Dynamic grasp and manipulation with the fingertips (""""""""precision pinch"""""""") depend on the mechanical ability to orchestrate fingertip motion and force. The muscle coordination patterns required to produce fingertip motion differ from those associated with exerting fingertip forces: fingertip motion depends on tendon excursions, whereas fingertip force relies on tendon tensions. Dexterous tasks like precision pinch require the coordination of muscles to fulfill two fundamental requirements: (i) the transition from fingertip motion to force in the same direction (e.g., to quickly pick-up a pencil); and (ii) combinations of fingertip motion and force in different directions (e.g., to roll the pencil). We will systematically explain how the musculature of the index finger is coordinated to fulfill these two fundamental requirements of precision pinch. This understanding will provide the biomechanical foundation for much needed studies on the degeneration of precision pinch in orthopedic and neurologic diseases and in aging. HYPOTHESIS I: The sequential transition from fingertip motion to force along the same direction involves interpolating between coordination patterns at the expense of motion and force accuracy, with the abruptness of the interpolation depending on the allowable margin of error. HYPOTHESIS II: The simultaneous production of submaximal fingertip motion and force in different directions can sufficiently constrain muscle interactions to eliminate muscle redundancy. To test these hypotheses, we propose an integrative approach combining behavioral studies, robotics analysis, biomechanical modeling, and cadaveric experiments.
AIM 1 : To describe the coordination patterns used by humans to produce a variety of index fingertip motion and force tasks. Muscle coordination patterns will be expressed as 7-dimensional vectors estimated from electromyograms (EMG).
AIM 2 : To explain how the pattern of muscle coordination satisfies the mechanical requirements of the task. A biomechanical model of the index finger will predict coordination patterns that reproduce the motions and forces observed in Aim 1 while evaluating the effects of variability in musculoskeletal parameters, and quantifying the degree of muscle redundancy.
AIM 3 : To validate the EMG and predicted coordination patterns by actuating the tendons of cadaveric fingers and comparing the mechanical output with the voluntary and model actions.
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