Dynamic stability and the FES gait

Average velocity and energy consumption are the main differences between the FES assisted gait of spinal cord injured (SCI) subjects and normal gait from biomechanical point of view. We demonstrate the inherent difference: a normal subject utilizes a dynamically stable gait while a SCI subject exercises a statically stable gait. The former one is mainly inertia and gravity driven and therefore the average gait velocity can not be arbitrary low. Contrary the statically stable gait can be arbitrary slow because the subject can remain in any gait phase for an arbitrary amount of time. We derived an index that enables us to calculate how stable/unstable is the subject at any moment during the gait. By calculating the index for the FES gait we showed not only that it consists of states that are both statically stable and dynamically stable. This results in slow gait because the SCI subject can perform each action arbitrary slow. Therefore, we believe that the FES gait can be improved particularly in terms of speed if it is converted to a dynamically stable gait where dynamically unstable states occur periodically.