Does Cardio Kill Your Gains? What the Science Actually Says

The fear that came from one study

In 1980, Robert Hickson published a paper in the European Journal of Applied Physiology that changed how strength athletes thought about cardio for the next four decades.

Hickson had three groups. One trained for strength only — thirty to forty minutes, five days per week. One trained for endurance only — forty minutes, six days per week. One group did both programmes simultaneously: eleven sessions per week in total.

For the first seven weeks, the combined group improved at both. Strength up. VO2max up. Then something happened. Strength began to plateau, then decline — while the endurance-only and strength-only groups continued improving. Hickson called this the interference effect, and the fitness world turned it into a rule: cardio kills gains.

Here is what the fitness world did not look at closely enough. The combined group was performing eleven high-intensity sessions per week. They were not concurrent training in any reasonable definition of the word. They were overtrained. The interference that emerged after week seven was almost certainly the result of accumulated fatigue and inadequate recovery — not some fundamental incompatibility between strength and endurance adaptations.

Forty-five years of subsequent research has gradually corrected this picture. What it shows is far less alarming than the original headline.

What the modern research actually says

A 2023 meta-analysis published in Sports Medicine examined the effects of concurrent training — specifically how sex and training status moderate the interference effect — and reached conclusions that every cardio-phobic strength athlete should read.

Whole muscle hypertrophy was not significantly reduced by concurrent training. When volume was equated and recovery was adequate, the size gains from lifting were preserved alongside the cardiovascular adaptations from aerobic work.

There is nuance. At the level of individual muscle fibres, there is a small negative effect on Type I fibre hypertrophy — particularly when running, rather than cycling, is the aerobic modality (SMD = −0.23). This effect exists. It is small. And it diminishes as training experience increases.

The same analysis found that trained strength athletes were the subgroup least affected by concurrent training. If you have been lifting seriously for years, your neuromuscular system has developed a tolerance for complex training stimuli that largely protects you from the interference effects seen in untrained participants.

The honest summary of the current evidence: the interference effect is real, small, and primarily a function of inadequate recovery — not an inherent conflict between the physiological adaptations themselves.

Why the adaptations are not actually opposed

Understanding why concurrent training works — when properly programmed — requires a brief look at the molecular signalling involved.

Endurance training activates AMPK, a metabolic sensor that promotes mitochondrial biogenesis and aerobic adaptations. Strength training activates mTOR, which drives muscle protein synthesis and hypertrophy. The early theory was that these pathways directly antagonise each other — that AMPK activation suppresses mTOR, and therefore cardio suppresses muscle growth.

The more complete picture is that these pathways operate on different timescales. The acute AMPK activation from a running session does not persist long enough to meaningfully suppress mTOR activation from a subsequent strength session — provided sufficient time separates them.

The interference effect appears when these sessions are stacked too closely, when total recovery is insufficient, or when fatigue from one modality carries into the other. The solution is not to eliminate one modality. It is to sequence them correctly and recover properly.

Zone 2: the modality that changes the equation

Not all cardio is the same training stress. This distinction matters a great deal for how you incorporate aerobic work into a strength programme.

Zone 2 training — steady-state aerobic work at 60 to 70 percent of maximum heart rate, an intensity at which you can sustain a conversation — is categorically different from high-intensity interval training in its recovery cost and its interaction with strength work.

Zone 2 primarily develops the aerobic base: mitochondrial density, fat oxidation capacity, cardiac output, and capillary density in working muscle. The fatigue it generates is modest. A 45-minute Zone 2 session on a bike will not meaningfully compromise your next strength training session, particularly if eight to twelve hours separate them.

The benefits for strength athletes are practical:

Recovery between sets. A higher aerobic base accelerates the rate at which your cardiovascular system recovers between working sets — use a rest timer to keep your rest periods consistent and stop guessing when you are ready for the next set. Better between-set recovery allows more total work in a session — more volume, more stimulus, more long-term adaptation.

Training density. As aerobic fitness improves, you can handle higher training volumes without the accumulated cardiovascular fatigue that limits less-fit athletes.

Body composition. Zone 2 training at sufficient duration (30 to 90 minutes) primarily uses fat as fuel, supporting body composition management without the recovery cost of high-intensity work.

Health longevity. The data here is striking enough to warrant its own section.

The number that may matter more than your one-rep max

In 2018, researchers published the long-term results of the Copenhagen Male Study in the Journal of the American College of Cardiology. Five thousand one hundred and seven middle-aged men were followed for 46 years. Over 90 percent of them died during the follow-up period — providing enough data to draw very precise conclusions about what predicted longevity.

The finding: every one metabolic equivalent (MET) increase in cardiorespiratory fitness corresponded to a 10 to 20 percent reduction in all-cause mortality. Every additional MET, sustained across a lifetime, was associated with approximately 45 additional days of life.

The effect was not subtle. Men in the lowest quartile of cardiorespiratory fitness had two to five times the risk of early death compared to those in the highest quartile. Moving from low fitness to below-average fitness — not fit, just less unfit — was associated with a 50 percent reduction in all-cause mortality risk.

VO2max is not a number that powerlifters discuss. It does not appear on a meet results sheet. But it is, by the evidence, one of the most powerful predictors of whether you will be lifting — or doing much of anything — at 70, 80, or beyond.

A lifter who adds two Zone 2 sessions per week and gradually builds their aerobic capacity may be doing more for their long-term health than any programming decision they make about squats.

How to programme concurrent training without interference

The research supports a set of practical principles for minimising the interference effect while capturing the benefits of both modalities.

Sequence strength before cardio within the same session. If you must combine them on the same day, lift first. Strength training requires maximal neural output. Performing it after exhausting aerobic work compromises your ability to recruit motor units and apply maximal force.

Separate hard sessions by at least six hours. The interference effect is primarily driven by residual fatigue from one session carrying into the next. A heavy squat session at 7 AM and a Zone 2 run at 7 PM is a reasonable split. A heavy squat session followed 90 minutes later by a sprint interval session is a recovery problem.

Keep cardio intensity low most of the time. The 80/20 principle from endurance sports applies here: approximately 80 percent of your aerobic work at low intensity (Zone 2 or below), 20 percent at higher intensity. This ratio produces better aerobic adaptation than equal amounts of moderate-intensity work, and it minimises interference with strength training.

Protect total weekly recovery. Hickson’s study failed its participants at the level of total weekly load. Eleven sessions per week without adequate recovery is overtraining — not concurrent training. A realistic structure for a serious lifter incorporating cardio looks like four strength sessions and two to three aerobic sessions per week, with at least one full rest day.

Day	Session
Monday	Strength (heavy)
Tuesday	Zone 2 cardio, 30–45 min
Wednesday	Strength (moderate volume)
Thursday	Rest or light Zone 2, 20–30 min
Friday	Strength (heavy)
Saturday	Zone 2 cardio, 45–60 min
Sunday	Rest

This structure provides four strength sessions with adequate recovery between hard efforts, and two to three Zone 2 sessions that build the aerobic base without compromising the lifting.

Running versus cycling: does the modality matter?

The 2023 meta-analysis noted that the small interference effect on Type I muscle fibre hypertrophy was more pronounced with running than with cycling. This is worth considering but not overthinking.

Running produces greater eccentric loading of the lower body — particularly the quads, hamstrings, and glutes — which creates additional mechanical stress that competes with the recovery demands from lower-body strength training. Cycling is a closed-chain, lower-impact activity with less eccentric loading and therefore less overlap with the damage profile of barbell training.

If you primarily train lower-body strength — squats, deadlifts — and you are choosing between running and cycling for your aerobic work, cycling is the more recovery-efficient choice. If you enjoy running and are willing to manage the additional fatigue, the difference in the evidence is small enough that preference is a reasonable tiebreaker.

Swimming is a third option that generates essentially no lower-body eccentric load and has been used for decades by strength athletes as a low-interference aerobic modality.

Frequently asked questions

If I add cardio, will my squat go down?

Not if you programme it correctly. The research shows that whole muscle hypertrophy and strength gains are preserved in concurrent training that allows adequate recovery. Your squat will decline if you add cardio on top of a programme that is already at your recovery limit, or if you are performing high-intensity cardio on days adjacent to heavy lower-body sessions without sufficient separation.

How much cardio is too much?

For strength-focused athletes, two to three Zone 2 sessions per week of 30 to 60 minutes each is a sustainable and evidence-supported amount. If your aerobic sessions are routinely leaving you too tired to train well in the gym, you have exceeded your recovery capacity for the current load.

Does HIIT interfere with strength training more than Zone 2?

Yes. High-intensity interval training generates significantly more fatigue — both cardiovascular and muscular — than Zone 2 work. It also produces a greater acute AMPK response. If interference is your concern, limit HIIT to one session per week and keep it well-separated from your heaviest strength sessions. Zone 2 is the safer default for managing concurrent training load.

I am a powerlifter. Do I need cardio?

The sport does not require it. You can be competitive in powerlifting without meaningful aerobic fitness. But the VO2max data does not make exceptions for competitive lifters. Your athletic goals and your long-term health are related but not identical, and the mortality data suggests that maintaining some aerobic capacity is one of the most powerful things you can do for your longevity. A few Zone 2 sessions per week will not cost you your total. They may add years to your life.

What if I genuinely hate cardio?

The evidence is clear enough that a reframe is worthwhile. Zone 2 cardio is not punishing interval work. It is 40 minutes on a stationary bike at a pace where you can hold a conversation — while watching something, listening to something, or being somewhere you find pleasant. If cycling feels like too much, a brisk walk is Zone 2 for most untrained individuals. The barrier is lower than the word “cardio” implies.

The full picture

Cardio does not kill gains. Poor recovery kills gains. Insufficient sleep kills gains. A caloric deficit severe enough to impair muscle protein synthesis kills gains.

Cardio, programmed correctly — primarily Zone 2, separated from hard strength sessions, kept within your overall recovery capacity — makes you a more durable, more resilient, longer-lived athlete. It improves your between-set recovery. It supports body composition. And by the Copenhagen data, it may be doing more for your life expectancy than any single decision you make about squats.

The interference effect is real. It is also small, and it is a programming problem — not a biological conflict.

Hickson was not wrong. He was observing what happens when you load athletes with eleven hard sessions per week without adequate recovery. The lesson was never “avoid cardio.” It was: manage your training load.

The steel that is only ever hammered eventually cracks. The cycle of stress, recovery, and varied stimulus — including aerobic work — is what builds something that lasts.

SteelRep’s programmes have deload weeks and progression built in to account for your total training load. If you are adding cardio alongside one of them, track how your recovery is responding — and adjust from there.