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The Journal of Neurophysiology Vol. 82 No. 3 September 1999, pp. 1187-1197
Copyright ©1999 by the American Physiological Society
1Department of Physiology, University of Western Ontario London, Ontario N6A 5C1; and 2Department of Physiology, University of Toronto, Toronto, Ontario M5S 1A8, Canada
Hore, J.,
S. Watts, and
D. Tweed.
Prediction and Compensation by an Internal Model for Back Forces
During Finger Opening in an Overarm Throw. J. Neurophysiol. 82: 1187-1197, 1999. Previous studies have
indicated that timing of finger opening in an overarm throw is likely
controlled centrally, possibly by means of an internal model of hand
trajectory. The present objective was to extend the study of throwing
to an examination of the dynamics of finger opening. Throwing a heavy
ball and throwing a light ball presumably require different neural
commands, because the weight of the ball affects the mechanics of the
arm, and particularly, the mechanics of the finger. Yet finger control
is critical to the accuracy of an overarm throw. We hypothesized that
finger opening in an overarm throw is controlled by a central mechanism that uses an internal model to predict and compensate for
movement-dependent back forces on the fingers. To test this idea we
determined whether finger motion is affected by back forces, i.e.,
whether larger back forces cause larger finger extensions. Back forces
were varied by having subjects throw, at the same fast speed,
tennis-sized balls of different weights (14, 55, and 196 g). Arm-
and finger-joint rotations were recorded with the search-coil
technique; forces on the middle finger were measured with force
transducers. Recordings showed that during ball release, the middle
finger experienced larger back forces in throws with heavier balls.
Nevertheless, most subjects showed proximal interphalangeal joint
extensions that were unchanged or actually smaller with the heavier
balls. This was the case for the first throw and for all subsequent
throws with a ball of a new weight. This suggests that the finger
flexors compensated for the larger back forces by exerting larger
torques during finger extension. Supporting this view, at the moment of ball release, all finger joints flexed abruptly due to the now unopposed torques of the finger flexors, and the amplitude of this
flexion was proportional to ball weight. We conclude that in overarm
throws made with balls of different weights, the CNS predicts the
different back forces from the balls and adjusts finger flexor torques
accordingly. This is consistent with the view that finger opening in
overarm throws is controlled by means of an internal model of the motor
apparatus and the external load.
This article has been cited by other articles:
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J. Hore and S. Watts Timing Finger Opening in Overarm Throwing Based on a Spatial Representation of Hand Path J Neurophysiol, June 1, 2005; 93(6): 3189 - 3199. [Abstract] [Full Text] [PDF]
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C. E. Lang and A. J. Bastian Cerebellar Damage Impairs Automaticity of a Recently Practiced Movement J Neurophysiol, March 1, 2002; 87(3): 1336 - 1347. [Abstract] [Full Text] [PDF]
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J. Hore, S. Watts, M. Leschuk, and A. MacDougall Control of Finger Grip Forces in Overarm Throws Made by Skilled Throwers J Neurophysiol, December 1, 2001; 86(6): 2678 - 2689. [Abstract] [Full Text] [PDF]
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D. Timmann, R. Citron, S. Watts, and J. Hore Increased Variability in Finger Position Occurs Throughout Overarm Throws Made by Cerebellar and Unskilled Subjects J Neurophysiol, December 1, 2001; 86(6): 2690 - 2702. [Abstract] [Full Text] [PDF]
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