Programming
Lifting and cardio: the interference effect, in proportion
The fear that cardiovascular training destroys strength gains is overstated, and the conditions under which it genuinely interferes are specific and avoidable.
The interference effect is real, well documented, and much smaller than gym culture suggests.
What the research shows
Studies on concurrent training — resistance training combined with endurance training — generally find that endurance adaptations are largely unaffected, while strength and particularly power adaptations can be blunted somewhat compared with resistance training alone.
The effect is dose-dependent. Higher volumes and frequencies of endurance work produce more interference. Modest amounts produce little or none.
The modality matters. Running appears to interfere more than cycling, probably because of the eccentric loading and consequent muscle damage.
And the effect is more pronounced for power and explosive strength than for maximal strength or hypertrophy.
The proposed mechanisms
Molecular signalling conflict. The pathways that drive endurance adaptation and those that drive muscle growth have some antagonism. This was the original explanation and it is probably less important than initially thought, since the acute signalling response does not predict long-term outcomes well.
Residual fatigue. The more mundane and probably more important explanation. Endurance work generates fatigue that degrades subsequent resistance training quality — fewer reps, lower loads, poorer sessions.
Energy availability. Adding substantial endurance work without adding food creates a deficit, and a deficit impairs muscle gain. A great deal of apparent interference is actually underfuelling.
How to minimise it
Separate the sessions. Ideally by at least six hours, or on different days. Doing hard intervals and then a heavy squat session an hour later is the worst case.
Do the priority first. If strength is the goal, lift when fresh. If endurance is the goal, run when fresh. Whatever comes second gets a compromised session.
Prefer cycling to running if the aim is to minimise interference, particularly for leg-focused strength work.
Manage the volume. Two to three moderate sessions a week produce essentially no interference for most people. Five hard sessions will.
Eat more. Additional training requires additional energy. Failing to add food converts a training decision into a deficit.
Avoid hard endurance work the day before a heavy lower body session.
Why you should do some anyway
The health case is overwhelming and it has nothing to do with lifting performance.
Cardiorespiratory fitness is one of the strongest predictors of all-cause mortality in the literature, with associations larger than most conventional risk factors. The relationship holds across populations and it is dose-responsive at the low end, where the biggest gains come from going from nothing to something.
Resistance training has its own independent benefits and it does not substitute for cardiovascular fitness.
There is also a practical training benefit: better aerobic capacity improves recovery between sets and between sessions, which supports higher training volumes.
A workable structure
For someone whose primary goal is strength or size:
Two or three cardiovascular sessions a week. One or two of them low intensity and reasonably long — thirty to sixty minutes of easy work, which is minimally fatiguing and builds the aerobic base. Optionally one shorter session of higher intensity intervals, placed away from heavy lower body training.
Cycling, rowing, incline walking, or swimming produce less residual soreness than running.
Total time commitment of two to three hours a week, which is enough for substantial health benefit and small enough that interference is negligible.
Walking, specifically
The most underrated intervention available. It produces essentially no interference, contributes meaningfully to energy expenditure, supports recovery, and has good evidence for health outcomes.
Eight to ten thousand steps a day, accumulated through ordinary activity, is worth more to most people than a structured cardio session they will not do.
The exception
If you are a competitive strength athlete in a peaking phase, reducing endurance work is reasonable, because the marginal interference matters when you are optimising for a specific day.
For everyone else — which is nearly everyone — the interference effect is not a good reason to avoid cardiovascular training. The health cost of avoiding it is far larger than the strength cost of doing it.
