Effect of slip on movement of body center of mass relative to base of support

Objective. The purpose is to investigate the effect of balance conditions a nd slippery perturbation on the position and velocity of the body's center of mass relative to the body's base of support. Design. Twenty-two young and healthful subjects were investigated while the ir walk was perturbed by a soap patch applied over a force plate. A safety harness was used to prevent the subject from falling on knee or buttock. Background. Appropriate postural response to meet physiological biomechanic al requirements is mandatory in restoration of balance upon slip. Methods. Twenty-two healthy subjects dressed with safety harness walked fir st without and then with slippery perturbation, guided by a metronome at 12 0 steps/min and 90 steps/min cadence. Data were collected from a motion ana lysis system and force plates. Results. For slippery perturbation, the displacement and velocity of center of mass with respect to base of support became smaller From heel strike to contralateral toe off. Subject's balance condition correlated significantl y to the displacement of center of mass with respect to base of support (r = -0.51 at 120 steps/min and r = -0.471 at 90 steps/min), as well as the ve locity (r = -0.834 at 120 steps/min, r = -0.673 at 90 steps/min) at contral ateral toe off. Conclusions. For slip during walking, smaller excursion and faster velocity of center of mass with respect to base of support were important for subje cts regaining balance from heel strike to contralateral toe off. The critic al time for subjects regaining stability is the first double support phase of the gait cycle.