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

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 activations.