Human posture control: preparation gait to avoid slips and falls

The purpose of the present study was to identify preparation gait patterns for avoiding slips and falls. Six experimental conditions combining two walking speeds and three internal gait models were presented. In order to investigate the initiation time of the preparation gait, subject walking speed was manipulated by using two step frequencies. In order to investigate the effect of a subject's intention and future external condition, the internal gait model was manipulated by combining two adaptation gait styles and two target surface conditions.

The hypotheses of this study were that (1) a preparation gait is observable and significantly different from a normal surface gait; (2) a preparation gait pattern is a function of an internal gait model; and (3) a preparation gait is initiated as a function of walking speed. To test the hypotheses, (1) kinetics and kinematics of gait were simulated using a two-link inverted pendulum model applying a system control theory; (2) the simulation results were used to derive specific hypotheses concerning how the preparation gait is generated by each of two adaptation gaits, that is, the walk-over adaptation gait and the slide-over adaptation gait; and (3) kinetic and kinematic data were collected from human subjects to identify the preparation gait.

It was found that a preparation gait involves several levels of motor programming. At the level of segment control, the overall motion patterns of segments are consistent regardless of the internal gait model. At the level of coordination control, different coordination patterns were found, corresponding to different internal gait models. Subjects attempted certain initial postures to adapt to a slippery surface, minimizing the foot and shank forward angles and maximizing the thigh and trunk forward angles. To do this, subjects reduced the angular velocity of distal segments more than that of the proximal segments. Subjects also changed the coordination strength of adjacent segments from the distal segments to the proximal segments as balance control became critical. Finally, preparation gait is initiated in the middle of the step cycle prior to contact with a slippery surface: preparation gait was a part of the gait rather than a unique movement.