Cardiovascular exercise is any activity that gets your heart rate up and keeps it elevated for a certain period of time, usually 20 to 30 minutes or more. Cardio includes physical endeavors like running, brisk walking, cycling, working out on an elliptical machine. Sustained cardio workouts can be mind-numbingly boring because you do the same movements over and over without much variation. No doubt, cardiovascular exercise has substantial benefits for one of the most important muscles in the human body, your heart, but what about your other muscles? If you focus too much on cardio, are you likely to lose muscle mass?
For years, trainers have argued back and forth about cardiovascular exercise – whether it can trigger muscle loss. According to most evidence, whether cardio is catabolic, depends on how much cardio you do and how you feed your body. At the extreme end of the spectrum, doing hours of cardio each week and eating a low-calorie or low-carb diet could lead to loss of lean body tissue. Whether your body is forced to break down muscle tissue during cardio has to do with how well fueled you are when you train. Your body taps into muscle glycogen and fat as a fuel source during cardiovascular exercise. Whether it mainly uses glycogen or fat as fuel depends on exercise intensity. If you’re working out at a moderate, steady-state pace, it will use a higher ratio of fat to glycogen. However, if you ramp up the intensity, your body uses more carbohydrates, or glycogen, as fuel.
Normally, protein is not a significant fuel source during cardiovascular exercise, as long as you have enough stored glycogen available. Where you get into trouble is when you don’t consume enough calories and carbs. That’s when you start breaking down protein and sending the amino acids to your liver to be turned into fuel. Your liver converts amino acids to glucose that your brain and body can use by a process called gluconeogenesis. This is a signal that you’ve entered a catabolic state. Yet, this typically only happens when you drop your calorie intake too low.
Now, if you were to do a few hours of cardio a day, it might be enough to create a catabolic state. On the other hand, if you’re only doing a few hours of cardio a week and eating a nutritionally balanced diet with sufficient calories, your body has no need to break down muscle tissue. So, you can’t make a blanket statement about whether cardio is likely to lead to muscle loss. It depends – but under “typical” circumstances it shouldn’t.
Does Cardiovascular Exercise Interfere with Muscle Gains?
A second question is whether too much cardio can interfere with muscle and strength gains when you do them on the same day. This is referred to as the “interference effect” and is based on the idea that aerobic exercise causes different adaptations than strength training and the two when done in close time proximity may interfere with one another.
A study looked at this issue. Researchers asked healthy men to do a cardio workout on an exercise bike, but to pedal with a single leg. Six hours later, the guys did bilateral leg extensions. They did it this way so that both legs were strength trained but only a single leg worked aerobically. When they biopsied the muscles they had worked, the muscle cells had similar responses, suggesting that there was no interference effect.
Some studies, however, suggest that the type of aerobic training is a factor. For example, cycling is less likely to interfere with strength gains than is running. Another study found that strength training alone was slightly better than combined training for gains in strength. However, doing aerobic exercise on the same day as strength training leads to similar increases in V02 max. So, if there is interference, aerobic exercise slightly impedes strength gains but not vice versa. Combined training doesn’t interfere with gains in aerobic capacity.
The Fatigue Factor
One possible caveat is that doing a long cardio session could fatigue the leg muscles enough to cause the subsequent strength-training session to be less productive. You could offset some of this by fueling adequately before each workout so that you have adequate glycogen muscle stores. However, unless scheduling precludes doing strength and cardio on different days, separate the training by at least 24 hours. This gives the muscles you worked aerobically time to recover before working them against resistance.
It’s also important to consider your goals. Is your main objective to build muscle size and strength? Then, strength training should take priority. That doesn’t mean you shouldn’t do cardio – your heart muscle is too important for that. Scale back the length of your cardio workouts and increase the intensity. With high-intensity interval training, workouts are shorter, yet you’re still getting aerobic and anaerobic conditioning.
If you must do both forms of training during the same session, focus on strength first while your muscles are rested, so you can maximize your lifts, especially if strength-training is a priority for you. On days that you have to train aerobically on the same day you lift, strength train your upper body and save lower body resistance exercises for days you’re not doing aerobics. You mainly use your lower body muscles when you work out aerobically and that shouldn’t interfere with upper body strength training. Also, keep in mind, you don’t have to do cardio every day to get the benefits.
The Bottom Line
Long periods of aerobic exercise without adequate calorie and nutritional support can lead to muscle loss due to the catabolic effect of unopposed aerobic activity. That’s why calorie restriction and long cardio sessions aren’t recommended. Strength training, being an anabolic activity, helps counteract muscle loss. If there’s an interference effect, in the setting of adequate calories and nutrition, it’s not as pronounced as many fitness gurus claim. Ideally, separate strength training and cardio by 24 hours and keep your cardio sessions short if strength and muscle gains are top priority.
Men’s Health. “Will Cardio Burn Muscle?”
Men’s Fitness. “Can Cardio and Weights Go Together?”
Skeletal Muscle Hypertrophy with Concurrent Exercise Training: Contrary Evidence for an Interference Effect. Murach KA, Bagley JR.Sports Med. 2016 Mar 1.