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The Journal of Neuroscience, July 8, 2009, 29(27):8784-8789; doi:10.1523/JNEUROSCI.0853-09.2009

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Brief Communications
Maximal Voluntary Fingertip Force Production Is Not Limited by Movement Speed in Combined Motion and Force Tasks

Kevin G. Keenan,^2,3,6 Veronica J. Santos,^4,6 Madhusudhan Venkadesan,^5,6 and Francisco J. Valero-Cuevas^1,2,6

¹Department of Biomedical Engineering and ²Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, California 90089, ³Department of Human Movement Science, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53211, ⁴Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona 85287, ⁵School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, and ⁶Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853

Correspondence should be addressed to Francisco J. Valero-Cuevas, University of Southern California, 3710 McClintock Avenue, RTH 404, Los Angeles, CA 90089-2905. Email: valero{at}usc.edu

Numerous studies of limbs and fingers propose that force–velocity properties of muscle limit maximal voluntary force production during anisometric tasks, i.e., when muscles are shortening or lengthening. Although this proposition appears logical, our study on the simultaneous production of fingertip motion and force disagrees with this commonly held notion. We asked eight consenting adults to use their dominant index fingertip to maximize voluntary downward force against a horizontal surface at specific postures (static trials), and also during an anisometric "scratching" task of rhythmically moving the fingertip along a 5.8 ± 0.5 cm target line. The metronome-timed flexion–extension movement speed varied 36-fold from "slow" (1.0 ± 0.5 cm/s) to "fast" (35.9 ± 7.8 cm/s). As expected, maximal downward voluntary force diminished (44.8 ± 15.6%; p = 0.001) when any motion (slow or fast) was added to the task. Surprisingly, however, a 36-fold increase in speed did not affect this reduction in force magnitude. These remarkable results for such an ordinary task challenge the dominant role often attributed to force–velocity properties of muscle and provide insight into neuromechanical interactions. We propose an explanation that the simultaneous enforcement of mechanical constraints for motion and force reduces the set of feasible motor commands sufficiently so that force–velocity properties cease to be the force-limiting factor. While additional work is necessary to reveal the governing mechanisms, the dramatic influence that the simultaneous enforcement of motion and force constraints has on force output begins to explain the vulnerability of dexterous function to development, aging, and even mild neuromuscular pathology.

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Received Feb. 19, 2009; revised May 18, 2009; accepted May 18, 2009.

Correspondence should be addressed to Francisco J. Valero-Cuevas, University of Southern California, 3710 McClintock Avenue, RTH 404, Los Angeles, CA 90089-2905. Email: valero{at}usc.edu

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