The preparation and production of isometric force in Parkinson's disease.

Subjects with Parkinson's disease (PD) and age-matched controls performed an isometric force production task, aiming at different target force levels without concurrent force feedback. Overall, PD subjects were as accurate as controls in attaining the target force levels, but executed the task differently. They had longer times to peak force and contraction durations, larger impulses and lower rates of force development, and force-time profiles with many more irregularities. They also initiated lower force contractions with longer latencies, unlike controls. The data suggest that PD subjects are deficient in the regulation of force and time parameters, rather than simply in force production. The ability to produce peak forces accurately limits the generality of previous assertions that PD subjects are heavily dependent on concurrent visual feedback.