Movement Directions Aligned in Joint Space Are Not Aligned in Muscle Space
Learned movements are thought to be represented in both extrinsic and intrinsic coordinate systems. Intrinsic representations have traditionally been characterized using joint-based coordinates, although the relationship between joint movements and muscle activation depends strongly on limb configuration. Consequently, movement directions aligned in joint space may not be aligned in muscle space, but the implications of this mismatch for motor learning have remained largely unexplored. We addressed this question by combining electromyographic (EMG) analysis with a visuomotor adaptation experiment. In Experiment 1, participants performed planar reaching movements in two workspaces separated by a 45{degrees} shoulder rotation while EMG activity was recorded from nine upper-limb muscles. Muscle-pattern similarity analysis revealed that movement directions aligned in joint space were not always aligned in muscle space and that the degree of misalignment varied systematically across movement directions. Based on these results, we predicted that visuomotor adaptation to clockwise (CW) and counterclockwise (CCW) rotations would produce different patterns of motor generalization, contrary to the prediction of conventional joint-space accounts. Experiment 2 confirmed this prediction, revealing a systematic shift between the CW and CCW generalization patterns that was consistent with the muscle-space prediction. These findings suggest that intrinsic representations of learned movements are not fully captured by joint-based coordinates alone and that muscle-based coordinates contribute to motor learning and its generalization. Together, these findings highlight the importance of considering underlying biomechanics when interpreting motor representations using generalization paradigms.