CENTER-OF-MASS VELOCITY-POSITION PREDICTIONS FOR BALANCE CONTROL

The purposes of this analysis were to predict the feasible movements d uring which balance can be maintained, based on environmental (contact force), anatomical (foot geometry), and physiological (muscle strengt h) constraints, and to identify the role of each constraint in limitin g movement. An inverted pendulum model with a foot segment was used wi th an optimization algorithm to determine the set of feasible center o f mass (CM) velocity-position combinations for movement termination. T he upper boundary of the resulting feasible region ran from a velocity of 1.1 s(-1) (normalized to body height) at 2.4 foot lengths behind t he heel, to 0.45 s(-1) over the heel, to zero over the toe, and the lo wer boundary from a velocity of 0.9 s(-1) at 2.7 foot lengths behind t he heel, to zero over the heel. Forward falls would be initiated if st ates exceeded the upper boundary, and backward fails would be initiate d if the states fell below the lower boundary. Under normal conditions , the constraint on the size of the base of support (BOS) determined t he upper and lower boundaries of the feasible region. However, frictio n and strength did limit the feasible region when friction levels were less than 0.82, when dorsiflexion was reduced more than 51%, or when plantar flexion strength was reduced more than 35%. These findings exp and the long-held concept that balance is based on CM position limits (i.e. the horizontal CM position has to be confined within the BOS to guarantee stable standing) to a concept based on CM velocity-position limits. (C) 1997 Elsevier Science Ltd.