DOES METHOCARBAMOL AFFECT FATIGUE MARKERS IN THE LOW-BACK ELECTROMYOGRAM

Many low-back patients undergo electromyography (EMG)-based evaluation s of muscle performance but present to the clinic after being prescrib ed muscle relaxants. The question that needed to be addressed was, do centrally acting muscle relaxants (methocarbamol; Robaxin(R)) affect t he EMG spectral indices of muscle fatigue that are often used to asses s muscle performance. Participants performed an isometric spine extens ion protocol involving a 30 s fatigue exertion trial, then 1 min rest, and finally a 10 s long repeat exertion trial, at a 60% maximum volun tary contraction (MVC) level of exertion. Seven men were tested on two separate days (approximately 3-7 days apart), one day while medicated (six doses) with Robaxin and on another while not medicated. Specific ally, the following parameters were studied in the bilateral multifidu s (L5), lower erector spinae (L3) and upper erector spinae (T9): the s lope of median power frequencies (MPFs) over the duration of the trial and the initial y-intercept of the MPF. The results generally suggest that methocarbamol (Robaxin) does not have any significant affect on the EMG median power frequency of the extensors during a fatiguing con traction followed by a repeat exertion, at least in normal people (one exception was observed-one side of multifidus at L5). However, given that this appears to be the first study of its kind, and that a relati vely small number of subjects were used in this study, further investi gation is needed to make a definitive conclusion about the effects of this drug on the several features of the electromyogram, over a broad spectrum of the clinical population performing a wider variety of task s. (C) 1998 Elsevier Science Ltd. All rights reserved.