In-step correlation of pelvis state to foot placement does not imply active control

Recent experimental studies of human walking have shown that within an individual step the pelvis (CoM) state can be used to predict the next foot placement. Here, we used a simple open-loop-stable 2D walking model to show that one cannot take this predictive ability, by itself, as evidence of active control. In our simulations, we too find a high correlation between the pelvis state and the foot placement, but entirely due to "passive" dynamics as our system has no control. Thus, such statistics should be used to characterize in-step active control in human walking only with caution, and likely require supporting evidence beyond the correlations themselves.