Myostatin Reduction Within the Myokine–Adipokine Network Predicts Aerobic Adaptation After High-Intensity Interval Training in Combat Athletes

High-intensity interval training (HIIT) is widely used to enhance aerobic performance in combat sports, yet the molecular mechanisms underlying training adaptation remain unclear. This study investigated whether changes in circulating myokine–adipokine profiles are associated with aerobic performance adaptation following sport-specific HIIT in trained combat athletes. Forty elite male kickboxers were randomly assigned to a HIIT group (n = 20) or a control group (n = 20). The HIIT group performed an eight-week sport-specific HIIT program in addition to regular training, whereas the control group maintained their usual training routines. Aerobic capacity was assessed using maximal oxygen uptake (VO2max). Fasting blood samples were collected before and after the intervention to determine circulating apelin, irisin, brain-derived neurotrophic factor (BDNF), myostatin, fibroblast growth factor-21 (FGF21), and adiponectin concentrations. VO2max increased significantly in the HIIT group compared with the control group (+2.10 ± 1.10 vs. +0.35 ± 0.80 mL·kg−1·min−1, p = 0.001). In addition, apelin, irisin, BDNF, FGF21, and adiponectin increased, whereas myostatin decreased following the intervention. Changes in myostatin were negatively correlated with improvements in VO2max (r = −0.55, p = 0.007), suggesting that reductions in myostatin may serve as a molecular indicator of aerobic adaptation in combat athletes.