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

The study had two goals: (1) to understand the role of peripheral and centr al factors in changes in the movement symmetry ratio (acceleration time div ided by deceleration time); and (2) to compare several theories of motor co ntrol with respect to their ability to predict changes in a number of kinem atic indices when movements are performed against expectedly and unexpected ly 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 ass ociated with moving a different expected load used in the previous block of trials. The main findings included: (a) the equifinality of movements in a ll conditions, irrespective of actual and expected loads; (b) a decrease in peak velocity and an increase in movement time when the actual load increa sed; (c) no significant changes in peak velocity and movement time when mov ing against a load while expecting a different load; and (d) symmetry ratio decreased with actual load and increased with expected load. Separate anal yses of the effects of changes of expected and actual loads showed differen t slopes of the relation between peak velocity and movement symmetry ratio. Based on the last findings we conclude that movement symmetry is defined b y both peripheral factors, possibly related to the role of damping forces, and by central control patterns. The equilibrium-point (EP) hypothesis is a ble to handle the findings better than torque-control models which make wro ng predictions with respect to findings (a), (c), and (d) or a model combin ing control of torque patterns with specification of a final equilibrium po sition which makes wrong predictions with respect to findings (c). (C) 1999 Elsevier Science B.V. All rights reserved.