Neuromuscular effort predicts walk–run transition speed in normal and adapted human gaits

ABSTRACT Often, humans and other animals move in a manner that minimizes energy costs. It is more economical to walk at slow speeds, and to run at fast speeds. Here, we asked whether humans select a gait that minimizes neuromuscular effort under novel and unfamiliar conditions, by imposing interlimb asymmetry during split-belt treadmill locomotion. The walk–run transition speed changed markedly across different gait conditions: forward, backward, hybrid (one leg forward, one leg backward) and forward with speed differences (one leg faster than the other). Most importantly, we showed that the human walk–run transition speed across conditions was predicted by changes in neuromuscular effort (i.e. summed leg muscle activations). Our results for forward gait and forward gait with speed differences suggest that human locomotor patterns are optimized under both familiar and novel gait conditions by minimizing the motor command for leg muscle activation. Summary: Interlimb asymmetry changes the walk–run transition speed on a split-belt treadmill with speed or direction differences between legs; leg muscle activation (neuromuscular effort) predicts both novel and normal transition speeds.