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J Neurophysiol (November 2, 2005). doi:10.1152/jn.00924.2005
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Submitted on September 2, 2005
Accepted on October 29, 2005

EMG changes in human thenar motor units with force potentiation and fatigue

Christine K. Thomas1*, Roland S. Johansson2, and Brenda R. Bigland-Ritchie3

1 The Miami Project to Cure Paralysis, Departments of Neurological Surgery, Physiology and Biophysics, University of Miami, Miami, FL, USA
2 Physiology Section, IMB, Umea University, Umea, Sweden
3 The Miami Project to Cure Paralysis, University of Miami, Miami, FL, USA

* To whom correspondence should be addressed. E-mail: cthomas{at}miami.edu.

Few studies have analyzed activity-induced changes in electromyographic activity (EMG) in individual human motor units. We investigated the changes in human thenar motor unit EMG that accompany the potentiation of twitch force and fatigue of tetanic force. Single motor unit EMG and force were recorded in healthy subjects in response to selective stimulation of their motor axons within the median nerve just above the elbow. Twitches were recorded before and after a series of pulse trains delivered at frequencies that varied between 5 Hz and 100 Hz. This stimulation induced significant increases in EMG amplitude, duration and area. However, in relative terms, all of these EMG changes were smaller than the potentiation of twitch force. Another 2 minutes of stimulation (13 pulses at 40 Hz each second) induced additional potentiation of EMG amplitude, duration and area, but the tetanic force from every unit declined. Thus, activity-induced changes in human thenar motor unit EMG do not indicate the alterations in force, or vice versa. These data suggest that different processes underlie the changes in EMG and force that occur during human thenar motor unit activity.




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