A technique for quantifying the response of seated individuals to dynamic perturbations

A technique is presented for monitoring the seated postural stability and control of human subjects. Estimates are made of the locations of the subject's center of pressure (CP(S)) and projection of the center of mass (CM(NP)) from moment balance equations using measured force and acceleration data. The CP(S) and CM(NP) indices describe the stability of the subject, independent of the chair, even in the presence of perturbations. The measurement system was evaluated for both rigid objects and human subjects situated in a wheelchair undergoing displacement. Estimated CM(NP) was within +/-5 mm of the actual value for static loads. For human subjects, the average correlation coefficient between the estimated CM(NP) signal and that computed from video data was 0.90; however, transient overestimation of displacement was seen during subject acceleration. The technique could help to better assess seated stability in dynamic environments, such as those experienced by wheelchair users in motor vehicles.