Does cardio kill muscle? The cardio and strength myth

"Don't do cardio, it will eat your muscle." I have heard it maybe a thousand times: in the gym, on social media, from guys who are scared that two sessions on the bike will wipe out a whole winter of hard work. The truth is calmer, and it takes a weight off. A sensible amount of cardio will not eat your muscle. Muscle is held by strength training, enough protein and a deficit that is not too aggressive. Cardio is an add-on in all of that, not the enemy. Let's look at where the fear comes from and where the problem really starts.

Short answer: No. A sensible amount of cardio will not eat your muscle. Lifting, enough protein and a moderate deficit hold it, and cardio is only an add-on. A real conflict shows up only with a huge endurance volume, for example marathon training, done at the same time as a maximal push for strength.

What actually holds muscle, and what does not

Start at the root, because this is where the myth falls fastest. You keep muscle with three things:

• Strength training. You give the muscle a reason to stay.
• Enough protein. You have the material to build and repair it. How much is in how much protein.
• A sensible deficit. You do not lose weight so fast that the body is pushed to take from muscle.

That is the load-bearing structure. A few walks or bike sessions a week do not hit this equation in any destructive way. Cardio does not override the strength signal "I need this muscle". It does not override the protein on your plate. It does not change how big your deficit is. You run that with your plate, not with the fact that you went for a walk. So no, easy cardio does not "eat" muscle.

What the "interference effect" is

The myth has a real core, or it would not stick around. The thing is called the interference effect. Simply put: endurance training and strength training pull the body in slightly different directions. Stack a huge volume of both side by side and they can slow each other's progress in strength and muscle growth a bit.

The catch is in the words "huge volume". We are talking about someone training for a marathon (or more) while trying to gain as much strength and muscle as possible at the same time. Two opposite maximums at once. There the conflict is real. A normal person doing a few cardio sessions a week plus lifting is nowhere near that zone. For them the interference effect is a fun fact, not a problem to solve.

The interference effect is real, but it starts at the extremes. A few cardio sessions a week next to lifting will not trigger it. That is not your league, unless you happen to be training for a marathon and powerlifting at the same time.

What really eats muscle

When people lose some muscle on a diet, the culprit is almost never cardio. It is one of these four. Notice that they are about food, training and recovery, not about a few bike rides.

• Too big a deficit. Cut so fast that the kilos drop one after another and the body cannot take energy from fat alone. It starts taking from muscle too. A slower pace protects muscle. More in how fast should you lose weight.
• Too little protein. Without building material the body has nothing to hold and repair muscle with.
• No strength training. Give the muscle no reason to stay and in a deficit it is the first thing written off. Lifting is your insurance here.
• Bad sleep. Short, poor sleep hurts recovery and works against keeping muscle. An underrated piece of the puzzle.

Do you see "a few walks" or "the bike" on that list? No. That is the whole point. Cardio does not belong in the dock.

What cardio is good for

Let me turn it around, because cardio does not only do no harm. It helps.

• Heart and fitness. You train your heart, your breathing, your endurance. For your health it is one of the best things you can do for yourself.
• Extra burn = an easier deficit. A few cardio sessions a week add a bit of room to your energy balance, so you do not have to cut food so hard. And a less drastic deficit protects muscle. So cardio actually helps you keep it. How much it really burns is in cardio burns less than you think.
• Recovery and your head. Easy cardio gets the blood moving, helps recovery and clears your head. After a hard day, a walk is sometimes the best workout.

Strength plus a bit of cardio is the ideal for most people. Not a choice between them. Both together.

How not to mess it up

To make strength and cardio work together and not against each other, keep a few simple rules:

• Strength training comes first. It is the engine for keeping and building muscle. Cardio fits around it, not the other way round.
• Keep cardio mostly low to moderate intensity, in a sensible amount. Walking, an easier bike, a brisker pace. You do not have to destroy yourself for it to count.
• Do not put hard cardio right before legs. Show up to squats with legs run into the ground and your lifting performance drops. And worse strength training is what really does not help your muscle.
• Eat enough protein. Again and again, because it is the most important insurance. Protein plus lifting = muscle is safe, whether you do cardio or not.

When cardio hurts muscle and when it is fine

So you have it in black and white, when to even talk about a problem and when to relax:

• A few cardio sessions or walks a week next to lifting: fine. No interference effect, muscle is not in danger. It even helps the deficit and your heart.
• Easy cardio, but after a hard lifting session or on another day: fine. Strength has priority and got what it needed. Cardio only adds to it.
• Hard cardio right before a leg session: better not. It lowers your lifting performance, and that is what actually harms muscle. Swap the order or the day.
• Huge endurance volume (marathon and up) alongside a maximal push for strength: here, yes. Two maximums at once really slow each other. But that is an extreme, not a normal routine.
• A few walks, but too big a deficit and too little protein: a problem, but not because of cardio. The culprit is the deficit and the food, not the walk. Fix the food.

What to do in practice

No science project, just a sensible routine that holds muscle and adds fitness:

• Strength training 2 to 4 times a week. That is the base and the priority. This is where muscle is decided. If time is short, see strength training one hour a week.
• Cardio 2 to 3 times a week, or simply your steps. Walking, an easier bike, anything you enjoy and will keep doing. Muscle is not afraid of it, the opposite.
• Watch the deficit and the protein. A weight loss pace that is not too aggressive and enough protein every day. These two protect muscle more than anything else.

So next time someone tells you cardio "eats muscle", translate it: cardio in a sensible amount is an add-on, not the enemy. Muscle is held by lifting, protein and a sensible deficit, and cardio rather helps with that. The scare is pointless. Build your routine on lifting, add some cardio, eat your protein and do not push the deficit to the extreme. Be Effective!

Frequently asked questions

What is the interference effect?

Endurance and strength training pull the body in slightly different directions. Stack a huge volume of both side by side and they can slow each other's progress in strength and muscle growth a bit. A few cardio sessions a week next to lifting will not trigger it.

What eats my muscle on a diet?

Rarely cardio. The culprit is usually too big a deficit, too little protein, no strength training or bad sleep.

Can cardio help me keep muscle on a diet?

Yes, indirectly. A few cardio sessions a week add a bit of room to your energy balance, so you do not have to cut food so hard. And a less drastic deficit protects muscle.

How do I combine cardio and lifting so they do not slow each other?

Strength training is the priority. Keep cardio mostly at low to moderate intensity. Easy cardio after lifting or on another day is fine. Hard cardio right before a leg session lowers your lifting performance, so swap the order or the day.

How many times a week should I lift and do cardio?

A sensible routine is strength training 2 to 4 times a week as the base, and cardio 2 to 3 times a week or simply your steps. Add a weight loss pace that is not too aggressive and enough protein every day.

Sources, so you can check:

1) Interference effect as described in the article: endurance and strength training pull the body in slightly different directions; a real conflict appears only with very large endurance volume (marathon training or more) alongside a maximal push for strength and muscle

2) Muscle retention in a diet as described in the article: strength training, enough protein, a moderate deficit and good sleep

Martin Barna

Online nutrition and fitness coach. Since 2013 I have helped 600+ people lose fat and keep the result. Science, not crash diets.

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This article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.