Isometric Wrist Research Articles

Coordinate Transformation is First Completed Downstream of Primary Motor Cortex

It was suggested previously that the transformation of action to muscle-based coding is completed in the primary motor cortex (M1). This is consistent with a predominant direct pathway leading from M1 to motoneurons. Accordingly, spinal segmental interneurons that are located downstream to M1 are expected to show muscle-like coding properties. We addressed this hypothesis using simultaneous recording of cortical and spinal activity in primates performing an isometric wrist task with multiple targets and two hand postures. Here we show that while the motor cortex follows an intermediate coordinate frame, spinal interneurons already follow a muscle-like coordinate frame. We thus suggest that the final steps in coordinate transformation of motor commands take place downstream of M1 via corticospinal interactions.

Isotonic and isometric contractions exert the same amount of corticomotor system excitability in healthy subjects and patients after stroke

Active, voluntary training of the centrally paretic upper limb is crucial for functional recovery after brain damage. The aim of this study was to determine whether the type of voluntary contraction has a differential influence on corticomotor system excitability in healthy subjects and patients after stroke. Experimental cross-sectional study. Fifteen healthy volunteers and 15 patients after stroke. Participants performed dynamic isotonic and isometric voluntary wrist extensions with the non-dominant or the paretic hand, respectively, with force levels of 10%, 20% and 30% of the maximum voluntary surface electromyogram. Excitability was measured by comparing the amplitude of motor evoked potentials elicited by transcranial magnetic stimulation. The type of contraction did not have any effect on the amplitude of motor evoked potentials, either in healthy subjects or in patients after stroke. Dynamic isometric and isotonic voluntary contractions seem to have the same effect on the excitability of the corticomotor system.

Muscle fatigue in relation to forearm pain and tenderness among professional computer users

BackgroundTo examine the hypothesis that forearm pain with palpation tenderness in computer users is associated with increased extensor muscle fatigue.MethodsEighteen persons with pain and moderate to severe palpation tenderness in the extensor muscle group of the right forearm and twenty gender and age matched referents without such complaints were enrolled from the Danish NUDATA study of neck and upper extremity disorders among technical assistants and machine technicians. Fatigue of the right forearm extensor muscles was assessed by muscle twitch forces in response to low frequency (2 Hz) percutaneous electrical stimulation. Twitch forces were measured before, immediately after and 15 minutes into recovery of an extensor isometric wrist extension for ten minutes at 15 % Maximal Voluntary Contraction (MVC).ResultsThe average MVC wrist extension force and baseline stimulated twitch forces were equal in the case and the referent group. After the fatiguing contraction, a decrease in muscle average twitch force was seen in both groups, but the decrease was largest in the referent group: 27% (95% CI 17–37) versus 9% (95% CI -2 to 20). This difference in twitch force response was not explained by differences in the MVC or body mass index.ConclusionComputer users with forearm pain and moderate to severe palpation tenderness had diminished forearm extensor muscle fatigue response. Additional studies are necessary to determine whether this result reflects an adaptive response to exposure without any pathophysiological significance, or represents a part of a causal pathway leading to pain.

Connected Corticospinal Sites Show Enhanced Tuning Similarity at the Onset of Voluntary Action

Corticospinal (CS) pathways provide the structural foundation for executing voluntary movements. Although the anatomy of these pathways is well explored, little is known about spinal decoding of parametric information transmitted via this route during voluntary movements. We addressed this question by simultaneously recording cortical and spinal activity in primates performing an isometric wrist task with multiple targets while measuring CS interactions. Single-pulse cortical stimulation effectively produced a short-latency (presumably monosynaptic) spinal response and thus revealed functionally connected CS sites. Spinal and cortical neurons recorded from connected CS sites showed alignment of directional-torque tuning that peaked at torque onset, consistent with the enhanced cortical drive active during this period. This increased tuning similarity was accompanied by an increased trial-to-trial covariability of firing. Whereas functional CS interactions were dynamic, the efficacy of cortical stimulation was unaffected by the motor state. These results suggest that around the onset of motor action there is a period of facilitated information transfer during which cortical command has greater efficacy in recruiting spinal neurons with matching tuning properties. Dynamic alignment of response properties may form the basis for a spinal readout mechanism of descending motor commands in which directional-torque is a parameter that is preserved across interacting CS sites.

Afferent Encoding of Central Oscillations in the Monkey Arm

We have investigated whether peripheral afferent fibers could encode the central oscillations that are commonly seen in the primate motor system. We analyzed 52 single afferent recordings from the C8/T1 dorsal root ganglia of two monkeys performing an isometric wrist flexion-extension task. Coherence and directed coherence were calculated between the afferent spikes and forearm EMG. Seven of 52 cells were identified as Group Ia afferents by the production of narrow postspike facilitation in spike-triggered averages of rectified EMG. These identified afferents showed significant coherence, and directed coherence, with EMG over a wide frequency range. By contrast, coherence was weak for a population that showed little directional preference for flexion or extension movements during task performance, and probably contained mainly cutaneous afferents. Oscillations are known to appear in muscle activity; their presence in afferent firing as well implies that central oscillations pass around a peripheral feedback loop and may be involved in sensorimotor integration.

Effects of focal hand dystonia on visually guided and internally guided force control

Background: A fundamental feature underlying many movement disorders is increased variability in the motor response. Despite abnormalities of grip force control in people with dystonia, it is not clear whether...

Joint-Angle–Dependent Neuromuscular Dysfunctions at the Wrist in Persons After Stroke

Hu X, Tong K, Tsang VS, Song R. Joint-angle–dependent neuromuscular dysfunctions at the wrist in persons after stroke. Objective To evaluate the joint-angle–dependent neuromuscular functions at the affected wrist in hemiplegic subjects after stroke while doing isometric maximal voluntary wrist flexion and extension across different wrist angles. Design We investigated torques during isometric maximal voluntary wrist flexions and extensions at 8 different wrist angles, ranging from −45° to 60°. We used the associated electromyographic activities of 2 agonist and antagonist muscle pairs related to wrist and elbow joints for the analysis of muscular coactivations. We compared the data obtained from poststroke subjects’ affected and unaffected sides. Setting A research laboratory in a rehabilitation center. Participants Eleven subjects with hemiplegia after stroke with passive range of motion (ROM) in the wrist from −45° to 60°. Interventions Not applicable. Main Outcome Measures Directly measured torques, torques after normalization during maximal isometric wrist contractions, and normalized moving average electromyographic signals of each muscle at the tested positions. Results The measured torques of the affected wrists were significantly lower than those of the unaffected wrists at all tested angles during wrist flexion and extension ( P<.05). The angle-dependent patterns of the normalized torque across the tested wrist angles varied from those of the unaffected wrists (2-way analysis of variance, P<.05). There were decreases in normalized torques during both flexion and extension at the extended positions in the affected group ( P<.05). Abnormal cocontractions were found in agonist and antagonist muscle pairs related to wrist and elbow joints, and between the elbow flexor and wrist extensor when subjects did the wrist contractions on the paretic side, especially at the wrist extended positions. Conclusions Wrist muscle weakness was distributed unevenly across the selected wrist ROM on the affected side, as represented by the varied patterns of the normalized torque-angle relationship, compared with the unaffected wrists. There were reductions in the selective control of muscle coactivating synergies both single-jointly and cross-jointly in the impaired nervous system during wrist contractions; the extent of these reductions was also related to the wrist angle configuration.

Co-contraction of the pronator teres and extensor carpi radialis during wrist extension movements in humans

In order to elucidate the functional significance of excitatory spinal reflex arcs (facilitation) between musculus (M.) pronator teres (PT) and M. extensor carpi radialis (ECR, longus: ECRL, brevis: ECRB) in humans, activities of the muscles were studied with electromyography (EMG) and electrical neuromuscular stimulation (ENS). In EMG study, activities of PT, ECRL, ECRB, and M. flexor carpi radialis during repetitive static (isometric) wrist extension and a series of a dynamic motion of wrist flexion/extension in the prone, semiprone, and supine positions of the forearm were recorded in 12 healthy human subjects. In the prone, semiprone, and supine positions, PT and ECR showed parallel activities during the static extension in all, eight, and eight subjects, respectively, and at the extension phase during the dynamic motion in all, eight and five subjects, respectively. These findings suggest that co-contraction of PT and ECR occurs during wrist extension movements at least with the prone forearm. The facilitation must be active during the co-contraction. In ENS study, ENS to PT was examined in 11 out of the 12 and that to ECRL was in the 12 subjects. Before ENS, the forearm was in the prone, semiprone, and supine positions. In all the subjects, ENS to PT induced a motion of forearm pronation to the maximum pronation. ENS to ECRL induced motions of wrist extension to the maximum extension and abduction (radial flexion) to 5–20 degrees of abduction regardless of the positions of the forearm. Moreover, it induced 30–80 degrees supination of the forearm from the prone position. Consequently, combined ENS to PT and ECRL resulted in motions of the extension and abduction while keeping the maximum pronation. These findings suggest that the co-contraction of PT and ECR during wrist extension movements occurs to prevent supinating the forearm. Forearm supination from the prone position should be added to one of the actions of ECRL.

Contribution of Intramuscular Oxidative Metabolism to Total ATP Production during Forearm Isometric Exercise at Varying Intensities

It is not fully clear whether intramuscular oxidative metabolism contributes to total adenosine triphosphate (ATP) production during forearm isometric exercise at varying intensities. We tested hypothesis that oxidative metabolism with intramuscular O2 contributes to lessen the dependence on anaerobic metabolism, in particular phosphocreatine (PCr) breakdown. Seven male subjects were tested for changes in muscle oxygenation (MO2) and high-energy phosphates in forearm flexor muscles at rest and during exercise under arterial occlusion by 31-phosphorus magnetic resonance spectroscopy (31P-MRS) and near infrared spectroscopy (NIRS). Isometric wrist flexion exercise was performed for 1 min or until exhaustion at intensities corresponding to 30%, 50% and 70% of maximal voluntary contraction (MVC) under intramuscular O2 (Intramuscular O2-Ex) and anaerobic (Anaero-Ex) conditions. Oxidative ATP production in Intramuscular O2-Ex was calculated as 0.05 +/- 0.01 mM/s for 30%MVC, 0.08 +/- 0.01 mM/s for 50%MVC and 0.11 +/- 0.01 mM/s for 70%MVC. At a lower intensity (30%MVC), PCr breakdown rate (0.17 +/- 0.02 mM/s) of Anaero-Ex was significantly higher than the rate (0.13 +/- 0.01 mM/s) of Intramuscular O2-Ex (p < 0.05). There was no significant difference in ATP production rates through PCr breakdown and glycolysis between Intramuscular O2-Ex and Anaero-Ex at the higher intensities (50% and 70%MVC). In conclusion, intramuscular oxidative metabolism plays a significant role in reducing the dependence on PCr breakdown during isometric exercise at a lower intensity (30%MVC).

Effects of Neuromuscular Electrical Stimulation Treatment of Cerebral Palsy on Potential Impairment Mechanisms: A Pilot Study

This pilot study examined the effects of neuromuscular electrical stimulation (NMES) therapy on upper limb impairment in children with cerebral palsy, specifically addressing spasticity, heightened passive resistance to wrist rotation, coactivation, and weakness. Eight subjects, aged five to 15 years, with spastic hemiparesis subsequent to brain injury, participated in three months of NMES therapy, targeting the wrist flexor and extensor muscles. Maximum voluntary wrist extension range of motion against gravity, spasticity, passive torque, maximum voluntary isometric torque, and coactivation were recorded prior to, during, and at the conclusion of the therapy. Seven of the eight subjects demonstrated a significant (>15 degrees) improvement in wrist extension range of motion against gravity following the NMES treatment, with an average gain of 38 degrees. Differences in spasticity (0.01 +/- 0.14 N-m, p = 0.80) and passive torque (0.03 +/- 0.11 N-m, p = 0.52) were not significant for these subjects. Isometric wrist extension torque, however, did increase significantly (p < 0.01), accompanied by a reduction in flexor coactivation (p < 0.01). Evidence suggests that the NMES treatment protocol affected wrist extension by improving the strength of the wrist extensor muscles, possibly through decreased flexor coactivation. Further studies are required, however, to determine whether electrical stimulation itself or other facets of the therapy paradigm played the key role in improvement.

The amplitude of Mmax in human wrist flexors varies during different muscle contractions despite constant posture

The amplitude of the maximal direct motor response (Mmax) elicited by supramaximal peripheral nerve stimulation can vary with time and with changes in muscle length. We sought to investigate the variability in the amplitude of Mmax in the human wrist flexors (flexor carpi radialis, FCR) at a constant joint position during different functional tasks. The subjects performed isometric wrist extension, radial deviation and gripping contractions matching either 10% or 50% of the EMG activity recorded in the extensor carpi radialis brevis (ECRB) during a wrist extension maximal voluntary contraction (MVC). Three supramaximal stimuli were delivered to the median nerve near the elbow during each task with 2–3 s between stimuli. Considerable variation was observed in the Mmax amplitude between the six tasks for individual subjects (coefficient of variation (CV) range, 6.0–31.4%). There was significantly greater variability between tasks at 50% MVC than at 10% MVC ( p = 0.017). However, there were no systematic differences in Mmax amplitude between the six tasks across the group ( p > 0.05). These results suggest that the amplitude of Mmax cannot be assumed as constant during experiments involving voluntary contractions even when subjects maintain the same posture.

Signs of muscle thixotropy during human ballistic wrist joint movements

A study was conducted on healthy subjects to determine whether voluntary ballistic wrist flexion movements are influenced by immediately preceding conditioning of the forearm muscles. Single rapid wrist flexion movements were made in response to an auditory "Go" signal. Rectified surface EMG was recorded from wrist flexors and extensors, and joint position was measured by a goniometer. The movements were preceded (2-3 s) by four different conditioning routines: 40-s rest (Rest), 10-s voluntary alternating wrist joint flexion and extension movements (Osc), and 10 s of 25 degrees weak isometric wrist extensor (Ext) or flexor contractions (Flex). When subjects made ballistic movements after Osc compared with Rest, peak velocity was higher (P = 0.02) and movement time shorter (P = 0.06), but there was no difference (P = 0.83) in motor reaction time (time between the onset of the first agonist burst and movement onset). If the movements were preceded by Ext compared with Flex, motor reaction time was longer (P = 0.01), indicating a longer electromechanical delay. There were no indications that postconditioning differences in agonist or antagonist muscle activity could explain the results. It was also demonstrated that, after Rest, peak velocity was lower (P < 0.01) for the first than for the second of a series of repetitive ballistic movements. The observations corresponded to results from passive experiments in which the median nerve was electrically stimulated. In conclusion, history-dependent (thixotropic) changes in skeletal muscle resistance seem to have implications for voluntary ballistic wrist movements. The study also provided evidence that muscle conditioning influences the central nervous reaction time preceding ballistic contractions.

Task dependent gain regulation of spinal circuits projecting to the human flexor carpi radialis

In humans, the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles act as antagonists during wrist flexion-extension and as functional synergists during radial deviation. In contrast to the situation in most antagonist muscle pairs, Renshaw cells innervated by the motor neurons of each muscle inhibit the motoneurons, but not Ia inhibitory interneurons, of the opposite motor pool. Here we compared gain regulation of spinal circuits projecting to FCR motoneurons during two tasks: flexion and radial deviation of the wrist. We also investigated the functional consequences of this organisation for maximal voluntary contractions (MVCs). Electromyographic (EMG) recordings were taken from FCR, ECR longus and ECR brevis using fine-wire electrodes and electrical stimulation was delivered to the median and radial nerves. Ten volunteers participated in three experiments. 1. To study the regulation of the Renshaw cell-mediated, inhibitory pathway from ECR to FCR motoneurons, forty stimuli were delivered to the radial nerve at 50% of the maximal M-wave amplitude for ECR brevis. Stimuli were delivered during both isometric wrist flexions and radial deviation actions with an equivalent EMG amplitude in FCR (approximately 5% wrist flexion MVC). 2. To explore the homonymous Ia afferent pathway to FCR motoneurons, 50 stimuli were delivered to the median nerve at intensities ranging from below motor threshold to at least two times that which evoked a maximal M-wave during wrist flexion and radial deviation (matched FCR EMG at approximately 5% wrist flexion MVC). 3. EMG amplitude was measured during MVCs in wrist flexion, extension and radial deviation. There was no significant difference in the inhibition of FCR EMG induced via ECR-coupled Renshaw cells between radial deviation and wrist flexion. However, the mean FCR H-reflex amplitude was significantly (P<0.05) greater during wrist flexion than radial deviation. Furthermore, EMG amplitude in FCR and ECR brevis was significantly (P<0.05) greater during MVCs in wrist flexion and extension (respectively) than radial deviation. ECR longus EMG was significantly greater during MVCs in radial deviation than extension. These results indicate that the gain of the Renshaw-mediated inhibitory pathway between ECR and FCR motoneurons is similar for weak flexion and radial deviation actions. However, the gain of the H-reflex pathway to FCR is greater during wrist flexion than radial deviation. Transmission through both of these pathways probably contributes to the inability of individuals to maximally activate FCR during radial deviation MVCs.

Activity patterns of wrist extensor muscles during wrist extensions and deviations

Wrist extensor muscles are prone to certain focal musculoskeletal disorders for which the activation pattern of the extensor carpi radialis (ECR) and ulnaris (ECU) muscles may be important risk factors. Surface and intramuscular EMG of these muscles were recorded during isometric low-force wrist extension in semipronation and pronation as well as for ulnar/radial deviation, and were analyzed using root mean square (RMS) and decomposition methods. Despite shorter ECR length at semipronation, higher amplitudes of intramuscular EMG and of motor unit action potentials (MUAPs) were found in pronation than in semipronation. However, these changes were not detectable in the surface EMG. Higher ECR activity levels were also found during wrist extension compared to ulnar/radial deviation, and differences in the motor unit (MU) properties were found during ulnar deviation compared to radial deviation and extension. Remarkably, the MUAPs of ECR were almost twice as large as those of the ECU. Overall, the ECR muscle did not respond as predicted from biomechanical considerations, and in general activity level was higher than expected. This may partly explain why the tendon of the ECR often is associated with lateral epicondylitis.

The after effects of motor cortex rTMS depend on the state of contraction when rTMS is applied

Objective To test whether the after effects of 5 Hz focal rTMS over the primary motor cortex are modulated by the state of muscle contraction when the rTMS is applied. Methods Ten healthy volunteers gave informed consent to the studies. Fifty stimuli at active motor threshold intensity were applied at 5 Hz to the forearm motor area during 10% MVC isometric wrist flexion, 10% MVC wrist extension and at rest. Short interval intracortical inhibition (SICI: interstimulus intervals of 2–5 ms) and intracortical facilitation (interstimulus intervals of 7 and 10 ms) were recorded at rest before and after rTMS from flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles with surface electrodes. Results rTMS during wrist flexion decreased SICI in the FCR whereas SICI was increased if rTMS was given during wrist extension. The opposite was true in ECR: SICI was increased if rTMS was applied during wrist flexion, whereas it was reduced if rTMS was given during wrist extension. rTMS applied at rest produced a small overall decrease of SICI in FCR and ECR but this was not individually significant in the FCR. The 10 s wrist flexion without any rTMS did not have any effect on SICI. Conclusions The after effects of motor cortex rTMS can be selectively modulated by the pattern of muscle contraction.

Measurement of isometric wrist muscle strength--a systematic review of starting position and test protocol.

Isometric strength of the wrist musculature is frequently used in the clinical setting as a primary outcome measure following a wrist disorder. A systematic search of the literature was undertaken to review published starting positions and test protocols used in assessing isometric strength of the wrist musculature. AMED, CINAHL, Current Contents, EMBASE and MEDLINE computerized databases were searched for literature published between January 1980 and September 2002 that described the methodologies used to assess isometric wrist muscle strength. Papers were excluded if they explored motor unit recruitment, strength was assessed via manual muscle testing or no information regarding starting position or test protocol was included. A total of 15 papers met the inclusion criteria. Starting position varied between papers, with little justification for the positions assumed. Test protocols used also were variably described, with some not meeting accepted standards for the assessment of isometric strength. Many of the reports lacked information regarding starting position and test protocol used, which precluded replication of the methodology. Further research is required to determine the optimum starting position and test protocol for the assessment of isometric wrist muscle strength. These should subsequently be assessed for reliability, to ensure that consistent results are produced over different occasions of testing.

Further insights into post-exercise effects on H-reflexes and motor evoked potentials of the flexor carpi radialis muscles.

The present study investigated the relative contribution of the cortical and spinal mechanisms for post-exercise excitability changes in human motoneurons. Seven healthy right-handed adults with no known neuromuscular disabilities performed an isometric voluntary wrist flexion at submaximum continuous exertion. After the subjects continued muscle contraction until volitional fatigue, the H-reflexes induced by an electric stimulation and motor evoked potentials (MEPs) induced by a transcranial magnetic stimulation (TMS) from a flexor carpi radialis (FCR) muscle were recorded 7 times every 20 s. The H-reflex was used to assess excitability changes at the spinal level, and the MEP was used to study excitability changes at the cortical level. Hreflexes showed a depression (30% of control value) soon after the cessation of wrist flexion and recovered with time thereafter. On the other hand, an early (short latency) MEP showed facilitation immediately after the cessation of wrist flexion (50% of control value) and thereafter decreased. A possible mechanism for the contradictory results of the 2 tests, in spite of focusing on the same motoneuron pool, might be the different test potential sizes between them. In addition, a late (long latency) MEP response appeared with increasing exercise. With regard to the occurrence of late MEP response, a central mechanism may be proposed to explain the origin-that is, neural pathways with a high threshold that do not participate under normal circumstances might respond to an emergency level of muscle exercise, probably reflecting central effects of fatigue.

THE ROLE OF THE EXTENSOR DIGITORUM COMMUNIS MUSCLE IN LATERAL EPICONDYLITIS

A common finding in tennis elbow is pain in the region of the lateral epicondyle during resisted extension of the middle finger (Maudsley's test). We hypothesized that the pain is due to disease in the extensor digitorum communis muscle, rather than to compression of the radial nerve or disease within extensor carpi radialis brevis. Thirteen human forearm specimens were examined. It was found that the extensor digitorum communis was separable into four parts. The part to the middle finger originated from the lateral epicondyle, but the muscle slips to the other fingers originated more distally. Pain ratings were measured in ten patients diagnosed with lateral epicondylitis during isometric finger and wrist extension tests. The results confirmed the high prevalence of a positive Maudsley's test in lateral epicondylitis, and also that the patients with tenderness at the site of origin of the extensor digitorum communis slip to the middle finger had the greatest pain during middle finger extension. These anatomical and clinical findings clarify the anatomy of extensor digitorum communis, and suggest that this muscle forms the basis for the Maudsley's test. The muscle may play a greater role in tennis elbow than previously appreciated.

Muscle Weakness and Cocontraction in Upper Limb Hemiparesis: Relationship to Motor Impairment and Physical Disability

The purpose of this article is to describe the relationship between poststroke upper limb muscle weakness and cocontraction, and clinical measures of upper limb motor impairment and physical disability. Electromyographic (EMG) activity of the paretic and nonparetic wrist flexors and extensors of 26 chronic stroke survivors were recorded during isometric wrist flexion and extension. The root mean square (RMS) of the EMG signal was used as a measure of strength of contraction. A ratio of RMS of antagonist and agonist muscles was used as a measure of cocontraction. Upper limb motor impairment and physical disability were assessed with the Fugl-Meyer motor assessment (FMA) and the arm motor ability test (AMAT), respectively. The strength of muscle contraction was significantly stronger in the nonparetic limb (P < 0.001). The degree of cocontraction was significantly greater in the paretic limb (P < 0.001). The strength of muscle contraction in the paretic limb correlated significantly with FMA (r = 0.62 to 0.87,P ≤ 0.001) and AMAT (r = 0.66 to 0.80, P ≤ 0.001) scores. Similarly, the degree of cocontraction correlated significantly with FMA (r = -0.70 to -0.64, P ≤ 0.001) and AMAT (r = -0.72 to -0.62, P ≤ 0.001) scores. Muscle weakness and degree of cocontraction correlate significantly with motor impairment and physical disability in upper limb hemiplegia. This relationship may provide insights toward development of specific interventions. However, additional studies are needed to demonstrate a cause and effect relationship.

Common Drive in Motor Units of a Synergistic Muscle Pair

The interaction among the motor units of the extensor carpi radialis longus (ECRL) and the extensor carpi ulnaris (ECU) muscles in man was studied during wrist extensions in which the two muscles acted as synergists. Intramuscular recordings were obtained using special quadrifilar needle electrodes. Isometric wrist extensions at 20-30% of the maximal effort were studied. The electromyographic (EMG) signals were decomposed into the individual motor-unit action potential trains comprising the signal. The interaction among motor units were characterized by the estimated time-varying mean firing rate and the cross-correlation between the time-varying mean firing rates of pairs of motor units. Pairs of motor units within each muscle as well as pairs of motor units across the muscles were considered. In-phase common fluctuations, termed common drive, were observed in the mean firing rates of motor units within each muscle, consistent with earlier work on other muscles. Common fluctuations were also observed between the firing rates of ECU and ECRL motor units albeit with a variable phase shift. The existence of common drive across synergistic muscles was interpreted as implying that the CNS considers the muscles as a functional unit when they act as synergists.