Human posture control: Preparation gait to avoid slips and falls

This study was to investigate a preparation gait for adaptation to a slippery surface. Previous slip and fall studies have mainly focused on inadvertent slips and falls, so the results mainly reflect the effects of the feedback control strategy. Few studies, if any, have considered the effects of feed-forward control strategy resulting from a preparation gait prior to stepping on a slippery surface. The hypotheses of the study were: (1) A preparation gait is observable and significantly different from a normal surface gait. (2) A preparation gait is a function of an adaptation gait modeling the future. (3) The time required to initiate the preparation gait is a function of walking speed.;To investigate the above hypotheses, this study employed the following procedures: (1) Kinetics and kinematics of preparation gaits were simulated with a two-link inverted pendulum model using a system control theory. (2) Simulation results were used to frame specific hypotheses concerning how the preparation gait is generated by each of two adaptation gaits: the walking-over adaptation gait and the sliding-over adaptation gait. (3) Kinetic and kinematic data collected from human experiment were used to identify the characteristics of the preparation gait.;The results showed that first, when approaching to a slippery surface, humans try to make a certain initial posture to adapt a slippery surface. The initial posture is minimizing the foot and shank forward angles and maximizing the thigh and trunk forward angles. In order to make the initial posture, humans reduce the angular velocity of distal segments more than that of the proximal segments. Second, humans use the same movement pattern as a preparation gait in slip and fall situations regardless types of internal gait model: There is no difference in overall patterns of angular motion of each segment between the slide-over and the walk-over conditions; in the phase planes of force, a pattern change always happens in the middle of step cycle. Third, different types of internal gait model produce different magnitudes of force or angular motion. As such, the slide-over condition produces the largest thrust force, while the walk-over produces the smallest thrust force; the foot and shank have a smaller range of motion in the slide-over condition and a larger range of motion in the walk-over, while the trunk and thigh have a larger range of motion in the slide-over and a smaller range of motion in the walk-over condition.