Hiroyuki TOIKAWA

Department of Rehabilitation Medicine, Keio University School of Medicine

(Received 11 October 2000; accepted 1 June 2001)

Abstract: To clarify electrically-induced muscle fatigue, the relationship between muscle tension and the myoelectrical signals as well as the spatial extent of the glycogen-depleted muscle fibers were investigated in the rat hindlimb muscles. Fifty-four male Wistar rats (12 to 16-week-old) were included. Under general anesthesia, tibialis anterior (TA) and soleus (SOL) muscles (20limbs respectively) were exposed and their distal tendons were attached to the force transducer to measure the isometric tension. A small electrode array consisting of stimulating needles and recording bars was placed over the muscle belly, and tetanic electrical stimulation was applied for 30s at a frequency of either 20Hz or 40Hz. Peak-to-peak amplitude and peak latency of the M-wave, and the muscle fiber conduction velocity (MFCV) were analyzed. The MFCVs were corrected to the temperature of 26°. Histochemical analysis was done for 6 TA muscles after the electrical stimulation. Serial cross-sections of the muscles were stained by the periodic-acid Schiff (PAS) reaction to show the glycogen-depleted muscle fibers near the stimulating electrodes. During tetanic electrical stimulation, progressive amplitude depression and latency increase of the M-waves as well as slowing of the MFCVs were found which were always preceded by decline of the isometric tension. The speed of these changes differed significantly between TA and SOL muscles and between 20 and 40Hz of stimulation frequencies. After the fatiguing 60s-stimulation, the glycogen depletion occurred in a wedge-shaped area with clear margins, which was thought to correspond to the electrically-activated muscle fibers. Monitoring the M-waves, especially their amplitudes, seems valuable for detecting muscle fatigue before actual tension drops. The results of PAS staining suggest that increasing stimulus intensity to take countermeasures against the tension drop may be of value for automatic control of functional electrical stimulation for the paralyzed muscle in the field of rehabilitation medicine.

Key words: rat skeletal muscle, electrical stimulation, muscle fatigue, muscle fiber conduction velocity, periodic-acid Schiff staining