Here’s why the common fear of gym-goers hoping to bulk up is largely a myth.
When he became the fastest American marathon runner with a 2:04:58 at the 2011 Boston Marathon, Ryan Hall weighed around 130 pounds and was as lean as a whippet. Now, Hall is nearing 200 and most of that new weight is muscle.
What changed? Hall retired from professional running in 2016. The short version of Hall’s story seems to confirm an oft-asked belief that cardio kills gains. But experts say it’s more complicated than that.
Casual running has been the beacon of weight-loss exercise since the jogging boom of the ’70s so it makes sense that it could be counterproductive if your goal is bulking up. Scientific studies—like this classic (and extremely old) one—seem to back up the idea. These don’t paint a complete picture though; if they did, researchers and trainers wouldn’t still be asking the question.
After sifting through newer studies and three experts, we waded through the nuance so you don’t have to. Hall, by the way, still does cardio. In fact, he recommends it (and he can still run a five-minute mile).
Tom Oddo, D.C., C.S.C.S., C.E.A.S. is the owner of City Integrative Rehabilitation in New York City.
Jenna Stangland, M.S., R.D.N., C.S.S.D., L.D.N., C.L.T. is a registered dietitian and specialist in sports dietetics.
Nichele Cihlar is a strength and conditioning coach and the director of training at GORUCK.
The short answer is no. The experts we spoke with agree that cardio doesn’t interfere with muscle growth in nearly all cases. Even in the exceptional cases that it does, strength training and cardio can dovetail nicely into a complete fitness program with proper nutrition and workout phasing (when you divide your workouts into mini three- to four-week cycles that have different goals).
Building cardiovascular capacity “will allow you to train longer and recover better,” says Nichele Cihlar, a strength and conditioning coach and director of training at GORUCK. Tom Oddo, D.C., C.S.C.S., C.E.A.S. agrees: “Moderate amounts of cardio actually increase your load tolerance for strength training.”
There’s scientific evidence to support this: A 2022 meta-analysis found that “concurrent aerobic and strength training does not compromise muscle hypertrophy and maximal strength development (1).” In other words, decent training volumes don’t appear to blunt your ability to build muscle—even when cardio and strength training are performed on the same day.
And if you aren’t seriously pushing it in the gym—i.e. hitting moderate volumes of both cardio and strength with adequate rest—concurrent exercise may even boost muscle growth (2).
Of course, any question involving the way the body works has a short answer, and a long, more nuanced answer. Dr. Oddo and Cihlar both make the caveat that a high load of cardio training can affect muscle gains. However, in most of the studies that reported these results, aerobic exercise was implemented at an intense and frequent level.
Another exception: If you’re training for power. Performing strength training and cardio in the same session might attenuate explosive strength gains (1). But simply separating them into two different sessions will clear up any issues.
If cardio doesn’t usually kill gains, why are people still worried that it will? Two reasons:
The basis behind the concern here is that cardio and weightlifting have opposing effects on the body. Resistance training promotes muscle hypertrophy via the mTOR (mammalian target of rapamycin) pathway, explains Oddo. Endurance training, on the other hand, works via the AMPK (adenosine monophosphate-activated protein kinase) pathway.
“These pathways can inhibit each other if not balanced properly, such that excessive cardio may impair muscle growth,” he says. Put simply, activating one dampens or turns off the other. This reciprocal relationship leads to the Chronic Interference Hypothesis, which states that resistance training builds muscle while aerobic exercise breaks it down.
“Chronic Interference Hypothesis is probably only relevant for highly trained athletes or bodybuilders with specific training goals,” says Oddo. That’s because these types of athletes are training at a very high frequency and high intensities, much more than the average gym-goer. Still, if you do fall into this category, separating the sessions—even by a few hours—can make a significant difference.
Another far more obvious effect of combining cardio and strength training is that we all operate with a limited amount of energy to give to any one workout. No matter how fit you are, it’s harder to do an all-out leg workout after a run than it is on fresh legs, and vice-versa.
That’s why being strategic in combining the two based on your preferred goal (cardio or strength) or the muscles being used is critical. For example, doing a run first may not impact an upper body lifting day as much as a leg day.
We wouldn’t be considering cardio if there weren’t clear benefits to doing it. Cardio is great for overall health and well-being, which can benefit your weightlifting regimen on its own—but there are a few specific reasons to prioritize it.
VO2 max—the maximum amount of oxygen your body can utilize during exercise—is a measure of how hard you can push yourself in and out of the gym. Cardio is the gold standard for increasing VO2 max because it pushes your heart rate into higher intensity zones (3).
Having a higher VO2 Max isn’t just good for more cardio or longevity, it’s good for strength work. “A high VO2 max allows for longer and more intense strength training workout sessions before reaching exhaustion,” says Dr. Oddo.
According to Dr. Oddo, cardio training increases mitochondrial density in muscle cells. This improves their ability to produce ATP (AKA the energy currency of your cells), theoretically improving recovery and allowing you to lift harder, for longer.
Cardio increases capillary density in muscle tissue, which improves the delivery of oxygen and nutrients and the removal of metabolic waste. Boosting blood flow also supports sustained muscle performance and faster recovery.
Improving your cardiovascular fitness will make you better positioned for whatever your other fitness goals are. Cardio increases the efficiency of the heart and lungs, notes Dr. Oddo, which helps with oxygen and nutrient transport during other activities. Improved blood flow also helps when doing high-rep sets and improves recovery, and improved recovery allows for higher training volumes.
Successfully combining cardio and resistance training without killing your gains doesn’t have to be complicated. Here’s what you should be thinking about.
Practically speaking, the easiest way to avoid the interference effect is to manage your training appropriately and limit high-volume cardio work, like hour-plus runs and bike rides. Aim for a moderate intensity and frequency—30 minutes two or three times a week, according to both Cihlar and Oddo. Oddo says not to rule out HIIT workouts either, as they can help improve cardiovascular capacity while supporting anaerobic power and muscular endurance.
It’s almost universal advice that if you’re going to do both cardio and strength in a single session, you should do strength first, cardio second. “This way, you are not pre-fatigued for your lifting session, which allows for heavier lifts and fosters muscle growth,” says Cihlar.
“Performing cardio before weight training can deplete glycogen stores and reduce performance in subsequent weight training sessions,” says Dr. Oddo. This allows maximum effort and energy to go toward lifting. Unless your main goal is cardio-based—then cardio should take precedence.
If you’re worried about meeting the post-workout nutrition window, don’t be. If you’re keeping cardio moderate, your adequate total daily intake is most important—assuming you’ve eaten something a couple hours before training, and plan to eat within an hour or two after.
Running might be the most accessible form of cardiovascular exercise out there, but it involves more impact on the joints and muscles than other types of exercise. The additional joint and muscle strain has the potential to take away from your resistance training (similarly to how doing cardio before strength can) though this is likely not an issue for seasoned runners.
Walking is lower impact, and if you’re on a treadmill you can turn up the incline to dial up the intensity. Rowing, swimming, and cycling are other low-impact options. Cihlar also recommends rucking—or walking with a weighted backpack—which elevates the heart rate without the impact of running.
“Periodizing cardio and resistance training can maximize both benefits without significant interference,” Dr. Oddo says. You might set yourself up with a conditioning phase that prioritizes cardiovascular fitness and then adjust to a strength phase, for instance. A cardio-focused phase doesn’t necessarily mean no strength training, just less volume (and vice-versa).
This can help mitigate interference effects, particularly if you’re training with highly specific goals and high training loads (like marathon training). In this case, careful programming is important.
Calories are required to support muscle growth and repair, plain and simple. “When you add cardio on top of a current resistance training program, there is going to be an even higher demand for calories,” says Stangland. If supply doesn’t meet demand, catabolism AKA muscle tissue breakdown can occur.
It’s not just about adding more calories though; cardio calls for carbs, Stangland says. Carbs are converted into muscle glycogen that’s used to fuel workouts, and it’s important to refill your body’s stores of the stuff to avoid muscle breakdown and be ready for future workouts.
Getting enough carbs throughout the day is important; but, whether you need to fuel with carbs during your workout depends on the length and intensity of the workout. For brief cardio (30 minutes) you probably don’t need to supplement with carbs during your workout, unless it’s after a long strength session and you’re tired.
For longer, more intense cardio over an hour, Stangland says that carb needs can be anywhere from 30 grams to over 60 grams per hour of cardio activity. Extended and intense endurance workouts call for even more—up to 90 grams per hour (4).
Foods with healthy fats are typically good at helping you meet calorie needs, too. “Fat is also an important macronutrient for building muscle because it supports hormone production and certain hormones fuel muscle growth,” explains Stangland. However, it’s slow to digest, so it’s best to avoid it right before a workout (if you do eat a high-fat meal, give yourself an hour and a half to two hours to digest).
Protein is essential for strengthening, building, and repairing muscle (not to mention other body tissues like bone, cartilage, and hair). To max out muscle gains and maintenance, aim for 1.4 to 2 grams of protein per kilogram of body weight per day (if you weigh 160 pounds that’s around 100 to 145 grams) (5). On a per-meal basis, shoot for around 0.4 grams per kilogram of body weight spread across four meals (6).
“Nutrient timing is also essential post-workout,” explains Stangland. “Because refueling is essential to muscle building, this applies to post-strength training and post-cardio.” The optimal carbohydrate-to-protein ratio for a post-workout meal is 3:1, she says. For example: 60 grams of carbohydrates and 20 grams of protein. That might look like one cup of plain Greek yogurt with a half cup of granola and one and a half cups of strawberries.
If you’re trying to be mindful of overall caloric intake, make your post-workout meal a part of a regular planned meal or snack. For weight or muscle gain, you’ll probably want to add this in addition to your typical meals or snacks.
We all know that cardio is sweaty business. This makes dehydration more of a risk when combining the two disciplines. “Dehydration can cause muscle cramps, muscle fatigue, and a decline in muscle performance,” says Stangland. Adjust your water intake accordingly, and consider using an electrolyte powder to balance fluid levels.
You’ve got questions about how cardio impacts your gains, we’ve got answers.
In most cases, no. Fasted cardio might risk muscle breakdown “if done excessively or without proper nutrition,” says Dr. Oddo. The idea behind fasted cardio is that it can help burn excess fat. But, one study involving two groups, one who ate before a workout and one who ate after, showed no difference in outcome (7).
If there’s no impact on fat loss and a small risk of muscle loss, why do it? (Unless you’re an early riser who can’t stomach a meal before a workout.) On the flip side, since you can work out harder with some fuel on board, it might be worth eating or drinking a little before your workout to put up a better effort.
It depends on your definition of high-intensity, and the type of exercise you’re doing. HIIT workouts are high-intensity, but they can actually drive muscle growth (8). Extended cardio workouts like long runs and bike rides that don’t involve resistance can lead to body mass loss, but when combined with resistance training may also increase gains (9). Additionally, cardio has been shown to prevent age-related muscle loss (10).
Only for those participating in high-intensity, extended training bouts, such as hour-and-a-half to two-hour long marathon training runs. A study involving ultramarathon runners participating in a 50k race showed a notable decrease in body mass before and after the race (11). Another showed that long-distance running leads to muscle damage (12). For those running shorter distances, and even those who do so at higher intensities, running might actually help you build muscle (8). The key is fueling properly before and after training.
The experts we consulted on the topic both recommended moderate cardio, 20- to 30-minute sessions two or three times per week, to avoid interference with muscle growth. If your cardio needs exceed this—if you’re training for a half marathon, say—interference is more of a risk but can still be managed through nutrition and careful workout planning. A realistic solution might be to shift your goals for the training period and return to a focus on hypertrophy after the race.
“Cardio can increase caloric expenditure, making maintaining the caloric surplus necessary for muscle growth harder,” says Dr. Oddo. “However, moderate cardio can improve cardiovascular health and aid recovery without significantly hindering muscle gains.” You can always eat more; those few hundred stationary bike or treadmill calories are worth the extra effort.
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