The idea that cardio “kills gains” is widespread in gym culture. The concern has a legitimate physiological basis — but the practical interference between aerobic and resistance training is more nuanced than the simplified version suggests.
The Interference Effect: What Hickson Found
The interference effect was formally described by Robert Hickson in a 1980 study. Subjects who trained for both endurance and strength simultaneously showed smaller strength gains than those who trained only for strength, despite identical resistance training protocols.
The proposed mechanism: endurance training activates the AMPK pathway (which signals cellular energy conservation and promotes mitochondrial biogenesis), while resistance training activates the mTOR pathway (which drives muscle protein synthesis and hypertrophy). These pathways have partially opposing effects — AMPK can suppress mTOR signaling when activated simultaneously or in close temporal proximity.
This became the theoretical foundation for the concern that cardio impairs strength and muscle building.
What Subsequent Research Shows
More recent meta-analyses have refined the original findings considerably.
Wilson et al. (2012) analyzed 21 studies and found that concurrent training did reduce hypertrophy and strength gains compared to resistance training alone — but the magnitude was modest:
- Hypertrophy was approximately 31% lower in concurrent vs. resistance-only groups
- Maximum strength was approximately 18% lower
- Power output was most affected
However, this analysis included studies with very high volumes of concurrent training and those where cardio and resistance training were performed in the same session.
A 2022 systematic review (Schumann et al.) found that when methodological factors were carefully controlled — particularly session order and training volume — the interference effect on hypertrophy was smaller than earlier estimates and in some conditions was not statistically significant.
The current evidence suggests interference is real but depends heavily on the specifics of how training is programmed.
What Actually Causes Interference
The research identifies several factors that amplify or reduce the interference effect:
Mode of cardio — Running (high mechanical loading, eccentric impact) appears to interfere more with lower-body hypertrophy than cycling, which has lower impact. Upper-body resistance training is less affected by lower-body cardio.
Volume and intensity of cardio — High-volume, high-intensity endurance work produces more interference than moderate cardio. Studies showing large interference effects often involve extreme concurrent training loads uncommon in recreational athletes.
Session order — Performing cardio before resistance training in the same session consistently produces more interference than the reverse order. When both must be done in the same session, resistance training first is the better-supported sequence.
Recovery time between sessions — The interference effect is substantially reduced when cardio and resistance training are separated by at least 6 hours, and further reduced at 24-hour separation. Separate-day programming largely eliminates interference at moderate cardio volumes.
Individual training status — Well-trained individuals appear to have more developed capacity for managing concurrent stimuli than beginners.
Practical Implications
For most people pursuing general fitness — not competing in either powerlifting or endurance sports — moderate concurrent training is not a meaningful obstacle to muscle gain.
The following general approach minimizes interference based on the available evidence:
Separate sessions by at least 6 hours when possible. Morning cardio and evening resistance training allows partial recovery between competing stimuli.
Choose lower-impact cardio modalities. Cycling, rowing, or swimming produce less lower-body mechanical interference than running when paired with lower-body resistance work.
Keep cardio volume proportional to goals. Two to three moderate cardio sessions per week alongside a resistance training program is unlikely to produce meaningful interference. High-mileage running programs alongside high-volume strength work will.
Prioritize session order. When both must be done in the same session, resistance training first is the more-supported approach.
Monitor recovery. If performance on resistance work is declining and fatigue is elevated, cardio volume may be exceeding recovery capacity.
Cardio and Body Composition
The interference question focuses primarily on hypertrophy. For body composition, moderate cardio contributes benefits that resistance training alone does not fully address.
Aerobic exercise improves cardiovascular health markers (VO2 max, resting heart rate, blood pressure) that resistance training affects less directly. It also contributes to energy expenditure, which can support fat loss without requiring equivalent dietary restriction.
For most people, the practical question is not whether to do both, but how to structure both without undermining one with the other. The research indicates this is achievable at moderate training volumes with sensible programming.
GYMRPG logs both resistance training sessions and cardio activity in the same interface, allowing users to track total weekly training load across both modalities.
Sources
- Wilson et al. (2012) — Concurrent training: A meta-analysis examining interference of aerobic and strength training. Journal of Strength and Conditioning Research, 26(8), 2293–2307.
- Hickson, R.C. (1980) — Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology, 45(2-3), 255–263.
- Schumann et al. (2022) — Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: An updated systematic review and meta-analysis. Sports Medicine, 52(3), 601–612.
- Murach, K.A. & Bagley, J.R. (2016) — Skeletal muscle hypertrophy with concurrent exercise training: Contrary evidence for an interference effect. Sports Medicine, 46(8), 1029–1039.