Action Progress Organizes Motor Memory
Many everyday actions, such as handwriting, can be performed fluently across a wide range of scales and speeds. This flexibility suggests that motor behavior relies on abstract representations that transcend low-level motor or sensory variables. However, how such abstract representations are formed from individual movement experiences remains unclear. Here, we test the hypothesis that motor memory is organized not only by physical state variables, but also by an abstract coordinate of action progress: the normalized progression of an intended action from initiation to completion. Using a series of motor adaptation experiments that dissociate action progress from kinematic variables and absolute time, we show that learned outputs are retrieved according to their position within an action. This progress-based coordinate spans the unfolding course of a planned action, whether a single reach or an action sequence, and operates continuously rather than as a simple early-late distinction. Together, these findings identify action progress as an abstract organizing coordinate through which the motor system may encode movement experiences as abstract motor memories, offering a framework for understanding the flexible generalization of human motor behavior.