Postural Control During Multi-directional Perturbations to the Arm in Standing Humans

Book excerpt: "Humans constantly interact physically with the world around them using their hands to manipulate objects and loads while standing. These interactions involve forces to the arm that not only disturb the focal task but can also destabilize balance. The present study provided a comprehensive investigation of the postural response to arm perturbations under a range of realistic conditions. Standing subjects held the handle of a joystick-type robot which was programmed to apply multi-directional force perturbations unilaterally to the right hand within the horizontal plane. Electromyographic, kinematic, and kinetic signals were recorded. In response to unpredictable perturbations, postural muscles were activated at latencies as short as 50-110 ms that suggest the response must be automatic. The amplitude of the feedback automatic postural responses (APRs) was highly tuned to the perturbation direction and systematically increased with the intensity of the perturbation, i.e. it was a function the peripheral sensory input. The activation of the lower limb muscles was not the result of local stretch reflexes and occurred even before the upright posture was significantly displaced. The cutaneous afferents of the palm of the perturbed hand are proposed as the primary sensors involved in encoding the perturbation and the genesis of the initial response in the caudal muscles. Lowering lateral stability by decreasing the stance width significantly increased displacement amplitude of the center-of-mass (COM) and the activity of the caudal muscles, but the hand kinematics and the activity of the arm muscles were invariant. Therefore, the processes involved in controlling the hand position and stabilizing balance in response to a transient perturbation may be independent. Increasing the stiffness of the arm, achieved by extending the elbow joint, significantly increased hand stability and propagation of the perturbations to the body, but it did not significantly influence the COM kinematics, likely because the variation of the arm mechanics was negligible compared to the overall dynamics of the body. Moreover, neither the activity of ankle muscles nor the associated center-of-pressure displacement was influenced by the elbow angle, suggesting that the ankle muscle activity primarily functioned to stabilize balance rather than to reposition the hand.When perturbations were predictable, anticipatory postural adjustments (APAs) were initiated 50-150 ms prior to the perturbation onset. APAs were tuned to both direction and amplitude of the predicted perturbation such that APAs generally functioned to accelerate the COM in the direction opposite to the forthcoming disturbance. The amplitude of APRs was also larger when perturbations were predictable, suggesting feedforward regulation of the response gain. The modulation of APRs with predictability could be partially explained by greater baseline activation of muscles due to APAs when the perturbation was predictable. The displacement produced by the postural disturbance significantly decreased in all directions when perturbations were predictable. In particular, the COM displacement was tightly regulated in the anteroposterior direction such that its displacement amplitude was relatively small and did not increase with the magnitude of the perturbation. Pathways similar to those of propriospinally mediated inter-limb reflexes demonstrated in the cat spinal cord which could originate from the cutaneous receptors of the palm of the perturbed arm might be responsible for genesis of the response in the lower limb muscles when the upper limb is perturbed." --