Does internal metabolic state determine human motor coordination strategy?

Abstract

Motor coordination requires the orchestration of multiple degrees of freedom in order to perform actions. Humans display characteristic and predictable reaching trajectories even though multiple trajectories are possible. Computational theories of motor control can explain these reaching trajectories by assuming that subjects orchestrate their movements to minimise a cost function, such as end-point variability or movement effort. However, how internal metabolic states influence decision making and sensorimotor control is not well understood. Here we measure human behaviour during a centre out reaching task in two distinct metabolic conditions in the morning - after having had breakfast and not. We find that humans alter their patterns of motor coordination according to their internal metabolic state and that this change in behaviour results in a 20% lower task-related energy expenditure when fasted. We suggest that movements are orchestrated according to different criteria in different metabolic states so that metabolic costs are reduced in low metabolic states. We also predict that motor coordination strategies take the metabolic costs of specific muscle groups into account when planning and executing movements. Thus, metabolic state may alter the computational strategies of decision making between animal-based experiments (in typically low metabolic conditions) and human psychophysics experiments (in typically high metabolic conditions).