Action tremor in Parkinson's disease.

Electromyographic activity of the biceps muscle was examined in 38 parkinsonian patients and 33 normal subjects during (i) rapidly alternating pronation-supination movements (RAM) of the forearm, and (ii) single pronation or supination movements in response to visual (light) or to kinesthetic signals (displacements of the hand). Biceps electromyography (EMG) displayed rhythmic activity over the supination phase of RAM in the majority of the parkinsonian patients, whereas continuous activity was evident in most normal subjects. Similar phenomena were observed when single movements were executed in response to visual or kinesthetic signals. Rhythmic activity could be triggered without any external displacements by voluntarily initiated supination of a previously quiescent limb even before actual movement of the limb occurred. Voluntary pronation (involving biceps inactivation) did not trigger any rhythmic biceps activity. The results are interpreted to support the hypothesis that parkinsonian patients have action tremor because voluntarily initiated activity leads to oscillations in internal feedback circuit involving descending pathways from motor cortex to spinal cord, and ascending pathways from the spinal cord back to the motor cortex.