Moderate-Intensity Interval Training Is Sufficient to Increase Muscle Mitochondrial Respiratory Capacity and Exercise Tolerance in Young Untrained Men

Introduction: There is considerable evidence that physical activity can stimulate mitochondrial biogenesis in muscles, enhance maximal mitochondrial oxygen consumption rate, improve physical fitness, and help maintain overall health in humans. On the other hand, there is growing evidence that excessive training load may have a detrimental effect on mitochondria and glucose homeostasis. Hypothesis: We hypothesized that moderate-intensity interval training (MIIT) would be sufficient to increase muscle mitochondrial oxidative capacity and exercise tolerance in untrained healthy people. Methods: Eleven young, healthy, untrained males took part in this study. They performed a 7-week endurance training program (MIIT) on a cycle ergometer, composed of 4 sessions per week, each lasting 40 min. Before and after the MIIT training program, a pulmonary maximal oxygen consumption (V̇O 2max ) as well as lactate threshold (LT) were determined and a muscle biopsy from the m. vastus lateralis was taken to evaluate the level of muscle proteins related to energy metabolism and mitochondrial biogenesis, and to measure the muscle's maximal mitochondrial respiration rate. Results: The training resulted in an increase of skeletal muscle homogenate V̇O 2max (by ~50%, P =0.001), due to increased mitochondrial mass and the enrichment of mitochondria with the individual respiratory chain complexes. The MIIT increased the power output at the lactate threshold (PO at LT) by ~43 W ( P =0.001) and the pulmonary V̇O 2max by 7% ( P =0.01) and had no adverse effect on blood glucose homeostasis. Conclusions: MIIT, which is a less intense form of training than the commonly used types of interval training, effectively induces a range of beneficial muscular and systemic adaptations that contribute to improvement of physical fitness and maintenances good health in humans.