Effects of the order of endurance and high-intensity interval exercise in combined training on mouse skeletal muscle metabolism

Endurance exercise (EE) mainly improves oxidative capacity, whereas high-intensity interval exercise (HIIE) also improves glycolytic capacity. There is growing evidence that suggests that combining EE with HIIE can lead to improved athletic performance and fitness outcomes compared with either form of exercise alone. This study aimed to elucidate whether the order in which EE and HIIE are performed in combined training affects oxidative metabolism and glycolysis in mouse skeletal muscle. Male ICR mice at 7 wk of age were divided into three groups: control (CON), EE-HIIE, and HIIE-EE. The total training period was 3 wk (3 times/week). Mice performed running on a treadmill as endurance exercise and swimming with a weight load of 10% of body weight as high-intensity interval exercise. EE before HIIE (EE-HIIE) improved running performance in the maximal EE capacity test (all-out test) and partly enhanced the expression levels of molecular signals involved in glycolysis compared with HIIE before EE (HIIE-EE). The order of exercise did not, however, impact the expression of proteins related to mitochondrial dynamics, including those involved in the morphological changes of mitochondria through repeated fusion and fission, as well as oxidative energy metabolism. The findings suggest that the order of exercise has no significant impact on the expression of proteins associated with glycolytic and oxidative energy metabolism. Nevertheless, our results indicate that the order of EE-HIIE may enhance running performance.