The Interference Effect: Can You Build Strength and Endurance Together?
The fear that cardio kills gains is half-true and half-myth. Here is what the concurrent-training research actually says — and how to schedule both so they barely interfere at all.
The Interference Effect: Can You Build Strength and Endurance Together?
"Cardio kills gains" is the most repeated and most misunderstood claim in the gym. The interference effect is real — but it's far smaller and far more manageable than the bro-science makes it sound. After years of running concurrent strength + endurance blocks (I do a spring half-marathon and still try to hold my strength), here's the honest read.
What the research actually shows
The foundational work is Hickson (1980), which found that training strength and endurance simultaneously blunted strength gains compared to strength alone. But the 2022 meta-analysis by Schumann et al. — pooling decades of better-controlled studies — found the interference is modest and specific: hypertrophy is largely unaffected, while maximal strength and explosive power take a small hit, and only when the endurance volume is high and the timing is poor.
Translation: a lifter doing 2-3 moderate cardio sessions a week is not meaningfully sacrificing muscle. A lifter training for a marathon while also trying to PR their squat in the same block — that's where the interference bites.
The two real mechanisms
1. Molecular signaling competition. Endurance training activates AMPK, which can transiently suppress mTOR — the pathway that drives muscle growth. This effect is acute (hours) and fades, which is why timing matters more than the cardio itself.
2. Residual fatigue. Hard cardio on the same muscles you just trained, or right before lifting, simply leaves you too fatigued to lift with quality. This is the bigger practical factor for most people.
The scheduling rules that minimize it
Everything I've read and lived comes down to four levers:
- Separate sessions by 6+ hours when possible. Lift in the morning, easy cardio in the evening (or vice versa). Same-session, cardio-before-lifting is the worst case for strength.
- Lift first if they must be back-to-back. Strength quality is more fragile than easy aerobic quality.
- Choose non-competing modalities. Cycling and rowing interfere with leg strength far less than running, which adds eccentric quad/hamstring damage.
- Keep most cardio easy (Zone 2). Low-intensity work barely triggers the AMPK competition. High-intensity intervals interfere more, so cap them at 1-2/week.
How to apply this
If you want both: lift 3-4x/week as your priority, add 2 easy Zone 2 sessions (bike/row) on upper-body or rest days, and at most one hard interval session away from leg day. Put cardio in a separate session from lifting whenever your schedule allows. Do this and the "interference" will cost you nearly nothing measurable — while your conditioning, recovery, and health all improve. The all-or-nothing framing was always wrong.