Does Cardio Kill Your Gains? What Interference Really Means
No, not for most people. Doing cardio alongside strength training can slightly blunt gains in strength and muscle, an effect researchers call interference, but it shows up mainly when cardio volume is high, sessions are crammed together, recovery is poor, or the cardio is a lot of running. For a busy adult doing two or three strength sessions and a moderate amount of cardio a week, the benefits to heart health, fitness and body composition far outweigh any small cost. The practical question is not whether to do cardio, but how much, what kind and when.
Where the worry comes from
The idea goes back to a 1980 study by Robert Hickson, in which people who combined demanding endurance training with strength training gained strength more slowly in the later weeks than people who only lifted. Since then, many studies have looked at “concurrent training”, and the picture is more reassuring than gym folklore suggests. Strength gains are usually similar. Muscle growth is sometimes slightly lower when endurance work is heavy. Explosive power is the quality most likely to suffer.
Look closely at the conditions in those studies and you often find large amounts of endurance training, several days a week, sometimes in the same session as the lifting. That is not what most people at a desk all day are doing.
Why interference happens
There are two broad explanations, and both are fairly intuitive:
- Fatigue. If your legs are tired from a long run, your squat session will be worse. Fewer good reps means less stimulus for strength and muscle.
- Competing signals. Endurance and strength training ask the muscle to adapt in somewhat different directions. At high volumes, the endurance message can partly dampen the strength one.
The first is by far the bigger factor for ordinary training. It is also the easier one to manage.
When cardio is most likely to get in the way
| Factor | Lower interference | Higher interference |
|---|---|---|
| Weekly volume | A few moderate sessions | Many long sessions |
| Type | Cycling, air bike, walking, swimming | Lots of running, especially downhill or long distance |
| Timing | Separate days or several hours apart | Hard cardio right before heavy lifting |
| Intensity mix | Mostly easy, a little hard | Everything hard |
| Recovery | Decent sleep and enough food | Short sleep, big calorie deficit |
Running is singled out because it involves a lot of impact and lengthening muscle work, which adds soreness and fatigue to the legs. Cycling-style cardio, including the air bike, is gentler on the legs’ recovery and tends to interfere less with lower-body strength work.
How to combine them in a real week
Here is a simple structure that works for most professionals who want to get stronger and fitter in four or five hours a week:
- Put strength first. Two or three full-body strength sessions, each built around a squat or hinge, a push and a pull.
- Keep most cardio easy. Two sessions of 20 to 40 minutes at a pace where you could hold a conversation. Our guide to zone 2 cardio explains how to judge it.
- Add a little hard work. One short interval session, or a few hard intervals at the end of a strength session, is enough for most goals.
- Space them out. Separate days are ideal. If they share a day, leave a few hours between them, or do the lifting first.
- Count your walking. Daily steps cost almost nothing in recovery and add up across a week.
If you have to do both in one visit, cardio before or after weights covers the order in more detail. In short: whatever matters most to you goes first.
What the guidelines ask for
The UK Chief Medical Officers recommend that adults do at least 150 minutes of moderate activity or 75 minutes of vigorous activity a week, plus muscle-strengthening activity on at least two days. The NHS exercise guidelines set this out plainly. That is a mix of both by design. Strength work protects muscle, bone and function as you age; cardio protects the heart and lungs. Abandoning one to protect the other is a poor trade for anyone whose main goal is health and a body that works well for decades.
Does cardio make you lose muscle when you are trying to lose fat?
This is usually the real concern behind the question. During a fat loss phase, the risk to muscle comes mainly from a large calorie deficit, too little protein, and stopping or going easy on strength training. Moderate cardio does not change that much. In fact it helps by increasing energy use without demanding the same recovery as extra lifting. What tends to cost muscle is the combination of aggressive dieting, lots of hard cardio, and lighter, less frequent lifting. Keep lifting heavy enough to challenge you, eat enough protein, and let cardio be a tool rather than a punishment.
Signs your cardio is eating into your strength
- Your main lifts have stalled for several weeks despite good sleep and food.
- Your legs feel heavy at the start of every strength session.
- Soreness from runs is still there when you next squat or deadlift.
- You are hungry and flat most of the time, and training feels like a chore.
The fix is usually small: swap one run for a cycle, move the hard cardio day further from leg day, or trim the total by 20 to 30 per cent for a few weeks and see what happens to your lifts.
How it is handled in a coached programme
In 1-1 personal training in Highgate, the session plan starts with your week, not with an ideal template. If you run twice a week with friends on Hampstead Heath, the programme accounts for it. The studio has an air bike for short, hard conditioning at the end of a strength session, which keeps intervals brief and easy to recover from. Easy cardio usually happens outside the studio, where it costs you no extra travel.
Check with your GP first if you have a heart condition, high blood pressure that is not controlled, or are returning to exercise after a long break.
Questions we get asked
Will running ruin my muscle gains?
Not at moderate volumes. Very high running mileage can slow muscle growth, especially in the legs, but a few runs a week alongside lifting is fine for most people.
How much cardio can I do while trying to build muscle?
Two or three sessions a week, mostly easy, is a sensible starting point. Watch your strength numbers and adjust if they stall.
Is HIIT better than steady cardio for lifters?
Short HIIT is time-efficient, but it is also fatiguing. A mix of mostly easy cardio and a small amount of hard work is easier to recover from.
Should I stop cardio completely when bulking?
No. Keeping some cardio supports heart health and recovery, and makes it easier to return to fat loss later.
See how programmes are built on the personal trainer in Highgate page, or visit The Highgate House homepage.
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