Anticipatory control of head posture.

OBJECTIVE: We tested a hypothesis on two patterns of anticipatory postural adjustments (APAs) in neck muscles, reciprocal and co-activation, that may be used in a task-specific way. We also explored possible relation of APAs in leg and trunk muscles to head stabilization. METHODS: Load perturbations (loading and unloading) were applied to the head, trunk, and head and trunk simultaneously using similar hand actions by standing persons. Electromyographic signals (EMGs) from 10 muscles were recorded. Shifts of the center of pressure and EMG indices were computed over typical time intervals for APA. RESULTS: Time-shifted (reciprocal) activation of neck flexor and extensor muscles during APAs was seen when perturbations were applied directly to the head. Simultaneous activation dominated when the perturbations were applied to the trunk. Minimal APAs were seen in the leg/trunk muscles during head perturbation tests. APAs during trunk perturbation were not different from those during trunk and head perturbation. CONCLUSIONS: The results confirm the existence of two different patterns of APAs in neck muscles. A time-shifted (reciprocal) pattern is more likely to be used in anticipation of a perturbation acting directly on the head. A simultaneous activation (co-activation) pattern is used when direction of head perturbation cannot be predicted with certainty. Leg/trunk APAs are unlikely to help stabilize head posture. SIGNIFICANCE: These results are important for better understanding of feed-forward mechanisms of the control of head posture with possible implications for neurological patients who suffer from impaired feed-forward postural control.