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First published online May 30, 2008
Journal of Experimental Biology 211, 1882-1892 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014407

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Motor unit recruitment patterns 1: responses to changes in locomotor velocity and incline

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, GA 30332, USA (e-mail: etole{at}gatech.edu)

Accepted 17 March 2008

Mammalian skeletal muscles are composed of a mixture of motor unit types, which contribute a range of mechanical and physiological properties to the muscle. For a muscle to effectively contribute to smooth, co-ordinated movement it must activate an appropriate number and combination of motor units to generate the required force over a suitable time period. Much evidence exists indicating that motor units are activated in an orderly fashion, from the slowest through to the fastest. A growing body of evidence, however, indicates that such a recruitment strategy does not always hold true. Here we investigate how motor unit recruitment patterns were influenced by changes in locomotor velocity and incline. Kinematics data and myoelectric signals were collected from three rat ankle extensor muscles during running on a treadmill at nine velocity and incline combinations. Wavelet and principal component analysis were used to simultaneously decompose the signals into time and frequency space. The relative frequency components of the signals were quantified during 20 time windows of a stride from each locomotor condition. Differences in signal frequency components existed between muscles and locomotor conditions. Faster locomotor velocities led to a relative increase in high frequency components, whereas greater inclines led to a relative increase in the low frequency components. These data were interpreted as representing changes in motor unit recruitment patterns in response to changes in the locomotor demand. Motor units were not always recruited in an orderly manner, indicating that recruitment is a multi-factorial phenomenon that is not yet fully understood.

Key words: electromyography, size principle, wavelet analysis, principal component analysis

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