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 (n=12) and females (n=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% (τ₄₀) and 100% (τ₁₀₀) 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). τ₁₀₀ (34.4±7.0 s) was greater than τ₄₀ (24.2±6.9 s, p<0.001), but the values were correlated (r=0.688; p<0.001). τ₄₀ (r=-0.692, p<0.001) and τ₁₀₀ (r=-0.488, p=0.016) correlated with myosin heavy chain I percentage as well as several markers of mitochondrial content, including COX II protein content in whole muscle (τ₄₀: r=-0.547, p=0.006; τ₁₀₀: r=-0.466, p=0.022), type I pooled fibres (τ₄₀: r=-0.547, p=0.006; τ₁₀₀: r=-0.547, p=0.006), and type II pooled fibres (τ₄₀: r=-0.516, p=0.009; τ₁₀₀: r=-0.635, p=0.001). τ₄₀ (r=-0.702, p<0.001), but not τ₁₀₀ (r=-0.378, p=0.083) correlated with CT; however, neither value correlated with W' (τ₄₀: r=0.071, p=0.753; τ₁₀₀: r=0.054, p=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.