Effect of temperature on muscle energy metabolism and endurance during successive isometric contractions, sustained to fatigue, of the quadriceps muscle in man.

1. The effects of altered tissue temperature on muscle metabolism during successive isometric contractions, sustained to fatigue, have been studied in the quadriceps muscle of man by combining biochemical analyses of metabolites in needle biopsy samples with measurements of endurance time with a force of 2/3 maximum voluntary contraction. Fatigue and recovery were observed repeatedly in a series of seven contractions at intervals of 20 sec, following immersion of the test leg in water at 12, 26 or 44 degrees C for 45 min. Muscle temperatures corresponding to these water temperatures were 22.5, 32.6 and 38.6 degrees C respectively.2. Increased levels of several glycolytic intermediates at rest in the heated muscle suggested an increased rate of glycolysis. ATP and phosphoryl creatine were lower at the end of the first contraction and the calculated rate of ATP utilization (including the contribution from anaerobic glycolysis) was highest in the heated nuscle.3. Significantly shorter endurance times were found for the heated muscle. These could not be attributed to depletion of local energy resources in muscle. Fatigue may be due to a reduction in the rate of regeneration of ATP from anaerobic glycolysis below that needed to maintain the contraction force. Lower values for the ratio of fructose 1,6-diphosphate: fructose 6-phosphate at the end of contractions, particularly at the highest temperature, are compatible with the hypothesis that there is partial inhibition of the rate controlling enzyme phosphofructokinase, possibly due to the accumulation of hydrogen ions in muscle.