Changes in movement kinematics during single-joint movements against expectedly and unexpectedly changed inertial loads

Abstract

The study had two goals: (1) to understand the role of peripheral and central factors in changes in the movement symmetry ratio (acceleration time divided by deceleration time); and (2) to compare several theories of motor control with respect to their ability to predict changes in a number of kinematic indices when movements are performed against expectedly and unexpectedly changed inertial loads. Subjects performed elbow flexion movements from a standard initial position to a fixed target "as fast as possible" against three different inertial loads. In some trials, prior to the movement, the load was changed unexpectedly for the subject. Then the load remained the same for a block of trials, then changed again, and so on. We assumed that the first trial of a block was performed using central control patterns associated with moving a different expected load used in the previous block of trials. The main findings included: (a) the equifinality of movements in all conditions, irrespective of actual and expected loads; (b) a decrease in peak velocity and an increase in movement time when the actual load increased; (c) no significant changes in peak velocity and movement time when moving against a load while expecting a different load; and (d) symmetry ratio decreased with actual load and increased with expected load. Separate analyses of the effects of changes of expected and actual loads showed different slopes of the relation between peak velocity and movement symmetry ratio. Based on the last findings we conclude that movement symmetry is defined by both peripheral factors, possibly related to the role of damping forces, and by central control patterns. The equilibrium-point (EP) hypothesis is able to handle the findings better than torque-control models which make wrong predictions with respect to findings (a), (c), and (d) or a model combining control of torque patterns with specification of a final equilibrium position which makes wrong predictions with respect to findings (c).