RELATIONSHIPS BETWEEN TIMING OF MUSCLE EXCITATION AND IMPAIRED MOTOR-PERFORMANCE DURING CYCLICAL LOWER-EXTREMITY MOVEMENT IN POSTSTROKE HEMIPLEGIA

We used an ergometer pedalling paradigm to relate abnormalities in the timing of muscle excitation to the impaired ability to perform mechan ical work in the plegic lower limbs of persons with hemiplegia. The EM Gs of seven leg muscles and pedal forces were measured bilaterally dur ing pedalling for 15 persons with hemiplegia and 12 neurologically int act age-matched control subjects. Subjects were asked to pedal at a mo derate workload (135 J) and cadence (40 r.p.m.). While intersubject va riability was high, the external mechanical work output of the plegic leg was significantly less (from 79.6% to -28.9% of the work produced by average leg of control subjects) as a result of less positive work and more negative work being done. The timing of EMG in individual ple gic limb muscles exhibited two distinct types of abnormalities that we re significantly correlated with this lesser work production: prolonge d excitation in the vastus medialis and phase-advanced excitation (bot h early initiation and early termination) in the rectus femoris and se mimembranosus. These results suggest that muscles were differently aff ected depending on their function, external power-producing muscles (e .g. vastus medialis) showing prolonged excitation and muscles that nor mally maintain crank progression during limb transitions (e.g. semimem branosus) showing phase-advanced excitation.