Contraction intensity affects NIRS-derived skeletal muscle oxidative capacity but not its relationships to mitochondrial protein content or aerobic fitness.

To further refine the near-infrared spectroscopy (NIRS)-derived measure of skeletal muscle oxidative capacity in humans, we sought to determine whether the exercise stimulus intensity affected the τ value and/or influenced the magnitude of correlations with in vitro measures of mitochondrial content and in vivo indices of exercise performance. Males ( = 12) and females ( = 12), matched for maximal aerobic fitness per fat-free mass, completed NIRS-derived skeletal muscle oxidative capacity tests for the vastus lateralis following repeated contractions at 40% (τ) and 100% (τ) of maximum voluntary contraction, underwent a skeletal muscle biopsy of the same muscle, and performed multiple intermittent isometric knee extension tests to task failure to establish critical torque (CT). The value of τ (34.4 ± 7.0 s) was greater than τ (24.2 ± 6.9 s, < 0.001), but the values were correlated ( = 0.688; < 0.001). The values of τ ( = -0.692, < 0.001) and τ ( = -0.488, = 0.016) correlated with myosin heavy chain I percentage and several markers of mitochondrial content, including COX II protein content in whole muscle (τ: = -0.547, = 0.006; τ: = -0.466, = 0.022), type I pooled fibers (τ: = -0.547, = 0.006; τ: = -0.547, = 0.006), and type II pooled fibers (τ: = -0.516, = 0.009; τ: = -0.635, = 0.001). The value of τ ( = -0.702, < 0.001), but not τ ( = -0.378, = 0.083) correlated with critical torque (CT); however, neither value correlated with W' (τ: = 0.071, = 0.753; τ: = 0.054, = 0.812). Overall, the NIRS method of assessing skeletal muscle oxidative capacity is sensitive to the intensity of skeletal muscle contraction but maintains relationships to whole body fitness, isolated limb critical intensity, and mitochondrial content regardless of intensity. Skeletal muscle oxidative capacity measured using near-infrared spectroscopy (NIRS) was lower following high-intensity compared with low-intensity isometric knee extension contractions. At both intensities, skeletal muscle oxidative capacity was correlated with protein markers of mitochondrial content (in whole muscle and pooled type I and type II muscle fibers) and critical torque. These findings highlight the importance of standardizing contraction intensity while using the NIRS method with isometric contractions and further demonstrate its validity.