CENTER OF MASS VELOCITY-POSITION PREDICTIONS
FOR BALANCE CONTROL
Yi-Chung Pai1 and
James Patton2
1Programs in Physical
Therapy,
Northwestern
University Medical School, Chicago, IL 60611
2Biomedical Engineering
Graduate Program,
Northwestern
University, Evanston, IL 60208 (j-patton@nwu.edu)
Journal of
Biomechanics, 30(4):347-354, 1997.
The purposes of this analysis were to predict the feasible movements
during which balance can be maintained, based on environmental (contact force),
anatomical (foot geometry), and physiological (muscle strength) constraints,
and to identify the role of each constraint in limiting movement. An inverted pendulum model with a foot
segment was used with an optimization algorithm to determine the set of
feasible center of mass (CM) velocity-position combinations for movement termination. The 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 the heel, to 0.45 s-1
over the heel, to zero over the toe, and the lower
boundary from a velocity of 0.9 s-1 at 2.7 foot lengths behind
the heel, to zero over the heel. Forward falls would be initiated if states
exceeded the upper boundary, and backward falls would be initiated if the
states fell below the lower boundary. Under normal conditions, the constraint on
the size of the base of support (BOS) determined the upper and lower
boundaries of the feasible region.
However, friction 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
expand 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.
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