TIME-VARYING MECHANICAL-BEHAVIOR OF MULTIJOINTED ARM IN MAN

1. The aim of this study was to describe the time-varying changes in t he mechanical parameters of a multijointed limb. The parameters we con sidered are the coefficients of stiffness, viscosity, and inertia. Con tinuous pseudorandom perturbations were applied at the elbow joint dur ing a catching task. A modified version of an ensemble technique was u sed for the identification of time-varying parameters. Torques at the elbow and wrist joints were then modeled with a linear combination of the changes in angular position and velocity weighed by the matrix of angular stiffness and the matrix of angular viscosity, respectively. C ontrol experiments were also performed that involved the stationary ma intenance of a given limb posture by resisting actively the applied pe rturbations. Different limb postures were examined in each such experi ment to investigate the dependence of the mechanical parameters on lim b geometry. 2. The technique for the identification of limb mechanical parameters proved adequate. The input perturbations applied at the el bow joint elicited angular oscillations at the wrist essentially uncor related with those produced at the elbow. The frequency of oscillation is much higher at the wrist than at the elbow, mainly because of the smaller inertia. The variance accounted for by the model was almost-eq ual-to 80% under both stationary and time-varying conditions; in the l atter case the value did not vary significantly throughout the task. I n addition, the model predicted values of the inertial parameters that were close to the anthropometric measures, and it reproduced the step wise increase in limb inertia that occurs at the time the ball is held in the hand. 3. The values of angular stiffness and viscosity estimat ed under stationary conditions did not vary significantly with joint a ngle, in agreement with previous results obtained under quasistatic po stural conditions. The matrix of the coefficients of angular stiffness was not symmetrical, indicating a prominent role for nonautogenic ref lex feedbacks with unequal gains for elbow and wrist muscles. 4. A com plex temporal modulation of angular stiffness and viscosity was observ ed during the catching task. The changes in the direct coefficients of angular stiffness tended to covary with those in the coupling coeffic ients from trial start up to almost-equal-to 30 ms before impact time. Around impact time, however, there was a complete dissociation: the d irect terms peaked, whereas the coupling terms dropped. The direct ter ms of angular viscosity also increased before impact, whereas the visc osity coupling terms remained close to zero throughout. 5. Neural corr elates of the changes in angular impedance were found by considering t he time course of the changes in net electromyographic activity and st retch reflexes during catching. Anticipatory muscle activity started 1 00-200 ms before impact and correlated qualitatively with anticipatory changes in angular stiffness. The peaks of the direct terms of stiffn ess and viscosity around impact could be accounted by the transient re versal of the direction of short-latency stretch reflex responses. The decrease of the coupling terms of stiffness around impact could be ex plained by a transient decrease of the gain of heteronymous stretch re flexes. 6. The matrices of the coefficients expressing stiffness and v iscosity in the Cartesian coordinates of the limb endpoint were also c omputed. From such matrices, the components corresponding to the vecto rs of resistance offered by the hand to a virtual vertical displacemen t were extracted. We found that the hand resistance is accurately modu lated relative to the impact time. The magnitude of hand resistance ve ctors increased consistently before impact, although with a different time course for hand stiffness and viscosity. Also before impact, the direction of viscous resistance vectors rotated closer to the vertical , indicating that a larger component of reactive force is exerted in t he direction of the expected perturbation. 7. The orientation of the v ectors of hand viscosity was variably correlated with the orientation of the vectors of hand inertia during catching. This result suggests t he existence of a parallel neural control of different components of h and impedance, that is inertia, stiffness, and viscosity. This paralle l control is predicated on the availability of accurate internal model s of the limb mechanical properties.