MUSCLE METABOLISM DURING EXERCISE IN YOUNG AND OLDER UNTRAINED AND ENDURANCE-TRAINED MEN

To examine effects of aging and endurance training on human muscle met abolism during exercise, P-31 magnetic resonance spectroscopy was used to study the metabolic response to exercise in young (21-33 yr) and o lder (58-68 yr) untrained and endurance-trained men (n = 6/group). Sub jects performed graded plantar flexion exercise with the right leg, wi th metabolic responses measured using a P-31 surface coil placed over The lateral head of the gastrocnemius muscle. Muscle biopsy samples we re also obtained for determination of citrate synthase activity. Rate of increase in P(i)-to-phosphocreatine ratio with increasing power out put was greater (P < 0.01) in older untrained [0.058 +/- 0.022 (SD) W- 1] and trained men (0.042 +/-0.010 W-1) than in young untrained (0.038 +/- 0.017 W-1) and trained men (0.024 +/- 0.010 W-1). Plantar flexor muscle cross-sectional area and volume (determined using H-1 magnetic resonance imaging) were 11-12% (P < 0.05) and 16-18% (P < 0.01) smalle r, respectively, in older men. When corrected for this difference in m uscle mass, age-related differences in metabolic response to exercise were reduced by approximately 50% but remained significant (P < 0.05). Citrate synthase activity was approximately 20% lower (P < 0.001) in older untrained and trained men than in corresponding young groups and was inversely related to P(i)-phosphocreatine slope (r = -0.63, P < 0 .001). Age-related reductions in exercise capacity were associated wit h an altered muscle metabolic response to exercise, which appeared to be due to smaller muscle mass and lower muscle respiratory capacity of older subjects. Endurance training was associated with 60-100% higher muscle citrate synthase activity and improved metabolic responses in young and older men but apparently could not prevent an age-related de crement in these variables or an age-related decrease in muscle mass.