Assessing Side-Differences in the Organization of Biceps Brachii Motor Units in Healthy Subjects and Stroke Patients. An Evaluation from Surface EMG and Incremental Electrical Stimulation

Studies have suggested a degeneration of lower motoneurons in muscles affected after stroke, with a possible collateral reinnervation from the surviving motoneurons to the denervated muscle fibers. If this assumption holds, each surviving motoneuron would innervate a greater amount of muscle fibers following stroke, i.e., motor units’ size would increase in muscles affected after stroke. By combining neuromuscular electrical stimulation with surface electromyography, the present PhD thesis aimed at investigating whether muscle reinnervation following stroke leads to greater variations in the amplitude of M waves elicited in muscles of the affected side of stroke patients, with respect to the contralateral, unaffected side. This issue was verified by applying current pulses at progressively greater intensities in the motoneurons that supply the biceps brachii muscle. Then, the size of increases in the amplitude of M waves elicited consecutively, hereafter defined as increments, was considered to evaluate structural adaptations in biceps brachii motor units following stroke. Changes in the amplitude of M waves evoked in a muscle is usually assumed to reflect changes in the number of motoneurons and, consequently, of muscle fibers activated. Hence, we hypothesized that for similar, relative increases in current intensity, greater increments in the M-waves amplitude would be observed in muscles of the affected than unaffected side of stroke patients. Before verifying this hypothesis, however, we investigated whether the size of increments in biceps brachii M waves differ between arms of healthy subjects. This question was motivated by the fact that, usually, humans tend to control more finely the muscle force production in dominant than non-dominant upper limbs. Once it is well established the recruitment of motor units is a key mechanism for which muscle force is controlled, we hypothesized that a relatively smaller number of motor units maybe recruited in muscles of dominant than nondominant limbs, for any given increase in synaptic input. Hence, we expected to observe smaller increments in the amplitude of M waves evoked in biceps brachii of dominant than non-dominant arms. This PhD thesis was, therefore, based on two main researches, entitled: (1) “Does the biceps brachii muscle respond similarly in both limbs during staircase, electrically elicited contractions?” and (2) “Assessing structural adaptation of biceps brachii motor units after stroke”. Both studies were investigated with the same methodological approach mentioned above. Our main findings showed that: (1) increments were significantly smaller in biceps brachii of dominant than non-dominant arms. These results suggest there was a more gradual motor units’ recruitment and, therefore, a broader spectrum of motor units’ recruitment thresholds in muscles of dominant than non-dominant arms, which may contribute for a finer regulation of force production; (2) there was a clear trend towards greater increments in the amplitude of M waves elicited in biceps brachii of the affected than unaffected arms of most of the stroke patients evaluated. Although for few of these patients it was not clear whether side-differences in the increments magnitude were an outcome of dominance or stroke, the results found corroborate with the notion that collateral reinnervation takes place after stroke, increasing the number of muscle fibers per unit and, therefore, the magnitude of the muscle responses. Overall, the findings of this PhD thesis strengthen the idea that the organization of the neuromuscular system may contribute to accounting for upper limb dominance and that stroke may lead to structural adaptations in motor units of affected muscles.