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Abstract
Skeletal muscle is one of the largest organs in the human body and its metabolic function is associated with both physical functioning and many pathologies. The mitochondrion is the main energy producing organelle in skeletal muscle cells, thus making its function vital for both health and disease. Purpose: The purpose of these studies was to validate a novel, non-invasive optical approach for assessing skeletal muscle (mitochondrial) oxidative capacity. Methods: Two experiments were performed. The first experiment utilized the well-known effects of endurance exercise, to induce a change in skeletal muscle oxidative capacity. Nine participants performed four weeks of forearm endurance exercise training, followed by five weeks of detraining. Skeletal muscle oxidative capacity was measured with near infrared spectroscopy (NIRS) every 5-7 days during both training and detraining. The seconds experiment was a cross validation between NIRS and the current gold standard in vivo approach, phosphorus magnetic resonance spectroscopy (31P-MRS). Sixteen participants were tested with both NIRS and 31P-MRS. Results: In the first study, the endurance exercise training resulted in a linear increase in oxidative capacity (NIRS rate constant) with a group average of 64 ± 37% improvement after four weeks of exercise training (p < 0.05). Oxidative capacity declined exponentially upon cessation of exercise training, with a mean half-time of ~7.7 days. In the second study, the average recovery time constant was 31.5 ± 8.5 s for PCr and 31.5 ± 8.9 s for mVO2 for all participants (p = 0.709). 31P-MRS correlated well with NIRS for both Channel 1 (Pearson's r = 0.88, p < 0.0001) and Channel 2 (Pearson's r = 0.95, p < 0.0001). Furthermore, both 31P-MRS and NIRS exhibit good repeatability between trials (CV = 8.1%, 6.9%, and 7.9% for NIRS Channel 1, NIRS Channel 2, and 31P-MRS respectively). Conclusion: NIRS measurements of skeletal muscle oxidative capacity increased with endurance exercise training, decreased with detraining, and showed good agreement with 31P-MRS. These findings support the validity of NIRS measurements of skeletal muscle oxidative capacity, and suggest that direct comparisons of NIRS and 31P-MRS recovery rates can be made.