Kinematic variability for dynamic stability in human gait

Abstract

Greater kinematic variability in human gait is often considered as negative impact on dynamic stability. Interestingly, findings from studies on human walking suggest that gait stability may be improved at the cost of local variability (Dingwell and Marin, 2006; Houdijk et al., 2007). To get more insight into kinematic and spatial-temporal variability in human gait, we looked at data previously collected of 21 human subjects walking and running on a treadmill. For direct comparison, we chose a range of speeds (0.5-2.6 m/s) where subjects could easily perform both gaits. Results demonstrate that walking is characterized by a remarkably steady sequence of alternating contact and swing phases indicated by hardly varying duty factors DF (Fig. 1A). In running, a lot higher variability of DF is observed. Additionally, high kinematic variability in running is found shortly after contact reflected by the timing of maximum ankle extension tφA max (Fig. 1B). At midstance in running, accurate timing of maximum ankle compression tφA min (Fig. 1B) is existent. The opposite is true for walking, where tφA min shows high variability, while tφA max is fairly precise. Both gaits show periods of high and periods of low variability on the joint level within the gait cycle, but time lines are gait-specifically different. Gait stability seems not to rely on continuous kinematic invariability but allows periods of high variability which are tolerated by the locomotor system. These findings might indicate that the double-support phase in walking has a similar function as the single-support phase in running: both diminish kinematic variability during loading of the system and then accelerate the center of mass (CoM) upward. Conversely, the flight phase in running corresponds to single-support in walking with increased kinematic variability during unloading of the system and subsequent downward acceleration of the CoM. However, the stabilizing mechanisms during ground contact are inherently different for both gaits with two legs simultaneously accelerating (trailing leg) and breaking (leading leg) the CoM in walking (Donelan et al., 2002) and one single leg first decelerating and finally accelerating the CoM in running (Blickhan, 1989).