Moving faster while preserving accuracy

Abstract

Achieving movements with accuracy despite the inevitable variability of the neuromuscular mechanisms is an important everyday life problem, which has to be solved for the production of any adapted motor act, such as walking, writing, catching, or pointing. To solve this problem when we have to make goal-directed movements as fast as possible, we systematically increase movement time when accuracy requirements increase, a ubiquitous phenomenon qualified as speed-accuracy trade-off. It has been proposed that this speed-accuracy trade-off reflects an optimal compromise between speed and accuracy in the presence of biological noise and that increasing movement speed inevitably leads to decreased motor accuracy. However, the recent finding that muscle cocontraction improves movement accuracy may challenge this view and begs the question of how movement speed control and cocontraction control coexist. Here, we show that humans are in fact able to move faster while preserving movement accuracy, by using a strategy where muscles are cocontracted around the joint. As this energetically costly cocontraction strategy was not naturally used, this result has two important implications. It first demonstrates that a speed modulation strategy is preferred to a cocontraction strategy for fast, accurate movements, and it also suggests that energy economy prevents us to execute accurate movements as fast as we could do. Consequently, we propose that the mechanisms underlying the speed-accuracy trade-off are more complex than previously thought, and suggest the existence of a previously unknown speed-energy-accuracy trade-off for goal-directed movements.