Circadian and individual variations in duration of spontaneous activity among ankle muscles of the cat.

This article concerns the spontaneous motor behavior of cat hindlimb muscles and muscle regions using 24-h electromyographic (EMG) recordings. Previously, we found marked differences in average daily "duty time" (i.e., the percentage of total sampling time filled with EMG activity) between different muscles, or muscle portions. We have now analyzed systematic differences in duty time between (i) highly active (midday) and relatively inactive (midnight) periods, and (ii) individual cats. Differences between cats seemed to be associated with differences in motor habits. The midnight reduction in activity was particularly striking for muscles with a high midday activity. Quantitative differences in spontaneous activity (duty time), as compared between active and inactive periods of the day or among individual cats, were associated with marked qualitative alterations in the distribution of activity among the sampled muscles, i.e., these quantitative differences could not be described as a simple up- or downscaling of general motor activity.