Quantifying instability during stepping via ideal trajectory analysis

We describe a quantitative method to assess repeated stair stepping stability. In both the mediolateral (ML) and anteroposterior (AP) directions, the trajectory of the subject´s center of mass (COM) was compared to an “ideal” trajectory. An ideal trajectory is a combination of two coupled sinusoids, which have been shown to approximate the COM trajectory of healthy subjects. The two identified sinusoids were unique in phase for each direction but the ML to AP oscillations were coupled by a two to one ratio respectively. Two dimensionless numbers, the ML instability index (IML) and AP instability index (IAP), were calculated using the COM trajectory and identified ideal sinusoids for each subject with larger index values resulting from less stable performance. The COM trajectories of nine healthy subjects and six patients diagnosed with unilateral or bilateral vestibular labyrinth hypofunction were analyzed. The average IML and IAP values of labyrinth disorder patients were respectively 127% and 119% greater than those of healthy subjects (p<0.014 and 0.006, respectively), indicating that the ideal trajectory analysis distinguishes labyrinth disorder patients from healthy subjects. COM trajectories also identify movement inefficiencies attributable to vestibulopathy