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Spatial Patterns in the Control of Human Arm Movement
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When people repeat a movement many times, they typically reproduce a stereotyped pattern (Atkeson & Hollerbach, 1985). This stereotypy is one way that the motor system solves the degrees of freedom problem (Bernstein, 1967). We asked 3 subjects to make pointing movements in 8 conditions representing trajectories along the sides of a square (Figure lA). The position of the index finger was measured, and resampled every centimeter. Spatial paths of 16 movements in Condition 1 from start (S) to target (f) are shown in Figure lB, and 16 movements in Condition 6 are shown in IC. Although the paths are fairly similar, there is both extrinsic variation in where the movements are located in space, and also intrinsic variation in their shapes.