We used an ergometer pedalling paradigm to relate abnormalities in the timing of muscle excitation to the impaired ability to perform mechanical work in the plegic lower limbs of persons with hemiplegia. The EMGs of seven leg muscles and pedal forces were measured bilaterally during pedalling for 15 persons with hemiplegia and 12 neurologically intact age-matched control subjects. Subjects were asked to pedal at a moderate workload (135 J) and cadence (40 r.p.m.). While intersubject variability 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 plegic limb muscles exhibited two distinct types of abnormalities that were significantly correlated with this lesser work production: prolonged excitation in the vastus medialis and phase-advanced excitation (both early initiation and early termination) in the rectus femoris and semimembranosus. These results suggest that muscles were differently affected depending on their function, external power-producing muscles (e.g. vastus medialis) showing prolonged excitation and muscles that normally maintain crank progression during limb transitions (e.g. semimembranosus) showing phase-advanced excitation.