Musculoskeletal Injuries Among Female Soldiers Working With Dogs

Musculoskeletal Injuries Among Female Soldiers Working With Dogs Abstract Introduction Female soldiers serving in the Israeli Defense Forces canine unit may be at increased risk of overuse injuries related to working with military dogs. We hypothesized that this particular type of occupational exposure may lead to an increased strain of the upper extremity due to such non-physiologic motions as pulling the dog’s strap or resisting the sudden pulling by the dog, and may result in an increased rate of overuse injuries. Materials and Methods We compared incidence of overuse injuries in a retrospective cohort of female soldiers who served either in the military working dogs’ unit (MWD), or in the light infantry battalions (Infantry) from 2005 to 2015. We compared injury incidence of both groups during two periods: 5 mo of basic training (neither worked with dogs) and 19 or more months of combat service. Incidence was calculated as number of diagnoses per person-months (rate ratios, RR); each diagnosis counted once per study subject. We used RR confidence intervals to compare incidence of injuries between groups. Results There were 3,443 person-months in the MWD group and 194,590 person-months in the Infantry group. There was no difference in injury incidence between groups during the initial period of basic training. During the second period, MWDs had higher incidence of upper limb (RR = 1.45, p = 0.048) and hip (RR = 3.6, p < 0.0001) injuries. The association between service with dogs and upper limb injuries remained significant (RR = 1.09, p = 0.005) after adjusting for confounding variables in the multivariate regression analysis. Other independent predictors of the upper limb overuse injuries included each additional month of service (RR = 1.003, p = 0.008), Eastern European origin compared with Israeli-born recruits (RR = 0.97, p = 0.03), increased body weight in increments of 10 kg (RR = 1.008, p = 0.03), anemia (RR = 1.02, p = 0.02) and fatigue (RR = 1.05, p ≤ 0.0001). Conclusions We found that service in the MWD unit was associated with increased risk of upper limb injuries. Identification of the exact mechanism of injury and targeted interventions, as well as treatment of anemia and fatigue may lead to reduction of injuries in this unit. INTRODUCTION Numerous studies have investigated the effect of dog ownership on emotional and physical well-being. It has been proposed that better health outcomes may be promoted by a healthier lifestyle associated with dog ownership.1 However, much of the evidence suggests that the better health outcomes of pet owners compared with people who have no pets, results from selection bias and can be explained by other factors, such as residential location and socio-economic status.2,3 Few studies, however, have attempted to measure pet owner musculoskeletal injuries associated with pets.4–7 We hypothesize that the cumulative mechanical load resulting from dog keeping activities, walking, squatting, handling a strap, sudden non-physiological motions provoked by the interaction with an active pet during training or walking may result in higher prevalence of overuse injuries and tendinopathies.8 This type of injury may be especially evident in the upper limbs, for which the additive “unnatural” biomechanical stresses related to dog keeping may be more significant than for lower limbs, mostly involved in locomotion and therefore accommodated to higher stresses. Holding a dog’s lead, when the dog walks before the owner, requires shoulder forward flexion with internal rotation, as well as elbow and wrist flexion and ulnar deviation. The upper extremity does not have to resist traction during the entire walk, as the dogs are known to accommodate their owner’s pace,9 but and the shoulder is more vulnerable to injury when a dog is startled and pulls or changes direction. In addition, the owner’s gait may change during dog walking due to pelvis rotation forward together with the hand holding the lead. We chose to investigate dog ownership as an occupational exposure, and approached a population group in which the exposure is relatively intensive and uniform: female soldiers serving in the military working dog’s unit (MWD). We chose this group for several reasons. First, the exposure is substantial significant, as MWD of Belgian Malinois and German Shepherd breeds are both active and heavy, undergoing many hours of weekly training and combat missions. On weekends, some soldiers receive days off, while others stay on the base and take care of several dogs, thus increasing their exposure to various dog-walking and keeping activities. Second, female soldiers were found to be especially prone to overuse injuries, compared with male soldiers.10 Moreover, male soldiers are assigned to different roles – they “send” the dogs to attack or pursue the target, or to find explosives. As a result, male soldiers may have lower exposure to dog walking than females. Third, the Israeli Defense Force (IDF) proposes unique research on the young adult population. Conscription is compulsory, making all social groups represented by the army recruit population. Physicians are available in every unit on a daily basis, possibly resulting in over-utilization of medical services. Therefore, even minor complaints are likely to be captured in the army medical records system. Finally, using the existing database, we were able to investigate the entire population reducing selection bias and strengthening internal validity, MATERIALS AND METHODS Study Population This retrospective cohort study included all female soldiers who served in IDF during 2005–2015 and underwent the initial 5-mo basic training in the same boot camp. Those who did not complete the basic training and those having less than 18 mo of follow-up (28%) were excluded. The remaining soldiers were divided into two groups: those who completed canine unit training and became soldiers working with MWDs (“MWD group”) and those who completed combat training and became light infantry or border guard soldiers (“INF-L group”, control). From the sixth month of service until the release from the army, both groups performed essentially the same military duties and used similar equipment and weapons. Working with dogs was the major difference of the MWD group from the INF-L group. For security purposes, we were not permitted to disclose the actual number of participating soliders, and reported the total of service periods per group in person-months (PM). Study variables included birthdate, weight, height, socio-economic status (measure employed by the Central Bureau of Statistics, designating socio-economic level per living area on a scale 1–10), place of birth (Israel, Western Europe and North America, Eastern Europe and former Soviet Union (FSU), Middle East and North Africa, South America and Asia, Sub-Saharan Africa and Ethiopia), and follow-up duration. Medical morbidity outcomes were retrieved from the military medical records. These included indicators of healthcare services utilization (number of physician visits, number of orthopedic surgeon referrals number of physiotherapist referrals), comorbid conditions (menstrual irregularities, fatigue and anemia diagnoses), and traumatic and overuse musculoskeletal conditions (searched by ICD codes relevant for each year of the study period). Unfortunately, relevant data from civilian medical records (e.g., musculoskeletal injuries before recruitment) were not accessible for this study. Statistical Analysis Distribution of nominal (origin and comorbidities) and ordinal (referrals to orthopedic surgeon and physiotherapy) variables was shown as percentages, while continuous variables (age, height, weight, number of physician visits, length of follow-up) were presented with mean and standard deviation. Incidence rate was calculated as number of new diagnoses per 1,000 PM. Comparison of incidence rates was presented as rate ratio (RR) with significance level. Alpha parameters were set at 0.05. Logistic regression was used to evaluate predictors of upper limb injury and adjust the effect of service as MDW for possible covariates. All variables that differed significantly (up to p = 0.1) between groups on the univariate analysis were included in the model after assessment for collinearity. Health service utilization measures were not included in the model, being a result of injuries and not their plausible cause. Predictors were reported as significant using lower p-value cut-off of 0.05. Some of the musculoskeletal diagnoses were essentially similar, and some where rarely encountered, which required their grouping according to mechanism of injury and anatomical locations. “Place of birth” was the only variable with data missing for 17 (0.2%) subjects. As previously mentioned, we have observed incidence of musculoskeletal injuries over two periods of service: boot camp (months 0–5) and special training and work as MWD or INF-L (month 6 until release from service). Comparison of injury incidences between groups during the first period was intended to test, whether the groups are comparable in a sense that they suffered from the same injury pattern during the similar exposure to physical activities of the boot camp. On the other hand, difference in injury rates during the second period would be attributed to differences of exposure, i.e., working with dogs vs not working with dogs. Each comorbid and musculoskeletal condition was counted once, even if appeared several times in the registry. Our intention was to sort out duplicate diagnoses that result from repeated referrals or other administrative encounters, and not necessarily from repeated injury. In addition, those conditions that a subject developed during the first period could not be counted again during the second period. The rationale for this decision was that if a soldier had a certain injury during the first period, then the same complaint during the second period had most probably resulted from the initial injury and not from the new exposure of working with dogs. The study was approved by the Israeli Defense Forces Medical Corps Institutional Review Board (No. 1652–2016). We adhered to the STROBE research reporting guidelines.11 RESULTS After application of exclusion criteria, the final study cohort consisted of 3,443 PM in the MWD group and 194,590 PM in INF-L group (Fig. 1). The actual numbers of soldiers may not be disclosed, but given the mean duration of service data (Table I), this corresponds to 106 MWD soldiers and 7,846 INF-L soldiers. The subjects’ baseline characteristics, comorbidities, and medical services utilization measures are presented in Table I. We found that MWD soldiers were older than INF-L soldiers (p < 0.001) and had longer mean period of service (p < 0.001). They had more physician visits over the period of service (p < 0.001) and more of them were referred to physiotherapists (p < 0.001). More MWDs were referred to orthopedic surgeons in total, but the percent of those having three or more orthopedic surgeon referrals was similar between groups. Prevalence of comorbid anemia and fatigue was significantly lower among MWD soldiers, as compared with Infantry (p = 0.02 and p = 0.002, respectively). FIGURE 1. View largeDownload slide Study flowchart. FIGURE 1. View largeDownload slide Study flowchart. Table I. Background Characteristics and Medical Services Utilization MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 aAfter exclusion of African origin. View Large Table I. Background Characteristics and Medical Services Utilization MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 aAfter exclusion of African origin. View Large All registered musculoskeletal injuries were grouped by anatomic location and pathophysiologic mechanism into nine groups: back overuse disorders and discopathies, anterior knee pain, upper limb overuse and trauma, foot and ankle overuse disorders, stress fractures of hip, leg and foot bones, ankle sprains, knee joint meniscal and ligamentous injuries, hip overuse disorders and neck overuse disorders and discopathies (Table II). An additional, and the largest group of unspecified complaints, such as “pain in limb” or “disorders of muscle, tendon and fascia”, accounted for about 30% of all injuries. There was no difference in incidence of injuries during the initial 5-mo training service between MWD and Infantry. Over the second period, however, MWD developed more upper limb complaints (RR = 1.45, p = 0.048) and hip joint overuse disorders (RR = 3.6, p < 0.0001) (Table II). Almost all specific diagnoses belonging to the “upper limb” injuries group were more prevalent in MWD group, though not all were significantly more frequent (Table II). Table II. Rates of Common Musculoskeletal Injuries Among MWD and Infantry Soldiers, According to Anatomical Region and Pathophysiology (per 1,000 PM) MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 *Significant at 0.05. Table II. Rates of Common Musculoskeletal Injuries Among MWD and Infantry Soldiers, According to Anatomical Region and Pathophysiology (per 1,000 PM) MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 *Significant at 0.05. Logistic regression showed that work in the MWD unit was predictive of upper limb injury even after for controlling for duration of follow-up, anemia, fatigue, weight, place of birth, and age at start of service. Of these, follow-up duration, anemia, fatigue, weight, and place of birth in Eastern Europe and the FSU were independent predictors of upper limb complaints (Table III). Table III. Logistic Regression Model Predictive of Upper Limb Injury Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 aIncrements of 10 kg. Table III. Logistic Regression Model Predictive of Upper Limb Injury Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 aIncrements of 10 kg. DISCUSSION Our results support our hypothesis that everyday exposure to working with dogs is associated with a higher incidence of upper limb injuries. In other reported soldier populations, upper limb injuries represent a relatively small fraction, only about 14%, of total military-service related injuries.12 In our study sample, upper limb injuries presented about 7.5% of all complaints, and in no sense were the dominant type of injury. However, MWDs had a 1.45-fold increased risk of any upper limb complaint and a 1.4- to 13.3-fold increased risk of the specific upper limb complaints (e.g., “shoulder region pain”, “sprain and strain of hand”), when compared with the control group (Table II). This overall increased risk of the upper limb injuries is robust and may be attributed to working with dogs, because other occupational exposures are essentially similar between groups. The exact activity that may have led to these increased injuries is holding the dog strap, which implies non-physiologic positions and motions of the upper limb joints and the repetitive forces pulling the arm, especially when the owner weighs only 15–20 kg more than the dog. Higher incidence of hip conditions among MWD soldiers as compared with the controls is an interesting finding, that remained significant after adjustment for the same variables as we did for the upper limb. It may result from greater stride length due to dog pulling and from ligamentous or cartilage damage due to jolts mid stride. We hope that our planned study of the biomechanical effects of walking the dogs will provide a comprehensive explanation to this difference. Only being born in Sub-Saharan Africa was a significant predictor of hip injuries, possibly implying a genetic effect. Anterior knee pain, foot and ankle overuse disorders, and lower limb stress fractures were less common among MWD compared with Infantry. Statistical significance was not achieved due to small size of the MWD group, and it is difficult to comment on this. However, if these differences result from overall lower activity load in the MWD unit, this would support the main finding of this study: despite lower physical activity, the exposure to working with dogs was associated with higher incidence of upper limb injuries. Higher number of physiotherapy and specialist referrals among MWD was an anticipated finding, for overall medical services are more available in the small special unit than in the battalions. This finding, however, deals with treatment of injuries and probably does not have any effect on incidence of injuries. We assessed for anemia and fatigue and included them in the regression model because of their possible association with an increased stress risk of stress fractures and other overuse injuries.13,14 In a previous study of overuse injuries among IDF light infantry female soldiers, Merkel and colleagues reported significantly lower serum iron and iron saturation values among those who sustained stress fractures, compared with those who did not have stress fractures.15 The authors proposed that both anemia and stress fractures could result either from insufficient dietary intake, or from systemic inflammatory response, which would negatively affect iron and bone metabolism. Both of the above mechanisms may also explain the association between anemia and upper limb overuse injuries observed in our study. General fatigue and overtraining have also been linked to overuse injuries, most likely because of impaired control of neuromuscular function and coordination between the intended activity and the afferent proprioceptive information.14 Co-ordinated muscular activity is required for donning equipment, holding a weapon and guiding a dog, all while walking on uneven surface. Neuromuscular fatigue may impede the adjustments required to maintain postural control and inhibit movements that may cause excess stress on soft tissues. With a cumulative effect over time, more overuse injuries are expected in subjects suffering from central fatigue.16,17 Alternatively, general fatigue may affect pain perception and elicit more musculoskeletal complaints. Additional explanations view general fatigue and overuse injuries as coexisting, and not causal, morbidities, both resulting from inadequate nutrition, low motivation (leading to more complaints) and systemic inflammatory reaction.18–20 Study Limitations The female military population of the IDF MWD unit may represent an optimal study population for our research question, for the reasons detailed in the introduction.10 Yet, this study has several limitations. A retrospective design could imply information bias and use of secondary data. We believe that physicians that inspected MWD and INF-L soldiers could differ in their diagnostic coding diligence, not to mention the proficiency in diagnosis of musculoskeletal conditions. Additionally, incidence of muskuloskeletal conditions that occurred during the second period may be underestimated, because when calculating the incidence rate of each injury we did not subtract from the denominator all follow-up periods of subjects who already had this condition during the first period. Finally, the danger of investigating combat soldiers’ research is that the effect of their routine strenuous physical activity may override the possibly more subtle effects of working with dogs. In other words, we suspected that differences between MWD and INF-L are not likely to be found in back and lower limb injuries that are caused by marches and drills, but rather in the incidence of upper limb injuries. Therefore, this study hypothesis is limited to upper limb injuries, and does not rule out the effect of working with dogs on other types of overuse injuries. In summary, this study has demonstrated a significant association between service in the Military Working Dogs Unit and upper limb overuse injuries in female recruits. In addition, this study demonstrated a significantly and substantially higher risk of hip injuries among soldiers working with dogs compared with other combat recruits. This finding will be further investigated by a planned biomechanical study. REFERENCES 1 Smith B. : The “pet effect” – health related aspects of companion animal ownership . Aust Fam Physician 41 ( 6 ): 439 – 42 . http://www.ncbi.nlm.nih.gov/pubmed/22675689. PubMed 2 Utz RL. : Walking the dog: the effect of pet ownership on human health and health behaviors . Soc Indic Res 2014 ; 116 ( 2 ): 327 – 39 . Google Scholar CrossRef Search ADS 3 Herzog H. : The impact of pets on human health and psychological well-being . Curr Dir Psychol Sci [Internet] 2011 Aug 8 [cited 2018 Jan 13]; 20 ( 4 ): 236 – 9 . http://journals.sagepub.com/doi/10.1177/0963721411415220. 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Google Scholar CrossRef Search ADS Author notes The views expressed are solely those of the authors and do not reflect the official policy or position of the U.S. Army, U.S. Navy, U.S. Air Force, the Department of Defense, or the U.S. Government. © Association of Military Surgeons of the United States 2018. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Military Medicine Oxford University Press

Musculoskeletal Injuries Among Female Soldiers Working With Dogs

Military Medicine , Volume 183 (9) – Sep 1, 2018

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Abstract

Abstract Introduction Female soldiers serving in the Israeli Defense Forces canine unit may be at increased risk of overuse injuries related to working with military dogs. We hypothesized that this particular type of occupational exposure may lead to an increased strain of the upper extremity due to such non-physiologic motions as pulling the dog’s strap or resisting the sudden pulling by the dog, and may result in an increased rate of overuse injuries. Materials and Methods We compared incidence of overuse injuries in a retrospective cohort of female soldiers who served either in the military working dogs’ unit (MWD), or in the light infantry battalions (Infantry) from 2005 to 2015. We compared injury incidence of both groups during two periods: 5 mo of basic training (neither worked with dogs) and 19 or more months of combat service. Incidence was calculated as number of diagnoses per person-months (rate ratios, RR); each diagnosis counted once per study subject. We used RR confidence intervals to compare incidence of injuries between groups. Results There were 3,443 person-months in the MWD group and 194,590 person-months in the Infantry group. There was no difference in injury incidence between groups during the initial period of basic training. During the second period, MWDs had higher incidence of upper limb (RR = 1.45, p = 0.048) and hip (RR = 3.6, p < 0.0001) injuries. The association between service with dogs and upper limb injuries remained significant (RR = 1.09, p = 0.005) after adjusting for confounding variables in the multivariate regression analysis. Other independent predictors of the upper limb overuse injuries included each additional month of service (RR = 1.003, p = 0.008), Eastern European origin compared with Israeli-born recruits (RR = 0.97, p = 0.03), increased body weight in increments of 10 kg (RR = 1.008, p = 0.03), anemia (RR = 1.02, p = 0.02) and fatigue (RR = 1.05, p ≤ 0.0001). Conclusions We found that service in the MWD unit was associated with increased risk of upper limb injuries. Identification of the exact mechanism of injury and targeted interventions, as well as treatment of anemia and fatigue may lead to reduction of injuries in this unit. INTRODUCTION Numerous studies have investigated the effect of dog ownership on emotional and physical well-being. It has been proposed that better health outcomes may be promoted by a healthier lifestyle associated with dog ownership.1 However, much of the evidence suggests that the better health outcomes of pet owners compared with people who have no pets, results from selection bias and can be explained by other factors, such as residential location and socio-economic status.2,3 Few studies, however, have attempted to measure pet owner musculoskeletal injuries associated with pets.4–7 We hypothesize that the cumulative mechanical load resulting from dog keeping activities, walking, squatting, handling a strap, sudden non-physiological motions provoked by the interaction with an active pet during training or walking may result in higher prevalence of overuse injuries and tendinopathies.8 This type of injury may be especially evident in the upper limbs, for which the additive “unnatural” biomechanical stresses related to dog keeping may be more significant than for lower limbs, mostly involved in locomotion and therefore accommodated to higher stresses. Holding a dog’s lead, when the dog walks before the owner, requires shoulder forward flexion with internal rotation, as well as elbow and wrist flexion and ulnar deviation. The upper extremity does not have to resist traction during the entire walk, as the dogs are known to accommodate their owner’s pace,9 but and the shoulder is more vulnerable to injury when a dog is startled and pulls or changes direction. In addition, the owner’s gait may change during dog walking due to pelvis rotation forward together with the hand holding the lead. We chose to investigate dog ownership as an occupational exposure, and approached a population group in which the exposure is relatively intensive and uniform: female soldiers serving in the military working dog’s unit (MWD). We chose this group for several reasons. First, the exposure is substantial significant, as MWD of Belgian Malinois and German Shepherd breeds are both active and heavy, undergoing many hours of weekly training and combat missions. On weekends, some soldiers receive days off, while others stay on the base and take care of several dogs, thus increasing their exposure to various dog-walking and keeping activities. Second, female soldiers were found to be especially prone to overuse injuries, compared with male soldiers.10 Moreover, male soldiers are assigned to different roles – they “send” the dogs to attack or pursue the target, or to find explosives. As a result, male soldiers may have lower exposure to dog walking than females. Third, the Israeli Defense Force (IDF) proposes unique research on the young adult population. Conscription is compulsory, making all social groups represented by the army recruit population. Physicians are available in every unit on a daily basis, possibly resulting in over-utilization of medical services. Therefore, even minor complaints are likely to be captured in the army medical records system. Finally, using the existing database, we were able to investigate the entire population reducing selection bias and strengthening internal validity, MATERIALS AND METHODS Study Population This retrospective cohort study included all female soldiers who served in IDF during 2005–2015 and underwent the initial 5-mo basic training in the same boot camp. Those who did not complete the basic training and those having less than 18 mo of follow-up (28%) were excluded. The remaining soldiers were divided into two groups: those who completed canine unit training and became soldiers working with MWDs (“MWD group”) and those who completed combat training and became light infantry or border guard soldiers (“INF-L group”, control). From the sixth month of service until the release from the army, both groups performed essentially the same military duties and used similar equipment and weapons. Working with dogs was the major difference of the MWD group from the INF-L group. For security purposes, we were not permitted to disclose the actual number of participating soliders, and reported the total of service periods per group in person-months (PM). Study variables included birthdate, weight, height, socio-economic status (measure employed by the Central Bureau of Statistics, designating socio-economic level per living area on a scale 1–10), place of birth (Israel, Western Europe and North America, Eastern Europe and former Soviet Union (FSU), Middle East and North Africa, South America and Asia, Sub-Saharan Africa and Ethiopia), and follow-up duration. Medical morbidity outcomes were retrieved from the military medical records. These included indicators of healthcare services utilization (number of physician visits, number of orthopedic surgeon referrals number of physiotherapist referrals), comorbid conditions (menstrual irregularities, fatigue and anemia diagnoses), and traumatic and overuse musculoskeletal conditions (searched by ICD codes relevant for each year of the study period). Unfortunately, relevant data from civilian medical records (e.g., musculoskeletal injuries before recruitment) were not accessible for this study. Statistical Analysis Distribution of nominal (origin and comorbidities) and ordinal (referrals to orthopedic surgeon and physiotherapy) variables was shown as percentages, while continuous variables (age, height, weight, number of physician visits, length of follow-up) were presented with mean and standard deviation. Incidence rate was calculated as number of new diagnoses per 1,000 PM. Comparison of incidence rates was presented as rate ratio (RR) with significance level. Alpha parameters were set at 0.05. Logistic regression was used to evaluate predictors of upper limb injury and adjust the effect of service as MDW for possible covariates. All variables that differed significantly (up to p = 0.1) between groups on the univariate analysis were included in the model after assessment for collinearity. Health service utilization measures were not included in the model, being a result of injuries and not their plausible cause. Predictors were reported as significant using lower p-value cut-off of 0.05. Some of the musculoskeletal diagnoses were essentially similar, and some where rarely encountered, which required their grouping according to mechanism of injury and anatomical locations. “Place of birth” was the only variable with data missing for 17 (0.2%) subjects. As previously mentioned, we have observed incidence of musculoskeletal injuries over two periods of service: boot camp (months 0–5) and special training and work as MWD or INF-L (month 6 until release from service). Comparison of injury incidences between groups during the first period was intended to test, whether the groups are comparable in a sense that they suffered from the same injury pattern during the similar exposure to physical activities of the boot camp. On the other hand, difference in injury rates during the second period would be attributed to differences of exposure, i.e., working with dogs vs not working with dogs. Each comorbid and musculoskeletal condition was counted once, even if appeared several times in the registry. Our intention was to sort out duplicate diagnoses that result from repeated referrals or other administrative encounters, and not necessarily from repeated injury. In addition, those conditions that a subject developed during the first period could not be counted again during the second period. The rationale for this decision was that if a soldier had a certain injury during the first period, then the same complaint during the second period had most probably resulted from the initial injury and not from the new exposure of working with dogs. The study was approved by the Israeli Defense Forces Medical Corps Institutional Review Board (No. 1652–2016). We adhered to the STROBE research reporting guidelines.11 RESULTS After application of exclusion criteria, the final study cohort consisted of 3,443 PM in the MWD group and 194,590 PM in INF-L group (Fig. 1). The actual numbers of soldiers may not be disclosed, but given the mean duration of service data (Table I), this corresponds to 106 MWD soldiers and 7,846 INF-L soldiers. The subjects’ baseline characteristics, comorbidities, and medical services utilization measures are presented in Table I. We found that MWD soldiers were older than INF-L soldiers (p < 0.001) and had longer mean period of service (p < 0.001). They had more physician visits over the period of service (p < 0.001) and more of them were referred to physiotherapists (p < 0.001). More MWDs were referred to orthopedic surgeons in total, but the percent of those having three or more orthopedic surgeon referrals was similar between groups. Prevalence of comorbid anemia and fatigue was significantly lower among MWD soldiers, as compared with Infantry (p = 0.02 and p = 0.002, respectively). FIGURE 1. View largeDownload slide Study flowchart. FIGURE 1. View largeDownload slide Study flowchart. Table I. Background Characteristics and Medical Services Utilization MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 aAfter exclusion of African origin. View Large Table I. Background Characteristics and Medical Services Utilization MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 MWD (3,443 PM) Infantry (194,590 PM) p-Value Mean SD Mean SD Background characteristics  Age, years 19.1 1 18.7 0.62 <0.001  Weight, kg 58 7.14 59.4 9.87 0.05  Height, cm 162 6.3 162.8 7.8 0.27  Mean follow-up duration, months 32.5 5.1 24.8 2.8 <0.001  Socio-economic   Low 18% 7% 0.85   Medium 55% 54%   High 27% 29%  Origin   Western Europe and North America 21% 9% 0.0009a   Middle East 33% 45%   Eastern Europe and former Soviet Union 28% 30%   Asia and South America 3% 5%   Israel 15% 11%   Africa 0 2% Medical services utilization  Physician visits 33.7 16.9 24.5 15.2 <0.001  Orthopedic referrals   1–2 21.6% 16% 0.3   3 or more 10% 10%  Physiotherapy referrals   1–2 23.6% 12% <0.001   3 or more 18.9% 5.2% Comorbid conditions  Anemia 4% 11% 0.02  Fatigue 15% 30% 0.002  Dysmenorrhea 19% 23% 0.34 aAfter exclusion of African origin. View Large All registered musculoskeletal injuries were grouped by anatomic location and pathophysiologic mechanism into nine groups: back overuse disorders and discopathies, anterior knee pain, upper limb overuse and trauma, foot and ankle overuse disorders, stress fractures of hip, leg and foot bones, ankle sprains, knee joint meniscal and ligamentous injuries, hip overuse disorders and neck overuse disorders and discopathies (Table II). An additional, and the largest group of unspecified complaints, such as “pain in limb” or “disorders of muscle, tendon and fascia”, accounted for about 30% of all injuries. There was no difference in incidence of injuries during the initial 5-mo training service between MWD and Infantry. Over the second period, however, MWD developed more upper limb complaints (RR = 1.45, p = 0.048) and hip joint overuse disorders (RR = 3.6, p < 0.0001) (Table II). Almost all specific diagnoses belonging to the “upper limb” injuries group were more prevalent in MWD group, though not all were significantly more frequent (Table II). Table II. Rates of Common Musculoskeletal Injuries Among MWD and Infantry Soldiers, According to Anatomical Region and Pathophysiology (per 1,000 PM) MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 *Significant at 0.05. Table II. Rates of Common Musculoskeletal Injuries Among MWD and Infantry Soldiers, According to Anatomical Region and Pathophysiology (per 1,000 PM) MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 MWD Infantry RR p-Value Major injury groups  Back overuse disorders and discopathies 26 24.6 1.06 0.6  Anterior knee pain 16.1 22.6 0.71 0.02*  Upper limb overuse and trauma 9.6 6.6 1.45 0.048*  Foot and ankle overuse disorders 8.9 12.5 0.71 0.09  Stress fractures of hip, leg and foot bones 8.2 10.1 0.82 0.33  Ankle sprains 4.8 5.4 0.9 0.64  Knee joint meniscal and ligamentous injuries 3.4 5.1 0.7 0.2  Hip overuse disorders 3.4 0.96 3.6 <0.001*  Neck overuse disorders and discopathies 3 2.97 1.04 0.9 Detailed list of upper limb complaints  Shoulder region pain 3.43 2.42 1.4 0.3  Pain in joint, hand 1.7 1.04 1.6 0.3  Sprain and strain of hand 0.69 0.2 3.3 0.08  Sprain and strain of shoulder 0.69 0.12 5.3 0.01*  Sprain and strain of wrist 0.69 0.16 4.3 0.03*  Bursitis of olecranon 0.34 0.026 13.3 0.003*  Contusion of upper limb, elbow 0.34 0.3 1.2 0.9  Sprain and strain of shoulder, rotator cuff 0.34 0.07 4.9 0.09 *Significant at 0.05. Logistic regression showed that work in the MWD unit was predictive of upper limb injury even after for controlling for duration of follow-up, anemia, fatigue, weight, place of birth, and age at start of service. Of these, follow-up duration, anemia, fatigue, weight, and place of birth in Eastern Europe and the FSU were independent predictors of upper limb complaints (Table III). Table III. Logistic Regression Model Predictive of Upper Limb Injury Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 aIncrements of 10 kg. Table III. Logistic Regression Model Predictive of Upper Limb Injury Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 Odds ratio 95% CI of Odds Ratio p-Value Service as MWD 1.09 1.03–1.16 0.005 Anemia 1.02 1.003–1.036 0.02 Fatigue 1.05 1.03–1.07 <0.0001 Weight (kg)a 1.008 1.0008–1.015 0.03 Length of follow-up (month) 1.003 1.0009–1.006 0.008 Place of birth Eastern Europe and former Soviet Union 0.97 0.94–0.997 0.03 aIncrements of 10 kg. DISCUSSION Our results support our hypothesis that everyday exposure to working with dogs is associated with a higher incidence of upper limb injuries. In other reported soldier populations, upper limb injuries represent a relatively small fraction, only about 14%, of total military-service related injuries.12 In our study sample, upper limb injuries presented about 7.5% of all complaints, and in no sense were the dominant type of injury. However, MWDs had a 1.45-fold increased risk of any upper limb complaint and a 1.4- to 13.3-fold increased risk of the specific upper limb complaints (e.g., “shoulder region pain”, “sprain and strain of hand”), when compared with the control group (Table II). This overall increased risk of the upper limb injuries is robust and may be attributed to working with dogs, because other occupational exposures are essentially similar between groups. The exact activity that may have led to these increased injuries is holding the dog strap, which implies non-physiologic positions and motions of the upper limb joints and the repetitive forces pulling the arm, especially when the owner weighs only 15–20 kg more than the dog. Higher incidence of hip conditions among MWD soldiers as compared with the controls is an interesting finding, that remained significant after adjustment for the same variables as we did for the upper limb. It may result from greater stride length due to dog pulling and from ligamentous or cartilage damage due to jolts mid stride. We hope that our planned study of the biomechanical effects of walking the dogs will provide a comprehensive explanation to this difference. Only being born in Sub-Saharan Africa was a significant predictor of hip injuries, possibly implying a genetic effect. Anterior knee pain, foot and ankle overuse disorders, and lower limb stress fractures were less common among MWD compared with Infantry. Statistical significance was not achieved due to small size of the MWD group, and it is difficult to comment on this. However, if these differences result from overall lower activity load in the MWD unit, this would support the main finding of this study: despite lower physical activity, the exposure to working with dogs was associated with higher incidence of upper limb injuries. Higher number of physiotherapy and specialist referrals among MWD was an anticipated finding, for overall medical services are more available in the small special unit than in the battalions. This finding, however, deals with treatment of injuries and probably does not have any effect on incidence of injuries. We assessed for anemia and fatigue and included them in the regression model because of their possible association with an increased stress risk of stress fractures and other overuse injuries.13,14 In a previous study of overuse injuries among IDF light infantry female soldiers, Merkel and colleagues reported significantly lower serum iron and iron saturation values among those who sustained stress fractures, compared with those who did not have stress fractures.15 The authors proposed that both anemia and stress fractures could result either from insufficient dietary intake, or from systemic inflammatory response, which would negatively affect iron and bone metabolism. Both of the above mechanisms may also explain the association between anemia and upper limb overuse injuries observed in our study. General fatigue and overtraining have also been linked to overuse injuries, most likely because of impaired control of neuromuscular function and coordination between the intended activity and the afferent proprioceptive information.14 Co-ordinated muscular activity is required for donning equipment, holding a weapon and guiding a dog, all while walking on uneven surface. Neuromuscular fatigue may impede the adjustments required to maintain postural control and inhibit movements that may cause excess stress on soft tissues. With a cumulative effect over time, more overuse injuries are expected in subjects suffering from central fatigue.16,17 Alternatively, general fatigue may affect pain perception and elicit more musculoskeletal complaints. Additional explanations view general fatigue and overuse injuries as coexisting, and not causal, morbidities, both resulting from inadequate nutrition, low motivation (leading to more complaints) and systemic inflammatory reaction.18–20 Study Limitations The female military population of the IDF MWD unit may represent an optimal study population for our research question, for the reasons detailed in the introduction.10 Yet, this study has several limitations. A retrospective design could imply information bias and use of secondary data. We believe that physicians that inspected MWD and INF-L soldiers could differ in their diagnostic coding diligence, not to mention the proficiency in diagnosis of musculoskeletal conditions. Additionally, incidence of muskuloskeletal conditions that occurred during the second period may be underestimated, because when calculating the incidence rate of each injury we did not subtract from the denominator all follow-up periods of subjects who already had this condition during the first period. Finally, the danger of investigating combat soldiers’ research is that the effect of their routine strenuous physical activity may override the possibly more subtle effects of working with dogs. In other words, we suspected that differences between MWD and INF-L are not likely to be found in back and lower limb injuries that are caused by marches and drills, but rather in the incidence of upper limb injuries. Therefore, this study hypothesis is limited to upper limb injuries, and does not rule out the effect of working with dogs on other types of overuse injuries. In summary, this study has demonstrated a significant association between service in the Military Working Dogs Unit and upper limb overuse injuries in female recruits. In addition, this study demonstrated a significantly and substantially higher risk of hip injuries among soldiers working with dogs compared with other combat recruits. This finding will be further investigated by a planned biomechanical study. REFERENCES 1 Smith B. : The “pet effect” – health related aspects of companion animal ownership . Aust Fam Physician 41 ( 6 ): 439 – 42 . http://www.ncbi.nlm.nih.gov/pubmed/22675689. PubMed 2 Utz RL. : Walking the dog: the effect of pet ownership on human health and health behaviors . Soc Indic Res 2014 ; 116 ( 2 ): 327 – 39 . Google Scholar CrossRef Search ADS 3 Herzog H. : The impact of pets on human health and psychological well-being . Curr Dir Psychol Sci [Internet] 2011 Aug 8 [cited 2018 Jan 13]; 20 ( 4 ): 236 – 9 . http://journals.sagepub.com/doi/10.1177/0963721411415220. 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Google Scholar CrossRef Search ADS Author notes The views expressed are solely those of the authors and do not reflect the official policy or position of the U.S. Army, U.S. Navy, U.S. Air Force, the Department of Defense, or the U.S. Government. © Association of Military Surgeons of the United States 2018. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Journal

Military MedicineOxford University Press

Published: Sep 1, 2018

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