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First published online May 30, 2008
Journal of Experimental Biology 211, 1893-1902 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014415

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Motor unit recruitment patterns 2: the influence of myoelectric intensity and muscle fascicle strain rate

Emma F. Hodson-Tole^1,* and James M. Wakeling²

¹ The Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Herts, AL9 7TA, UK
² School of Kinesiology, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada

^* Author for correspondence at present address: The School of Applied Physiology, Georgia Institute of Technology, Atlanta, Georgia, USA (e-mail: etole{at}gatech.edu)

Accepted 17 March 2008

To effectively meet the force requirements of a given movement an appropriate number and combination of motor units must be recruited between and within muscles. Orderly recruitment of motor units has been shown to occur in a wide range of skeletal muscles, however, alternative strategies do occur. Faster motor units are better suited to developing force rapidly, and produce higher mechanical power with greater efficiency at faster shortening strain rates than slower motor units. As the frequency content of the myoelectric signal is related to the fibre type of the active motor units, we hypothesised that, in addition to an association between myoelectric frequency and intensity, there would be a significant association between muscle fascicle shortening strain rate and myoelectric frequency content. Myoelectric and sonomicrometric data were collected from the three ankle extensor muscles of the rat hind limb during walking and running. Myoelectric signals were analysed using wavelet transformation and principal component analysis to give a measure of the signal frequency content. Sonomicrometric signals were analysed to give measures of muscle fascicle strain and strain rate. The relationship between myoelectric frequency and both intensity and muscle fascicle strain rate was found to change across the time course of a stride, with differences also occurring in the strength of the associations between and within muscles. In addition to the orderly recruitment of motor units, a mechanical strategy of motor unit recruitment was therefore identified. Motor unit recruitment is therefore a multifactorial phenomenon, which is more complex than typically thought.

Key words: electromyography, size principle, strain rate, locomotion

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				E. F. Hodson-Tole and J. M. Wakeling Motor unit recruitment patterns 1: responses to changes in locomotor velocity and incline J. Exp. Biol., June 15, 2008; 211(12): 1882 - 1892. [Abstract] [Full Text] [PDF]