Concurrent training with long-interval HIIT does not impair skeletal muscle protein synthesis or hypertrophy: little...

Concurrent training is commonly associated with blunted muscle hypertrophy compared with resistance training alone, but the underlying physiological mechanisms remain unclear. This study aimed to investigate the acute and chronic effects of concurrent versus resistance training on muscle protein synthesis, satellite cell dynamics, myonuclear content, myogenic regulatory factor expression, muscle fiber hypertrophy, strength, and aerobic capacity. Nineteen previously untrained young men were randomly assigned to either concurrent or resistance training for 16 wk. Muscle biopsies were collected before and 48 h after a standardized exercise session at weeks 4 and 16. Samples were analyzed for myofibrillar protein synthesis via deuterium oxide incorporation, satellite cell content, myonuclear number, and gene expression. Strength, aerobic capacity, and muscle fiber cross-sectional area were measured at baseline and postintervention. Muscle protein synthesis increased 48 h postexercise at both weeks 4 and 16 (P = 0.0105), with no group differences. Satellite cell content increased over time in type II fibers only (P = 0.0021). Myonuclear number increased in both fiber types (type I: P = 0.0301 and type II: P = 0.0009), with higher values in type I fibers in the concurrent training group (P = 0.0027). MYF5 and MYF6 expression increased over time (P = 0.0141 and P = 0.034, respectively), and MYOD1 was elevated postexercise only in concurrent training (P = 0.0009). Type II fiber size increased (P = 0.016). Strength gains were greater in resistance training (P = 0.016), whereas aerobic capacity improved only in concurrent training (P < 0.001). Sixteen weeks of concurrent training did not inhibit molecular mechanisms associated with muscle hypertrophy in previously untrained individuals.NEW & NOTEWORTHY Sixteen weeks of concurrent training with long-interval HIIT preserved key molecular adaptations related to muscle hypertrophy, including protein synthesis, satellite cell activity, and gene expression. Both concurrent and resistance training increased type II fiber cross-sectional area, but only concurrent training improved V̇o_2peak. Although strength gains were lower with concurrent training, molecular and cellular remodeling remained intact, supporting it as an effective strategy to enhance both muscle growth and aerobic fitness simultaneously.