Can Cardio Really “Kill” Your Gains? Exercise Science Experts Weigh In

You don’t need to be deep in the fitness world to have run into the idea that cardio “kills” muscle gains—as if every cycling class and or 3-mile run sends a little army of molecules through your body to chomp away at hard-earned muscle tissue. While it’s an entertaining image and a potentially convincing theory, the reality is that it’s not exactly true. 

“Still, if you observe the body types and body compositions of elite distance runners as compared with elite strength athletes, it’s understandable that the concept rapidly grew in popularity,” says Sara Rosenkranz, exercise physiologist and associate professor in kinesiology and nutrition sciences at the University of Nevada, Las Vegas.

Keep reading to hear what science has to say about whether cardio is bad for muscle growth and how to optimize your routine for gains—just don’t come at us for taking away your excuse to skip cardio.  

Does Cardio Actually Kill Gains? 

The answer is complicated: “Yes and no,” says Alex Rothstein, program coordinator of exercise science at New York Institute of Technology. This idea of doing both resistance training and aerobic training (cardio) is called “concurrent training,” and the research on the topic is inconclusive, he says.

In most cases, cardio shouldn’t prevent hypertrophy (muscle growth) nor cause you to lose the muscle you have. “This is a common misconception implying that lean muscle tissue is broken down by excessive cardio activities,” explains Samantha Coogan, a registered dietitian nutritionist and director of the didactic program in nutrition and dietetics at the University of Nevada, Las Vegas. 

The theory that cardio can impede strength gains—called “adaptation interference”—wasn’t just pulled out of thin air. The idea took root when a 1980 study compared leg strength gains after participants did either 10 weeks of concurrent training (cardio and leg resistance exercise performed in the same training program) or 10 weeks of resistance training alone. They found that strength improvements were greater in the group that didn’t do any cardio. “From that point on, the idea that aerobic exercise was counterproductive to achieving gains in muscle mass and/or strength took off,” Rosenkranz explains. Experts have continued to study the topic and pose a few different theories that could explain why adaptation interference might happen, though we still don’t have definitive proof it exists.

In fact, newer studies have shown quite the opposite could be true: cardio doesn’t inhibit muscle gain and may even help it. “In recent years, the body of research evidence indicates that doing concurrent training does not interfere with hypertrophy following resistance training,”  Rosenkranz says. “In fact, some evidence suggests that concurrent exercise may actually augment muscle growth in some circumstances.” 

Cardio can also result in gains of its own. “Aerobic activity helps increase blood flow, improves muscle endurance, and can help increase muscle protein synthesis,” Coogan says. (Muscle protein synthesis is the process by which skeletal muscle makes new proteins; it plays a crucial role in how your body responds to and adapts as a result of resistance training.) Research suggests that repeated, consistent aerobic activity enhances acute and chronic muscle protein synthesis in skeletal muscle and increases the size of muscles and muscle fibers. “Both aerobic activity and strength/resistance training play a role in muscle hypertrophy, and incorporating a healthy balance between the two is key,” Coogan says.

So, no, cardio likely isn’t negating your strength work, though there may be exceptions to these rules. After all, the results you get from any exercise will depend largely on how you train, along with many other factors. “The potential for ‘killing gains’ is going to be dependent on the amount of aerobic training as well as the intensity and other lifestyle and genetic factors,” Rosenkranz says. Someone who’s trying to gain muscle while marathon training, for example, may have very different results from someone who’s jogging a 5K twice a week.

For example, for highly specialized athletes (like powerlifters or sprinters), research suggests that doing aerobic and resistance exercise in the same session may negatively affect gains in explosive strength, Rosenkranz says. Still, “both types of training could still be done, even on the same day (one in the morning, and one in the early evening, for example; ideally more than six hours apart), just not during the same session,” she says. And for most other people, meeting the Office of Disease Prevention and Health Promotion’s physical activity guidelines for Americans (150 minutes per week of moderate-to-vigorous aerobic activity or 75 minutes of vigorous aerobic activity) is not likely to interfere with muscle mass or strength gains, particularly if the two types of workouts are done separately, Rosenkranz says. 

Why You Should Do Cardio and Weights

No matter your fitness goals, both cardio and resistance training are non-negotiables in your workout routine. “Cardio is essential for cardiorespiratory fitness and cardiometabolic function,” Coogan says. “Heart health is a key factor in overall health, and even more so in your ability to perform exercise, period.” Consider the recent groundbreaking study published in the British Journal of Sports Medicine that found that an increased cardio fitness level can reduce your risk of death from any cause by 11 to 17 percent.

There’s a reason the physical activity guidelines for Americans outline specific recommendations for aerobic physical activity, muscle-strengthening activities, and flexibility training: They’re all important for maintaining good health. “If the goal is just strength or muscle growth, it’s not a requirement to do cardio,” Rothstein says. “But if the goal is overall health and wellness, it is important to still include cardio as the cardiorespiratory systems are extremely important for a long and healthy life.” 

Not to mention, cardio may offer some mental health benefits that strength training doesn’t always deliver. “For me, cardio not only serves as something that keeps me physically healthy, but it’s also very good for my mental health. I run to be centered with myself,” says Peloton instructor Camila Ramón. “I can't say that I’ve felt the rush or the high of being in that meditative state from weight training. I think there's a different kind of power that you find in the mundanity of cardio workouts.”

Cardio and Strength Workouts: Does Order Matter for Muscle Gains?

It does, but it depends on your goals. In general, whichever workout is your priority is the one that should go first.

For example, if your goal is to gain muscle and you’re doing both cardio and strength training in one session, you’ll want to do weights before cardio. “This will allow you to be less fatigued, and you'll be able to put all of your energy and effort into the weight-training portion,” Camila says. The other experts agree. “If you start with heavy cardio, you could go into your strength training session fatigued and unfocused,” Coogan adds. “Fatiguing muscles during strength training could lead to poor form and injury.” 

Putting cardio before resistance training could also impact your performance. One systematic review showed that when cardio preceded resistance exercise in the same session, lower-body one-rep max performance was diminished compared with when resistance exercise came before cardio, Rosenkranz says. (Reminder: One-rep max, or 1RM, is the maximum amount of weight you can lift for one repetition.)

When does it make sense to do cardio before strength training? For starters, it’s A-OK to use light cardio as a warm-up before strength work. “Starting with light-intensity cardio can help warm up your muscles and reduce your risk for injury during strength training,” Coogan says. If your overall fitness goal is cardio-focused (think: training for a triathlon) or the goal for the given workout is cardio-based (i.e. it’s a cardio day, but you want to do some core work afterward), it can also make sense to do cardio first. 

In general, it’s OK to order your workouts either way, but ”you have to make sure that the aerobic training doesn’t interfere with the quality of the resistance training,” Rothstein says. TL;DR: Don’t tucker yourself out doing cardio first, especially if your goal is to gain strength or muscle.

How to Combine Cardio and Weightlifting for Maximum Gains

Repeat: Cardio probably won’t be the thing that stands between you and bigger biceps or glutes. Still, if you’re looking to minimize any chance of cardio eclipsing your strength gains, following these guidelines can help you maximize your chances of hypertrophic results.

Go high-intensity. “Sprint or high-intensity interval training (HIIT), for example, can lead to muscle hypertrophy,” Rosenkranz says, so higher-intensity training may be better than longer, slower endurance training for maximizing gains.

Consolidate your cardio. “Most recommendations suggest that it’s best to meet the aerobic exercise guidelines on fewer days as compared to more frequent bouts,” Rosenkranz says. “One specific recommendation suggests two (or at most three) days per week of short-duration aerobic training, done as interval cycling.” 

Space out different types of training. Doing your cardio workouts completely separately from your resistance ones can help reduce any risk of shortchanging your hard work. Rosenkranz recommends spacing them out by a minimum of three hours, though “some evidence says six hours would be better, and then 24 hours even better,” she says. “Those sorts of scheduling issues can be challenging, though, and it’s worth noting that evidence does not support the need to separate the workouts for most people.”

Make sure you’re challenging yourself. To see any gains in strength or muscle mass, you need to make sure you’re performing resistance training with enough intensity to challenge yourself, Rothstein adds. Progressive overload, a training principle that entails strategically increasing the difficulty of your workout, can help you continue to make progress and avoid plateaus.

When Cardio Can Hinder Your Muscle Gains

There are instances where cardio can play a role in hindering muscle gains, though this often isn’t a result of the cardio itself but a lack of adequate nutrition and recovery to support your training.

Overall training load, combined with under-fueling (i.e. not eating enough protein, carbs, or calories), can contribute to muscle loss and/or prevent you from achieving gains, Rosenkranz says. “When the training load is high, and therefore caloric expenditure is high (like with marathon training), the muscle damage incurred through both resistance and aerobic training is difficult to repair, and without that proper recovery, gains are less likely. This is also a situation that may lead to overtraining and injury.”

If you’re regularly performing both strength training and cardio and seeing little to no results, it could be a sign of undereating, Coogan says. “Underfueling both pre- and post-workout could potentially impact muscle growth and strength.” To facilitate muscle growth, you need a minimum of 1.2 grams of protein per kilogram of body weight, and the range can go up to 2 or even 2.5 g/kg of protein (the latter potentially being appropriate for collegiate or pro athletes). “Carbohydrates are also essential to restore muscle glycogen and help maintain weight, especially for endurance athletes,” Coogan says. Carbs are a primary fuel source during exercise, so it's critical to ensure athletes and active individuals aren’t restricting their carb intake, she adds. 

Why Recovery Is Key for Gaining Muscle 

It might sound counterintuitive, but the time you spend recovering from training can be just as important for muscle growth as the reps and sets you put in. “Rest days are necessary for muscle growth and repair,” Coogan says. “Without adequate rest, muscle tissue cannot recover and repair from the damage induced during strength training. While muscle damage is normal during a strength training session, the recovery phase is where that muscle has the most potential for growth.”

Two to three active rest days per week should suffice, Coogan says. This can include yoga, stretching, or walking, which will allow your muscles to recover while receiving blood flow to help them repair and optimize muscle growth.

Takeaway

What goes on at a microscopic level in your muscles is a result of much more than just your resistance training routine. For most people, cardio shouldn’t keep you from gaining strength and size as long as you prioritize resistance training and fuel the repair and creation of new muscle tissue with adequate nutrition and recovery.

And even if your main goal is to grow as big as the Hulk, don’t discount cardio for its life-changing benefits for your health. “You want to make sure that you're taking care of your body long-term, and the way that you will reap the most benefits is by doing different types of training,” Camila adds. So, sorry, you shouldn’t skip cardio. At least now you can be sure it’s worth your time—and not messing with your gains.