There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
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.