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. Muscle is held by strength training, enough protein and a deficit that is not too aggressive. In all of that, cardio is an add-on.

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. The interference effect grows with the amount of endurance work and shows up mainly in explosive strength. For someone lifting and doing a few moderate cardio sessions a week, the effect on muscle and maximal strength is small or none.

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 adds a bit of extra burn, but the size of your deficit is decided mostly by your plate. 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". In studies, the effect grows with how often and how long you do endurance work. It is more noticeable with running than with cycling, and it shows up mainly in explosive strength. It matters most for someone with a big endurance load, like marathon training, who is also chasing maximal strength and muscle. For someone lifting and doing a few moderate cardio sessions a week, the effect on muscle and maximal strength is small or none. For them the interference effect is just a fun fact.

The interference effect is real, but it grows with the dose. A few moderate cardio sessions a week next to lifting usually have little or no effect on your progress. 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. They are all about food, training and recovery.

• Too big a deficit. Cut too fast and a bigger share of what you lose comes from muscle, not fat. 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 you lose more of it than you need to. Lifting is your insurance here.
• Bad sleep. Short, poor sleep hurts recovery and works against keeping muscle.

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

Cardio actually 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. If you have a heart condition, high blood pressure or have not trained in years, check with your doctor before you add hard cardio.
• 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.

For most of my clients, lifting plus a bit of cardio works best.

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. Together with lifting, it is your main insurance. Protein plus lifting, with a deficit that is not too aggressive, protects muscle well, with or without cardio.

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. Little or 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. Fix the food.

What to do in practice

A simple 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. It will not cost you muscle.
• Watch the deficit and the protein. A weight loss pace that is not too aggressive and enough protein every day. Together with lifting, these two protect muscle the most.

So next time someone tells you cardio "eats muscle", translate it: cardio in a sensible amount is just an add-on. 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 moderate cardio sessions a week next to lifting usually have little or no effect on 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 coached 600+ clients through fat loss. Evidence-based, sustainable plans.

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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.