The effect of load on variances of object replacing arm movements

Abstract

A three-dimensional (3D) dynamic upper limb model was developed to simulate muscle forces in slightly restricted point-to-point movements. Healthy subjects performed point-to-point movements repetitively with an object held in the hand. The object was either a very light (0.06kg) or a heavier one (2kg). Joint coordinates were recorded. Using joint coordinates, and muscle attachment sites taken from the literature, virtual muscle forces acted during the execution of the movement were calculated for 4 arm muscles. Variances of hand position trajectories, joint configuration trajectories and muscle activities (measured EMG and muscle forces of biceps, triceps, delta anterior and delta posterior) were calculated for both object conditions. There were no significant differences for hand position and arm configuration variances considering the two object conditions while muscle activity variances (for all muscles except deltoid posterior) increased significantly by executing the movement with heavier object. Since high muscle activity variances didn't result equally high increments in kinematic variances we suggest that the stabilization of the outer descriptors (kinematic properties) of the arm is resulted by the enhanced muscle cooperation through synergies when the motor task was performed with heavier object in the hand.