EXPERIMENTALLY BASED NONLINEAR VISCOELASTIC MODEL OF JOINT PASSIVE MOMENT

Previous investigations have not converged on a generally accepted mod el of the dissipative part of joint passive moment. To provide a basis for developing a model, a series of measurements were performed to ch aracterize the passive moment at the metacarpophalangeal joint of the index finger. Two measurement procedures were used, one in moment rela xation over a range of fixed joint angles and the other at a series of constant joint velocities. Fung's quasi-linear viscoelastic theory mo tivated the development of the passive moment model. Using this approa ch, it was not necessary to make restrictive assumptions regarding the viscoelastic behavior of the passive moment. The generality of the fo rmulation allowed specific functions to be chosen based on experimenta l data, rather than finding coefficients which attempted to fit a pres elected model of the data. It was shown that a nonlinear viscoelastic model described the passive stiffness. No significant frictional effec ts were found. Of particular importance was the nonlinear behavior of the dissipative part of the passive moment which was modeled by joint speed raised to a power less than one. This result could explain the d iffering findings among previous investigations, and may have importan t implications for control of limb movement.