Motor unit synchronization of the vasti muscles in closed and open chain tasks

OBJECTIVES: To investigate motor unit synchronization between medial and lateral vasti and whether such synchronization differs in closed and open chain tasks. DESIGN: Electromyographic recordings of single motor unit action potentials were made from the vastus medialis obliquus (VMO) and multiunit recordings from vastus lateralis during isometric contractions at 30 degrees of knee flexion in closed and open chain conditions. SETTING: Laboratory. PARTICIPANTS: Five volunteers with no history of knee pain (age, 30+/-3.32 y). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURE: The degree of synchronization between motor unit firing was evaluated by identifying peaks in the electromyographic averages of the vastus lateralis, triggered from motor unit action potentials in the VMO, and the proportion of power in the power spectral density of the triggered average at the firing frequency of the reference motor unit. The proportion of cases in which there was significant power and peaks in the triggered averages was calculated. RESULTS: The proportion of trials with peaks in the triggered averages of the vastus lateralis electromyographic activity was greater than 61.5% in all tasks, and there was a significantly greater proportion of cases where power in the spectrum was greater than 7.5% ( P =.01) for the closed chain condition. CONCLUSIONS: There was a high proportion of synchronized motor units between the 2 muscles during isometric contractions, with evidence for greater common drive between the VMO and vastus lateralis in closed chain tasks. This has implications for rehabilitation because it suggests that closed chain tasks may generate better coordination between the vasti muscles.     

闭链与开链运动对髌股疼痛综合征股四头肌作用的表面肌电图研究

摘要 目的：利用表面肌电图(sEMG)对髌股疼痛综合征（patellofemoral pain syndrome, PFPS）患者双足半蹲伴或坐位伸膝动作下股四头肌进行评估,比较不同开链与闭链动作对股四头肌的作用差异。 方法：PFPS病例组及正常对照组各30例,分别在双足半蹲以及坐位伸膝动作时检测股外侧肌(vastus lateralis, VL)及股内斜肌（vastus medialis oblique, VMO）表面肌电图,分析时域、频域指标,比较其平衡关系。 结果：病例组双足半蹲动作与坐位伸膝动作时各肌电指标的VL/VMO比值在时域、频域指标均有显著性差异,在坐位单侧完全伸膝动作时VL/VMO肌电比值大于双足半蹲动作且>1。 结论：VL、VMO表面肌电指标在双足半蹲动作时较坐位单侧完全伸膝动作时更接近平衡,提示适当屈膝时的闭链运动能够更有选择性地激发VMO,这为PFPS的康复治疗的动作选择提供参考。     

表面肌电仪对髌股疼痛综合征患者膝关节肌电活动的分析

目的利用表面肌电仪分析髌股疼痛综合征患者康复训练前后股内侧斜肌（VMO）和股外侧肌（VL）肌电活动的变化。方法26例髌股疼痛综合征患者随机分为神经肌电生物反馈训练组（A组）和单纯训练组（B组）各13例，均接受相同的家庭训练，但A组患者在神经肌电生物反馈仪配合下进行训练。所有患者在开始训练前和训练8周后，利用表面肌电仪对VMO和VL的神经肌肉电活动进行连续6h评定，同时对膝关节疼痛程度进行评定。结果治疗后，B组患者的VMO／VL神经肌电比率与治疗前比较差异无显著性意义（P〉0．05），而A组患者的VMO／VL神经肌电比率与治疗前比较差异有显著性意义（P〈0．05）。结论采用神经肌电生物反馈仪配合髌股疼痛综合征患者的康复训练可促进股内侧斜肌的募集。     

Electromyographic activity of the quadriceps components during the final degrees of knee extension

An imbalance in the recruitment patterns of the quadriceps muscle has been implicated in patellofemoral knee pain; however ambiguity remains in the normal recruitment patterns of this muscle. We investigated the activity of the quadriceps muscle during the final degrees of knee extension, specifically in the medial and lateral components. Sixteen healthy subjects (age; 22.5 (± 3.4) years, body mass; 67.5 (± 12.1) kg and height; 173.9 (± 10.4) cm) participated.Torque and electromyographic (EMG) activity of the quadriceps muscle were measured during isokinetic and isometric knee extensions. Torque and EMG activity reduced in all component muscles during the final degrees of isokinetic extension, but this did not occur during isometric contractions at similar angles. Normalised activity of rectus femoris (RF) was greater than that of vastus lateralis (VL) and vastus medialis obliquus (VMO) during the final degrees of isokinetic knee extension at 30°/sec; there were no differences between any component muscles at the higher velocities. VMO:VL and vastus medialis longus (VML):VL ratios were unchanged during knee extensions, except that VML:VL ratio increased significantly during the final degrees of extension at 30°/sec. Future work should compare these results with people with anterior knee pain, and explore this further during activities of daily living.     

Vastus medialis obliquus and vastus lateralis activity in open and closed kinetic chain exercises in patients with patellofemoral pain syndrome: an electromyographic study

OBJECTIVES: To evaluate the electromyographic activities of vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles in open and closed kinetic chain exercises in subjects with patellofemoral pain syndrome (PFPS). DESIGN: Case-controlled study. SETTING: Rehabilitation science center in a tertiary medical center. PARTICIPANTS: Ten patients with bilateral knee pain diagnosed with PFPS and 10 healthy volunteers. INTERVENTIONS: Subjects performed open kinetic chain exercise on an isokinetic dynamometer and closed kinetic chain exercise by squat-to-stand and stand-to-squat tasks. Surface electromyography was done for the VMO and VL muscles. MAIN OUTCOME MEASURES: VMO/VL ratios were calculated after normalization of muscle activities. RESULTS: The VMO/VL ratios of PFPS subjects were significantly lower than were those of unimpaired subjects during knee isokinetic closed kinetic chain exercises (p = .047). However, there was no statistical difference in VMO/VL ratio between subjects with and without PFPS during closed kinetic chain exercises (p = .623). Maximum VMO/VL ratio was obtained at 60 degrees knee flexion in closed kinetic chain exercise. CONCLUSION: In closed kinetic chain exercises, more selective VMO activation can be obtained at 60 degrees knee flexion. Maximal VMO/VL ratio was observed at this knee flexion angle, and muscle contraction intensity was also greatest.     

Delayed onset of electromyographic activity of vastus medialis obliquus relative to vastus lateralis in subjects with patellofemoral pain syndrome

OBJECTIVE: To determine whether electromyographic (EMG) onsets of vastus medialis obliquus (VMO) and vastus lateralis (VL) are altered in the presence of patellofemoral pain syndrome (PFPS) during the functional task of stair stepping. DESIGN: Cross-sectional. SETTING: University laboratory. PATIENTS: Thirty-three subjects with PFPS and 33 asymptomatic controls. INTERVENTIONS: Subjects ascended and descended a set of stairs-2 steps, each 20-cm high-at usual stair-stepping pace. EMG readings of VMO and VL taken on middle stair during step up (concentric contraction) and step down (eccentric contraction). MAIN OUTCOME MEASURES: Relative difference in onset of surface EMG activity of VMO compared with VL during a stair-stepping task. EMG onsets were determined by using a computer algorithm and were verified visually. RESULTS: In the PFPS population, the EMG onset of VL occurred before that of VMO in both the step up and step down phases of the stair-stepping task (p <.05). In contrast, no such differences occurred in the onsets of EMG activity of VMO and VL in either phase of the task for the control subjects. CONCLUSION: This finding supports the hypothesized relationship between changes in the timing of activity of the vastimuscles and PFPS. This finding provides theoretical rationale to support physiotherapy treatment commonly used in the management of PFPS.     

Comparison of vastus medialis obliquus: vastus lateralis muscle integrated electromyographic ratios between healthy subjects and patients with patellofemoral pain

The purpose of this study was to compare vastus medialis obliquus:vastus lateralis muscle (VMO:VL) integrated electromyographic (IEMG) ratios of healthy subjects and patients with unilateral patellofemoral pain (PFP) under isotonic and isometric quadriceps femoris muscle contraction conditions. Subjects ranging in age from 18 to 35 years (mean = 28.06, SD = 5.97) were assigned to one of three groups on the basis of type of knee condition. In group 1, which consisted of seven healthy control subjects with no history of knee pathology, both knees were tested. In group 2, which consisted of nine patients with unilateral PFP, only the painful knee was tested. In group 3, which consisted of the same nine patients who comprised group 2, only the nonpainful knee was tested. Nonnormalized and normalized VMO:VL IEMG ratios were computed for ascending stairs, descending stairs, submaximal isometric contraction, and maximal isometric contraction (nonnormalized only). A two-way analysis of variance for repeated measures indicated VMO:VL ratios for isotonic stair-climbing activities were significantly greater than VMO:VL ratios for isometric contractions. Nonnormalized VMO:VL ratios in group 1 were significantly greater than nonnormalized VMO:VL ratios in the other two groups. Patients with PFP may have abnormal VMO:VL activation patterns, and isotonic quadriceps femoris muscle exercise may elicit more favorable muscle activation patterns than isometric exercise for patients with PFP.     

Activation of vastus medialis oblique is not delayed in patients with osteoarthritis of the knee compared to asymptomatic participants during open kinetic chain activities

This study investigated whether the onset of electromyographic (EMG) activity of vastus medialis oblique (VMO) was delayed relative to that of vastus lateralis (VL) in patients with osteoarthritis (OA) of the knee compared to asymptomatic participants during open kinetic chain activities. An exploratory observational cross sectional study was carried out. Two groups were tested, symptomatic OA knee patients, diagnosed by an orthopaedic surgeon, (n=17), mean (SD) age 66.0 (7.6) years, and asymptomatic participants (n=17), 56.7 (8.6) years. Surface EMG activity of VMO and VL was measured, during concentric contractions extending the knee from 90 degrees flexion, and during maximal voluntary isometric contractions at 60 degrees knee flexion. The EMG onset times of VMO and VL were determined visually and by algorithm. The onset timing difference (OTD) between the two muscles was calculated for each subject, by subtracting the onset time of VL from VMO. Mann-Whitney U-tests revealed that the OTD between VMO and VL was not significantly different between the groups during either contraction type (both p>0.05). The results of this exploratory study may have implications for rehabilitation programmes aimed at developing preferential activation of VMO compared to VL in OA knee patients.     

Ankle positioning and knee perturbation affect temporal recruitment of the vasti muscles in people with patellofemoral pain

Objectives

To compare the temporal recruitment of the vastus medialis obliquus (VMO) and vastus lateralis (VL) during voluntary ankle movements and perturbed standing in people with patellofemoral pain, and to determine the effects of different reflex and voluntary postural exercise tasks on VMO facilitation.

Design

Repeated-measures design.

Participants

Twenty-three subjects with patellofemoral pain.

Interventions

Quadriceps reflex contraction in response to postero-anterior knee perturbations was measured with three crural muscle contraction conditions and three postural exercises (semi-squatting, tip-toeing and heel standing).

Main outcome measures

The electromyographic (EMG) onset time of the VMO and VL during each task was measured and compared across the different tasks.

Results

The mean EMG onset time of the VMO was later than that of the VL in the voluntary tasks such as tip-toeing (VMO 95.3 ms vs VL 36.4 ms, mean difference 58.9 ms, 95% confidence interval −33.7 to 151.5 ms), whereas earlier VMO activation was found in the perturbation tests such as toe standing (VMO 17.6 ms vs VL 22.9 ms, mean difference −5.3 ms, 95% confidence interval −25.3 to 14.7 ms).

Conclusion

These findings suggest the potential benefits of unexpected perturbation activities for facilitating VMO activation. The clinical applications of perturbation tasks in rehabilitation exercise programmes and the underlying mechanisms warrant further investigation.     

Vastus Lateralis Motor Unit Firing Rate Is Higher in Women With Patellofemoral Pain

Objective

To compare neural drive, determined from motor unit firing rate, in the vastus medialis and lateralis in women with and without patellofemoral pain.

Electromyographic fatigue characteristics of the quadriceps in patellofemoral pain syndrome

This study compared the fatigue characteristics of the vastus medialis oblique (VMO), vastus lateralis (VL) and rectus femoris (RF) muscles. Ten healthy subjects with 10 patients with patellofemoral pain syndrome (PFPS) performed an isometric leg press for 60 seconds at 60% MVIC with data collected using surface EMG. The power spectrum was analyzed and the extracted median frequency normalised to calculate a linear regression slope for each muscle. A repeated measures ANOVA revealed no significant differences, neither between the groups (P=0.592) nor the muscles (P=0.434). However, the slopes for the VMO and VL were different between the two groups with similar slopes for the RF. There was much larger variability of MF values in the PFPS group. The VMO:VL ratio calculated from these slopes for the healthy subjects was 1.17 and for the PFPS group was 1.78. These results may indicate unusual features in the fatigue indices of the quadriceps in PFPS.     

Effects of Open and Closed Kinetic Chains of Sling Exercise Therapy on the Muscle Activity of the Vastus Medialis Oblique and Vastus Lateralis

[Purpose] The muscle strength of the quadriceps muscle is critical in patellofemoral pain syndrome. The quadriceps muscle supplies the power for dynamic patellar movement, and the vastus medialis oblique (VMO) and vastus lateralis (VL) enable the patella to stabilize during tracking. We followed the theories about open and closed kinetic chain exercises to design two exercises, sling open chain knee extension (SOCKE) exercise and sling closed chain knee extension (SCCKE) exercise. The purpose of our study was to research the changes in quadriceps muscle activity during both exercises. [Methods] Electromyographic analysis was used to explore the different effects of the two exercises. The MVC% was calculated for the VMO and VL during exercise for analysis. [Results] We found that the mean MVC% values of the VMO and VL during the SOCKE exercise were higher than those during the SCCKE exercise. The ratio of the VMO to VL was 1.0 ± 0.19 during the SOCKE exercise and 1.11 ± 0.15 during the SCCKE exercise. [Conclusions] The SOCKE exercise is targeted at quadriceps muscle training and has a recruitment effect on the VMO. The beneficial effect of the SOCKE exercise is better than that of the SCCKE exercise.Key words: Patellofemoral pain syndrome, Sling exercise therapy, Electromyography     

Electromyographic comparison of vastus medialis obliquus facilitatory versus vastus lateralis inhibitory kinesio taping in athletes with patellofemoral pain: A randomized clinical trial

BackgroundPatellofemoral pain (PFP) is among the most common orthopedic complications afflicting active young people. Diminished coordination of the vastii, delayed activation of the vastus medialis obliquus (VMO), and decreased VMO-to-vastus lateralis (VL) activity ratio are well documented as underlying causes of PFP. This study compares the effects of VMO facilitatory kinesio tape (KT) versus VL inhibitory KT on electromyographic (EMG) activity of the vastii, balance, and pain in athletes with PFP.MethodsIn this single-blind randomized clinical trial, thirty-two female athletes with PFP (mean age 26.33 ± 5.93 years) were randomly assigned to VMO facilitatory KT (n = 16) or VL inhibitory KT (n = 16) groups. In the facilitatory group, a Y-shaped strip of KT at 25% of its available tension was attached from the origin of the VMO to its insertion and in the inhibitory group, an insertion-to-origin Y-shaped strip of KT at 15% of its available tension was applied on the VL. Pain intensity, dynamic balance, and EMG data were assessed respectively with a visual analogue scale, the modified Star Excursion Balance Test, and an EMG telemetry system, before and immediately after KT application.ResultsPain intensity decreased and dynamic balance improved significantly after taping in both groups, and VMO: VL activity ratio increased significantly in the VL group. However, none of the parameters differed significantly between groups.ConclusionsBoth VMO facilitatory and VL inhibitory KT can improve pain and balance, while the inhibitory technique might be more effective in regulating the VMO to VL activity ratio in athletes with PFP.     

Patellar kinematics, part I: the influence of vastus muscle activity in subjects with and without patellofemoral pain

BACKGROUND AND PURPOSE: Reduced motor unit activity of the vastus medialis muscle relative to the vastus lateralis muscle has been implicated as a cause of lateral patellar subluxation. The purpose of this study was to assess the influence of vastus muscle motor unit activity on patellar kinematics. SUBJECTS: Twenty-three women (mean age=26.8 years, SD=8.5, range=14-46) with a diagnosis of patellofemoral pain and 12 women (mean age=29.1 years, SD=5.0, range=24-38) without patellofemoral pain participated. Only female subjects were studied because of potential biomechanical differences between sexes. METHODS: Patellar kinematics (kinematic magnetic resonance imaging) and vastus muscle electromyographic (EMG) activity using indwelling electrodes were measured during resisted knee extension. Measurements of medial and lateral patellar displacement and tilt obtained from magnetic resonance images were correlated with normalized vastus lateralis:vastus medialis oblique muscle and vastus lateralis:vastus medialis longus muscle EMG ratios at 45, 36, 27, 18, 9, and 0 degrees of knee flexion using a stepwise regression procedure. RESULTS: The vastus lateralis:vastus medialis longus muscle EMG ratio contributed to the prediction of lateral patellar glide at 27 degrees of knee flexion (r=-.48), with increased vastus medialis longus muscle activity being associated with greater lateral patellar displacement. A similar inverse relationship was evident with lateral patellar tilt at 36, 27, 18, and 9 degrees of knee flexion. CONCLUSION AND DISCUSSION: These results suggest that increased motor unit activity of the vastus medialis muscle appears to be associated with abnormal patellar kinematics in women, but it is not necessarily a cause of abnormal patellar kinematics.     

Contribution of components of the quadriceps femoris muscle for producing external torque in patients with patellofemoral pain syndrome

IntroductionDifferent muscular activities of the quadriceps components for producing necessary torque may change in patients with patellofemoral pain syndrome (PFPS). The aim of the current study, therefore, was to assess the contribution of each component of the quadriceps femoris muscle for producing external torque in patients with PFPS.MethodTwelve females with PFPS (24.7 ± 2.3 years) and twelve healthy matched females (25.4 ± 2.4 years) performed three consecutive knee flexion and extension movements with maximum effort at 45°/s and 300°/s using a Biodex system 3 dynamometer. Simultaneously, electromyographic (EMG) activities of the vastus medialis oblique (VMO), RF (rectus femoris) and vastus lateralis (VL) muscles were recorded using a DataLog instrument. Standard multiple regressions were used to assess the ability of EMG activities of the VMO, RF and VL muscles to predict normalized quadriceps femoris isokinetic concentric and eccentric torques at 45°/s and 300°/s in the normal and patient groups.ResultsIn the normal group, the VL and the VMO were the good predictors of quadriceps concentric torque at 45°/s and 300°/s, respectively. The VL and the RF were the good predictors of quadriceps eccentric torque at 300°/s in the patient group. No other conditions showed a considerable prediction for quadriceps torque in the normal or patient group.ConclusionFemales with PFPS differ with normal females in terms of the contribution of each component of the quadriceps femoris for producing external torque. Training the VMO for concentric contraction at both high and low velocities should be included in the management of the patients with PFPS.     

膝骨关节炎患者股内侧肌、股直肌和股外侧肌间的协调性

目的应用表面肌电图和等速肌力测定方法比较膝骨关节炎患者与正常人股内侧肌、股直肌和股外侧肌间的协调性改变。方法34例膝骨关节炎患者和34名膝关节健康者在膝关节屈曲10°、60°、100°等长伸膝和等速60°/s、180°/s伸膝运动测试模式下,进行股内侧肌(VM)、股直肌(RF)和股外侧肌(VL)的表面肌电图和股四头肌肌力的评测,包括肌肉激活启动顺序和VM/VL神经肌电比值。结果与正常人对比,膝骨关节炎患者在等速180°/s伸膝运动时VM相对于VL启动延迟（P＜0.05）;在膝屈10°等长伸膝运动时,VM/VL神经肌电比值降低（P＜0.05）。结论膝骨关节炎患者患侧股四头肌的协调性减退。     

Physiotherapy education in a digital era: blending and flipping

Abstract

The evidence base for changes in electromyographic (EMG) onset and intensity of vastus medialis (VM) and vastus lateralis (VL) in patients with patellar instability is systematically reviewed. The databases AMED, CINAHL, the Cochrane Library, EMBASE, Ovid Medline, Physiotherapy Evidence Database (PEDro), Pubmed and Zetoc were searched from their inception to March 2008, in addition to a manual search of relevant journals. All English-language papers assessing the relative EMG onset and intensity of vastus medialis oblique (VMO) and VL, in patients with patellar dislocation, subluxation and general instability were included. Five papers, consisting of 73 knees with patellar instability, were reviewed. No studies were identified assessing VMO–VL onset in patients with patellar instability. Four studies reported no difference in relative EMG intensity of VMO and VL in patients with patellar instability compared with asymptomatic control subjects. One study reported some evidence of a difference in VM to VL EMG intensity in one cohort of patients with patellar subluxation. The CASP appraisal of the evidence base highlighted a number of methodological weaknesses. There was no robust evidence for any difference in the relative intensity of EMG activity between the VMO and VL in patients with patellar instability. There is no good-quality research evidence to suggest that abnormal vastii EMG intensity or onset are aetiological factors associated with patellar instability.     

Altered vastii recruitment when people with patellofemoral pain syndrome complete a postural task

OBJECTIVE: To investigate the recruitment of the vastus medialis obliquus (VMO) and vastus lateralis during voluntary tasks that challenge the stability of the knee and to evaluate whether there is a change in the coordination of the postural response by the central nervous system in subjects with patellofemoral pain syndrome (PFPS). DESIGN: Cross-sectional. SETTING: University laboratory in Australia. PARTICIPANTS: Thirty-seven subjects with PFPS and 37 asymptomatic sex-matched controls. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Recordings of electromyographic activity of the VMO, vastus lateralis, tibialis anterior, and soleus were made by using surface electrodes. Subjects rose onto their toes (rise task) or rocked back on their heels (rock task) in a visual choice-reaction time task. Electromyographic onsets were determined by using a computer algorithm and were verified visually. RESULTS: Our results confirm that, in asymptomatic subjects, contraction of the VMO and vastus lateralis occurs as part of the feed-forward postural response associated with ankle movements in standing, and the contraction of these separate heads of the quadriceps group occurs simultaneously. However, when subjects with PFPS perform identical tasks, the electromyographic onset of the vastus lateralis occurs before that of the VMO. CONCLUSION: These findings indicate a difference in motor control in subjects with PFPS. They also support the hypothesized relation between changes in the timing of activity of the vastii and PFPS and provide the theoretic rationale to support physiotherapy treatment commonly used in the management of PFPS.     

The effects of squatting with visual feedback on the muscle activation of the vastus medialis oblique and the vastus lateralis in young adults with an increased quadriceps angle

[Purpose] The purpose of this study was to identify the effects of performing squat exercises with visual feedback on the activation of the vastus medialis oblique (VMO) and vastus lateralis (VL) muscles in young adults with an increased quadriceps angle (Q-angle). [Subjects] This study used a motion analysis program (Dartfish, Switzerland) to select 20 young adults with an increased Q-angle, who were then divided into a squat group that received visual feedback (VSG, n=10) and a squat group that received no visual feedback (SG, n=10). [Methods] The intensity of exercises was increased every two weeks over a six-week exercise period in both groups. A visual marker was attached to the patella of the subjects in the VSG, and they then performed squat exercises with a maximum of 90° of knee flexion within a route marked on a mirror. The SG performed squat exercises with a maximum 90° of knee flexion without attaching a visual feedback device. [Results] Analysis of the muscle activation due to 90° squat exercises indicated that both groups had statistically significant increases in activation of the VL. The VSG exhibited statistically significant increases in activation of the VMO. [Conclusion] This study confirmed that squat exercises with visual feedback are effective in activation of the VMO and VL muscles. The findings are meaningful in terms of preventing the occurrence of patellofemoral pain.Key words: Q-angle, Visual feedback squat exercise, VMO     

Changes in Motor Unit Firing Rate in Synergist Muscles Cannot Explain the Maintenance of Force During Constant Force Painful Contractions

The firing rate of low threshold motor units is decreased in constant force contractions during experimental pain. However, as firing rate is a determinant of force, it is unclear how force is maintained. Increased synergist muscle activity may compensate. This was investigated by evaluation of motor unit firing rate in synergist ankle plantar flexor muscles (triceps surae). Single motor unit action potentials were recorded in medial gastrocnemius and soleus muscles with fine wire electrodes in 10 subjects. Gross muscle activity was estimated from surface electromyographic (EMG) recordings. Bolus injections of 5% hypertonic saline were injected into lateral gastrocnemius to induce pain (low intensity, 0.5 mL; high intensity, 1.5 mL). Subjects gently plantar-flexed the ankle to recruit 1 to 4 motor units and performed 3 20-second contractions to this target before, during, and after pain. Firing rate decreased ∼12% in synergist heads of triceps surae during pain and recovered after pain. Despite reduced firing rate, root-mean-square surface EMG amplitude did not change. The effect of nociceptor stimulation is not restricted to painful muscles but reduces motor unit firing in synergist muscles. Changes in synergist muscles cannot explain the maintenance of muscle force. Maintenance of surface EMG amplitude suggests recruitment of additional motor units.PerspectiveThis study showed that activity of synergist muscles can be affected by muscle pain. However, the changes in activity of synergist muscles may not compensate for changes in the painful muscle. This finding provides evidence of more widespread effects of pain on muscle control.