41 The musculoskeletal system can move in more ways than are strictly necessary, allowing 42 many tasks to be accomplished with a variety of limb configurations. Why some configurations 43 are preferred has been a focus of motor control research, but most studies have focused on 44 shoulder-elbow or whole-arm movements. This study focuses on movements involving forearm 45 pronation-supination (PS), wrist flexion-extension (FE), and wrist radial-ulnar deviation (RUD), 46 and elucidates how these

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... three degrees of freedom (DOF) combine to perform the common task 47 of pointing, which only requires two DOF. Although pointing is more sensitive to FE and RUD 48 than to PS and could be easily accomplished with FE and RUD alone, subjects tend to involve a 49 small amount of PS. However, why we choose this behavior has been unknown and is the focus 50 of this paper. Using a second-order model with lumped parameters, we tested a number of 51 plausible control strategies involving minimization of work, potential energy, torque, and path 52 length. None of these control schemes robustly predicted the observed behavior. However, an 53 alternative control scheme hypothesized to control the DOF that were most important to the task 54 (FE and RUD) and ignore the less important DOF (PS), matched the observed behavior well. In 55 particular, the behavior observed in PS appears to be a mechanical side effect caused by 56 unopposed interaction torques. We conclude that moderately-sized pointing movements 57 involving the wrist and forearm are controlled by ignoring forearm rotation even though this 58 strategy does not robustly minimize work, potential energy, torque, or path length. 60 New and Noteworthy 61 Many activities require us to point our hands in a given direction using wrist and forearm 62 rotations. Although there are infinitely many ways to do this, we tend to follow a stereotyped 63 pattern. Why we choose this pattern has been unknown and is the focus of this paper. After 64 testing a variety of hypotheses, we conclude that the pattern results from a simplifying strategy in 65 which we focus on wrist rotations and ignore forearm rotation. 66 Keywords 67 Redundancy, pointing, wrist, forearm, Donders 68 69 109 1 Donders' Law is an alternative description of how redundant DOF are combined during rotation. Instead of expressing the pattern as a relationship between joint angles, Donders' Law expresses the pattern as a relationship between the coordinates of the total rotation vector (due to rotation in all DOF). Consequently, Donders' Law states that the total rotation vector only occupies a subspace of the total space it could occupy.