PURPOSE: Previous studies have used near-infrared spectroscopy (NIRS) to measure skeletal muscle mitochondrial capacity. This study tested the hypothesis that NIRS-measured mitochondrial capacity would improve with endurance exercise training and decline with detraining. METHODS: Nine young participants performed 4 wk of progressively increasing endurance exercise training of the wrist flexor muscles followed by approximately 5 wk of inactivity. The rate of recovery of muscle oxygen consumption (mV̇O2) was measured with NIRS every 3-7 d, indicating mitochondrial oxidative capacity. RESULTS: A linear increase in mitochondrial capacity (NIRS rate constant) was found with a group average of 64% ± 37% improvement after 4 wk of exercise training (P < 0.05). Mitochondrial capacity declined exponentially upon cessation of exercise training, with a mean half-time of approximately 7.7 d. CONCLUSIONS: Both the magnitude and the time course of mitochondrial adaptations to exercise training and detraining measured with NIRS was consistent with previous studies using both in vitro and in vivo techniques. These findings show that NIRS-based measurements can detect meaningful changes in mitochondrial capacity. Copyright © 2013 by the American College of Sports Medicine.