Humans use minimum cost movements in a whole-body task

Abstract

Humans have elegant bodies that allow gymnastics, piano playing, and tool use, but understanding how they do this in detail is difficult because their musculoskeletal systems are extraordinarily complicated. Nonetheless, common movements like walking and reaching can be stereotypical, and a very large number of studies have shown their movement cost a major factor. In contrast, one might think that general movements are very individuated and intractable, but a recent study has shown that in an arbitrary set of whole-body movements used to trace large-scale closed curves, near-identical posture sequences were chosen across different subjects, both in the average trajectories of the bodys limbs and in the variance within trajectories. The commonalities in that result motivate explanations for its generality. One possibility could be that humans also choose trajectories that are economical in energetic cost. To test this hypothesis, we situate the tracing data within a fifty degree of freedom dynamic model of the human skeleton that allows the computation of movement cost. Comparing the model movement cost data from nominal tracings against various perturbed tracings shows that the latter are more energetically expensive, inferring that the original traces were chosen on the basis of minimum cost.