Skeletal muscle metabolism, contraction force and glycogen utilization during prolonged electrical stimulation in humans.

Muscle metabolism and contraction force were examined in the quadriceps femoris muscles of seven volunteers during 45 min of electrical stimulation. Intermittent stimulation was used, with tetanic trains at 20 Hz lasting 1.6 s, separated by pauses of 1.6 s. Muscle biopsies were taken at rest and during stimulation (80 s, 15, 30 and 45 min). During the initial 80 s of stimulation contraction force decreased to 72% of initial force. The glycogenolytic rate was 40.9 mmol glucosyl units kg-1 dry muscle min-1 and glycolytic intermediate levels increased several fold. Muscle phosphocreatine decreased to 26% of resting concentration and the ATP turnover rate from anaerobic sources was 4.99 mmol kg-1 dry muscle s-1. With continued stimulation from 80 s to 15 min, force decreased to 43% of the initial value at 5 min and 31% at 15 min. Glycogenolysis fell to 5.4 mmol kg-1 dry muscle min-1 and glycolytic intermediate levels decreased suggesting that anaerobic glycolysis contributed progressively less ATP for force production. The final 30 min of stimulation was characterized by a low rate of glycogenolysis (1.35-1.67 mmol kg-1 dry muscle min-1) and a constant force production (25.5% of initial). The ATP turnover rate, assuming glycogen was metabolized aerobically, was 1.86 mmol kg-1 dry muscle s-1. Phosphocreatine, ATP and glycolytic intermediates returned to near resting levels, indicating that anaerobic energy pathways were not reactivated.(ABSTRACT TRUNCATED AT 250 WORDS)