Validity of NIR spectroscopy for quantitatively measuring muscle oxidative metabolic rate in exercise

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Abstract
The purpose of this study was to examine the validity of the quantitative measurement of muscle oxidative metabolism in exercise by near-infrared continuous-wave spectroscopy (NIRcws). Twelve male subjects performed two bouts of dynamic handgrip exercise, once for the NIRcws measurement and once for the 31P-magnetic resonance spectroscopy (MRS) measurement as a standard measure. The resting muscle metabolic rate (RMRmus) was independently measured by 31P-MRS during 15 min of arterial occlusion at rest. During the first exercise bout, the quantitative value of muscle oxidative metabolic rate at 30 s postexercise was evaluated from the ratio of the rate of oxyhemoglobin/myoglobin decline measured by NIRcws during arterial occlusion 30 s after exercise and the rate at rest. Therefore, the absolute values of muscle oxidative metabolic rate at 30 s after exercise [V˙o2NIR(30)] was calculated from this ratio multiplied by RMRmus. During the second exercise bout, creatine phosphate (PCr) resynthesis rate was measured by 31P-MRS at 30 s postexercise [Q(30)] under the same conditions but without arterial occlusion postexercise. To determine the validity of NIRcws, V˙o2NIR(30) was compared with Q(30). There was a significant correlation betweenV˙o2NIR(30), which ranged between 0.018 and 0.187 mM ATP/s, and Q(30), which ranged between 0.041 and 0.209 mM ATP/s (r = 0.965, P < 0.001). This result supports the application of NIRcws to quantitatively evaluate muscle oxidative metabolic rate in exercise.

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