Concurrent training with long-interval HIIT does not impair skeletal muscle protein synthesis or hypertrophy: Little evidence of an 'interference effect'

Concurrent training is commonly associated with blunted muscle hypertrophy compared to 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 fibre hypertrophy, strength, and aerobic capacity. Methods: Nineteen previously untrained young men were randomly assigned to either concurrent or resistance training for 16 weeks. Muscle biopsies were collected before and 48 hours 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 fibre cross-sectional area were measured at baseline and post-intervention. Results: Muscle protein synthesis increased 48 hours post-exercise at both weeks 4 and 16 (P=0.0105), with no group differences. Satellite cell content increased over time in type II fibres only (P=0.0021). Myonuclear number increased in both fibre types (type I: P=0.0301; type II: P=0.0009), with higher values in type I fibres in the concurrent training group (P=0.0027). MYF5 and MYF6 expression increased over time (P=0.0141; P=0.034), and MYOD1 was elevated post-exercise only in concurrent training (P=0.0009). Type II fibre size increased (P = 0.016). Strength gains were greater in resistance training (P = 0.016), while aerobic capacity improved only in concurrent training (P<0.001). Conclusion: Sixteen weeks of concurrent training did not inhibit molecular mechanisms associated with muscle hypertrophy in previously untrained individuals.