Control strategies for the transition from multijoint to single-joint arm movements studied using a simple mechanical constraint

dChanges were studied in neuromotor control that were evoked by constrainin g the motion of the elbow joint during planar, supported movements of the d ominant arm in eight normal human subjects. Electromyograph (EMG) recording s from shoulder and arm muscles were used to determine whether the normal m ultijoint muscle activity patterns associated with reaching to a visual tar get were modified when the movement was reduced to a single-joint task, by pinning the elbow to a particular location in the planar work space. Three blocks of 150 movements each were used in the experiments. Subjects were pr esented with the unconstrained task in the first and third blocks with an i ntervening block of constrained trials. Kinematic, dynamic, and EMG measure s of performance were compared across blocks. The imposition of the pin con straint caused predictable changes in kinematic performance, in that near-l inear motions of the hand became curved. This was followed by changes in li mb dynamic performance at the elbow. However, changes in EMG activity at th e shoulder lagged the kinematic changes substantially (by about 15 trials). The gradual character of the changes in EMG timing does not support a prim ary role for segmental reflex action in mediating the transition between mu ltijoint and single-joint control strategies. Furthermore, the scope and ma gnitude of these changes argues against the notion that human motor perform ance is driven by the optimization of muscle- or joint-related criteria alo ne. The findings are best described as reflecting the actions of a feedforw ard adaptive controller that has properties that are modified progressively according to the environmental state.