Temporal patterns of muscle activation for arm movements in three-dimensional space.

Little is known about the patterns of muscle activation that subserve arm movement in three-dimensional space. In this study, activation patterns of seven arm muscles were related to the spatial direction of human arm movement. Twenty movement directions defined two orthogonal vertical planes in space. The arm movements were moderately paced; each movement lasted approximately 500 msec. New techniques of EMG analysis were developed to describe the temporal pattern of muscle activation. For each muscle, a principal component analysis revealed a common phasic and tonic waveform for all directions of movement, within one plane. A temporal shifting procedure based on best covariance values revealed activation delays associated with different movement directions. The results show a consistent pattern of temporal shifting of the common waveform for movements in different directions. Coupled with past results showing that activation amplitude is a function of the cosine angle of movement or force direction, the present results suggest a relatively simple control strategy for mechanically complex arm movements: neural circuits produce a common phasic and tonic activation waveform that is scaled in amplitude and delayed in time, depending on the desired movement direction.