Background: A sound with a frequency of around 40 Hz can be heard, with a stethoscope, in the muscles of the hand and forearm during tonic contraction or movement. This sound is the acoustic correlate of the Piper rhythm seen in electromyographic records of muscle activity. The way in which motor activity is organised at the cortical level may determine the frequency of the sound heard.
Methods: The wrist extensor muscles in the forearm of seven patients with idiopathic Parkinson's disease, studied with and without antiparkinsonian treatment, and a control group of seven healthy men were auscultated during very strong extension of the wrist and during repeated slow flexion and extension of this joint.
Findings: In healthy people, the Piper-band sound made by muscle activity is 40-50 Hz. In all seven parkinsonian patients, when off medication, the normal Piper-band sound was replaced by a pulsatile muscle activity of 10 Hz during strong wrist extension, and in four patients during repeated flexion and extension of the wrist. After treatment with dopaminergic drugs, the Piper mode of muscle discharge was restored and motor performance was improved.
Interpretation: The Piper mode of muscle discharge is dependent on the normal functioning of pallidal projections to the motor areas of the cortex. In abnormal function a muscle discharge of 10 Hz dominates, which is suboptimal when fast or powerful contractions are necessary.