One hypothesis for the generation of spatially oriented arm movements by the central nervous system is that a desired joint position is determined by the ratio of the tensions of agonist and antagonist muscles. According to this hypothesis, the transition between equilibrium states should be solely a function of the contraction time of the motor units and the mechanical properties of the arm. We tested this hypothesis in intact and deafferented rhesus monkeys by holding the forearm and measuring the accelerative transient after release of the forearm and by directly measuring the time course of the increase in torque during the movement. Both methods indicated an average time of 400 msec for attaining peak torque in a movement with a duration of 700 msec. In addition, by displacing the arm from its normal trajectory during the movement, we observed that the arm returned neither to the initial nor to the final equilibrium positions, but to points intermediate between them. We conclude that the processes underlying trajectory formation must be more complex than a simple switch between one equilibrium position and another.

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The formation of forearm trajectories of moderate velocities (0.3-1.3 rad/s) was studied in monkeys performing a simple visuomotor task. The experiments were designed to test the hypothesis that the transition from one position to another is subserved by a rapid shift to a final equilibrium of forces in agonist and antagonist muscles. This idea is attractive because it suggests the possibility that in simple movements the trajectory is determined by the inherent inertial and viscoelastic properties of the limb and muscles around a joint. The results indicate that these moderate speed movements are controlled by a gradual, and not a step-like, shift to the final equilibrium position.

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1. A threatening visual stimulus frequently elicits the defense response (DR) in crayfish, a behavior that comprises orienting the body to face the stimulus, raising the thorax, and extending and opening the claws. Although this behavior has been reported previously, its kinematics have not been characterized. This work employs kinematic analysis to provide a quantitative description of the claw (cheliped) as it is moved during the DR. 2. The cheliped was modeled as an open kinematic chain with three segments and 4 df. Simulations employing the model were compared with actual cheliped trajectories during the DR to ascertain the applicability of the model. The model was then employed to demonstrate the effects of individual joint rotations on the overall trajectory of the claw. 3. The individual segments of the cheliped were monitored during the DR, and spatial trajectories, tangential velocities, and intersegmental joint angles were calculated. 4. The joint angles assumed at the final position of the DR were highly stereotyped. This constancy in joint angle at the endpoint of the movement stands in contrast to the variability in both the angular and spatial trajectories of the cheliped as it was moved towards the final position. 5. Movement time was relatively constant. Larger amplitude movements were performed with a proportional increase in velocity similar to arm movements in primates. 6. The DR positions the cheliped in a fixed location in the workspace. Unlike the primate arm during reaching, the cheliped does not proceed towards the endpoint with a smooth controlled trajectory characteristic of a system with fine interjoint coordination. Instead, it appears that individual joint rotations are performed independently, thus precluding trajectory control although permitting an accurate specification of the endpoint.