Electromyographic and contractile properties of rabbit masseter motor units during fatiguing stimulation

Trigeminal motoneurons were electrically stimulated in order to investigate the electromyographic (EMG) behavior in relation to the contractile properties of motor units of the masseter muscle. A total of 80 motor units were studied in situ in male New Zealand White rabbits ( n=46). The motor units were separated into two groups, each exposed to a specific fatiguing stimulation regimen. Motor unit action potential (MUAP) features, which comprised the amplitude (AMP) and inter-peak time (IPT), and the tetanic force were measured. All motor units were classified as fast (F) units. Forty-one motor units underwent a prolonged standard fatigue regimen of 40-Hz trains at 1 Hz for 20 min. While the MUAP showed an immediate decrease of mean AMP at the beginning of the stimulation, the mean force and IPT increased. After 2 min, the force declined, while the IPT continued to increase until 20 min. Only after 3 min of stimulation, did the degree of force decrease parallel the decline in MUAP AMP. After 20 min of stimulation, the majority of motor units ( n=34) still generated a force larger than 50% of the initial value, but only 17 motor units showed MUAP AMP of less than 50% of the initial EMG response. A more intensive fatigue regimen (40-Hz trains at 1.5 Hz) was applied to another group of 39 motor units. A rapid decline of force and MUAP amplitude to almost 50% was observed within the first 5 min of stimulation. After 20 min, only four motor units were still able to produce a tetanic force of more than 50% of the initial. Most strikingly, motor units with twitch contraction times faster than 22 ms exhibited a decrease in force more than in MUAP AMP, whereas the reverse was seen for units slower than 22 ms; motor units with a twitch contraction time of 22 ms showed equal decrease in AMP and force. This finding is suggestive of a division of fast masseter motor units into two classes, those which fatigue more rapidly mechanically and those which fatigue more readily electrically.