Humans use minimum cost movements in a whole-body task

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, although movements can be very individuated, some common movements like walking and reaching can be stereotypical, with the movement cost a major factor. A recent study has extended these observations by showing that in an arbitrary set of whole-body movements used to trace large-scale closed curves, near-identical posture changes were chosen across different subjects, both in the average trajectories of the body’s limbs and in variations within trajectories. The commonality of that result motivates explanations for this generality. One 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. Moreover, the computational approach used to establish minimum cost principle suggests a refinement of what is known about cortical movement representations.Author SummaryAlthough motor cortical areas have been extensively studied, their basic response properties are still only partially understood, and it remains controversial whether neural activity relates to muscle commands or to abstract movement features. We provide a new perspective of how movements may be resented in the brain by showing that humans chose trajectories with minimum energy cost while accomplishing goal-directed tasks. Furthermore, most of the current neural control studies are experimental. Our computational methodology coupled with a minimum energy principle suggests a refinement of the brain’s storage of remembered movements.