It has been observed for nearly 100 years that visually guided human movements appear to be composed of submovements, intermittently executed overlapping segments. This paper presents experiments to investigate the pervasiveness of movement intermittency and, in particular, whether it is exclusively due to visual feedback. With and without visual feedback, human subjects were asked to 1) move with constant velocity and 2) draw elliptical figures on a phase-plane display (showing velocity vs. position) that required cyclic movements at different frequencies. In both tasks, we found that removal of visual feedback did not significantly change movement intermittency. Subjects were unable to generate movements at constant speed. In addition, subjects moved less smoothly when drawing slower phase-plane ellipses. Furthermore, elliptical phase-plane figures were not always drawn at the frequency suggested by the center of the display. Instead, subjects moved more slowly than the tall (fast) ellipse displays suggested, and faster than the wide (slow) displays suggested. These results show that 1) movement intermittency is not exclusively due to visual feedback and 2) may in fact be a fundamental feature of movement behavior.

The "classical" body-powered above elbow arm prosthesis continues to be used by a large majority of arm prosthesis users, even though many more modern devices are available. This paper presents a set of experiments designed to compare performance of unimpaired arms and body-powered prostheses of six unilateral amputees. The experiments were designed to measure quantitatively how well the body-powered prosthesis can be used to perform free-motion tasks, as well as to study the qualitative features of movement common to both the prosthesis and unimpaired arm. It was found that regular peaks in velocity were common to both the unimpaired arm and prosthesis movements, suggesting that movements were composed of a sequence of successive actions. In addition, it was found that the body-powered prosthesis generally required more movements than the unimpaired arm to meet an accuracy constraint and could not keep up with the unimpaired arm when a speed constraint was imposed, even though the body-powered prosthesis was able to match the unimpaired arm in a simple nondynamic task.

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Physically interacting with kinematic constraints is commonplace in everyday actions. We report a study of humans turning a crank, a circular constraint that imposes constant hand path curvature and hence should suppress variations of hand speed due to the power-law speed-curvature relation widely reported for unconstrained motions. Remarkably, we found that, when peripheral biomechanical factors are removed, a speed-curvature relation reemerges, indicating that it is, at least in part, of neural origin.

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