Is Cardio Bad for Muscle Growth? What 12 Studies Says

Does cardio kill your gains? Or can you mix cardio and weights without putting the brakes on muscle growth? Here’s everything you need to know.

Not long ago, I was putting together a training program for a guy who hadn’t been to the gym for a couple of months. He’d lost some muscle, and was on a mission to get it back as fast as humanly possible.

Not only that, he also wanted to get a lot fitter, but without interfering with muscle growth. He had four days a week to train, and wanted my help to make sure that his program was set up properly.

Does Cardio Kill Your Gains?

Some folks take the view that this can’t be done. If you want to maximize muscle growth, they say, endurance training should be kept to an absolute minimum.

Are they right?

While you don’t want to cut out cardio completely, you don’t want to end up hurting muscle growth.

How much cardio, if any, should you do when you’re trying to build muscle? At what point does it start interfering with the adaptations to resistance training?

Sprinters vs Marathon Runners

You’ll often see pictures of a marathon runner next to a sprinter, the comparison between the two physiques supposedly offering definitive proof that too much steady-state cardio will cause muscle mass to be lost faster than Samson losing his strength after a trip to the barbers.

The argument goes something like this:

“When you perform increasingly longer bouts of cardio, your body gets rid of its muscle and stores fat to prepare for each bout of cardio, while also down-regulating anabolic hormones such as testosterone and growth hormone, as these are paradoxical to the demands being placed on your body. The classic example is to look at the comparison between a sprinter and a marathoner… which would you rather look like?”

However, this ignores the fact that endurance athletes aren’t trying to gain muscle. Many will restrict their calorie intake to keep their weight down. Beyond a certain point, adding extra weight, be it in the form of muscle or fat, will slow them down.

For a sprinter, gaining additional muscle mass – which requires lifting weights and eating plenty of calories – will often help rather than hurt their performance.

For an endurance athlete, restricting calorie intake, doing very little resistance training and running long distances is going to improve their performance. But it’s the exact opposite of what you should be doing to gain muscle.

That’s why the physiques of sprinters and marathoners are so different, not because steady-state cardio is inherently bad for muscle growth.

What happens if someone follows a diet and training program geared towards hypertrophy and then throws some cardio into the mix?

Here’s a closer look at what science has to say on the subject of aerobic exercise and muscle growth.

Where the Cardio Slows Muscle Growth Story Got Started

Much of the concern about cardio workouts slowing muscle growth started back in 1980, when a researcher by the name of Robert Hickson published a paper titled Interference of Strength Development by Simultaneously Training for Strength and Endurance.

Hickson was a powerlifter, who’d been doing his post-doctoral work in the laboratory of Professor John Holloszy. Seen by many as the father of endurance training research, Holloszy would go for a daily run through the local park. 

Wanting to make a good impression with his new boss, Hickson joined in with the afternoon runs.

However, despite the fact he was still doing the same amount of strength training, Hickson found that his muscles were shrinking and he was getting weaker.

When Hickson told his boss what was going on, Holloszy told him that “this should be the first study you do when you have your own lab.”

When Hickson moved to Chicago, and opened a new laboratory at the University of Illinois, that was exactly what he did.

Endurance Training + Weights = Smaller Strength Gains

In his study, Hickson found that subjects who combined resistance and endurance training made smaller strength gains than those who lifted weights and nothing else [1]. 

However, the training routine Hickson devised was not for the faint of heart. The quads were trained five days a week with squats, leg presses and leg extensions, for a total of 30 work sets per week.

The endurance training side of things was equally brutal. It involved interval training (a total of six 5-minute bouts at a work rate approaching VO2max) on an exercise bike, three times a week.

On the other three days, subjects were told to run “as fast as possible” for 30-40 minutes.

There were no recovery days. No easy weeks. It was pretty much flat out for the entire 10-week study.

But even then, strength in the concurrent strength and endurance and resistance-only groups increased at much the same rate during the first 6–7 weeks of the study.

While the resistance-only group carried on getting stronger, gains in the concurrent strength and endurance group hit a plateau, before taking a dip in weeks 9 and 10.

How Does Cardio Interfere With Muscle Growth?

Cardio workouts do have the potential to put the brakes on muscle growth, especially if your training program looks like the one Hickson had his subjects doing back in the day. 

It does so in a number of ways.

First, cardio can compromise the “quality” of a lifting session, which in turn reduces the strength of the muscle-building stimulus generated by a given workout. 

For example, let’s say you go to the gym, do a hard high-intensity interval training (HIIT) session on the bike, then train your legs immediately afterwards.

Chances are it’s not going to go well. 

You won’t be able to do as many reps, or lift as much weight, because of the residual fatigue from the HIIT. As a result, the intensity of the “growth signals” sent to your muscles will be weaker than they otherwise would be, which in turn means a slower rate of muscle gain [2]. 

Cardio can also interfere with your ability to recover from lifting weights. 

Let’s say that you’re following a 5×5 program. You lift weights on Monday, Wednesday and Friday, then rest on the other days.

Adding cardio on several of your rest days means that some of the resources used by your muscles to recover and grow are now spread more thinly than they were before. This can bring muscle growth to a halt, or at least slow it down considerably.

Cardio workouts can also mess around with some of the molecular signalling pathways involved in muscle hypertrophy [5].

That is, the strength of the “make me bigger” signal being sent to muscle fibers in the hours and days after training is going to be reduced, slowing the rate at which muscle protein is synthesized.

However, saying that cardio has the potential to interfere with hypertrophy doesn’t mean that it will, and the extent to which cardio interferes with muscle growth depends on a number of factors.

• The proximity of the cardio to your lifting sessions.
• The order in which the training is done.
• The type of cardio you’re doing (i.e. jogging vs cycling).
• How often it’s done.
• How hard it is.
• How long your cardio sessions last.
• How well you recover from it, which in turn is affected by age, diet, and sleep, as well as any crap you have going on in your life that leaves you stressed, anxious, or worried.

How Much Cardio Should You Do?

There are four main training variables that you can tweak and play around with. Those variables are frequency, intensity, time and type.

What you do to one changes what happens to the others.

Let’s say you turn frequency up to the max, and decide that you want to do some form of cardio every day. In that case, you’ll need to play around with the intensity, time, and type of exercise you do to avoid interfering with muscle growth.

On the flip side, a reduction in training frequency allows you to ramp up the intensity levels, as well as giving you more choice about the type of cardio sessions you do.

You might find that running five days a week, for example, is too much for your body, but 2-3 days a week is fine.

How Age and Stress Affect Recovery

There are a few other things to think about, most notably your age, as well as any stuff you have going on in your life that’s causing you an undue amount of anxiety, worry or some other form of psychological stress that you don’t want to feel.

Physical and psychological stress make withdrawals from the same account. Too much of one (psychological stress) can make it harder to recover from the other (the physical stress of training).

If you’re under a lot of pressure at work, have no idea how you’re going to pay the rent next month, and happen to live next door to someone who enjoys playing loud, thumping music at three in the morning, you have a lot of non-training related “stress” going on in your life. 

As a result, you’re not going to recover as well from workout to workout, and the adaptive response to your training program won’t be all that it might have been.

The number of times you’ve traveled around the sun will also affect your results. Things you could get away with in your twenties and thirties will have a much bigger impact on your ability to recover at the age of 40 or 50.

In one study, a group of triathletes in their fifties recovered more slowly than triathletes in their twenties in the days following a 30-minute downhill run [12].

Ten hours after the run, there was also a trend for masters triathletes to produce a slower time trial performance than their younger counterparts.

Cardio competes directly with some of the resources that muscles use to maintain their size and strength. As you get older, those resources become increasingly scarce, and you’ll need to be a lot more careful about how they’re allocated.

How Much Cardio Is Too Much for Building Muscle?

There are no strict rules that lay out exactly how much cardio you should do and when you should do it. But I do have some general guidelines that will minimize the extent to which cardio interferes with muscle growth.

1. First, cap the amount of moderate- to high-intensity cardio you do to a couple of hours a week. 

2. Avoid any intense cardio immediately before lifting weights. You’re better off doing it once the heavy training is out of the way, or even on a separate day. 

3. I’d also suggest that you focus mainly on low-impact cardio, such as cycling, swimming, climbing stairs, rowing or even incline treadmill walking, rather than running.

Does Running Kill Gains?

Running tends to cause a lot more muscle damage than something like swimming, cycling, or even incline treadmill walking. There’s also a lot more wear and tear on your joints.

As a result, running has a much greater potential for impeding recovery and interfering with hypertrophy [7].

A review of the research on concurrent training and muscle growth, published in the Journal of Strength and Conditioning Research, reports that “resistance training concurrently with running, but not cycling, resulted in significant decrements in both muscle hypertrophy and strength.” [6].

How Do You Do Cardio Without Slowing Muscle Growth?

Let me give you a few examples of how to put it all together.

The first option is to do some form of low-intensity cardio, such as walking, every day.

While walking isn’t going to have much of an impact on your cardiovascular fitness (unless you’re completely out of shape), it is going to change your health for the better.

A brisk walk first thing in the morning is also a great way to clear your head and set you up for the day.

Option two is 30-40 minutes of moderate-intensity (60-70% of maximum heart rate) cycling 2-3 times a week, on the days you don’t train with weights. 

Interestingly, the addition of cycling to a resistance training program can actually improve rather than impairs gains in muscle mass [3].

In fact, there are various studies to show that cycling alone can stimulate muscle growth in the lower body, albeit in untrained novices rather than seasoned lifters [8, 9, 10, 11].

With cycling, there’s also far less muscle damage and joint stress compared to running, which means there’s less potential to interfere with the adaptations to weight training.

If you want something that doesn’t take up as much time, go for HIIT. This can be done on separate days, or even after you’ve trained your lower body – the stimulus isn’t that much different from doing a few burnout sets of squats or leg presses.

When it comes to improving cardiovascular fitness, most studies show that HIIT is superior to steady-state cardio, on a time-equated basis at least.

In one study, 10 minutes of interval training, done three times a week, delivered many of the same benefits as 150 minutes (50 minutes, three times a week) of traditional cardio [4].

By the end of the three-month study, subjects in the continuous group had ridden for 27 hours. However, the interval group spent a little over five hours on the bike.

Despite a five-fold lower time commitment, various measures of cardiometabolic health improved to a similar extent in both the HIIT and steady-state groups.

VO2 peak, a measure of aerobic fitness, increased by 15-20% in both groups. Insulin sensitivity, a marker of blood sugar control, improved to a similar extent in both groups. Muscle mitochondrial content also increased similarly after HIIT and steady-state exercise.

If you’re already very fit, this type of program isn’t going to get you a lot fitter. But it takes less work to maintain a given level of fitness than it does to build it in the first place, and you don’t need a lot of training just to keep things ticking over. 

Frequently Asked Questions

Does cardio affect muscle gain?

Provided you don’t go overboard on the volume, frequency and intensity, there’s no need to worry about cardio negatively affecting muscle growth. Cardio seems to interfere a lot more with the development of power than it does strength or muscle mass.

Does lifting after cardio slow muscle growth?

That depends a lot on what type of cardio you’re doing, and how long you’re doing it for.

Using low-intensity steady-state cardio as part of a general warm-up for weight training has the potential to improve your performance in the gym. That’s because a warm muscle tends to perform better than a cold muscle, as well as being more resilient to injury.

But it’s a different story with more intense and/or longer duration cardio.

In one study, performance during the high pull, squat, bench press, deadlift and push press was adversely affected when lifting was performed after cardio, with high-intensity interval exercise leading to the largest decline in exercise performance.

Over time, this dip in performance is likely going to mean slower gains in strength and size.

Should I do cardio if I want to gain muscle?

There’s no rule that says you have to do any cardio at all if you want to gain muscle.

However, there is some evidence to suggest that cardiovascular exercise has the potential to help rather than hurt your gains if its used in the right way.

For one, some types of cardio may help recovery by promoting blood flow to the muscles without causing further damage.

Cycling at a low-to-moderate intensity for 20-30 minutes the day after a heavy leg day in the weight room, for example, will often reduce post-exercise muscle soreness, and may accelerate the rate at which muscle damage is repaired.

There’s also some interesting research to show that differences in capillary density go some way towards explaining why why some people make gains faster than others.

Capillaries are tiny blood vessels, which deliver oxygen, nutrients and hormones to muscle cells, clear the metabolites that build up during a workout, as well as helping with the repair and recovery process from one workout to the next.

In one review, researchers report that men with a higher capillary density saw faster gains in muscle mass than individuals with a lower capillary density after six months of resistance training.

And one of the best ways to go about boosting capillary density is to do some cardio.

How long should I do cardio so I don’t lose muscle?

Limit the amount of moderate- to high-intensity cardio you do to 2 hours a week, and avoid any intense cardio immediately before hitting the weight room. You’re better off doing cardio once the heavy lifting is done and dusted, ideally on a separate day.

Can I do cardio everyday and gain muscle?

If you want to do cardio every day, you’re far better off with some kind of low-intensity exercise, such as walking, or even riding a bike. With daily high-intensity cardio, you’ll just end up digging yourself into a hole that can take weeks to recover from.

Does cardio lower testosterone levels?

Large amounts of endurance exercise and being very lean are both associated with lower testosterone levels.

In fact, some research on male athletes participating in endurance sports, such as marathons and long-course triathlons, reports testosterone levels that are just 50% of those found in comparable age-matched, non-exercising men.

The common characteristics and traits of men displaying the so-called Exercise-Hypogonadal Male Condition (EHMC for short) include:

– They have low resting basal testosterone levels, typically 50–75% that of normal, healthy, age-matched sedentary men.

– Their low testosterone levels do not appear to be a transient phenomenon related to the acute stress-strain of exercise.

– In many cases, it appears an adjustment in the regulatory axis (to allow a new lower set-point for circulating testosterone) has occurred.

– They typically have a history of early involvement in organized sport and exercise training. This has resulted in these men having many years of almost daily exposure to physical activity.

– The type of exercise training history most frequently seen in these men is prolonged, endurance-based activities such as distance running, cycling, race walking, and triathlon training.

However, there are several reasons why EHMC is something most people don’t need to worry about.

For one, it’s a phenomenon that’s typically observed in endurance athletes who are very lean and training for events like triathlons, ultramarathons, that type of thing.

Rather than being caused by steady-state cardio per se, it has more to do with low energy availability, a result of the highly restrictive diet necessary to maintain such a low body fat percentage.

If you get someone to diet for long enough, their T levels will dip sooner or later, irrespective of how much cardio they’re doing.

For example, there was an interesting case-study that looked at a professional drug-free bodybuilder while he was dieting down for a contest.

By the time of the show, testosterone had dropped to just 25% of its normal resting level.

That’s despite the fact he was doing mainly resistance training (5 hours per week) and HIIT (40 minutes per week), with only 30 minutes devoted to steady-state cardio.

In most cases, that amount of exercise by itself wouldn’t be a problem. The real issue was the reduced energy intake combined with a very low level of body fat (this guy was down to 4.5% by the time of the contest).

Even with no exercise at all, such an extreme level of leanness will cause T levels to take a dive.

In short, too much exercise, be it steady-state cardio, HIIT, or resistance training, especially when it’s combined with prolonged dieting, can have an adverse effect on hormone levels. 

However, it’s not cardio by itself that’s causing the problem, but large amounts of exercise combined with a restrictive diet and low body fat percentage.