Simulation of exercise-dependent difference in metabolism with a mathematical model for analyses of measurements using near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a useful technique for noninvasive measurement of muscle oxygenation. However, analyses of the dynamic changes in muscle metabolism based only on experimental observations of NIRS are difficult. Therefore, we constructed a mathematical model of muscle metabolism, comprising of the ATP synthesizing systems and O₂ diffusion system, to identify the mechanisms responsible for those observations. A customized NIRS instrument was used to measure the changes in muscle oxygenation of the forearm flexor muscles during intermittent and continuous isometric flexion exercises when healthy male subjects participated in exercises tests. The exercise-dependent difference in changes could be distinguished and the simulated results agreed well with that measured experimentally. Although the contraction intensity for both exercises was identical, the magnitude of energy needed to perform the respective exercises was different. This difference was reflected by the changes in the ATP synthesizing systems, in which the energy needed during the latter-half of continuous exercise was mostly supplied by anaerobic system, whereas that during intermittent exercise was supplied by the aerobic and anaerobic systems that operated synergistically. From the results, we conclude that the model could be a useful tool for the elucidation of the relationship between experimental observations of NIRS and muscle metabolism