Citius, Maius, Melius: Mitochondria and Endurance Performance.

Endurance performance relies on a range of physiological adaptations classified as central or peripheral depending on whether the remodeling occurs in the cardiovascular system or in the skeletal muscle, respectively. One of the goals of these adaptations is to enhance oxygen transport and its utilization by mitochondria to sustain ATP resynthesis. While a link between skeletal muscle mitochondrial characteristics and endurance performance may seem obvious, there is no consensus on whether mitochondrial characteristics are key determinants of endurance performance. In this narrative review, we examine cross-sectional, correlational, and intervention studies conducted in humans that support or challenge the role of mitochondria in endurance performance. Cross-section studies suggest that individuals with superior endurance performance exhibit greater mitochondrial content and respiratory function than those with lower fitness levels. Correlation studies have shown positive associations between multiple mitochondrial characteristics and markers of endurance performance. However, a lack of correlation between training-induced changes in mitochondrial characteristics and endurance performance has also been reported. Intervention studies indicate that changes in mitochondrial characteristics following training, phlebotomy, or detraining are often associated with changes in markers of endurance performance. Conversely, increasing oxygen delivery to the working muscle (i.e., via increasing oxygen concentration) has been shown to improve performance markers, suggesting these improvements are not limited by mitochondrial characteristics. In conclusion, while substantial evidence associates mitochondrial characteristics with endurance performance, this relationship is not universal; central factors display an equally strong influence independently of mitochondrial characteristics. We propose that enhanced mitochondrial characteristics represent an important and often necessary, but not sufficient, adaptation that is required to support endurance performance.