Control of dynamic stability during gait termination on a slippery surface.

There are three common ways by which to successfully terminate gait: decreased acceleration of whole-body center of mass (COM) through a flexor synergy in the trail leg, increased deceleration of whole-body COM through an extensor synergy in the front limb, and an energy/momentum transfer to dissipate any remaining momentum if the first two strategies are unsuccessful. Healthy individuals were asked to stop on a slippery surface while we examined their unexpected response to the slippery surface. Kinetic data from the forceplates revealed lower braking forces in the slip trials compared with normal gait-termination trials. Subjects were unable to control their center of pressure (COP) to manipulate the COM as revealed by increased deviations and maximum absolute ranges of COP movement. Subject COM deviated farther in both horizontal planes and lowered further during the slip compared with normal gait-termination trials. Arm movements were effective in dissipating forward COM movement. In addition, there likely was a transfer of forward to lateral momentum to stop forward progression. All recorded muscle activity in the lower limbs and back increased during the slip to provide support to the lower limbs and correct upright balance. The trailing limb shortened its final step to provide support to the lowering COM. The balance-correction response seen here resembles previous reactions to perturbations during locomotion suggesting there is a generalized strategy employed by the nervous system to correct for disturbances and maintain balance.