Investigator: Carolynn Patten, PhD PT Mentors: Felix E. Zajac, PhD and Kevin C. McGill, PhD Project Category: Stroke - 2000 Objectives: To elucidate the neuromuscular mechanisms responsible for impairment in regulation of muscular force in post-stroke hemiplegia through identification of abnormalities of motor unit recruitment and discharge rate modulation. Research Plan: Two groups of subjects at distinct levels of recovery from hemiplegia (Brunnstom Levels III-IV (CVA-I) & V-VI (CVA-II)) are compared against control subjects. All subjects perform three types of contraction tasks: production of isometric force to 40% of maximal voluntary force, facilitation of isometric force by flexion or extension of the contralateral homologous limb, and dynamic force production including both concentric and eccentric conditions. Methods: Using needle and fine-wire electrodes, EMG signals are recorded from the vastus medialis during isometric and anisometric contractions performed on an instrumented pedaling ergometer. EMG signals are decomposed into their constituent parts, the discharge patterns of individual motor unit (MU) action potential trains identified, and the control properties of groups of concurrently active motor units compared between subject groups and across task conditions. Findings: To date, 19 subjects have been studied: 7 controls, 7 CVA-I and 5 CVA-II. Motor unit discharge rates observed in the isometric 40% MVC contractions are significantly increased in hemiparetic persons (Controls mn=12.04 pps, CVA-I mn=12.63 pps (paretic), mn=11.73 pps (non-paretic), CVA-II mn=13.98 pps (paretic), mn=20.94 pps (non-paretic) p < .0001). Facilitated activity of the contralateral limb produces dramatic effects on the involved leg but only minimal effects to the uninvolved leg of more severely impaired hemiparetic persons (CVA-II). The joint findings of these two experiments suggest a significant spinal level adaptation occurs which alters excitability of the motor neuron pool. In dynamic contractions transitional motor unit activity, including substitution of motor units across repeated cycles of locomotor activity and motor units which fire only at the transitions between static and dynamic force regulation has been observed. From our early experimental observations, it appears that such transitional activity is reduced and often absent in hemiparetic persons. Clinical Relevance: Compensatory spinal segmental adaptations which appear to occur in hemiparetic persons would serve to improve the likelihood of producing effective motor output in the face of reduced descending motor drive and reduction in the pool of functioning motor units. Such compensation, however, exerts deleterious effects on the sensitivity of force regulation and leads prematurely to fatigue. Poor control of movement and undue fatigue are frequent complaints of hemiparetic persons. This research is important then for its role in identifying physiologic mechanisms associated with functional weakness in hemiparetic persons and for its potential to provide a sensitive, scientifically-based means for assessment of rehabilitation potential and efficacy of therapeutic interventions. Funding Source: VA RR&D Career Development Award |
