Does tremor pace repetitive voluntary motor behavior in Parkinson's disease?

In patients with Parkinson's disease and in normal subjects, the influence of tremor on repetitive voluntary movement was investigated in the index finger by comparing frequency of isometric force tremor with frequency of voluntary alternating isometric contractions. Tremor frequency, measured over the range from 0 to 70% maximum voluntary force, usually increased with force. The tremor frequency band was lower and more often overlapped with the upper voluntary frequency range in patients than in normal subjects. Normal subjects could accurately produce voluntary contractions at all cue frequencies from 1 to 5 Hz. Patients could produce auditory-paced frequencies of 1 and 2 Hz, but at higher cue frequencies, their voluntary contractions were often faster or slower than the cue. The faster or "hastened" voluntary frequencies were within the tremor frequency band, whereas the slowed voluntary frequencies were below it. Maximal voluntary frequency was often greater than the lowest but always less than the highest tremor frequency. It is concluded that parkinsonian tremor may pace voluntary repetitive movements to go faster than intended with the highest tremor frequency being an upper limit for voluntary frequency. Similar mechanisms may underlie the hastened repetitive vocal responses that were also observed in the parkinsonian patients.