred trotters and its correlation with performance. for funding and staff and students at Wången for management of the Equine Vet J. 1985;17:292–6. horses. We are also grateful to Scandivet AB and Polar Finland for material 21. Magnusson LE, Thafvelin B. Studies on the conformation and related traits support and Trioplast AB and Dow Chemicals for financial support. We of standard-bred trotters in Sweden. J Anim Breed Gen. 1990;107:135–48. would also like to thank Claudia von Brömssen for statistical support. A great 22. Essengustavsson B, Lindholm A. Muscle-fiber characteristics of active and in- thank also to Stig H Johansson, Thomas Uhrberg, Roger Persson, Lars-Åke active standardbred horses. Equine Vet J. 1985;17:434–8. Svärdfeldt and Jan Halberg for contribution to the design of the training 23. Stefansdottir GJ, Ragnarsson S, Jansson A. A comparison of a portable blood program. lactate analyser and laboratory plasma analysis of blood samples from exercised horses. Comp Exerc Physiol. 2012;8:227–31. Author details 24. Marlin D, Nankervis K. Equine exercie physiology. Oxford, United Kingdom: Department of Animal Nutrition and Management, Swedish University of Blackwell Science Ltd.; 2002. 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Effects of training on blood lactate running speed relationship in thoroughbred racehorses. J Appl Physiol. 1994;77:298–302. 51. Dubreucq C, Chatard JC, Courouce A, Auvinet B. Reproducibility of a standardised exercise test for Standardbred trotters under field conditions. Equine Vet J. 1995;27:108–12. 52. Courouce A, Geffroy O, Barrey E, Auvinet B, Rose RJ. Comparison of exercise tests in French trotters under training track, racetrack and treadmill conditions. Equine Vet J. 1999; 31 Suppl 30: 528–532. Submit your next manuscript to BioMed Central 53. Gottlieb M, EssÉN-Gustavsson B, Lindholm A, Persson SGB. Circulatory and and take full advantage of: muscle metabolic responses to draught work compared to increasing trotting velocities. Equine Vet J. 1988;20:430–4. • Convenient online submission 54. Art T, Lekeux P. Training-induced modifications in cardiorespiratory and • Thorough peer review ventilatory measurements in thoroughbred horses. Equine Vet J. 1993;25:532–6. • No space constraints or color figure charges 55. Butler PJ, Woakes LS, Andersson LS, Smale K, Roberts CA, Snow DH. The • Immediate publication on acceptance effect of cessation of training on cardiorespiratory variables during exercise. • Inclusion in PubMed, CAS, Scopus and Google Scholar In: Persson SGB, Lindholm A, Jeffcott LB, editors. Equine Exercise Physiology 3. Davis, CA: ICEEP Publications; 1991. p. 71–6. • Research which is freely available for redistribution Submit your manuscript at www.biomedcentral.com/submit http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Acta Veterinaria Scandinavica Springer Journals

Reduced high intensity training distance had no effect on VLa4 but attenuated heart rate response in 2-3-year-old Standardbred horses

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Springer Journals
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Copyright © 2015 by Ringmark et al.; licensee BioMed Central.
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Medicine & Public Health; Veterinary Medicine
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10.1186/s13028-015-0107-1
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Abstract

Background: Training of Standardbred race horses aims to improve cardiovascular and metabolic functions but studies on the effects of different training strategies from breaking till racing are lacking. Sixteen horses with the goal to race as 3-year-olds were studied from breaking (1-year-olds) to December as 3-year-olds. Horses were allocated to either a control (C) or reduced (R) training program from 2 years of age. The aim was to evaluate the effect of reducing the distance of high intensity exercise by 30% with respect to velocity at lactate concentration 4 mmol/l (V ), blood La4 lactate and cardiovascular response. All training sessions were documented and heart rate (HR) was recorded. A standardized exercise test of 1,600 m was performed 10 times and a V test was performed five times. La4 Results: C horses initially exercised for a longer time with a HR >180 beats per minute compared to R horses (P <0.05) but after 6–9 months, time with HR >180 bpm decreased in C and were similar in the two groups (P >0.05). Over the 2-year period, recovery HR after the 1,600 m-test decreased in both groups but was within 2 months lower in C than in R(P < 0.05). C horses also had lower resting HR as 3-year-olds (P < 0.01) than R horses. In C, post exercise hematocrit was higher than in R (P < 0.05). There was a tendency (P < 0.1) towards a larger aortic diameter in C as 3-year-olds (C: 1.75 ± 0.05, R: 1.70 ± 0.05 cm/100 kg BW). Left ventricle diameter and blood volume (in December as 2-year-olds) did not differ between groups. There were no differences between groups in post exercise blood lactate concentration or in V . Both groups were equally successful in reaching the goal of participation in races. La4 Conclusions: Horses subjected to a reduced distance of high intensity training from the age of 2 showed an attenuated heart rate response, but were able to maintain the same V and race participation as horses La4 subjected to longer training distances. Keywords: Anaerobic threshold, Cardiovascular response, Hematocrit, Lactate, Heart rate Background and glycogen [5]. While all physiological systems are Standardbred horse race performance is positively corre- stressed in order to improve exercise performance and lated to V [1-3] and, in general, European Standardbred quality of musculoskeletal tissues, changes within each La4 horses are subjected to high intensity training for at least may also contribute to fatigue during acute exercise and 1 year before the competition career can begin. Training training [5,6] and (chronic) overload of musculoskeletal is associated with improvements in cardiovascular re- tissues. Amongst sport horses, lameness is a common sponses [4], thermoregulation, aerobic and glycolytic me- cause of a defaulted race career [7,8]. Interestingly, there tabolism, as well as reductions in the accumulation of have been no studies comparing the effects of different extracellular potassium, reduced depletion of muscle ATP training strategies applied from breaking to the start of the racing career on performance related variables in race * Correspondence: [email protected] horses. However, the volume of high intensity training is Deceased positively correlated with increased risk of musculoskeletal Department of Animal Nutrition and Management, Swedish University of problems in race horses [9-11]. Accordingly, one strategy Agricultural Sciences, SE-75007 Uppsala, Sweden Full list of author information is available at the end of the article © 2015 Ringmark et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 2 of 13 to reduce the risk of musculoskeletal overload injuries Training could be a reduction of the volume (distance, duration) of The training programs were designed together with a high intensity training. group of experienced professional trainers and included The aim of this study was to evaluate the effect on heat training, flat interval training and uphill interval V blood lactate and cardiovascular response of two training, which is common practice in Sweden. The La4, training distances (control and reduced by 30%) of high training track mainly used was a 1,000 m oval banked intensity training over a 21-month period in 2 year old sand track, 3 m incline that during wintertime was fro- Standardbred horses with the goal to race as 3-year-olds. zen and covered with crushed snow and ice. Throughout The control training program was designed together the study, the speed was set by the trainer and was the with group of professional trainers and estimated to be same in both groups (see below). Based on the correl- sufficient to get horses in to a condition to race by the ation between post-exercise lactate response and 10-min age of 3 years. recovery HR (y = 4,1513x + 55,944, R = 0.54) observed on these horses in March 2011, a maximal 10 min post- Methods exercise HR of 80 bpm was aimed for (corresponding to The study was performed at the Swedish National a blood lactate concentration less than 6 mmol/L; Trotting School, Wången, Sweden, and the horse use plasma lactate ≤ 8 mmol/L; [23]) and speed was adjusted and testing protocols approved by the Umeå Local according to this in all training sessions as 2-year-olds Ethics Committee according to the Swedish law of but not as 3-year-olds. This meant that if HR10 min post animal welfare. exercise were > 80 bpm in an individual the speed during the next session was decreased for that individual. As Horses there were individual variations of HR10 within training th Sixteen Standardbred geldings born between 19 of groups, two sub-training groups were created with th March and 15 of July in 2009 were used. The horses equally many horses from each training group. These were from four Swedish breeders, were progeny from groups were exercised at a speed adjusted so that all eight sires and were chosen with the criteria to be of horses should have a HR10 of max 80 bpm. The speed mainly American ancestors. The horses were taken was also adjusted due to track conditions (muddy track, care of and trained by high school students with super- lower speed). As 3-year-olds, speed was adjusted accord- vision from professional trainers. Prior to the study, all ing to the trainer’s judgement of the horses’ capability horses had received the same training which started but was always the same for group C and group R. with breaking in September 2010 as 1-year-olds [12]. In January 2011 as 2-year-olds, horses had recently All horses had been at the national training facility for been castrated and therefore training for 2–3 weeks was 2–4 months before the study started, and kept on the mostly walk and slow trot ~3 times/week. In February same diet and management system [12]. The study 2011, exercise in heats of 1,600 m on the oval banked started in January 2011 as 2-year-olds and from January to race track was introduced mixed with jogging 1–2 th the middle of March 2011 all horses were subjected to times/week. Beginning on the 18 of March 2011, all ex- thesametrainingprogram.Inthe middle of March ercise sessions that were expected to cause a heart rate 2011 horses were divided in two groups and one group (HR) > 180 beats per minute (bpm) had the exercise dis- was allocated to a control (C) training program and the tance reduced by approximately 30% for horses allocated other to a reduced (R) training program until December to the R-group. The HR limit of 180 bpm was chosen 2012 as 3-year-olds. The groups were balanced with since workloads producing a HR above that will stress respect to breeder and factors known to affect race both aerobic and anaerobic systems [24]. performance in Swedish Standardbred trotters. These High intensity training was performed ~2 times/week. include sire [13] estimated genetic potential (breeding Heat exercise (C: 1–3 times 1,600 m/training session, R: index calculated from the average of the parents [14,15], 1–3 times 1,100 m/training session, 10 min walk/jog be- percentage of French ancestors [16], inbreeding coeffi- tween bouts until October 2011 as 2-year-olds and cient [17], age [18,19], abnormal radiographic findings therefrom 4 min walk between bouts) was performed in metacarpal metatarsal and tarsal joints, conformation throughout the study (Table 2). Before heat exercise (subjectively determined through visual examinations: horses in both groups were warmed up by jogging x-shaped legs, asymmetric pelvis, tied in below the knee, 3,000 m at an approximate speed of 6 m/s. The mean i.e. leg circumference smaller below knee than above speed during heat exercise when horses had a fetlock), height at withers [20,21] and proportion of type HR > 180 bpm was 8.7 ± 0.1 m/s as 2-year-olds and 9.5 ± IIA / type IIB muscle fibers in musculus gluteus medius 0.1 m/s as 3-year-olds (P< 0.0001). After heat exercise [22] from a muscle biopsy taken in December 2010 as horses were jogged 1,200 m at an approximate speed of 1-year-olds (Table 1). 6 m/s before return to the stable. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 3 of 13 Table 1 Parameters used to balance training groups Group Horse Breeder Breeding Sire French, % Inbreeding Age, Height, cm X-ray Conformation IIA/IIB index coefficient, % days findings remark C 1 A 106.5 1 20.6 7.2 580 153 c 0 0.73 C 2 A 101 1 0 15.8 653 152 d 2 1.34 C 3 A 107.5 1 0 13.5 535 155 0 1.58 C 4 A 109.5 3 26.5 7 630 157.5 1 1.27 C 5 B 115.5 2 0 13.5 611 156 0 1.86 C 6 B 114 2 10.3 8.5 600 157 b 1 1.58 C 7 B 106.5 4 11.0 7.7 567 153 d 2 1.50 C 8 C - 5 10.2 11.1 587 154 a 1 0.83 Mean C-group 108.6 9.8 10.5 595 154.7 7* 1.34 R 9 A 107 1 0 1.8 585 155.8 c 1 1.23 R 10 A - 1 15.3 9.6 612 160 0 1.29 R 11 A - 1 26.5 5.7 623 150.5 a 1 1.31 R 12 D 113.5 6 0 12.6 569 152 a 1 0.95 R 13 B 113.5 2 0 9.2 631 152.5 d 0 2.35 R 14 B 105 2 10.3 8.4 605 161 1 1.08 R 15 B 115.5 7 0 14.9 565 152 0 0.98 R 16 C 103.5 8 0 11.5 604 158 b 1 1.56 Mean R-group 109.7 6.5 10.7 599 155.2 5* 1.34 X-ray performed in November as 1-year-olds on tarsal, metacarpal and metatarsal joints, horses with defects validated to similar severity received the same letter and were separated between the groups. * = Total number of remarks. Information used to allocate horses in to 2 groups (control: C and reduced: R). Groups were balanced to be as equal as possible with respect to breeder (A-D), breeding index (average of breeding index of parents, − if no breeding index for parents available), sire, part of French ancestors in pedigree, inbreeding st 1 coefficient, age (days in the 1 of January as 2-year-olds), height at withers (cm), x-ray findings conformation remarks (0 = no serious remark, 1–2 = remarks) and quote of percentage of type IIA / type IIB muscle fibers in musculus gluteus medius. From August 2011 as 2-year-olds, interval exercise and Saltin [6] (interval exercise: 5.3 ± 3.3 mmol/L after the th th (performed at straight parts of the oval track, 4 interval and 7.2 ± 5.6 mmol/L after the 6 interval, up- th 500 m/interval in August and September and there- hill interval exercise: 10.0 ± 1.4 mmol/L after the 4 inter- th after 700 m/interval, 1 min walk between each inter- val and 8.8 ± 1.2 mmol/L after the 6 interval). val [C: 6 times 500–700 m, R: 4 times 500–700 m]) On a few occasions, as a preparation for participation was also performed. The mean speed during interval in official races, heat exercise of 2,000 m was performed exercise on flat ground for both groups when horses by all horses, since most official races in Sweden are had a HR > 180 bpm were 8.3 ± 0.2 m/s as 2-year-olds 2,140 m. and 9.3 ± 0.2 m/s as 3-year-olds (P< 0.001). The warm Between training sessions both groups were also up before and jog after the intervals were the same as jogged on a mostly flat jog track (2-year-olds: 47 ± 8 ses- for heat exercise. Also, from February 2012 as 3-year- sions, 169 ± 175 s/session with HR > 180 bpm [n = 20], olds, uphill intervals (600 m, 5% incline, 30 m rise per 3-year-olds: 35 ± 12 sessions 38 ± 34 s/session with interval, 4 min walk/jog downhill between each interval HR > 180 bpm [n = 13] [mean ± SD per horse]) and at a [C: 6 intervals, 180 m rise, R: 4 intervals, 120 m rise]) cross country jog track (2-year-olds: 16 ± 4 sessions, were used. Before uphill intervals horses were jogged in 289 ± 122 s with HR > 180 bpm [n = 28], 3-year-olds: a hilly terrain at an approximate speed of 5 m/s for 3 ± 2 sessions, 524 ± 132 s with HR > 180 bpm [n = 26] 5,500 – 6,000 m. After the last interval horses were [mean ± SD]) depending on track conditions. walked 500 m back to the stable. During all interval ex- Equally many training sessions were planned for horses ercise, horses from both groups were exercised together in both groups and all training sessions (type and distance) for the first 4 intervals and then C horses continued for were documented in a protocol and summarized in eight 2 more intervals after the R horses had finished. For the 3-month periods with the exception of the first period th uphill interval exercise when HR > 180 bpm the speed was of 2.5 months that ended on the 17 of March 2011 as 7.3 ± 0.9 m/s. Lactate concentrations were used to confirm 2-year-olds (Table 2). Horses that according to the workloads recommended for interval training by Lindholm head trainer’s opinion were unhealthy were left out of Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 4 of 13 Table 2 Number of training sessions 2-year-olds 3-year-olds Group January-March April-June July-September October-December January-March April-June July-September October-December Heat training C 7.0 ± 0.8 15.9 ± 2.5 11.8 ± 1.3 6.6 ± 0.6 3.6 ± 0.5 5.1 ± 1.0 10.0 ± 1.3 9.3 ± 1.3 R 6.1 ± 0.8 15.6 ± 2.5 16.1 ± 1.3 8.8 ± 0.6 4.5 ± 0.5 7.0 ± 1.0 10.5 ± 1.3 6.3 ± 1.3 Intervals, flat C 4.0 ± 0.5 6.6 ± 1.1 1.8 ± 0.1 1.8 ± 0.3 1.1 ± 0.1 0.4 ± 0.2 R 5.0 ± 0.5 8.1 ± 1.1 1.9 ± 0.1 2.6 ± 0.3 1.0 ± 0.1 0.3 ± 0.2 Uphill intervals C 6.1 ± 1.5 6.6 ± 1.3 6.3 ± 0.7 1.9 ± 0.4 R 9.4 ± 1.5 8.9 ± 1.3 7.8 ± 0.7 2.1 ± 0.4 Cross country jog C 5.5 ± 0.3 6.9 ± 0.6 3.9 ± 1.4 2.3 ± 0.5 0.3 ± 0.2 0.5 ± 0.2 R 6.0 ± 0.3 5.5 ± 0.6 4.6 ± 1.4 2.8 ± 0.4 0.25 ± 0.2 0.0 ± 0.2 Total training C 7.0 ± 0.8 21.4 ± 2.4 22.6 ± 1.0 17.1 ± 2.1 13.8 ± 1.7 13.8 ± 2.3 17.9 ± 1.8 11.5 ± 1.6 R 6.1 ± 0.8 21.6 ± 2.4 26.6 ± 1.0 21.5 ± 2.1 18.5 ± 1.7 18.8 ± 2.3 19.3 ± 1.8 8.6 ± 1.6 Total training All 6.6 ± 0.6 21.5 ± 1.7 24.6 ± 0.7 19.3 ± 1.4 16.1 ± 1.2 16.3 ± 1.6 18.6 ± 1.3 10.1 ± 1.1 Number of sessions performed per horse as heat training (including 1–3 times 1,600 m [C-group] or 1–3 times 1,100 m [R-group], 2,000 m tests, V –tests [4 times 1,000 m] and races [bonus race, qualification race La4 and official races]), intervals on track (500–700 m [C: 6 times, R: 4 times]), uphill intervals (5% incline, 600 m [C: 6 times, R: 4 times]), cross country jogs and total number of training sessions including heat training, intervals on track, uphill intervals and cross country jogs for 8 horses subjected to a control training program (C) and 8 horses subjected to a reduced (by 30% distance, R) training program from March as 2-year-olds to December as 3-year-olds (lsmeans ± SE). High intensity training was scheduled ~2 times/week but horses could miss training due to health problems, lack of staff or poor weather and track conditions. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 5 of 13 training for as long as he thought was needed. Training October and December 2012). Before the test started, could also be cancelled due to poor weather and track horses warmed up with 3,000 m of slow trot (5.6 m/s) conditions or lack of staff. According to a routine that finished with a ~ 200 m fast trot, followed by slow trot check list including drivers’ opiniononfitness and for another 1,000 m. The V -test comprised 4 intervals La4 general condition, horses were sent for a veterinary of 1,000 m with increasing speed (bout 1: 10.3 ± 0.4 m/s, check. After longer pauses (>3 weeks) training then bout 2: 10.7 ± 0.4 m/s, bout 3: 11.1 ± 0.4 and bout 4: started with light exercise until the horse was consid- 11.4 ± 0.4 m/s [mean ± SD]) with the aim of determining ered ready to join the training program of the group V between bouts 2 and 3. Within 1 min after each La4 where it belonged. Planned periods of rest for all bout a blood sample was collected from the jugular vein horses due to practical reasons for the trainer and staff in 7 mL lithium heparin-tubes . The mean ± SD blood occurred for one week in April and 1.5 weeks in July and lactate concentrations were for bout 1: 2.8 ± 1.2 mmol/l, October 2011 as 2-year-olds and one week in January, bout 2: 3.6 ± 1.5 mmol/l, bout 3: 5.6 ± 1.9 mmol/l and March, June and December 2012 as 3-year-olds. for bout 4: 8.7 ± 2.0 mmol/l. Immediately after the blood To reflect the Swedish Standardbred racing industry, sampling, the horses were jogged for 750 m, ~3 min, an overall goal for horses from both groups was to start and then started the next bout. The tests were per- competing in races at latest in the last 6 months of the formed on a 1,000 m race track and the horses were 2012 year’s season as 3-year-olds. When horses were tested in groups of 2–7 horses, balanced between C and considered to be adequately trained (trainer’s opinion) Rhorses. they participated in a preparation race (2,140 m, velocity standardized to 10.5 - 11.4 m/s) as 2-year-olds, a qualifi- Blood samples and analyses cation race (required for permission to participate in Blood samples were immediately stored cooled and official races 2,140 m, > 11.8 m/s) as 3-year-olds and analyzed within 20 min for lactate with a Lactate Pro also in official races (1,600 - 2,140 m) as 3-year-olds. For analyzer , validated for use in horses at lactate concen- the purposes of this study, these races were classified as trations ≤ 12 mmol/L [23]. Hematocrit was determined heat exercise when calculating the amount of different in the last blood sample in the V -test and all samples La4 types of training. from the 1,600 m-test by centrifugation of capillary tubes for 2 minutes at 14,000 X g at ~15°C. Standardized exercise tests Two types of standardized exercise tests were performed. Heart rate recording The first was a single bout of 1,600 m (1,600 m-test), since Heart rate and track speeds during all training sessions the shortest race distance in Sweden is 1,640 m, with the and exercise tests were measured using heart rate purpose to monitor training response [25]. The test was monitors set to record at 1 Hz and synchronized with a performed 5 times when horses were 2 years old (March, GPS (Polar CS600X and G5 GPS sensor, Polar Electro, May, July, August and December 2011) and 5 times when Finland). Data was downloaded and an analysis provided they were 3 years old (April, June, August, October and by Polar ProTrainer 5 Equine Edition Software (Polar December 2012). Before each test the horses did a warm Electro, Finland) was used. From HR recordings, only sec- up consisting of 3,000 m of slow trot (5.6 m/s), ~ 200 m tions where HR was > 180 bpm were used and total time fast trot (free speed, to make sure the horse was alert), with HR > 180 bpm, total distance with HR > 180 bpm and then slow trot for 1,000 m before the test started. The and average speed when HR > 180 bpm were calculated. test aimed for a steady speed of 10.8 m/s, with the excep- Also, HR10 min after the intensive exercise and the tion of the first test which was performed in March 2011 highest recorded HR for each horse and period was prior to the reduced training intensity for R horses which determined from the HR curves. Curves with missing was run at a speed of 10.0 m/s (near maximal speed that values of more than 20 s in the span > 180 bpm were could be maintained at trot for many individuals at that discarded. After removal of records of poor quality, 1,093 time). There were no differences between groups for speed curves remained. in the 1,600 m-test (10.6 ± 0.0 m/s for C and 10.7 ± 0.0 m/s Data were categorized by the type of exercise; heat for R). Within 1 min after the test was finished, a blood training, interval training on flat ground and uphill sample was collected in 7 mL lithium heparin-tubes and interval training. After the 1,600 m-test an average HR th thehorsesjoggedfor 1,000mandthenreturnedtothe for the 10 min after passing the finish line was calcu- stable. lated from the HR-meter output (HR10). Heart rate at The second exercise test was performed to determine 10 min was chosen because of the very rapid decline in the velocity at blood lactate concentration of 4 mmol/L HR within 10 min after exercise and for practical rea- (V ) for each individual [26]. The V -test was per- sons, i.e. horses had returned to the stable and HR mea- La4 La4 formed 5 times as 3-year-olds (May, July, August, surements could be made indoors, while standing still in Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 6 of 13 a calm environment. HR at V was estimated from the used mainly consisted of Meadow fescue, Timothy and La4 last 30 s of each 1,000 m bout in the V -tests and cal- Ryegrass. In total 5 batches were used during the study La4 culated using exponential regression. (47–71% DM content, 10.3-11.7 MJ ME and 93–164 g CP st nd Resting HR was recorded by 2 different methods in per kg of DM), both 1 and 2 cuts. The diet was com- January as 2- and 3-year-olds and in December as 3- plemented with 0.25 - 1 kg (depending on haylage nutri- year-olds. In method 1, HR was determined by ausculta- ent content) of a pelleted Lucerne product to enhance tion with a phonendoscope in the evening with no other mineral supplement intake and meet the CP requirements activities in the stable, thus securing a calm and well [32]. A commercial mineral supplement was used to meet known environment for the horses. In method 2, horses vitamin and mineral requirements except for between were fitted with HR meters (Polar CS600X) and HR re- September 2011 as 2-year-olds and May 2012 as 3-year- corded during night time when quiet. From the HR- olds when the only supplement used was a selenium and meter output the lowest mean of at least 10 min of a Vitamin E product due to sufficient content of Ca, P and consecutive recording were used as resting HR. Mg in haylage during this period. Sodium chloride was al- ways provided from salt blocks placed in a feed box within Cardiovascular assessments and plasma aldosterone each stall and from September year as 2-year-olds an add- When horses were at rest, measurement of blood vol- itional 15 g of NaCl was fed together with lucerne and ume were performed in December 2011as 2-year-olds other minerals due to expected increased sweat losses. using Evans blue dye dilution as described previously Water was offered from two 20 L buckets in the stall that [27] and a hematocrit value obtained from the last V were refilled twice a day, and also provided from a big tub La4 sample (V test performed the same day). Echocardio- in the outdoor paddock. La4 d 2 grams were recorded using ultrasound and a 1.7-3 mHz Horses were stabled individually in 9 m boxes for ap- probe in January as 2-year-olds, December as 2-year-olds proximately 8 hours / day and spent the rest of the time and December as 3-year-olds. Left Ventricular Internal in an outdoor paddock (approximately 20 000 m ) with Diameter during diastole (LVIDd) and aorta diameter access to shelters. Wood shavings were used as litter in were recorded and calculated as recommended by boxes and shelters. Hoof trimming and shoeing was th Pattesson [28]. performed every 5 - 6 week, and during wintertime Respiratory rate (RR) was recorded at the same time (October/November-March) ice chalks were mounted as resting HR by auscultation with a phonendoscope and to the shoes permanently. visual inspection. Blood pressure (BP) was measured at rest with high definition oscillometric (HDO) technique Calculations and statistical analyses (S + B med VET GmbH, Germany) with a cuff placed at All statistical analyses were performed using Statistical the root of the tail. Analysis Systems package 9.3 (SAS Institute Inc., Cary, Plasma aldosterone concentration was analysed to NC, USA). From HR-recordings of training sessions, time, assess sodium status which could affect cardiovascular distance and speed where HR > 180 bpm, HR10 min post responses. The concentration was determined after ex- exercise and maximum HR were analyzed for differences traction of fat and proteins (acetone and petroleum between groups (control or reduced), periods (1–8asde- ether extraction) by use of a radioimmunoassay kit scribed in the training section) and interaction of group (Coat-a-Count, DPC, Los Angeles, CA), earlier used in and period using a mixed model (proc mixed). We also horses [29,30]. The samples for the standard curve modelled correlations between repeated measurements of were extracted in the same way. The quality control individuals using the model: was run using MultiCalc software version 2.0 (Wallac, Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ e ij k i ij k j k jk Turku, Finland). The within and between assay variation was < 10%. Recovery in diluted samples was 95.3%. There where Y is the observation, μ the mean value, a the ij k i was no difference in plasma aldosterone concentration be- effect of individual, β the effect of group, γ the effect of j k tween groups (C-group: 244 ± 37 pmol/L and R-group: period, and e the residuals with a spatial power cor- ij k 264 ± 37 pmol/L, P> 0.05). relation structure between periods (SP-POW). The number of training sessions performed by each Feeding and management horse was also analyzed in a mixed model for the same Horses were fed grass haylage ad libitum and had been effects using an unstructured covariance structure (UN): adapted to ad lib feeding for 4 months prior to the study. Average body condition score according to Henneke et al. Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ e ij k i ij k j k jk [31] (scale 1–9) were maintained around 5 for the whole study period. From the end of April until October some where Y is the observation, μ the mean value, a the ij k i grass pasture was available in the paddock. The haylage effect of individual, β the effect of group, γ the effect of j k Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 7 of 13 period, and e the residuals where all covariance’s be- errors, P values in text are from ANOVA tables if nothing ij k tween periods are estimated separately. else is declared. To assess the effect of training status on exercise test variables, the number of training sessions that each indi- Results vidual performed during the last 3 weeks before each Training with HR > 180 bpm exercise test was summarized and classified as 1: 0–2 As expected, C horses initially exercised for a longer dis- training sessions, 2: 3–5 training sessions and 3: ≥ 6 tance with a HR > 180 bpm in heat exercise (P< 0.05), training sessions. Training status was then included as a flat ground interval exercise (P< 0.0001) and uphill fixed effect in the models for V and 1,600 m-test. interval exercise (P< 0.0001) but after 6–9 months, time La4 From the V -test, blood lactate concentration (mmol/L) with HR > 180 bpm decreased in C and was no longer La4 was plotted against speed (m/s) and exponential regres- significantly different from R (P > 0.05). Overall, the C sion analyses were used to estimate individual V (MS horses exercised for a longer time (Figure 1) with La4 Office Excel 2010). V and hematocrit from the last HR > 180 bpm in the interval training sessions (flat; La4 blood sample in the V -test, and blood lactate, HR10 P< 0.001 and uphill; P< 0.01) but there were no overall La4 and hematocrit from the 1,600 m-tests were analyzed for differences between groups in the heat exercise or cross differences between groups and occasions when the test country jogs (P > 0.05). However, for heat exercise there was performed by the following model: was an interaction of group and period where C horses exercised with a HR > 180 bpm for a longer time in period Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ δ th ij klm i l j k jk 2and 3 (19 of March-end of September 2011as 2-year- þ η þ e olds) than R. The speed when HR >180 bpm was the same ijk l m for both groups during interval exercise, but during heat where Y is the observation, μ the mean value, a ijk l m i exercise C horses had a higher speed (9.2 ± 0.1 m/s vs. the effect of individual, β the effect of group, γ the j k 8.9 ± 0.1 m/s, P< 0.001). effect of occasion, δ the effect of speed (1,600 m-test The highest recorded HR did not differ between only) η the effect of training status and e the m ijklm groups (225 ± 2 bpm vs. 227 ± 2 bpm for C and R, respect- residuals following an ARH(1), i.e. an autoregressive ively, P> 0.05), and peak HRs were lower in the first structure with heterogeneous variances, for the different 3 months than in the last 18 months (216 ± 3 and occasions. 227–231 ± 3 bpm, respectively, P< 0.01). In heat exer- Resting HR, RR and BP were analyzed for differences cise, highest recorded HR were in C 213 ± 1 bpm as between groups using the first value as in January 2011 2-year-olds and 223 ± 1 bpm as 3-year-olds and in R as 2-year-olds as a covariate in the following model: 216 ± 1 bpm as 2-year-olds and 223 ± 1 bpm as 3-year- olds. In the interval exercise highest recorded HR were Y ¼ μ þ a þ β þ γ þ ðÞ βγ þ δ þ e ij kl i l ij kl j k jk 216 ± 2 bpm in C and 219 ± 1 bpm in R and in the uphill where Y is the observation, μ the mean value, a the interval exercise it was 221 ± 1 bpm in C and 224 ± 1 bpm ijkl i effect of individual, β the effect of group, γ the effect of in R, respectively. j k age, δ the effect of starting value as 1-year-olds and e l ijkl the residuals following an ARH(1). Lactate response and hematocrit For all the above analyses, if the overall effect of There were no effects of group, interaction of group and interaction between group and occasion was P < 0.1, a occasion or training status (number of training sessions separate analysis with this excluded was run including 3 weeks prior to test) on blood lactate concentration fol- the effects of age and group alone. If no overall effect of lowing the 1,600 m test (Figure 2). However, there was a interaction was present, P values for effect of group is significant effect of occasion and lactate was lower in reported from this analysis. For differences between October 2012 as 3-year-olds for both groups and also in periods for all horses, data were also analyzed with the December 2012 for C but not R compared with the first effect of group excluded. test in March 2011 as 2-year-olds. There was no differ- Differences between groups in plasma volume, aldos- ence in V between groups and the highest V was La4 La4 terone and mean HR at V for 13 horses (due to miss- observed in July to August 2012 (Table 3). La4 ing values in 3) were analysed using GLM procedure. The hematocrit increased during the study (Figure 2 Also, a Pearson’s correlations test was performed where and Table 3) and was higher overall in C than in R all horses were treated as one group to study possible both after the V -test (56 ± 0 and 55 ± 1, respectively, La4 correlations between lactate, HR10 and hematocrit for P< 0.05) and after the 1,600 m-test (Figure 2, P< 0.05). In the 1,600 m-tests. the 1,600 m-test there was also an effect of occasion Differences were considered as significant if P< 0.05. where haematocrit in all horses increased from 50 ± 1% in Values are presented as least squared means ± standard March 2011 as 2-year-olds to 59 ± 1% in December 2012 Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 8 of 13 A B C Figure 1 High intensity training time and distance. Mean time and distance per training session with heart rate (HR) > 180 bpm in 16 horses subjected either to a control training program (squares) or a reduced (by 30% distance) training program (triangles) from the middle of March as 2-year-olds until December as 3-year-olds. Training was performed as heat training (A), interval training on flat track (B) or uphill interval training (C). * Indicates a difference (P < 0.05) between the control and reduced group. Unfilled dots indicate a significant difference within group from (A) January-June as 2-year-olds, (B) from July-September as 2-year-olds and (C) from January-March as 3-year-olds. as 3-year-olds but there was no interaction between group There was no difference in HR at V between groups La4 and occasion and no effect of training status in either of (C: 211 ± 3 bpm; R: 217 ± 3, P >0.05). the tests (Figure 2 and Table 3). Blood volume, echocardiogram and blood pressure Recovery heart rate, resting HR and HR at V Blood volume was similar in both groups (C: 55.9 ± 1.5 L La4 The HR10 following the 1,600 m-test was lower in C and R: 54.8 ± 1.5 L, P> 0.05). than in R (P< 0.0001) and in August, October and There was a tendency (P< 0.1) for an interaction of December 2012 as 3-year-olds compared with the group and age for diameter of aorta /100 kg BW but no starting value in March 2011 as 2-year-olds (Figure 3). overall effect of group or age (Table 4). The left For both groups, HR10 following the 1,600 m-test, ventricular internal diameter during diastole (LVIDd) compared to values in March as 2-year-olds, was lower was not affected by group or age. There was no differ- in October as 3-year-olds and in December as 3-year- ence in RR between groups, but in R the RR was lower olds forConly. The HR10 afteruphillinterval exercise as 2-year-olds than as 1-year-olds (Table 4). There were was higher for R (P< 0.05). The HR10 after heat exercise no differences between groups in systolic BP, diastolic was higher in R (P < 0.0001) with significant differences BP or mean arterial pressure (Table 4). When all horses for all periods except 1, 2 and 5. In the 1,600 m-test, were analyzed as one group, the RR and diastolic BP HR10 was positively correlated with blood lactate concen- decreased from the age of 1 year until the age of tration (r =0.28, P< 0.05) and negatively correlated with 2 years (11 ± 1 and 9 ± 0 breaths/min and 72 ± 2 and hematocrit (r = − 0.41, P< 0.0001). There were no effects 66 ± 2 mmHg, respectively). of training status on HR10 following any of the exercise regimens. Discussion The resting HR was higher in R than in C, both over- General all and as 3-year-olds (P< 0.05) when both method 1 This study shows that reducing the training distance by (auscultation) and method 2 (HR-meter) were used. In 30% did not affect muscle aerobic capacity, assessed as both groups, resting HR decreased from 1-year-olds to V , by the time horses were expected to race as 3- La4 2- and 3-year-olds when measured with method 1 but year-olds. The training programs used were also efficient only in C when measured with method 2 (Table 4). in terms of getting horses fit for race as horses from Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 9 of 13 Figure 2 Lactate and hematocrit response. Lactate (mmol/L blood) and hematocrit (%) within 1 minute after a 1,600 m-test at an oval racetrack in 16 horses divided in a control (squares) and a reduced (by 30% distance, triangles) training program since March as 2-year-olds. * Indicate difference (P < 0.05) between the control and reduced group. Unfilled dots indicate a significant difference from the starting point in March as 2-year-olds within group. Number of participating horses in each test: 12, 8, 15, 11, 15, 12, 11, 13, 13, 12. both groups were able to pass a national qualification race and preparation race to a higher extent than the rest of the cohort of 2009 in training. Experimental horses also participated in true races as 3-year-olds to the same extent as the rest of the cohort [15]. However, the C horses showed, already within the first 2–8 months of training, more pronounced cardiovascular training responses indicated by the increased hematocrit Table 3 Velocity at blood lactate concentration 4 mmol/l (V ) and haematocrit in V -tests La4 La4 Month as 3-year-olds N V (m/s) Hematocrit (%) La4 a a May 14 10.7 ± 0.1 54 ± 1 b a July 10 11.0 ± 0.1 55 ± 1 Figure 3 Recovery heart rate. Heart rate 10 min after finishing b a August 11 11.0 ± 0.1 56 ± 0 1,600 m-tests (A), number of participating horses in each test: 12, 8, 15, 11, ab b October 10 10.8 ± 0.1 57 ± 1 15, 12, 11, 13, 13, 12,), after training sessions performed as heat training a a (B), interval training on flat track (C) and uphill interval training (D) in 16 December 9 10.6 ± 0.1 55 ± 1 horses divided in a control (squares) and a reduced (by 30% distance, P-value 0.0215 0.2504 triangles) training group since March as 2-year-olds * Indicate difference a, b Means within a column with different superscript letters differ (P < 0.05). (P < 0.05) between the control and reduced group. Unfilled dots indicate V (m/s) and hematocrit (%) after four intervals of 1000 m in 16 3-year-old La4 a significant difference within group from (A) the starting point in March horses trained in either a control (C-group) or reduced (by 30% distance, R-group) as 2-year-olds, (B) from January-June as 2-year-olds, (C) from July- training program since March as 2-year-olds (lsmeans ± SE). N = number of horses September as 2-year-olds and (D) from January-March as 3-year-olds. participating in each test. There were no differences between groups within occasions. Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 10 of 13 Table 4 Left ventricle- and aortic diameter, blood pressure and resting heart rate Age January 2 year December 2 year December 3 year Group C R C R C R N8 8 8 8 8 7 LVIDd 2.34 ± 0.09 2.33 ± 0.09 2.40 ± 0.09 2.28 ± 0.09 Aortic diameter 1.70 ± 0.05 1.75 ± 0.05 1.75 ± 0.05 1.70 ± 0.05 2 a b b RR 11 ± 1 12 ± 1 9± 0 9 ±0 10 ± 1 11 ± 1 Systolic BP 116 ± 4 117 ± 4 116 ± 5 114 ± 5 120 ± 4 109 ± 5 Diastolic BP 72 ± 3 73 ± 3 64 ± 3 67 ± 3 66 ± 4 67 ± 4 Mean arterial BP 87 ± 3 89 ± 3 83 ± 4 84 ± 4 86 ± 4 82 ± 4 4 A a B b B b Resting HR1 47 ± 2 50 ± 2 41 ± 2 41 ± 2 35 ± 1 41 ± 1 * 4 A a B a B a Resting HR2 39 ± 1 38 ± 1 36 ± 1 39 ± 1 36 ± 1 40 ± 1 * Cm/100 kg BW. Breaths per minute. mmHg. beats per minute. *indicates a difference between groups within age (P< 0.05). a,A Different superscript letters indicates a difference (P< 0.05) within group between occasions, upper case letters = difference in C-group, lower case letters = difference in R-group. Left ventricle inner diameter diastole (LVIDd) and aortic diameter in 16 1- and 3-year-old Standardbred horses allocated to either a control (C) or a reduced (R) (by 30% distance) training program and resting heart rate determined with phonendoscope (Resting HR1) and HR-meters (Resting HR2), respiratory rate (RR), systolic blood pressure (BP), diastolic BP and mean arterial pressure at 1, 2 and 3 years of age in the same horses (lsmeans ± SE). and lower HR during and after exercise. These cardio- the size of the left ventricle in the present study is unclear vascular differences appear however, not to have any but may be due to a low number of observations. How- major impact on the goal to be fit for racing which is in ever, the aortic diameter showed a tendency to be larger accordance with earlier observations that V has a in C compared to R indicating increased central circula- La4 stronger correlation to race performance than HR dur- tory conductance. Reports of aortic adaptations to training ing exercise [3]. in horses are scarce but a larger aortic dimension has earl- ier been observed in human endurance athletes compared Effects on circulation to strength training athletes [41]. Amongst the tested cardiovascular variables, recovery There was also a more rapid adaptation of cardiovas- HR responded most rapidly to training distance and was cular response in C than R with respect to HR during within 2 months lower in C than in R. However, a sig- exercise. Within 6–9 months of training the time spent nificant decrease within groups was not observed until at HR > 180 bpm did not differ between groups, despite June 2012 as 3-year-olds and by the end of the study, the longer exercise distance in C. With heat training, both groups showed an improvement (reduction) of the C horses trotted at a higher speed than the R when 17–19 beats (~20%) following the 1,600 m-tests. The HR > 180 bpm, which shows the greater aerobic capacity rapid cardiovascular responses observed, especially in in C, than R, horses. This agrees with previous studies C, were consistent with earlier studies. Sixteen weeks showing that HR during exercise may be reduced by of training may increase maximal oxygen consumption 10–20 bpm in horses subjected to 4–10 weeks of train- by as much as 19% [33]. Part of the improvement ing with HR 150 bpm for 20–30 min [42] or 4–10 min might be due to the increased hematocrit where a 10% of interval training [43]. The comparatively fast and increase was associated with a 5% higher circulating marked effects on HR during exercise observed in these O volume [34]. Possible reasons for a decreased HR studies might be due to a low initial level of fitness, include increased stroke volume [35] and cardiac output since the rate of improvement seems to decline the [36], increased plasma volume [37], increased capillary more fit the horse gets [26,44]. Evans and Rose [36] density of skeletal muscles [38] and increased muscle oxi- showed no change in exercise HR response during dative capacity with increased fiber type IIA/IIB ratio [39]. 7 weeks of training at mean velocities of 4–8.3 m/s and In the present study there was no support for an increased Lindner et al.[45]did notfindany improvementinthe cardiac volume based on the size of the left ventricle, nor velocity at HR 180 bpm in horses trained at a blood of blood volume. This is in contrast to Buhl and Ersbøll lactate concentration of 10 mmol/L for 6 weeks, in [40] where an increase in the size of the left ventricle with detrained and untrained horses. In the present study, training and age was reported for Standardbred horses at horses had been in training for 6 months prior to the the age of 2–3.5 years. The reason for lack of response in first test and a more marked HR reduction could Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 11 of 13 probably not be expected. The present study also to follow the development of a horse. Measuring recov- showed a clear effect of training on HR at rest. A reduc- ery HR may also provide a more reliable measurement tion in HR at rest with increased fitness is a well-known than submaximal heart rate as HR-meters may lose con- effect in human athletes [46] but the same response has tact for periods during exercise. In contrast, measure- not until very recently [47] been documented in horses ment of lactate in response to our 1,600 m-test appears and never in horses at the same age submitted to differ- to be of limited value to assess short-term training ent levels of training. For measurement of resting HR effects of young Standardbred horses since the concen- the use of HR meters during night time can be recom- tration did not decrease significantly until October 2012 mended since excitement, especially when horses were as 3-year-olds. This is similar to observations by Ronéus 1 year old, probably affected the manual measurements et al. [25] who measured lactate responses to a submaxi- with a stethoscope. mal work in Standardbred trotters at 24, 26, 29 and There was no effect of training on BP and RR which 40 months of age where a significant decrease was ob- agrees with earlier observations on horses subjected to served only at 40 months. It shall also be noted that the shorter training periods of 10–16 weeks [48,49]. post-exercise sample may not represent the maximal blood lactate response to the test since peak plasma lac- Effects on lactate response tate concentration may occur 0–10 min post exercise The ability to achieve a similar V with both training [53]. To test V as in the present study appears to be La4 La4 volumes demonstrates that the rapidity of skeletal muscle too imprecise to indicate training response. submaximal metabolic adaptation (i.e. oxidative capacity) Interestingly, lost training three weeks prior to exercise was not impaired by reduced distance of high intensity tests did not affect metabolic and cardiovascular re- training. This implies that the volume (duration) of the sponses significantly since there were no effects of train- training sessions may not be a crucial point in improving ing status (quantity of training three weeks prior to the V once volume is above an as yet undetermined thresh- tests) on the HR and lactate responses. Earlier studies La4 old. Once above the volume threshold, intensity (speed or on the effect of reduced activity on oxygen uptake, HR load; i.e. total muscle fibre recruitment) or repetitions and lactate responses to exercise are contradictory (number of training sessions [50]) are likely of greater [43,54-57]. The variation in responses between studies importance. might be due to the level of fitness in the horses, to the When training as 3-year-olds, there was an increase in volume of reduction and to a low number of observa- V in all horses during the summer and autumn period tions in each study. La4 compared to the first test performed in the spring but V then decreased in the last test performed in Conclusions La4 December. In the same month, higher lactate concentra- Horses subjected to a reduced distance of high intensity tion, hematocrit and recovery HR was observed with the training from the age of two showed an attenuated train- 1,600 m-test. These effects are probably attributed to add- ing effect on the cardiovascular system, but were able to itional stress associated with poorer training conditions maintain the same muscle metabolic system responses to during winter (altered track surface, heavier shoes on submaximal exercise and race participation as horses sub- horses and clothing of drivers increasing weight and air jected to a longer training distance. This implies that the friction for example), and possibly other reasons such as duration of training sessions, at least within the interval impaired health status. In general, field tests have been used in the present study, may not be a crucial point for shown to have a good reproducibility in French Standard- achieving race fitness and future studies need to investi- bred trotters when performed on the same track [51], such gate the importance of exercise intensity and number and as occurred in the present study. However, differences in frequency of training sessions. As is now common prac- the physiological responses to different tracks have been tice, measurement of recovery HR after a standardized test reported [52] which may be comparable to seasonal is recommended to monitor training progress. changes in track conditions in the present study. By test- ing equally many horses from each group at the same time Endnotes (all horses exercised in group), the risk of unbalanced con- Venosafe, Terumo Europe, Leuven, Belgium. ditions were minimized. Arkray Factory Inc., Koji Konan-cho, Koka, Shiga, Japan. Assessment of training progress Hemokrit 4, Lic Instruments, Stockholm, Sweden. This study confirms that both working and recovery HR Vivid 3, GE Healthcare, General Electrics, UK. are valuable measurements to detect training progress. 95% Lucerne, 5% molasses, Krafft AB, Sweden. Recovery HR is easy to measure with a stethoscope and 50-150 g of Miner Röd, content/kg: Ca, 110 g; P, 17 g; can therefore be used by trainers as part of their routine Mg, 60 g; NaCl, 125 g; Cu, 1 200 mg; Se, 15 mg; vitamin Ringmark et al. Acta Veterinaria Scandinavica (2015) 57:17 Page 12 of 13 A, 200,000 IU; vitamin D , 10,000 IU; and vitamin E, 7. Dyson PK, Jackson BF, Pfeiffer DU, Price JS. Days lost from training by two- and three-year-old Thoroughbred horses: A survey of seven UK 15,000 mg or 150 g of Miner Vit content/kg: Ca 55 g; P training yards. Equine Vet J. 2008;40:650–7. 65 g; Mg 60 g; NaCl 125 g; Cu 900 mg; Se 15 mg; vita- 8. Vigre H, Chriel M, Hesselholt M, Falk-Ronne J, Ersboll AK. Risk factors for the min A 100,000 IU; vitamin D 10,000 IU; and vitamin E hazard of lameness in Danish Standardbred trotters. Prev Vet Med. 2002;56:105–17. 5,000 mg, Krafft AB, Sweden. 9. Cogger N, Perkins N, Hodgson DR, Reid SWJ, Evans DL. Risk factors for Protect E-Selen, Lantmännen Lantbruk, Sweden. musculoskeletal injuries in 2-year-old Thoroughbred racehorses. Prev Vet Med. 2006;74:36–43. Abbreviations 10. Estberg L, Gardner IA, Stover SM, Johnson BJ, Case JT, Ardans A. Cumulative BP: Blood pressure; bpm: Beats per minute; C: Group of horses submitted to racing-speed exercise distance clusters as risk factor for fatal musculoskeletal a control training program; HR: Heart rate; LVIDd: Left ventricle internal injury in thoroughbred racehorses in California. Prev Vet Med. diameter at diastole; R: Group of horses submitted to a reduced distance 1995;24:253–63. training program; Resting HR1: HR determined by phonendoscope; Resting 11. Hamlin MJ, Hopkins WG. Retrospective trainer-reported incidence and HR2: HR determined from HR-meters; RR: Respiratory rate; V : Velocity at La4 predictors of health and training-related problems in standardbred blood lactate concentration of 4 mmol/litre. racehorses. J Equine Vet Sci. 2003;23:443–52. 12. Ringmark S, Roepstorff L, Essen-Gustavsson B, Revold T, Lindholm A, Competing interests Hedenstrom U, et al. Growth, training response and health in Standardbred All authors declare that they have no competing interests. yearlings fed a forage-only diet. Animal. 2013;7:746–53. 13. Gaustad G, Kjaersgaard P, Dolvik NI. Lameness in 3-year-old standard-bred Authors’ contributions trotters influence of parameters determined during the first year of life. SR participated in study design, collection and analysis of HR, lactate, and J Equine Vet Sci. 1995;15:233–9. hematocrit data, performed statistical analyses and drafted the manuscript. 14. Arnason T. Genetic evaluation of Swedish standard-bred trotters for racing AL participated in study design, collection of data and drafted the performance traits and racing status. J Anim Breed Gen. 1999;116:387–98. manuscript. UH participated in study design and data collection. ML 15. Swedish Trotting Association Database. [www.travsport.se] participated in the measurements of blood volume and has critically revised 16. Blodbanken Database. [www.blodbanken.nu] the manuscript. CK carried out the measurements of blood pressure, resting 17. Arnason T. Genetic evaluations, genetic trends and inbreeding in HR, left ventricle and aorta and the interpretation of these results. KD Scandinavian trotter populations. In: Annual meeting of EAAP; 29 August - 2 participated in the blood volume and aldosterone measurements. AJ September Stavanger, Norway. The Netherlands: Wageningen Academic conceived and designed the study, participated in data collection, data Publishers; 2011. p. 132. analysis and drafted the manuscript. All authors read and approved the final 18. Collinder E. Födelsetidens inverkan på travhästars avelsvärde. Sv Vet Tidning. manuscript. AL approved the manuscript in August 2014 before he passed 1999;15:703–5. away. 19. Saastamoinen MT, Ojala MJ. Influence of birth-month on age at 1st start and racing performance in young trotters. Acta Agr Scand. 1991;41:437–45. Acknowledgements 20. Dalin G, Magnusson LE, Thafvelin BC. Retrospective study of hindquarter The authors thank National Centre for Trotting Education, Wången, Sweden, asymmetry in standardbred trotters and its correlation with performance. for funding and staff and students at Wången for management of the Equine Vet J. 1985;17:292–6. horses. We are also grateful to Scandivet AB and Polar Finland for material 21. Magnusson LE, Thafvelin B. Studies on the conformation and related traits support and Trioplast AB and Dow Chemicals for financial support. We of standard-bred trotters in Sweden. J Anim Breed Gen. 1990;107:135–48. would also like to thank Claudia von Brömssen for statistical support. A great 22. Essengustavsson B, Lindholm A. Muscle-fiber characteristics of active and in- thank also to Stig H Johansson, Thomas Uhrberg, Roger Persson, Lars-Åke active standardbred horses. Equine Vet J. 1985;17:434–8. Svärdfeldt and Jan Halberg for contribution to the design of the training 23. Stefansdottir GJ, Ragnarsson S, Jansson A. A comparison of a portable blood program. lactate analyser and laboratory plasma analysis of blood samples from exercised horses. Comp Exerc Physiol. 2012;8:227–31. Author details 24. Marlin D, Nankervis K. Equine exercie physiology. Oxford, United Kingdom: Department of Animal Nutrition and Management, Swedish University of Blackwell Science Ltd.; 2002. 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Acta Veterinaria Scandinavica – Springer Journals

Published: Mar 20, 2015

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