During voluntary activity in humans, motor units are exposed to a number of descending drives that tend to synchronize motor unit activity at particular frequencies. In particular, the contralateral motor cortex drives muscle discharge in the beta (15-30 Hz) and Piper (30-60 Hz) bands. The cortical activity in these bands is task-specific, somatopicly distributed and generally precedes muscle discharge by an interval appropriate for conduction in fast pyramidal pathways. Coherence between cortex and muscle in the beta band is found during isometric contractions of weak to moderate strength. Thus oscillations within the beta band seem to coincide with a stable, relatively immutable state--a free running mode of the motor cortex that may maintain stable motor output with a minimum of computational effort. In contrast, coherence between cortex and muscle in the Piper band is most evident during strong isometric contractions or during movement. Demands on the motor cortex are likely to be greater and more mutable under these circumstances. Synchronisation in the gamma band may provide a means of binding together those particular, often spatially distributed, cortical elements involved in movement execution under conditions that vary from moment to moment and require some attention. Mechanisms both intrinsic and extrinsic to the cortex determine the pattern of rhythmic cortical activity. The basal ganglia have a pivotal role in this regard, and inadequate output from these nuclei leads to a disappearance of the beta and Piper drives to muscle. This may in turn contribute to slowness and weakness in Parkinson's disease.