Skeletal muscle metabolism during exercise : an in vivo ³¹P nuclear magnetic resonance study

The metabolic and biochemical adaptations which set the endurance limit in skeletal muscle and are modified by physical training, and those which set the fatigue limits in conditions of chronic hypoxia, are not completely understood. Therefore, the purpose of this study was to measure the key metabolites involved in the control of oxidative and glycolytic metabolism, during the elevated metabolic demands of exercise, in subject groups which were separated by distinct differences in their training status or by their exposure to chronic hypobaric hypoxia. Since repeated measures of the key metabolites involved in energy metabolism (PCr, Pi, ATP) and intracellular pH (pHm) would be exceedingly difficult using the conventional muscle needle biopsy technique, ³¹P NMR was selected as an appropriate, noninvasive method for measuring these metabolites. Two separate exercise models were developed for use within a 1.0 m bore NMR machine. An electrical stimulation model using the rectus femoris muscle was developed and the factors which influenced reliability and reproducibility of the data were determined. In addition, a dynamic exercise model was developed in which the gastrocnemius muscle was exercised in a mechanical calf ergometer. The results of the experiments using the electrical stimulation model indicate that RF coil geometry, stimulation intensity and duty cycle, electrode placement, and subject tolerance require very close control for the model to be reliable. It is felt that this model is best suited for experiments which require a within-subject design and is ideally suited for experimental or therapeutic intervention studies. The calf ergometer was used to compare sedentary lowlanders, marathon and ultramarathon runners, power trained athletes, and Quechua Indians, native to altitudes of 4,200 m in the Andes, before and after deacclimation to sea level. It was found that the Andean natives did not possess a standard physiological phenotype with respect to aerobic and anaerobic capacities. In addition, given the Andean's very low anaerobic capacity and intermediate aerobic capacity, this group performed calf work equivalent to that of highly trained endurance and power athletes. Moreover, pHm, PCr, Pi, and ATP showed equivalent perturbation at fatigue and in recovery compared with the marathon runners but considerably less perturbation than was found in the power trained athletes who possess equivalent aerobic capacities but far greater anaerobic capacities. NMR derivable estimates of the phosphorylation potential in this study support the theory that closer coupling between ATP supply-ATP demand may be responsible for reduced kinetic and thermodynamic activation of mitochondrial metabolism seen in the Andean natives.Science, Faculty ofZoology, Department ofGraduat