Motor learning adapts to effort-dependent uncertainty

Adaptive behaviour depends on continuously updating motor control when action outcomes differ from expectations. Because action execution and sensory feedback are noisy, the value of an error depends on how reliably it reflects the consequences of an action. Most studies have manipulated the reliability of sensory feedback or action outcomes rather than uncertainty generated by the action itself. Here we show that motor adaptation is calibrated to effort-dependent uncertainty arising during action generation. Using a force-based motor adaptation paradigm, we demonstrate that greater effort progressively increased uncertainty in action execution and proprioceptive feedback, with corresponding changes in adaptation rate that closely matched normative predictions. Reward enhanced adaptation by improving proprioceptive precision rather than directly changing adaptation rate. Supporting this framework, patients with major depressive disorder exhibited elevated execution and proprioceptive uncertainty, impaired reward-dependent improvement in proprioceptive precision, and slower motor adaptation. These findings establish intrinsic uncertainty as a general determinant of adaptive behaviour, linking action generation, sensory evaluation, and motivation.