Our nervous systems are lazy.

Selinger et al. show that the subconscious nervous processes that regulate our movement are constantly adapting to minimize the energy required for movement in a given situation. To show this, they had subjects wear robotic exoskeletons that could increase or decrease resistance to the knees, to change the difficulty of swinging the legs during walking. Within minutes, gate was adjusted to be energetically more optimal.

Highlights

•People readily adapt established gait patterns to minimize energy use 

•People converge on new energetic optima within minutes, even for small cost savings 

•Updated predictions about energetically optimal gaits allow re-convergence within seconds 

•Energetic cost is not just an outcome of movement, but also continuously shapes it

Summary

People prefer to move in ways that minimize their energetic cost. For example, people tend to walk at a speed that minimizes energy use per unit distance and, for that speed, they select a step frequency that makes walking less costly. Although aspects of this preference appear to be established over both evolutionary and developmental timescales, it remains unclear whether people can also optimize energetic cost in real time. Here we show that during walking, people readily adapt established motor programs to minimize energy use. To accomplish this, we used robotic exoskeletons to shift people’s energetically optimal step frequency to frequencies higher and lower than normally preferred. In response, we found that subjects adapted their step frequency to converge on the new energetic optima within minutes and in response to relatively small savings in cost (less than 5%). When transiently perturbed from their new optimal gait, subjects relied on an updated prediction to rapidly re-converge within seconds. Our collective findings indicate that energetic cost is not just an outcome of movement, but also plays a central role in continuously shaping it.