S. Jaric et al., Changes in movement kinematics during single-joint movements against expectedly and unexpectedly changed inertial loads, HUMAN MOVE, 18(1), 1999, pp. 49-66
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.