The meta-analysis by Wilson and colleagues (2012) remains a landmark in concurrent training research, pooling 21 studies to estimate the interference effect of adding aerobic exercise to resistance training. Their primary finding was a small-to-moderate impairment in lower-body strength (standardized mean difference ≈ −0.3) and power (−0.3 to −0.4), while hypertrophy was not significantly affected. The effect was most pronounced when running served as the endurance mode, compared with cycling, and when sessions were performed on the same day without adequate recovery. These data established a dose-response pattern: interference scales with weekly aerobic volume and proximity to resistance exercise.
Mechanisms Underlying the Interference Effect
The molecular basis of interference centers on conflicting signaling pathways. Resistance training primarily activates the mTOR pathway to drive protein synthesis and hypertrophy, while endurance exercise upregulates AMPK, which can inhibit mTOR via TSC2 phosphorylation. Nader (2006) articulated this paradigm, though subsequent work has shown it is oversimplified. Chronic adaptations may also involve fiber-type shifts: concurrent training can promote a transition toward a more oxidative phenotype, potentially compromising maximal force production. However, these mechanisms are not deterministic; the magnitude of interference varies with nutrition, training history, and the specific programming of each modality.
Evidence Summary: Effect Sizes and Moderators
A recent systematic review and meta-analysis by Huiberts and colleagues (2024) examined sex and training status as moderators of concurrent training adaptations. In trained individuals, the addition of endurance training attenuated strength gains compared with resistance-only training, with a small effect size (g ≈ −0.2 to −0.3). Untrained participants showed negligible interference, suggesting that the window for adaptation is broad enough to accommodate both stimuli simultaneously. Schumann and colleagues (2022) updated the evidence base with a focus on skeletal muscle size and function, finding no significant differences in hypertrophy or explosive strength between concurrent and strength-only groups when training frequency and volume were matched. Their analysis of log variability indicated no increase in individual response heterogeneity, challenging the notion that some people are uniquely susceptible to interference.
A network meta-analysis by Murlasits and colleagues (2024) compared different concurrent training types on lower-limb strength and hypertrophy. Intrasession sequencing—performing resistance before endurance or vice versa—did not produce consistent differences in outcomes. The key moderator was the mode of endurance exercise: running elicited greater interference than cycling, likely due to eccentric loading and muscle damage. High-intensity interval training (HIIT) showed less interference than continuous moderate-intensity exercise, possibly because HIIT’s metabolic demands are more compatible with strength adaptations.
Practical Application
For practitioners, the evidence supports several programming strategies to minimize interference. First, separate endurance and resistance sessions by at least 6 hours to allow molecular signaling to resolve. Second, prioritize the modality most aligned with the primary training goal early in the session or on dedicated days. Third, limit weekly running volume—especially if it exceeds 3 sessions or 90 minutes per session—when maximal strength is the priority. Cycling or rowing may be preferable to running for those concerned about lower-body strength loss. Finally, ensure adequate protein intake and caloric surplus to support simultaneous adaptations. For youth athletes, a meta-analysis by Gäbler and colleagues (2018) found that concurrent training improved both strength and endurance without significant interference, making it a safe and effective approach for developing athleticism.
Caveats and Limitations
The meta-analytic evidence is constrained by heterogeneity in study designs, populations, and outcome measures. Most studies are short-term (8–12 weeks) and may not capture long-term adaptations. The interference effect appears to be specific to lower-body strength and power; upper-body outcomes are rarely affected. Individual differences in recovery capacity, nutrition, and stress likely modulate the response, but these factors are poorly controlled in the literature. Additionally, the practical significance of a small effect size (e.g., a 5% difference in strength gain) must be weighed against the health and performance benefits of concurrent training. For athletes in sports requiring both strength and endurance, the net benefit of concurrent training typically outweighs the modest interference.
Readers should consult a physician or qualified healthcare professional before beginning any new exercise program, particularly if they have pre-existing health conditions or are returning from injury.
References
- Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises — PubMed
- Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status — PMC
- Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis — PMC
- Comparative efficacy of concurrent training types on lower limb strength and muscular hypertrophy: A systematic review and network meta-analysis — PMC
- The Effects of Concurrent Strength and Endurance Training on Physical Fitness and Athletic Performance in Youth: A Systematic Review and Meta-Analysis — PMC
