The fear of cardio in strength training circles has become almost cultural. Mention a 5k run to the wrong person in a powerlifting gym and you'll get a lecture about cortisol. But the interference effect — the actual physiological mechanism behind this concern — is real, well-documented, and almost universally misunderstood.
Steady state cardio, done sensibly, is not your enemy. Randomly bolting it onto a strength programme with no thought about timing, volume, or modality? That's a different conversation entirely.
Where the Interference Effect Actually Comes From
The interference effect was first described by Hickson in 1980 — a study showing that concurrent strength and endurance training produced smaller strength gains than strength training alone. That finding has been replicated and refined considerably since, most comprehensively in a 2012 meta-analysis by Wilson et al. covering over 20 years of concurrent training research.
The mechanisms are real. Endurance training upregulates AMPK (AMP-activated protein kinase), which promotes mitochondrial biogenesis and fat oxidation. Strength training drives mTOR signalling, which promotes muscle protein synthesis. These pathways are not perfectly compatible — AMPK can inhibit mTOR activity — and if both are activated simultaneously, you get a blunted response to both stimuli.
Here's what that meta-analysis also found: the interference effect is highly dependent on how concurrent training is structured. It is not an inevitable consequence of doing both.
Modality Matters More Than You Think
Running produces significantly greater interference with strength adaptations than cycling. This is not a small difference. Wilson et al. found that running-based concurrent programmes produced meaningful reductions in hypertrophy and strength, while cycling-based programmes showed considerably less interference — to the point where the effect was often negligible.
The likely reason is impact and eccentric loading. Running generates substantial mechanical stress in the lower limbs that cycling does not. When that stress overlaps with a lower body strength session, recovery is compromised and the adaptive signal from the strength work gets diluted.
This distinction matters practically. A lifter who swaps their treadmill sessions for the bike — same duration, same intensity — may solve most of their interference problem without changing anything else.
Sequencing Is the Variable Nobody Talks About
Research by Robineau et al. (2016) looked specifically at the order and proximity of strength and endurance sessions, and the findings are worth taking seriously. Performing endurance work immediately before a strength session produced the worst outcomes — acute fatigue from the cardio directly compromised force production during lifting, reducing the training quality of the strength session.
Performing cardio immediately after strength work was better but still suboptimal. The best outcomes — for both strength and endurance — came from separating the two by at least six hours, or training them on different days entirely.
Most people doing concurrent training are not doing this. They're finishing a heavy lower body session and jumping on a treadmill. Or doing 20 minutes on the bike as a warmup before squatting. Both are worse than the alternative.
Volume and Intensity of the Cardio Itself
This is where the "cardio kills gains" crowd have got one thing right — they've just applied it too broadly. High-volume, high-intensity endurance work does produce meaningful interference. Long runs at threshold pace, multiple high-intensity interval sessions per week, significant weekly mileage alongside heavy strength work: these create competition for recovery resources that the body cannot easily resolve.
Low-to-moderate intensity steady state cardio, kept to a reasonable weekly volume, does not carry the same cost. A 30-40 minute Zone 2 session on a bike, separated from strength work by several hours or a full day, contributes to cardiovascular health, aids recovery through improved circulation, and does not meaningfully suppress hypertrophy or strength when programmed sensibly.
The problem is not the modality. It is the lack of intention behind how it is placed in the week.
What Actually Good Concurrent Programming Looks Like
There is no universal prescription — it depends on your goals, training age, and the demands of your sport. But some principles hold across the evidence:
- Separate strength and cardio sessions by at least six hours where possible, or train them on different days
- Prioritise the quality that matters most to your goal — do that first in the day
- Favour cycling or rowing over running if lower body strength is a priority
- Keep cardio intensity moderate; save high-intensity work for phases where strength development is secondary
- Account for cardio volume in your overall recovery load — it is not free
The Real Problem
The strength community's fear of cardio has produced a generation of athletes who avoid it entirely, develop poor cardiovascular bases, and then wonder why their work capacity in the gym is limited and their recovery between sessions is slow. Aerobic fitness is not separate from strength performance — it underpins recovery, supports work capacity, and contributes to long-term athletic longevity.
Avoiding cardio because someone told you it would shrink your muscles is not evidence-based. Programming it carelessly because you assumed it didn't matter is equally indefensible. Neither approach reflects a serious engagement with how adaptation actually works.
If you want both qualities — and most people training for health and performance should — the answer is to programme them properly. Not to pick one and fear the other.
If you're not sure how to structure both in your week, book a free consultation and we'll work it out based on your actual goals and schedule.