Motor unit behavior in Parkinson's disease.

This article reviews the literature related to motor unit behavior in Parkinson's disease (PD). The focus is on bradykinesia, or slowed movement. There is sparse literature on muscular performance in PD, as PD is regarded as a disease of higher motor centers. Nevertheless, a decrease in muscle activation has been demonstrated, and motor unit behavior is altered so that (1) the discharge patterns of motor units are irregular and intermittent, (2) a greater number of motor units are recruited at low thresholds as compared with the findings for age-matched control subjects, and (3) antagonist muscles are abnormally coactivated. Possible reasons for these changes include imbalances in excitatory and inhibitory inputs to motor neurons, adaptations in motor neurons secondary to disuse, or deviations in the normal aging process. For the physical therapy of persons with PD, we propose a greater emphasis on strength-training exercises.