A new perspective on the walking margin of stability

There remains a pressing need for a stability metric that can reliably identify fall susceptibility during walking, enabling more effective gait rehabilitation for reduced fall incidence. One available metric is the maximum margin of stability (MOS(max)), which is calculated using the body's center of mass (COM) position and velocity along with the location of the maximum center of pressure (COP(max)). However, MOS(max) has several limitations that may limit stability assessment. Specifically, the assumptions of a fixed COP and constant ground reaction force (GRF) are not applicable to gait. To address these limitations, a modified MOS equation that allows for a variable COP and is not dependent on a constant GRF is presented here. The modified MOS was significantly lower than MOS(max) throughout a significant portion of single limb support for normal walking gait. This finding indicates the MOS(max) metric may lack sensitivity to instability as it may still be positive when the actual MOS indicates existing or impending instability. This comparison also showed that the MOS might offer additional information about walking stability relevant to gait assessment for fall prevention and rehabilitation. However, like other stability metrics, this capability must be established with further investigations of perturbed and pathological gait.