Neck muscles in the rhesus monkey. II. Electromyographic patterns of activation underlying postures and movements

Electromyographic (EMG) activity was recorded in less than or equal to 12 n eck muscles in four alert monkeys whose heads were unrestrained to describe the spatial and temporal patterns of neck muscle activation accompanying a large range of head postures and movements. Some head postures and movemen ts were elicited by training animals to generate gaze shifts to visual targ ets. Other spontaneous head movements were made during orienting, tracking, feeding, expressive, and head-shaking behaviors. These latter movements ex hibited a wider range of kinematic patterns. Stable postures and small head movements of only a few degrees were associated with activation of a small number of muscles in a reproducible synergy. Additional muscles were recru ited for more eccentric postures and larger movements. For head movements d uring trained gaze shifts, movement amplitude, velocity, and acceleration w ere correlated linearly and agonist muscles were recruited without antagoni st muscles. Complex sequences of reciprocal bursts in agonist and antagonis t muscles were observed during very brisk movements. Turning movements of s imilar amplitudes that began from different initial head positions were ass ociated with systematic variations in the activities of different muscles a nd in the relative timings of these activities. Unique recruitment synergie s were observed during feeding and head-shaking behaviors. Our results emph asize that the recruitment of a given muscle was generally ordered and cons istent but that strategies for coordination among various neck muscles were often complex and appeared to depend on the specifics of musculoskeletal a rchitecture, posture, and movement kinematics that differ substantially amo ng species.