Methods are described for the determination of ATP, CP, Pi, glycogen, G-6-P, and lactate concentrations in muscle specimens (15 mg) obtained with a needle biopsy technique. Muscle biopsy specimens were obtained in two materials (n = 13; maximal oxygen uptake (max Vo2) 61 ml x min-l x kg-I and n = 15; average max VO, 48 ml x min-l x kg-I), at submaxi-ma1 (55-95 per cent of max 3 0~) . and at short time exhaustive work levels. A depletion of the ATP and CP stores in relation to the relative work load was observed while the muscle lactate concentration increased faster at higher work loads concomitant with a tendency to an increased glycolysis. The ATP and CP stores (the phosphagens) could maximally account for an alactacid anaerobic energy output equivalent to 1.2-1.3 liters of oxygen. The lactacid anaerobic energy output, however, seemed to start when the phosphagens were lowered to approximately 70 % of the basic values which occurred during steady-state work at work loads in excess of 50 yo of maximal oxygen uptake.
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