How fast is a person moving during split-belt treadmill walking? Insights from center of mass velocity and stride...

Split-belt treadmill walking has contributed to our understanding of human locomotor adaptation and motor learning. On the split-belt treadmill, each limb walks on a belt moving at a different speed. An obvious question is: how fast is a person walking on a split-belt treadmill? This question has implications for metabolic and mechanical energy consumption, stability, and neuromotor adaptation. There are two interpretations of this question that provide different answers and implications. First, how fast a person is walking can be interpreted as the center of mass velocity, which is a vector of the rate of displacement in the three directions of an inertial reference frame. The center of mass velocity is zero on average in a global reference frame and oscillates throughout the gait cycle, providing information about mechanical and stability demands. Second, how fast a person is walking can be interpreted as stride speed, which is a scalar defined as stride length divided by stride time. Stride speed is constrained by the split-belt treadmill to the average belt speed, subtracted by the interaction between belt speed difference and step time asymmetry. Thus, stride speed is slower than the average speed of the belts due to modifications in step time asymmetry. Here, we provide a framework for calculating and contextualizing these two quantities, highlighting their implications for recent work in split-belt adaptation. We suggest that the design and interpretation of future split-belt adaptation studies can benefit from careful consideration of the analyses presented here.