Adherence and perceptions of a home sports video gaming program in persons with spinal cord injuries: A pilot study.

OBJECTIVE: Sports video-gaming can facilitate increased activity levels in persons with limited exercise options. Understanding how persons with spinal cord injuries (SCI) participate in home-based video-gaming and its potential impact on maintaining or enhancing physical function remains largely unexplored. The purpose of this study was to evaluate adherence, perceptions, and potential physical effects of a home sports video-game program for persons with chronic SCI. PARTICIPANTS: Fourteen individuals with chronic SCI (9 tetraplegia, 5 paraplegia). DESIGN: This was a prospective study. Wii video-gaming systems that included four sports games were provided to participants for home use. Participants were instructed to play for 8 weeks 3-4 days/week. The video consoles recorded the time and number of sessions played. OUTCOME MEASURES: At baseline and at the conclusion of the program, measurements were made of upper extremity strength, perceived exertion, active heart rate, pain, balance, and a functional upper extremity (UE) test. The total time and number of sessions were compared to the prescribed game play as a measure of adherence. Scores from a self-reported survey were used to gauge participants' satisfaction and perceptions of their gaming experience. RESULTS: Overall mean gaming adherence was 85% during the first 4 weeks and 69% for the second 4 weeks. There were no significant changes in upper extremity strength, active heart rate, balance, pain, or functional UE test for either group. All of the participants rated video gaming as enjoyable and 85.7% perceived that it could be used as a form of exercise. CONCLUSION: The Wii sports home video-gaming intervention elicited overall high adherence rates and was well received by study participants indicating that it may have value as an adjunctive tool for increasing physical activity for individuals with SCI.

Toward Consensus in Assessing Upper Limb Muscle Strength and Pinch and Grip Strength in People With Tetraplegia Having Upper Limb Reconstructions.

OBJECTIVES: To reach agreement on standardized protocols for assessing upper limb strength and grip and pinch force for upper limb reconstructive surgery for tetraplegia. METHODS: Selected members of an expert panel composed of international therapists formed at the 2018 International Congress for Upper Limb Surgery for Tetraplegia conducted a literature review of current practice that identified gaps and inconsistencies in measurement protocols and presented to workshop attendees. To resolve discrepancies, a set of questions was presented to workshop attendees who voted electronically. Consensus was set at 75% agreement. RESULTS: For manual muscle testing, consensus was reached for using the Medical Research Council scale, without plus or minus, and the use of resistance through range when testing grade 4 and grade 5 strength. Pectoralis major and serratus anterior should be routinely tested, however there was no consensus on other shoulder muscles. Grip and pinch strength should be tested according to the American Society of Hand Therapists positioning. For grip strength, either the Jamar or Biometrics dynamometer expressed in kilograms should be used. For grip and pinch strength, three measurements should be performed at each testing. No consensus was reached on a device for pinch strength. CONCLUSION: This work is an important step to enable comparable data in the future. Further consensus methods will work toward developing more comprehensive guidelines in this population. Building international consensus for pre- and postoperative measures of function supports objective evaluation of novel therapies and interpretation of multicenter studies.

Upper limb muscle activation during sports video gaming of persons with spinal cord injury.

OBJECTIVE: Video gaming as a therapeutic tool has largely been studied within the stroke population with some benefits reported in upper limb motor performance, balance, coordination, and cardiovascular status. To date, muscle activation of upper limb muscles in persons with spinal cord injuries (SCI) has not been studied during video game play. In this paper, we provide descriptive and comparative data for muscle activation and strength during gaming for players with tetraplegia and paraplegia, as well as, compare these results with data from traditional arm exercises (ie, biceps curl and shoulder press) with light weights which are commonly prescribed for a home program. PARTICIPANTS: Fourteen individuals with chronic SCI (9 tetraplegia, 5 paraplegia). DESIGN: We measured upper limb muscle activation with surface electromyography (EMG) during Wii Sports video game play. Muscle activation was recorded from the playing arm during 4 selected games and normalized to a maximum voluntary contraction (MVC). OUTCOME MEASURES: Heart rate and upper limb motion were recorded simultaneously with EMG. Wilcoxon signed rank tests were used to analyze differences in muscle activation between participants with paraplegia versus tetraplegia and compare gaming with traditional arm exercises with light weights. A Friedman 2-way analysis of variance identified key muscle groups active during game play. RESULTS: Overall muscle activation across the games was not different between those with paraplegia and tetraplegia. Heart rate during video game play for tennis and boxing were on average 10 to 20 beats/minute above resting heart rate. The magnitude of EMG was relatively greater for traditional arm exercises with light weights compared with game play. CONCLUSION: The selected Wii games were able to elicit upper extremity muscle activation and elevated heart rates for individuals with SCI that may be used to target therapeutic outcomes.

Multicenter Survey of the Effects of Rehabilitation Practices on Pinch Force Strength After Tendon Transfer to Restore Pinch in Tetraplegia.

OBJECTIVE: To identify key components of conventional therapy after brachioradialis (BR) to flexor pollicis longus (FPL) transfer, a common procedure to restore pinch strength, and evaluate whether any of the key components of therapy were associated with pinch strength outcomes. DESIGN: Rehabilitation protocols were surveyed in 7 spinal cord injury (SCI) centers after BR to FPL tendon transfer. Key components of therapy, including duration of immobilization, participation, and date of initiating therapy activities (mobilization, strengthening, muscle reeducation, functional activities, and home exercise), were recorded by the patient's therapist. Pinch outcomes were recorded with identical equipment at 1-year follow-up. SETTING: Seven SCI rehabilitation centers where the BR to FPL surgery is performed on a routine basis. PARTICIPANTS: Thirty-eight arms from individuals with C5-7 level SCI injury who underwent BR to FPL transfer surgery (N=34). INTERVENTION: Conventional therapy according to established protocol in each center. MAIN OUTCOME MEASURES: The frequency of specific activities and their time of initiation (relative to surgery) were expressed as means and 95% confidence intervals. Outcome measures included pinch strength and the Canadian Occupational Performance Measure (COPM). Spearman rank-order correlations determined significant relations between pinch strength and components of therapy. RESULTS: There was similarity in the key components of therapy and in the progression of activities. Early cast removal was associated with pinch force (Spearman ρ=-.40, P=.0269). Pinch force was associated with improved COPM performance (Spearman ρ=.48, P=.0048) and satisfaction (Spearman ρ=.45, P=.0083) scores. CONCLUSIONS: Initiating therapy early after surgery is beneficial after BR to FPL surgery. Postoperative therapy protocols have the potential to significantly influence the outcome of tendon transfers after tetraplegia.

Evaluation of a Task-Based Intervention After Tendon Transfer to Restore Lateral Pinch.

OBJECTIVE: To quantify changes in pinch force and brachioradialis (BR) activation after a task-based training program designed to improve pinch force after BR to flexor pollicis longus (FPL) transfer. DESIGN: One-group repeated-measures design compared pinch force and BR activation pre- and posttraining. Significant differences were tested with Wilcoxon signed-rank tests for pairwise comparisons at the P≤.05 level. SETTING: Testing occurred in a Veterans Affairs Medical Center research laboratory and training was in a home setting. PARTICIPANTS: Participants with cervical spinal cord injury (SCI) and previous BR to FPL transfer were enrolled in the study (N=8). Six patients completed the training program and posttraining measures. INTERVENTIONS: The 10-week training was a home program that included novel activities to increase BR activation and practice producing pinch force in a variety of upper limb postures. Participants were provided with the task-based training equipment and instructed to practice 3 times per week. MAIN OUTCOME MEASURES: Fine-wire electromyography of the transferred BR was recorded in maximum effort pinch force (N). Secondary measures included the strength and activation of the antagonist elbow extensor. RESULTS: Pinch force increased 3.7N (.38kg) and BR muscle activation increased 10% (P≤.05) after the training. There was no increase in elbow extension strength, but participants with previous posterior deltoid to triceps transfer achieved greater activation of the antagonist elbow extensor. CONCLUSIONS: The findings from this pilot study suggest that outcomes of tendon transfer and conventional therapy can be improved for patients with chronic cervical SCI.

Measurement of Outcomes of Upper Limb Reconstructive Surgery for Tetraplegia.

Reconstructive arm/hand surgery for tetraplegia is performed to improve arm/hand function and therefore personal well-being for individuals who accept such elective surgeries. However, changes at an impairment level do not always translate into functional or quality of life changes. Therefore, multiple outcome tools should be used that incorporate sufficient responsiveness to detect changes in arm/hand function, activity and participation, and quality of life of the individuals involved. This narrative review aims to assist clinicians to choose the most appropriate tools to assess the need for reconstructive surgery and to evaluate its outcomes. Our specific objectives are (1) to describe aspects to consider when choosing a measure and (2) to describe the measures advised by an international therapist consensus group established in 2007. All advised measures are appraised in terms of the underlying construct, administration, and clinical relevance to arm/hand reconstructions. Essentially there are currently no criterion standard measures to evaluate the consequences of reconstructive arm/hand surgery. However, with judicious use of available measures it is possible to ensure the questions asked or tasks completed are relevant to the surgical reconstruction(s) undertaken. Further work in this field is required. This would be best met by immediate collaboration between 2 outcome's tool developers and by analysis of pre- and postoperative data already held in various international sites, which would allow further evaluation of the measures already in use, or components thereof.

Rehabilitation After Surgical Reconstruction to Restore Function to the Upper Limb in Tetraplegia: A Changing Landscape.

Upper limb reconstructive surgical procedures for individuals with tetraplegia are performed in many centers internationally. Most recipients of surgery return to local communities and nonsurgical centers for postoperative rehabilitation and long-term follow-up. This supplement focuses on the clinical significance of upper extremity reconstruction, addressing issues related to the availability and choice for surgery, preoperative assessments, postoperative training paradigms, and appropriate outcome measures. Comprehensive intervention protocols are described in terms of dose, timing, specific activities, modalities, and related outcomes. Shared knowledge of current rehabilitation practice, as it relates to reconstructive surgery, can expand treatment options communicated to patients, increase the availability of postoperative muscle reeducation programs, and motivate long-term follow-up assessments.

Triceps Brachii in Incomplete Tetraplegia: EMG and Dynamometer Evaluation of Residual Motor Resources and Capacity for Strengthening.

BACKGROUND: Candidates for activity-based therapy after spinal cord injury (SCI) are often selected on the basis of manual muscle test scores and the classification of the injury as complete or incomplete. However, these scores may not adequately predict which individuals have sufficient residual motor resources for the therapy to be beneficial. OBJECTIVE: We performed a preliminary study to see whether dynamometry and quantitative electromyography (EMG) can provide a more detailed assessment of residual motor resources. METHODS: We measured elbow extension strength using a hand-held dynamometer and recorded fine-wire EMG from the triceps brachii muscles of 4 individuals with C5, C6, or C7 level SCI and 2 able-bodied controls. We used EMG decomposition to measure motor unit action potential (MUAP) amplitudes and motor unit (MU) recruitment and firing-rate profiles during constant and ramp contractions. RESULTS: All 4 subjects with cervical SCI (cSCI) had increased MUAP amplitudes indicative of denervation. Two of the subjects with cSCI had very weak elbow extension strength (<4 kg), dramatically reduced recruitment, and excessive firing rates (>40 pps), suggesting profound loss of motoneurons. The other 2 subjects with cSCI had stronger elbow extension (>6 kg), more normal recruitment, and more normal firing rates, suggesting a substantial remaining motoneuron population. CONCLUSIONS: Dynamometry and quantitative EMG may provide information about the extent of gray matter loss in cSCI to help guide rehabilitation strategies.

Comparison of wrist and elbow stabilization following pinch reconstruction in tetraplegia.

PURPOSE: Individuals with spinal cord injuries resulting in tetraplegia may receive tendon transfer surgery to restore grasp and pinch function. These procedures often involve rerouting the brachioradialis (Br) and the extensor carpi radialis longus tendons volar to the flexion-extension axis of the wrist, leaving the extensor carpi radialis brevis (ECRB) muscle to provide wrist extension strength. The purpose of this study was to determine whether externally stabilizing the wrist after transfer procedures would improve the ability to activate the transferred Br and resulting pinch force, similar to the effect observed when the elbow is externally stabilized. METHODS: We used a one-way repeated-measures study design to determine the effect of 3 support conditions on muscle activation and lateral pinch force magnitude in 8 individuals with tetraplegia and previous tendon transfer surgeries. Muscle activation was recorded from Br and ECRB with intramuscular electrodes and from biceps and triceps muscles with surface electrodes. We quantified pinch strength with a 6-axis force sensor and custom grip. We recorded measurements in 3 support conditions: with the arm self-stabilized, with elbow stabilization, and with elbow and wrist stabilization. Pairwise differences were tested using Wilcoxon signed-rank tests. RESULTS: Maximum effort pinch force magnitude and Br activation were significantly increased in both supported conditions compared with the self-supported trials. The addition of wrist stabilization had no significant effect compared with elbow stabilization alone. CONCLUSIONS: A strong ECRB has adequate strength to extend the wrist, even after multiple transfers that contribute an additional flexion moment from strong activation of donor muscles. Anatomical and functional differences between the wrist and elbow musculature are important determinants for self-stabilizing joints proximal to the tendon transfer. The ability to increase Br activation and resulting pinch force may be determined, in part, by the individual's ability to develop new coordination strategies.

A simulation analysis of the combined effects of muscle strength and surgical tensioning on lateral pinch force following brachioradialis to flexor pollicis longus transfer.

Biomechanical simulations of tendon transfers performed following tetraplegia suggest that surgical tensioning influences clinical outcomes. However, previous studies have focused on the biomechanical properties of only the transferred muscle. We developed simulations of the tetraplegic upper limb following transfer of the brachioradialis (BR) to the flexor pollicis longus (FPL) to examine the influence of residual upper limb strength on predictions of post-operative transferred muscle function. Our simulations included the transfer, ECRB, ECRL, the three heads of the triceps, brachialis, and both heads of the biceps. Simulations were integrated with experimental data, including EMG and joint posture data collected from five individuals with tetraplegia and BR-FPL tendon transfers during maximal lateral pinch force exertions. Given a measured co-activation pattern for the non-paralyzed muscles in the tetraplegic upper limb, we computed the highest activation for the transferred BR for which neither the elbow nor the wrist flexor moment was larger than the respective joint extensor moment. In this context, the effects of surgical tensioning were evaluated by comparing the resulting pinch force produced at different muscle strength levels, including patient-specific scaling. Our simulations suggest that extensor muscle weakness in the tetraplegic limb limits the potential to augment total pinch force through surgical tensioning. Incorporating patient-specific muscle volume, EMG activity, joint posture, and strength measurements generated simulation results that were comparable to experimental results. Our study suggests that scaling models to the population of interest facilitates accurate simulation of post-operative outcomes, and carries utility for guiding and developing rehabilitation training protocols.

The innervation and organization of motor units in a series-fibered human muscle: the brachioradialis.

We studied the innervation and organization of motor units in the brachioradialis muscle of 25 normal human subjects. We recorded intramuscular EMG signals at points separated by 15 mm along the proximodistal muscle axis during moderate isometric contractions, identified from 27 to 61 (mean 39) individual motor units per subject using EMG decomposition, and estimated the locations of the endplates and distal muscle/tendon junctions from the motor-unit action potential (MUAP) propagation patterns and terminal standing waves. In three subjects all the motor units were innervated in a single endplate zone. In the other 22 subjects, the motor units were innervated in 3-6 (mean 4) distinct endplate zones separated by 15-55 mm along the proximodistal axis. One-third of the motor units had fibers innervated in more than one zone. The more distally innervated motor units had distinct terminal waves indicating tendonous termination, while the more proximal motor units lacked terminal waves, indicating intrafascicular termination. Analysis of blocked MUAP components revealed that 19% of the motor units had at least one doubly innervated fiber, i.e., a fiber innervated in two different endplate zones by two different motoneurons, and thus belonging to two different motor units. These results are consistent with the brachioradialis muscle having a series-fibered architecture consisting of multiple, overlapping bands of muscle fibers in most individuals and a simple parallel-fibered architecture in some individuals.

Activation of brachioradialis muscles transferred to restore lateral pinch in tetraplegia.

PURPOSE: Surgical transfers of muscles are used to restore lateral pinch in tetraplegia; however, outcomes are variable. The purpose of this study was to compare activation of the brachioradialis (Br) after transfer to the flexor pollicis longus during maximum effort in its primary function (elbow flexion) with maximum effort in its postoperative function (lateral pinch) and to record Br activation during functional tasks. METHODS: Fine-wire electrodes recorded activation of the Br in 11 arms with tetraplegia. Subjects produced maximum lateral pinch force with and without elbow stabilization and were classified according to elbow strength. The elbow was stabilized by supporting the arm and limiting elbow motion. A force sensor mounted on a custom grip recorded the pinch force. Electromyographic (EMG) signals recorded during lateral pinch were expressed as a percentage of the maximum voluntary contraction recorded during maximum-effort elbow flexion. RESULTS: The EMG activation was significantly lower during lateral pinch compared with resisted elbow flexion. The mean EMG during lateral pinch in the self-supported elbow condition was 34% of the maximum voluntary contraction; with the elbow stabilized the EMG increased to 55% of the maximum voluntary contraction. Postoperative pinch-force magnitude was 14 N with self-support and 20 N with the elbow stabilized. Subjects with weak elbow extension strength produced significantly lower pinch forces compared with subjects with strong elbow extension but had similar ability to activate the Br. The Br activation was higher when the pinch tasks were performed successfully. CONCLUSIONS: These findings suggest a reduced ability to activate the transferred muscle fully in lateral pinch function after surgery, even with the addition of elbow support. The Br activation is linked to successful performance of lateral pinch tasks. The subjects' inability to activate the transferred muscle fully may be affected by postoperative muscle re-education and contribute to postoperative weakness.

Amplitude threshold criteria improve surface electrode specificity during walking and functional movements.

Contamination of electromyographic (EMG) data due to crosstalk in recordings from surface electrodes can lead to misinterpretation of results. The purpose of this study was to determine if removing a portion of the EMG signal normalized to a maximum voluntary contraction (MVC) would improve the specificity of surface electrode recordings. We hypothesized that setting an amplitude threshold to define when a muscle was active would remove that part of the myoelectric signal most likely to include crosstalk, without affecting the intensity or the onset and cessation times. Surface and intramuscular electrodes recorded signals from the same muscles of adults performing cyclic ankle movements and walking at self-selected speeds. Signals identified as crosstalk were eliminated when 15% and 18% of the amplitude of the normalized signal was removed and muscle timing or intensity was not significantly changed in most cases.

A comparison of gait with solid and hinged ankle-foot orthoses in children with spastic diplegic cerebral palsy.

This study compared the effects of solid and hinged ankle-foot orthoses (AFOs) on the gait of children with spastic diplegic cerebral palsy (CP) who ambulate with excessive ankle plantar flexion during stance. Twelve children with spastic diplegic CP wore no AFOs for an initial 2-week period, solid AFOs for 1 month, no AFOs for 2 weeks, and hinged AFOs for 1 month. Lower extremity muscle timing, knee and ankle joint motions, moments and powers, and temporal-distance characteristics were measured during ambulation for an initial barefoot baseline test, and with solid and hinged AFOs for the other two tests. Both orthoses increased stride length, reduced abnormal ankle plantar flexion during initial contact, midstance and terminal stance (TST), and increased ankle plantar flexor moments closer to normal during TST. Hinged AFOs increased ankle dorsiflexion at TST and increased ankle power generation during preswing (PSW) as compared to solid AFOs, and increased ankle dorsiflexion at loading compared to no AFOs. No other significant differences were found for the gait variables when comparing these orthoses. Either AFO could be used to reduce the excessive ankle plantar flexion without affecting the knee position during stance. The hinged AFO would be recommended to produce more normal dorsiflexion during TST and increased ankle power generation during PSW in children with spastic diplegic CP.

Validation of a computer-aided EMG decomposition method.

This paper presents an objective assessment of the accuracy of EMGLAB, a computer-aided EMG decomposition program that we developed. EMG signals were recorded simultaneously using monopolar needle and fine-wire electrodes from nearby sites in the tibialis anterior muscle during moderate isometric contractions. The signals were decomposed independently by an experienced operator, yielding 3-12 (mean 8.7) motor-unit action potential (MUAP) trains per signal. Decomposition accuracy was estimated by crosschecking 83 pairs of trains from different signals that corresponded to the same motor units. The results show that EMGLAB was able to decompose large MUAPs (peak amplitudes greater than 2.5 times rms signal amplitude) with 98-100% accuracy, and smaller MUAPs with 80-100% accuracy. Many of the errors involved misalignment of small MUAPs within superimpositions and amounted to less than 5 ms. These results validate the accuracy of EMGLAB for decomposing EMG signals of moderate complexity.

Increased jitter and blocking in normal muscles due to doubly innervated muscle fibers.

Increased jitter and intermittent impulse blocking in electromyographic (EMG) signals are considered evidence of transmission abnormality and are not usually associated with normal muscle. However, motor unit action potentials (MUAPs) that exhibit increased jitter and blocking have recently been shown to occur in the brachioradialis muscles of neurologically healthy subjects. The jitter and blocking result from collisions, refractoriness, and conduction-velocity variability in long muscle fibers that are innervated by two different motoneurons at widely separated endplates. We analyzed MUAPs obtained by decomposing EMG signals from the brachioradialis muscles of four normal subjects. The rate of blocking of some MUAP components was as high as 28%, the jitter between some components exceeded 300 micros (mean consecutive difference), and the mean incidence of irregular MUAPs was 14%. These values would be considered abnormal in many other muscles. Jitter from doubly innervated fibers can be distinguished from other types of pathological jitter because one component exhibits amplitude variability. Clinical neurophysiologists should be aware that increased jitter and blocking do not necessarily indicate pathology in brachioradialis and perhaps other long, parallel-fibered muscles.

Towards a realistic biomechanical model of the thumb: the choice of kinematic description may be more critical than the solution method or the variability/uncertainty of musculoskeletal parameters.

A biomechanical model of the thumb can help researchers and clinicians understand the clinical problem of how anatomical variability contributes to the variability of outcomes of surgeries to restore thumb function. We lack a realistic biomechanical model of the thumb because of the variability/uncertainty of musculoskeletal parameters, the multiple proposed kinematic descriptions and methods to solve the muscle redundancy problem, and the paucity of data to validate the model with in vivo coordination patterns and force output. We performed a multi-stage validation of a biomechanical computer model against our measurements of maximal static thumbtip force and fine-wire electromyograms (EMG) from 8 thumb muscles in each of five orthogonal directions in key and opposition pinch postures. A low-friction point-contact at the thumbtip ensured that subjects did not produce thumbtip torques during force production. The 3-D, 8-muscle biomechanical thumb model uses a 5-axis kinematic description with orthogonal and intersecting axes of rotation at the carpometacarpal and metacarpophalangeal joints. We represented the 50 musculoskeletal parameters of the model as stochastic variables based on experimental data, and ran Monte Carlo simulations in the "inverse" and "forward" directions for 5000 random instantiations of the model. Two inverse simulations (predicting the distribution of maximal static thumbtip forces and the muscle activations that maximized force) showed that: the model reproduces at most 50% of the 80 EMG distributions recorded (eight muscle excitations in 5 force directions in two postures); and well-directed thumbtip forces of adequate magnitude are predicted only if accompanied by unrealistically large thumbtip torques (0.64+/-0.28Nm). The forward simulation (which fed the experimental distributions of EMG through random instantiations of the model) resulted in misdirected thumbtip force vectors (within 74.3+/-24.5 degrees from the desired direction) accompanied by doubly large thumbtip torques (1.32+/-0.95Nm). Taken together, our results suggest that the variability and uncertainty of musculoskeletal parameters and the choice of solution method are not the likely reason for the unrealistic predictions obtained. Rather, the kinematic description of the thumb we used is not representative of the transformation of net joint torques into thumbtip forces/torques in the human thumb. Future efforts should focus on validating alternative kinematic descriptions of the thumb.

The unoperated hand: the role of passive forces in hand function after tetraplegia.

Passive forces play a large role in hand function after tetraplegia. Most individuals with tetraplegia choose not to undergo surgical reconstruction of hand function and, therefore, depend on the passive properties of their musculoskeletal system to perform functional tasks. Knowledge of the levels of force needed to perform many of these tasks is lacking. Understanding the mechanics of producing passive force is important for designing adaptive tools and other devices for tetraplegic individuals. Knowledge of the passive properties of the upper extremity is important in forming treatment strategies. The passive forces produced for change to the tenodesis grasp are small but useful to the individual. Since these forces arise from basic anatomy and muscle function, they are important even after surgical restoration of hand function. Compensatory strategies for the unoperated hand probably play a role in the operated hand. The approach to surgical restoration of grasp must consider how passive forces contribute to functional outcome.

Electrophysiological evidence of adult human skeletal muscle fibres with multiple endplates and polyneuronal innervation.

Electromyographic (EMG) signals were recorded using intramuscular electrodes at six different sites in the brachioradialis muscles during voluntary isometric contractions in four subjects. The potential waveforms and discharge patterns of up to 12 simultaneously active motor units were identified from each signal using computer-aided decomposition. Out of a total of 301 motor unit potentials identified, 23 potentials exhibited behaviour consistent with having been generated by muscle fibres that were innervated by two different motoneurons at widely separated endplates. These potentials discharged in association with two different motor units, but were blocked or delayed whenever the two motor units discharged within a few milliseconds of one another. The blocking was consistent with a collision or refractoriness when one motoneuron tried to excite the fibre while it was already conducting an action potential initiated by the other motoneuron. The delays were consistent with decreased conduction velocity associated with incomplete recovery of the fibre after a preceding action potential. From the temporal separation between the discharges of the two motoneurons that resulted in blocking, the spatial separation between the endplates was estimated to be between 26 and 44 mm. These findings challenge the classical concept of the motor unit as an anatomically distinct and functionally independent entity. It is suggested that the human brachioradialis muscle may contain both long, polyneuronally innervated fibres and short, serially linked, singly innervated fibres.