Diminished stepping responses lead to a fall following a novel slip induced during a sit-to-stand

While neuromotor responses associated with successfully restoring balance have been widely characterized, little is known of how responses employed to regain balance fail. We identified biomechanical factors that characterized the unsuccessful recovery responses to a novel perturbation. An unexpected forward slip was induced, using a low-friction platform, just after seat-off of a sit-to-stand in 41 young subjects. Kinematic variables describing the recovery responses were compared between fallers and those who recovered; "falls" into the fall-arrest system were identified based on center of mass (COM) descent. Twelve "single-step" falls, four "multi-step" falls, and 11 recoveries resulted. Single-step fallers exhibited a more posterior COM at slip onset, support (nonstepping) limb collapse resulting in hip vertical descent, and a shortened protective step backward. A factor analysis indicated that six common factors explained 88% of the variance in 22 kinematic variables describing the slip and recovery response. Single-step falls were associated with two factors, one related to initial or imposed instability and one to the reactive response. The former factor indicated that step length and COM position at step touchdown were related to COM position at slip onset. Support limb collapse comprised the latter factor. Multi-step fallers, by appearance, exhibited a shortened initial backward step that was ineffective at enabling balance recovery. As a whole, the results indicated that unsuccessful recovery was associated with a diminished stepping response. Fallers may have inadequately scaled an appropriate reflexive stepping response or might, inappropriately, have reflexively attempted to recover without stepping.