Overarm Throws with the Nondominant Arm: The Kinematics of Accuracy.
Hore, J., S. Watts, D. Tweed and B. Miller.
Physiology Department, University of Western Ontario, London, ON, N6A 5C1,
APStracts 3:0181N, 1996.
SUMMARY AND CONCLUSIONS
1. Overarm throws made with the nondominant arm are usually less accurate than
those made with the dominant arm. The objective was to determine the errors in
the joint rotations associated with this inaccuracy, and thereby to gain
insight into the neural mechanisms that contribute to skill in overarm
throwing. 2. Overarm throws from both left and right arms were recorded on
different occasions as 6 right- handed subjects sat with a fixed trunk and
threw 150 tennis balls at about the same speed at a 6-cm square on a target
grid 3 m away. Joint rotations at the shoulder, elbow, wrist and finger, and
arm translations, were computed from recordings of arm segment orientations
made with the magnetic-field search-coil technique. 3. All subjects threw less
accurately in this task with their left (nondominant) arm. For throws
made with the left arm the height of ball impact on the target grid was
related to hand trajectory length, and to hand orientation in space at ball
release, but not to hand trajectory height. 4. Two hypotheses were proposed to
explain the decreased ball accuracy in the high-low direction when throwing
with the nondominant arm: that it was caused by increased variability in the
velocity or timing of onset of rotations at proximal joints (which determine
the path of the hand through space) or, increased variability in the velocity
or timing of onset of finger extension (which determine the moment of ball
release). 5. A prediction of the first hypothesis was that proximal joint
rotations should be more variable in throws with the left arm. This was the
case for the majority of proximal joint rotations in the 6 subjects when
variability was examined in joint space. However, some proximal joint
rotations were more variable in the right arm. 6. The first hypothesis was
directly tested by determining whether hand angular position in space (which
represents the sum of all proximal joint rotations) was related to ball impact
height on the target grid, at a fixed translational position in the throw. No
relation was found between these variables for throws with the left arm in 4
subjects, while a weak relation was found for 2 subjects. It was concluded
that, considering all subjects, the first hypothesis could not explain the
results. 7. In contrast, in agreement with the second hypothesis, a strong
relation (P<0.001) was found in all subjects between ball impact height on the
target grid and time of ball release for throws with the left arm, and with
time of onset of finger extension. 8. Across all 6 subjects the timing
precision (windows) for 95% of the throws was for ball release: 9.3 ms right
arm, 22.5 ms left arm; and for onset of finger extension: 13.7 ms right arm,
26.7 ms left arm. 9. Timing of onset of finger extension was no less accurate
than timing of onset of other joint rotations for both left and right arms.
However simulations of throws showed that, for the same error in timing,
finger extension had twice as large an effect on ball direction as any other
joint rotation. Timing errors at the fingers have a greater effect than errors
at other joints because finger errors are scaled by the higher angular
velocity of the hand in space rather than by the smaller angular velocities of
the individual joints. 10. It is concluded that although rotations were in
general more variable at both proximal and distal joints of the non-dominant
(left) arm, the major cause of its decreased throwing accuracy was increased
variability at the distal joints, i.e., in the timing of onset of finger
extension. This may be due to a lack of precision in the commands from the
right hemisphere to the left fingers in right-handed throwers.
Received 26 January 1996; accepted in final form 6 August 1996.
APS Manuscript Number J5-96.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 19 September 1996