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 and 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 their 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 biomechanical requirements is mandatory in restoration of balance upon slip.
METHODS: Twenty-two healthy subjects dressed with safety harness walked first without and then with slippery perturbation, guided by a metronome at 120 steps/min and 90 steps/min cadence. Data were collected from a motion analysis 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 significantly 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 velocity (r=-0.834 at 120 steps/min, r=-0.673 at 90 steps/min) at contralateral 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 subjects regaining balance from heel strike to contralateral toe off. The critical time for subjects regaining stability is the first double support phase of the gait cycle. RELEVANCE: It is confirmed that two variables, the displacement and the velocity of center of mass with respect to base of support, are valuable biomechanical factors and provide quantifiable determination for investigation of the balance condition in slipping.