Motor unit control in Parkinson's disease and the influence of levodopa.

Patients with Parkinson's disease are unable to adjust the firing rate of motor units that initiate contraction from zero to higher rates; the frequency modulation of motor units is not normal, but motor units recruit normally as effort is increased. Treatment with levodopa makes these motor units accessible to activation and frequency control. Elderly subjects also have difficulty in the activation of minimal contraction and in the maintenance of firing but to a significantly lesser degree than do parkinsonian patients.