Concurrent Training with Long-Interval HIIT Preserves Muscle Growth Mechanisms
The "interference effect" only affects strength.
A new 16-week study from researchers in Brazil and Australia shows that combining resistance training with high-intensity interval training (HIIT) does not suppress the molecular machinery driving muscle growth, challenging the enduring rumour that doing both types of training simultaneously hurts hypertrophy. Never mind the fact that Crossfit athletes exist. The research, published in the Journal of Applied Physiology, reveals that muscle protein synthesis, satellite cell activity, and myonuclear accumulation all proceeded normally in lifters doing concurrent training, but strength gains took a hit compared to those lifting alone.
The “interference effect,” a phenomenon first described in the early 1980s, where cardio seems to blunt strength and muscle gains from resistance training, has been a contentious topic in sports science for decades. This study provides the clearest molecular explanation yet: the interference doesn’t operate through the biological pathways that build muscle. Instead, it appears to work through neural mechanisms that suppress the development of maximal strength.
Aim
The researchers set out to determine whether 16 weeks of concurrent training (resistance training combined with long-interval HIIT) would impair the fundamental cellular and molecular processes that underpin muscle growth, or whether the interference effect (if it occurred) operated through a different mechanism entirely. Specifically, they wanted to track rates of muscle protein synthesis, satellite cell dynamics, myonuclear content, and gene expression patterns across both acute exercise sessions and the full training span.
Methods
Nineteen previously untrained young men were split into two groups: one performing resistance training alone (10 participants, average age 25 years) and one doing concurrent training (9 participants, average age 23 years). The resistance training group completed two sessions per week on the 45-degree leg press and leg extension, starting with 2 sets of 9-12 repetitions and progressing to 3 sets after week 4, with 60 seconds rest between sets. The concurrent training group did identical resistance sessions plus aerobic HIIT: two additional HIIT sessions weekly for a total of four weekly sessions. The HIIT protocol consisted of 1-minute sprints at peak oxygen consumption velocity with 1-minute passive recovery between bouts, totalling 10 sprints initially (weeks 1-4), progressing to 15 sprints (weeks 5-12), and ending with 10 sprints of 2-minute duration at the new adjusted velocity (weeks 13-16).
Muscle biopsies were collected from the vastus lateralis at baseline, then at weeks 4 and 16 of training, with each testing window including samples taken 24 hours before exercise, immediately before exercise, and 48 hours after exercise. Researchers used deuterium oxide incorporation to directly measure rates of myofibrillar protein synthesis. They analysed satellite cell content and myonuclei number via immunofluorescence microscopy. Gene expression of myogenic regulatory factors (MYOD1, MYOG, MYF5, and MYF6) was measured by quantitative PCR. Type I and Type II muscle fiber cross-sectional area was quantified from immunostained muscle sections. One-repetition maximum strength on the leg press was tested at baseline and post-training. Peak oxygen uptake (V02peak) was measured via a graded treadmill test at baseline, week 8 (for training adjustment), and post-training. All participants consumed 30 grams of whey protein immediately after each training session, maintained their habitual diet, and were instructed to avoid other strenuous activity during the study.
Results
Both groups showed substantial increases in muscle protein synthesis 48 hours after exercise at both week 4 and week 16, with no meaningful difference between concurrent and resistance training. At week 4, concurrent training showed a 0.425 increase in protein synthesis above pre-exercise levels, while resistance training showed 0.379. At week 16, concurrent training maintained a robust 0.276 increase, whereas resistance training declined to a non-significant 0.124 increase.
Satellite cell content increased significantly in Type II fibers only from week 4 to week 16, independent of which training group. Type I fiber satellite cells showed no time or group effects. Myonuclear number increased in both fiber types over the 16 weeks. Notably, Type I fibers in the concurrent training group showed higher myonuclear content compared to resistance training alone.
For gene expression, MYF5 and MYF6 mRNA increased significantly over time. MYOD1 was acutely elevated 48 hours after exercise, but only in the concurrent training group. MYOG showed no significant changes.
Type II muscle fiber cross-sectional area increased by approximately 674 square micrometers from baseline to week 16 across both groups. Type I fibers showed no significant growth in either group.
Leg press strength increased significantly in both groups over 16 weeks, but the resistance training group gained substantially more than concurrent training. Resistance training showed an 88 kg increase in 1-RM, while concurrent training showed only a 55 kg increase. Post-training, resistance training had significantly higher absolute strength values compared to concurrent training.
Peak oxygen uptake increased only in the concurrent training group by 24.9 mL/kg/min, while resistance training showed essentially no change. Post-training, concurrent training tended toward higher V02peak, though this difference was not statistically significant.
