Force development during target-directed isometric force production in Parkinson's disease.

Performing various daily activities requires precise application and control of forces, which has been well addressed in neurologically healthy individuals. Recent experiments have demonstrated that in young, normal subjects generating rapid force pulses over various force amplitudes was accomplished by linearly increasing the rate of force development while keeping time to peak force approximately constant (i.e., a pulse-height control strategy). Using Parkinson's disease (PD) patients the present study examined whether PD patients use a pulse-height control strategy during rapid and accurate isometric force production. Subjects were instructed to produce force pulses to three different target amplitudes (15, 35, and 55% of their maximal voluntary contractions) at their preferred speed and as fast as possible. When the task was performed as fast as possible, PD patients differed from controls by producing reduced rates of force development and prolonged times to peak force as a function of force amplitude. During performance of the task at preferred speed, which leaves the rate of force production unconstrained, PD patients did not show improved regulation of time to peak force in scaling different force amplitudes compared to controls. These results suggest that PD patients have a difficulty in utilizing a pulse-height control strategy and that such impairments are not dependent on speed.