Disentangling the effects of metabolic cost and accuracy on movement vigor

AO_SCPLOWBSTRACTC_SCPLOWOn any given day, we make countless reaching movements to objects around us. While such ubiquity may suggest uniformity, each movement is unique in the speed with which it is made. Some movements are slow, while others are fast. These variations in reach speed have long been known to be influenced by accuracy constraints; we slow down when accuracy demands are high. However, in other forms of movement like walking, metabolic cost is the primary determinant of movement speed. Here we ask, how do metabolic cost and accuracy interact to determine speed of reaching movements? First we systematically measure the effect of increasing mass on the metabolic cost of reaching across a range of movement speeds. Next, in a sequence of three experiments, we examine how added mass affects preferred movement speeds in a simple reaching task with increasing accuracy requirements. We find that mass consistently increased metabolic cost and led to slower movements. Yet, intriguingly, preferred reach speeds were slower than metabolically optimal. We then demonstrate how a single model that, critically, considers both accuracy and metabolic cost can explain preferred movement speeds across the range of accuracy and effort requirements tested. Together, these findings provide a unifying framework to explain the combined effects of metabolic cost and accuracy on movement speed, and also highlight the integral role metabolic cost plays in determining reach speed.