Muscle mitochondrial capacity in high- and low-fitness females using near-infrared spectroscopy

Abstract

The recovery of muscle oxygen consumption (m $\dot{V}$ O₂ ) after exercise measured using near-infrared spectroscopy (NIRS) provides a measure of skeletal muscle mitochondrial capacity. Nevertheless, due to sex differences in factors that can influence scattering and thus penetration depth of the NIRS signal in the tissue, e.g., subcutaneous adipose tissue thickness and intramuscular myoglobin and hemoglobin, it is unknown whether results in males can be extrapolated to a female population. Therefore, the aim of this study was to measure skeletal muscle mitochondrial capacity in females at different levels of aerobic fitness to test whether NIRS can measure relevant differences in mitochondrial capacity. Mitochondrial capacity was analyzed in the gastrocnemius muscle and the wrist flexors of 32 young female adults, equally divided in relatively high ( $\dot{V}$ O₂ peak ≥ 47 ml/kg/min) and relatively low aerobic fitness group ( $\dot{V}$ O₂ peak ≤ 37 ml/kg/min). m $\dot{V}$ O₂ recovery was significantly faster in the high- compared to the low-fitness group in the gastrocnemius, but not in the wrist flexors (p = 0.009 and p = 0.0528, respectively). Furthermore, $\dot{V}$ O₂ peak was significantly correlated to m $\dot{V}$ O₂ recovery in both gastrocnemius (R² = 0.27, p = 0.0051) and wrist flexors (R² = 0.13, p = 0.0393). In conclusion, NIRS measurements can be used to assess differences in mitochondrial capacity within a female population and is correlated to $\dot{V}$ O₂ peak. This further supports NIRS assessment of muscle mitochondrial capacity providing additional evidence for NIRS as a promising approach to monitor mitochondrial capacity, also in an exclusively female population.

Keywords: $\dot{\mathrm{V}}$ O2peak; NIRS; fitness; mitochondria; oxidative metabolism.