Maintaining standing balance by handrail grasping.

Maintaining balance while standing on a moving bus or subway is challenging, and falls among passengers are a significant source of morbidity. Standing passengers often rely on handrail grasping to resist perturbations to balance. We conducted experiments that simulated vehicle starts, to examine how handrail location (overhead or shoulder-height), perturbation direction (forward, backward, left or right), and perturbation magnitude (1 or 2m/s(2)) affected the biomechanical effort (peak centre-of-pressure (COP) excursion and hand force) and muscle activations (onset and integrated EMG activity) involved in balance maintenance. COP excursions, hand forces and muscle activations were altered in a functional manner based on task constraints and perturbation characteristics. Handrail position affected normalized values of peak COP and hand force during forward and backward, but not sideways perturbations. During backward perturbations, COP excursion was greater when grasping overhead than shoulder-height. During forward perturbations, hand force was greater when grasping shoulder-height than overhead. Biceps activations were earlier during shoulder-height than overhead grasping, while tibialis anterior activity was higher during overhead than shoulder-height grasping. Our results indicate that, when facing forward or backward to the direction of vehicle motion, overhead grasping minimizes hand force, while shoulder-height grasping minimizes COP excursion. In contrast, grasping with a sideways stance eliminates the effect of handrail location, and was associated with equal or lower biomechanical effort. This suggests that, at least for vehicle starts, the most reasonable strategy may be to stand sideways to the direction of the vehicle movement, and grasp either at shoulder-height or overhead.