Effect of Intermittent Electrical Stimulation on Low‐Frequency Muscle Fatigue

Abstract

Skeletal muscles exposed to sustained, low-frequency (LF) stimulation experience a long-lasting reduction in force production known as low-frequency fatigue (LFF). The purpose of this study was to investigate the effect on LFF of a single high-frequency (100 Hz) pulse (i.e., doublet), added to the beginning of repeated, LF (20 Hz)) trains of pulses. Paired sartorii muscles (60–70 mg) from male R. pipiens (N=10) were dissected-free and vertically-mounted in water-jacketed chambers containing oxygenated (100%), Ringer solution (25°C; pH 7.2). One muscle from each animal was stimulated using individual trains of supramaximal, square-wave pulses (20 Hz, 0.2 ms) of 4 sec. duration. Twenty such trains were repeated with a duty-cycle of 4 sec. on/5 sec. off. The remaining muscle was treated identically except that a high-frequency (HF) pulse (100 Hz) was inserted prior to every 4th pulse of the 4 sec. train. Average tension (AT) for each train was determined and normalized relative to the first train. Analysis indicated a −76.4% overall decrease in AT in response to repeated LF stimulation. By comparison, the addition of a single HF pulse at the beginning of each 4 sec. train resulted in a significantly (p<.01) lesser decline (−35.7%). These results suggest that LFF could perhaps be mediated by a manipulation of the neural activation pattern to include intermittent HF and LF components. This result may be due to a catch-like property in skeletal muscle, as original described by Burke et al. (Science, 173:372–376, 1970).