M. Elise Johanson

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.

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.

Forearm muscle activation during power grip and release

The position of the hand during power grip is well-described, but the normal phasic activity of the extrinsic forearm muscles during power grip and release is unknown. People with neurologic impairment may have inadequate power grip or release because of abnormal muscle timing. This study describes the timing of the forearm muscles in 10 normal subjects during power grip and release, which was evaluated using electromyography. During power grip, subjects had consistent timing patterns for extrinsic finger motors and different but individually consistent patterns for wrist motors. This finding supports our hypothesis that different individuals habitually use a specific motor strategy and an intact central nervous system allows them to change their motor strategy to adapt to new environmental parameters.

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.

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 μs (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. Muscle Nerve 28: 423–431, 2003

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 outcomes 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.

EMG investigation of the effects of peripheral feedback on goal-directed wrist movements

The objective of this investigation was to determine whether timing of the wrist musculature could be altered when changing the goal of the movement or the sensory feedback needed to complete the movement. Electromyographic (EMG) data from seven wrist and finger muscles as well as simultaneous wrist and finger joint positions were recorded from five normal adults. Subjects performed wrist flexion and extension through different arcs of motion, as fast as possible against a physical stop, and as accurately as possible using visual feedback for speed and joint position. Next, the tonic vibratory reflex was elicited and topical anaesthetic spray was applied; no changes were observed afterwards. The subjects then repeated the wrist movements, and it was evident that activation of agonist and antagonist muscle groups varied according to the goal of the movement.

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.

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.