The Effects of Concurrent Aerobic and Strength Training on Muscle Fiber Hypertrophy: A Systematic Review and Meta-Analysis.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of controlled intervention trials
PubMed 35476184 · doi:10.1007/s40279-022-01688-x
What was done
A systematic review and random-effects meta-analysis was conducted across six databases up to March 2021 following PRISMA guidelines. The review included studies of healthy adults of any sex and age undergoing at least 4 weeks of supervised concurrent aerobic and strength training compared to an identical strength training protocol alone. The evaluated outcome was muscle fiber hypertrophy (overall, type I, and type II). Subgroup analyses examined aerobic modality (running vs. cycling), exercise order, training frequency, age, and training status.
What was found
A total of 15 studies were analyzed. Concurrent training showed a small negative effect on overall muscle fiber hypertrophy compared to strength training alone (standardized mean difference [SMD] -0.23, 95% CI -0.46 to -0.00, p = 0.050). For type I fibers, SMD was -0.34 (95% CI -0.72 to 0.04, p = 0.078), and for type II fibers, SMD was -0.13 (95% CI -0.39 to 0.12, p = 0.315). A negative effect was observed for type I fibers when aerobic exercise was running (SMD -0.81, 95% CI -1.26 to -0.36) but not cycling. Other subgroup comparisons (frequency, status, modality, order) showed no significant differences.
Why it matters
While concurrent training generally preserves whole-muscle hypertrophy, this review shows small interference effects at the individual muscle fiber level, specifically highlighting running-induced impairment in type I fiber growth.
Limits
The total number of individual human participants is not reported in the abstract. The main overall effect was borderline significant (p = 0.050), and fiber-specific main effects did not reach statistical significance. The PROSPERO protocol was modified after registration to restrict the analysis solely to fiber-level hypertrophy.
Cited by
- supports Concurrent aerobic and resistance training does not cause an inherent molecular interference effect that blunts muscle adaptations when session quality is maintained.