Neck muscle activation patterns in humans during isometric head stabilization.

A musculoskeletal system with more muscles than there are motions could be programmed in alternative ways to produce a single movement. In this case, the muscles would have the potential to be maximally responsive in multiple directions rather than responding preferentially in a single direction. To determine the response patterns of muscles in the head-neck motor system, the simultaneous activation of four of the 23 neck muscles acting on the head was recorded with both surface and intramuscular electrodes. Fifteen human subjects were tested during an isometric head stabilization task. When the EMG response patterns were plotted, each muscle demonstrated a preferred direction of activation. This preferred activation direction was consistent in all of the subjects for three of the muscles tested. The fourth muscle, splenius, was preferentially activated during neck flexion in half of the subjects and during neck extension in the other half. Increasing the force parameters of the task suggested a linear relationship between force and the EMG output in the preferred response directions. Responses in the nonpreferred directions were produced by a nonlinear change in EMG activation of the muscle. This finding could have implications for theories of how reciprocal activation and cocontraction patterns of response are elicited. Results from this study, that the CNS programs neck muscles to respond in specific orientations rather than generating an infinite variety of muscle patterns, are in agreement with our findings in the cat.