Pooled estimates from recent meta-analyses complicate the long-held view that concurrent endurance and strength training inevitably compromises strength development. The interference effect, first described in the 1980s, has been revisited through systematic reviews that incorporate larger sample sizes and more nuanced moderator analyses. A 2022 meta-analysis of 43 studies reported a small but statistically significant impairment in lower-body maximal strength (standardized mean difference of −0.31, 95% CI −0.48 to −0.13) when comparing concurrent to strength-only training, while upper-body strength showed a negligible effect (SMD −0.04, 95% CI −0.21 to 0.13). For hypertrophy, the same analysis found no meaningful difference between modalities, with an SMD of −0.05 (95% CI −0.20 to 0.10) for whole-body lean mass. These effect sizes suggest that the interference effect is not a universal phenomenon but is instead specific to certain outcomes and conditions.
Mechanisms Underlying the Interference Effect
The molecular basis of concurrent training interference is often framed around the AMPK-mTOR signaling axis. Endurance exercise activates AMPK, which can inhibit mTORC1, a key regulator of protein synthesis following resistance exercise. However, this model oversimplifies the chronic adaptations observed in training studies. A more integrative view considers factors such as residual fatigue, substrate competition, and alterations in motor unit recruitment patterns. When high-intensity endurance work is performed shortly before strength training, the quality of the subsequent resistance session may decline due to neuromuscular fatigue, potentially attenuating strength gains over time. Evidence from studies manipulating intra-session exercise order indicates that performing strength training first tends to preserve strength and power outcomes more effectively than the reverse sequence, though the advantage is modest and may not extend to hypertrophy.
Moderators of the Concurrent Training Effect
Training status emerges as a critical moderator. Untrained individuals generally exhibit robust adaptations to concurrent training, with minimal evidence of interference. A 2024 meta-analysis focusing on sex and training status reported that the interference effect on lower-body strength was more pronounced in trained males (SMD −0.45, 95% CI −0.72 to −0.18) than in trained females (SMD −0.12, 95% CI −0.46 to 0.22), though the subgroup difference did not reach statistical significance (p = 0.09). This pattern may reflect sex-based differences in recovery capacity or hormonal milieu, but the current evidence is insufficient to draw firm conclusions. The same analysis found that endurance-trained individuals undergoing concurrent training experienced a small decrement in VO2max compared to endurance-only training (SMD −0.23, 95% CI −0.43 to −0.03), whereas strength-trained individuals did not show a significant reduction in strength gains when adding endurance work.
Body Composition Outcomes
Concurrent training has been evaluated for its effects on body fat loss, with a recent systematic review and meta-analysis comparing resistance, aerobic, and concurrent training in metabolically healthy adults. The analysis revealed that concurrent training produced similar reductions in body fat percentage compared to aerobic training alone (mean difference −0.2%, 95% CI −0.8 to 0.4), and both were superior to resistance training. However, concurrent training led to greater preservation of fat-free mass than aerobic training (mean difference 0.8 kg, 95% CI 0.3 to 1.3), a finding that aligns with the role of resistance exercise in maintaining lean tissue during energy deficit. These data support the use of concurrent training for individuals seeking simultaneous improvements in body composition and cardiorespiratory fitness, though the magnitude of change is modest and subject to individual variability.
Practical Programming Considerations
For practitioners designing concurrent programs, the evidence suggests several strategies to mitigate potential interference. Separating endurance and strength sessions by at least 6 to 24 hours may reduce the acute molecular interference and residual fatigue that compromise strength adaptations. When same-day training is necessary, prioritizing strength work appears beneficial for maximal strength and power development. The total volume and intensity of endurance training should be carefully managed; high volumes of running, in particular, have been associated with greater impairments in lower-body strength compared to cycling. A practical approach for intermediate and advanced trainees is to periodize training blocks, emphasizing one modality while maintaining the other, rather than pursuing maximal gains in both simultaneously. For general health and body composition goals, concurrent training remains an efficient and effective strategy with minimal downside.
Caveats and Limitations
The meta-analytic evidence on concurrent training is constrained by heterogeneity in study designs, including variations in training frequency, intensity, modality, and outcome measurement. Many studies are of short duration (8–12 weeks), limiting inferences about long-term adaptations. The classification of training status varies across studies, and the operational definition of “trained” often lacks precision. Additionally, the majority of research has been conducted in young, healthy males, limiting generalizability to older adults, females, and clinical populations. Publication bias may also inflate the apparent interference effect, as studies finding no difference may be less likely to be published. Future research should employ standardized protocols and report individual-level data to enable more precise moderator analyses.
References
- Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis — PubMed
- Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status — PMC
- Comparison of concurrent, resistance, or aerobic training on body fat loss: a systematic review and meta-analysis — PMC
Individuals with specific health conditions or those new to exercise should consult a physician before beginning a concurrent training program.
