Objective: The sit-to-stand (STS) is a simple test to evaluate the functional performance of the quadriceps muscle in patients with knee osteoarthritis (OA). The aim was to evaluate the electromyographic (EMG) activity of the ipsilateral quadriceps during STS task at different seat heights and feet positions in patients with severe unilateral OA. The EMG activity was recorded in a group of eight participants with unilateral OA during the performance of STS task in four conditions: (1) knee-height seat with feet together, (2) knee-height seat with feet askew (feet side by side and heel- to-toe), (3) low-height seat (25% lower than knee-height seat) with feet together, and (4) low-height seat with feet askew. Results: There was a statistically significant difference among the four conditions in the EMG activity ( p =0.004). Particularly, the EMG activity of the quadriceps was significantly higher when participants rose from the low height with their feet askew than when they rose from the knee height with their feet placed together (p =0.004) or askew (p =0.002). These results recommend considering initial feet position and seat height when evaluating the functional activity of the quadriceps in patients with unilateral OA using STS test. Keywords: Knee, Osteoarthritis, Quadriceps muscle, Sit-to-stand, Electromyography Introduction The assessment of the quadriceps muscle is warranted The knee joint is the joint most commonly affected by to determine the functional status of patients with OA. osteoarthritis (OA) . Patients with knee OA usually The sit-to-stand (STS) test is a performance-based meas - suffer from pain, limited range of motion, stiffness and ure frequently used in patients with knee OA to measure muscle weakness . Therefore, knee OA has been recog - the functional performance of the quadriceps muscle. nized as a major source of disability and physical impair- Sufficient quadriceps force is required to complete the ment in older adults . STS movement. Therefore, quadriceps weakness was Quadriceps muscle weakness is a common clinical fea- found to have a significant impact on STS performance ture of knee OA [4–6]. Persistent weakness of the quadri- [11–13]. ceps plays a major role in increasing the stress over the Electromyography (EMG) is commonly used to obtain knee joint and progression of joint damage . Hence, information about the effects of chair seat height and ini - improving the functional strength of the quadriceps in tial feet positions on the activity of the lower limb mus- patients with knee OA has received great attention in the cles during STS movement [14–17]. Measuring EMG literature [8–10]. activity of the knee extensors during these tasks would reflect the amount of loading applied to the quadriceps. This is a key muscle to be targeted during the rehabili - tation program of patients with knee OA. Therefore, the *Correspondence: firstname.lastname@example.org purpose of this study was to evaluate the EMG activity Department of Physical Therapy, Faculty of Applied Medical Sciences, of the quadriceps muscle, specifically the vastus lateralis University of Tabuk, Tabuk 71491, Saudi Arabia Full list of author information is available at the end of the article (VL), during STS task at different seat heights and feet © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/ publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Al Amer et al. BMC Res Notes (2018) 11:356 Page 2 of 6 positions in individuals with severe unilateral OA. The electrode was affixed over the fibular head. The VL was data presented in this study is a side product of another chosen in this study as a representative of the quadriceps unpublished research project investigating the activity of muscles based on its several unique characteristics. The thigh musculatures during selected functional activities VL is considered the largest among the four quadriceps before and after total knee arthroplasty (TKA). muscles  and the main generator of extension torque at the knee . Although there is no difference in time Main text of onset among the four muscles of the quadriceps during Methods closed-chain movements, the VL has the largest amount Eight participants (five males and three females) volun - of EMG activity during that type of movement . teered for the study 1–2 weeks before undergoing uni- The participants performed STS tasks in the follow - lateral elective TKA. The mean age of the participants ing order: STS at normal height (knee-height seat) with was 64.61 ± 11.01 years and the body mass index was feet together (Fig. 1a), STS at normal height with feet 34.06 ± 8.89 kg/m . Participants were included based on askew (feet side by side, heel-to-toe with foot of arthritic the following criteria: no other musculoskeletal disorders knee behind the other) (Fig. 1b), STS at low height (25% or neurological pathologies; and no previous hip, knee, lower than knee-height seat) with feet together (Fig. 1c), spine or neck surgery within the past year. and STS at low height with feet askew (Fig. 1d) (tasks EMG activity was sampled at 1000 Hz and sweep speed hereafter will be identified as NHFT, NHFA, LHFT and of 100 points/s. using the Myosystem 1200 version 2.11 LHFA, respectively). The starting position was sitting on (Noraxon USA, Inc., Scottsdale, AZ) via the Telemyo an armless, backless chair, and maintaining feet flat on 900 telemetry unit (Noraxon USA, Inc., Scottsdale, AZ). the floor and thighs at hip width. To perform the tasks, Two adhesive surface electrodes were placed over the participants were instructed to stand while holding arms mid-muscle belly of the VL of the arthritic knee. The across the chest with weight equally distributed on both electrodes were placed longitudinally in a bipolar config - feet in NHFT and LHFT. In NHFA and LHFA, they had uration with inter-electrode distance of 2 cm. A ground the chance to load their feet as they wish to complete the Fig. 1 The four conditions of sit-to-stand task. a Normal height feet together (NHFT ). b Normal height feet askew (NHFA). c Low height feet together (LHFT ). d Low height feet askew (LHFA) Al Amer et al. BMC Res Notes (2018) 11:356 Page 3 of 6 task. Two trials of each STS task were performed and EMG activity would fall below or equal to 100% of the used for analysis. maximum signal . The normalized peak EMG activity For normalizing the EMG data, participants completed of VL during the two trials of each task was averaged and two maximal voluntary isometric contractions (MVIC) used as the final outcome score for the performed task. using an electromechanical dynamometer (Biodex Inc., Statistical analysis included one independent variable: Shirley, NY) while EMG was recorded. The examined the STS task, with four levels (NHFT, NHFA, LHFT limb was stabilized with the hip and knee flexed to 90° and LHFA). The dependent variable was the normalized and 15°, respectively. Two 10-s trials were recorded with peak EMG activity of the quadriceps muscle measured 2 min of rest in between. in percent of maximum activation. A one-way analysis To analyze EMG signals of quadriceps, raw signals were of variance (ANOVA) for repeated measures (univari- full-wave rectified and smoothed with a 10 ms window. ate approach) was conducted to test the main effect of The obtained linear envelope was analyzed to determine the independent variable with alpha level set at 0.05. The the peak EMG signal amplitudes during the STS task tri- univariate approach was selected due to the small sam- als and during MVIC trials. Because it is common to find ple size. Regarding the sphericity assumption, Geisser- high levels of EMG activity during dynamic tasks ver- Greenhouse epsilon hat ( ε ˆ ) of 0.764 was found. Therefore, sus MVIC , the two trials of each STS task were also the degrees of freedom (factor and error) were adjusted examined for the maximum EMG signal. Subsequently, according to this value to prevent inflation of alpha. the peak EMG signal obtained during the STS tasks trials was normalized to the maximum EMG signal obtained either during the MVIC or during the STS trials, which- Results ever had higher activity. This normalization method is The means and standard deviations of the normalized common in EMG studies [13, 21, 22] and was suggested EMG for each condition are illustrated in Fig. 2. The as a more accurate method since all the normalized peak result of the one-way ANOVA showed a statistically Fig. 2 The means and standard deviations in each condition. NHFT: normal height feet together, NHFA: normal height feet askew, LHFT: low height feet together, LHFA: low height feet askew Al Amer et al. BMC Res Notes (2018) 11:356 Page 4 of 6 significant difference among the four conditions in the same compensatory approach during the performance normalized EMG activity, F (2.29, 16.04)=7.54, p =0.004. of the STS task. Nevertheless, quadriceps weakness is a All pairwise comparisons were conducted to exam- common feature in patients with knee OA and patients ine the difference among tasks using Bonferroni tests. with TKA in the early phases following the surgery . Alpha level was adjusted to .05/6 (number of compari- Repositioning the foot of the unaffected side anterior sons) = 0.0083 to prevent type I error. All pairwise com- to the foot of the arthritic side significantly increased parisons revealed the normalized EMG activity was the activity of the quadriceps. Generally, placing the significantly higher in LHFA (M = 76.93%) than in NHFT feet posteriorly moves the ground reaction force vector (M = 51.16%) or NHFA (M = 59.44%) (Fig. 2). No further further posteriorly with respect to the knee, leading to differences were found among the rest of conditions. a higher external flexion moment applied on that knee Table 1 displays the t-statistic, degrees of freedom and p . In this study, the relatively posterior position of the value for each comparison. arthritic knee’s foot produced higher demand on the ipsi- lateral quadriceps to overcome the increase in the ground Discussion reaction force. Additionally, this position retained the This study was conducted to evaluate the effect of four arthritic knee closer to the center of gravity . As a different conditions of STS task on the EMG activity of result, the arthritic knee was the principal leg to perform the quadriceps muscle in individuals with severe uni- the upward displacement of the body. For that reason, the lateral OA. The findings showed that during STS move - relatively posterior position of the foot of the arthritic ment, the activity of the quadriceps is modulated by the knee required the participants to use that knee instead chair height and feet position. of the unaffected one as compensation, due to possible A possible explanation of the difference in EMG activ - weakness or pain avoidance of the affected side. ity of the quadriceps between the LHFA and NHFT posi- The reported increase in the EMG activity of the tions is that the participants could be trying to avoid quadriceps during STS movement from a low height in loading the arthritic knee during the latter due to possible comparison to those with knee height (with feet askew in weakness of the quadriceps muscle, pain, or both. When both tasks) suggests increasing the demand on the knee the arthritic and unaffected sides’ feet were parallel, par - extensors. This finding is in agreement with previous ticipants had the chance to compensate for the arthritic research [14, 15, 25]. Arborelius et al.  examined the side which might be weaker than the unaffected side. This effect of rising from two different seat heights in healthy phenomenon has been observed in patients with unilat- individuals and found a significant increase in the activ - eral TKA [12, 13]. Farquhar et al. . found the activity ity of the VL muscle with rising from a lower seat height of the quadriceps muscle on the involved side to be sig- in comparison to higher seat height. As the seat height nificantly lower than the uninvolved side during STS task decreases, the knee flexion angle and the knee flexion up to 3 months following the surgery. Due to significant moment will increase. This would lead to higher demand weakness in the knee extensors, patients avoided load- on the quadriceps muscle to extend the knee in lifting the ing of the involved limb by shifting the load to the unin- body weight [15, 26]. volved limb. However, because the EMG activity of the Performing the STS test with placing both feet together quadriceps at the uninvolved side was not recorded in the provides an opportunity for the patients to use the unin- present study, we are not sure if the participants used the volved side to compensate for the possible weakness of the arthritic side. This may not reflect the true status of the quadriceps performance on the affected side. Con - versely, repositioning the unaffected side anterior to the Table 1 Results of all pairwise comparisons arthritic side imposes more demand on the patient to use for the normalized EMG during different sit-to-stand tasks the involved side instead of compensating with the unin- Pairwise comparison t df p-value volved side. This task better demonstrates the true func - tional performance of the ipsilateral quadriceps muscle. NHFT vs. NHFA 1.47 7 0.186 Furthermore, lowering the seat height will add greater NHFT vs. LHFT 2.62 7 0.035 difficulty to the test as it places more demand on the side NHFT vs. LHFA 4.25 7 0.004 being tested. Therefore, starting positions with regard to NHFA vs. LHFT 1.09 7 0.311 chair height and initial feet position need to be standard- NHFA vs. LHFA 4.63 7 0.002 ized in order to avoid misleading results. LHFT vs. LHFA 1.71 7 0.130 To conclude, the results of this study indicate that the NHFT normal height feet together, NHFA normal height feet askew, LHFT low modification of seat height and feet position during STS height feet together, LHFA low height feet askew movement plays an important role in clinically evaluating Indicates significant difference at α = 0.0083 Al Amer et al. BMC Res Notes (2018) 11:356 Page 5 of 6 Human Subjects. Subjects’ approvals for participation were obtained using patients with knee OA. Lowering the seat height and written informed consent prior procedures. placing the foot of the unaffected side anterior to the foot of the arthritic side increase the demand on the quadri- Funding This study was supported by grants from the Methodist Hospital Research ceps muscle of the arthritic knee. This starting position Institute. would prevent patients from utilizing some strategies to avoid using their arthritic side to complete the STS task. Publisher’s Note This, in turn, may reflect the true functional condition Springer Nature remains neutral with regard to jurisdictional claims in pub- of the knee extensors in patients with knee OA and the lished maps and institutional affiliations. potential need of additional intervention. Received: 22 March 2018 Accepted: 1 June 2018 Limitations A limitation of this study is the small sample size, which may have affected the significance of the results. Particu - References 1. Dieppe P, Cushnaghan J, Tucker M, Browning S, Shepstone L. The bristol larly when Bonferroni adjustment was used. Another ‘OA500 study’: Progression and impact of the disease after 8 years. Osteo- potential source of type II error is the high variability of arthr Cartil. 2000;8(2):63–8. the EMG data. In fact, some electrophysiological studies 2. Barker K, Lamb SE, Toye F, Jackson S, Barrington S. 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