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J Appl Physiol (July 19, 2002). doi:10.1152/japplphysiol.00008.2002
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Articles in PresS, published online ahead of print July 19, 2002
J Appl Physiol, 10.1152/jap.00008.2002
Submitted on January 8, 2002
Accepted on July 16, 2002

Characteristics of surface mechanomyogram are dependent on development of fusion of motor units in humans

Yoshitake Yasuhide1*, Shinohara Minoru2, Ue Hidetoshi1, and Moritani Toshio1

1 Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Kyoto, Japan
2 Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA

* To whom correspondence should be addressed. E-mail: yoshitake{at}oita-nhs.ac.jp.

The purpose of this study was to test if surface mechanomyogram (MMG) recorded on the skin reflects the contractile properties of individual motor units in humans. Eight motor units in the medial gastrocnemius muscle were identified and trains of stimulation at 5, 10, 15 and 20 Hz were delivered to each isolated motor unit. There was a significant positive correlation between the duration of MMG and twitch duration. MMG amplitude decreased with increasing stimulation frequency. Reductions in MMG amplitude were in parallel with the reductions in force fluctuations, and the rate of change in both was positively correlated across the motor units. Rate of change in MMG amplitude against force was negatively correlated to half relaxation time and twitch duration. Similar negative correlations were found between force fluctuations and contractile properties. These results provide evidence supporting a direct relation between MMG and contractile properties of individual motor units within the gastrocnemius muscle, indicating that surface MMG is dependent on the contractile properties of the activated motor units in humans.






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