A balanced workout program should include some form of cardio, even if your main objective is to build muscle size and definition. The most common form of cardiovascular exercise still today is moderate-intensity cardio, like jogging and cycling for at least 30 minutes. Despite its popularity, moderate-intensity cardio is not necessarily the best type for improving your aerobic capacity, especially if you’re trying to build muscle strength and size. In fact, unless you’re exercising for stress relief, high-intensity interval training, or HIIT, is better for accomplishing a variety of fitness goals.
Cardio and Weight Training: Are They Incompatible?
For years now, there’s been a debate about “concurrent training,” whether or not aerobic exercise interferes with the adaptations to strength training. What most research shows is the degree of interference depends on the type of training you’re doing. One study found that running interferes more with muscle strength gains than cycling does. How frequently you train and for how long are other factors that determine the degree of interference you get. Longer periods of moderate-intensity cardio seem to impede gains in strength and muscle size more than shorter periods of high-intensity cardio.
It’s not surprising that long periods of cardio interfere with the ability to build strength and muscle size. The adaptations to the two forms of training are different. Sustained cardio burns calories and taps into a significant amount of stored energy. As energy stores dwindle, the activity of a molecule known as AMPK increases. Think of AMPK as your body’s energy sensor. When the amount of ATP in your cells drops and your glycogen stores are depleted, AMPK is turned on. There are some benefits to turning on AMPK. For one, it promotes fat loss. It also improves insulin sensitivity, which is good for your metabolic health.
The downside is AMPK also blocks the activity of another important molecule called mTOR (mammalian target of rapamycin). That’s a problem if you’re trying to build muscle size since mTOR is an important stimulus for the synthesis of new muscle proteins. It turns on the proper pathways for new proteins to be built.
Long periods of cardio are only one stimulus that activates AMPK and blocks the activity of mTOR. Any type of physical stress, including excessive calorie or nutrient restriction, does the same. It makes sense, doesn’t it? When you’re well-fed and energy supplies are high, your body can shift some of its energy to building new muscle proteins. When energy is in short supply, due to too few calories or too much exercise, mTOR switches off to conserve energy.
Based on this, it’s easy to see how spending lots of time doing moderate-intensity cardio interferes with muscle gains. Plus, if you’re spending a significant amount of time doing aerobic exercise, you’re depleting the energy reserves that you have available for resistance training. How hard can you push yourself lifting if you’re exhausted from doing an hour of cardio?
Increase the Intensity and Shorten the Duration
What’s the solution? High-intensity interval training, because it’s short, and because it increases the release of hormones like growth hormone and testosterone that help build muscle, doesn’t block the mTOR pathway in the same way. Instead, it creates a more anabolic environment that promotes muscle growth.
Of course, if you do high-intensity exercise too often, you can exhaust yourself and make it harder to build muscle. That’s why it’s best to limit HIIT training to twice a week if your focus is to build muscle strength and size. You can also add a moderate-intensity session of no more than 30 minutes to the mix if you enjoy it. Regardless of what form of cardio you’re doing, HIIT or moderate-intensity, make sure you’re consuming enough calories and protein, to keep your body out of an energy deficient state.
It’s also best to separate your cardio and weight training by 24 hours to avoid the negative effects of concurrent training. You could do cardio in the morning and lift weights in the evening or do cardio and weight training on different days. It’s especially important to separate your lower body workout from your cardio sessions, by at least 24 hours, since you’re using the same muscles as you use for cardio.
How about Fat Loss
You might argue that moderate-intensity exercise is still better for fat loss since you’re burning calories for a longer period of time. Not necessarily. High-intensity interval training, despite its shorter duration, increases the levels of fat-burning hormones to a greater degree. It also upsets your body’s equilibrium so that it needs to expend more energy to recover. We know this as the after-burn effect. You’re essentially burning more calories AFTER a HIIT workout than after a moderate-intensity cardio session.
Another reason moderate-intensity exercise may NOT be better for fat loss has to do with AMPK. Remember how AMPK goes up in response to low energy stores? AMPK also serves another purpose, it tells your body that you’re energy depleted and needs to eat. In response, your appetite increases. One reason some people don’t lose weight, or even gain weight when they exercise is that they “out eat” their workout. As their energy reserves fall, AMPK gives them the munchies and they eat more to compensate.
Moderate versus High-Intensity Exercise for Improving Aerobic Capacity
There’s another reason to add more HIIT sessions to your routine. A 2008 study found that gains in aerobic capacity (V02 max) were greater with high-intensity exercise than with moderate, sustained aerobic exercise. Other studies have come to a similar conclusion. Vigorous exercise places more demands on your heart and forces it to adapt more than moderate intensity exercise. It’s the intensity that counts that matters most in terms of fitness gains, not exercise duration.
The Bottom Line
You need cardio but moderate-intensity cardio, especially when you do it frequently or for long periods of time, may interfere with strength and muscle gains. Keep your cardio sessions short and vigorous. If you do moderate-intensity cardio, don’t do it on the same day strength train, unless you’re doing circuit training with lighter weights. If you devote a day to moderate-intensity cardio, make sure your body has enough protein and calories to keep your body from becoming catabolic.
References:
Med Sci Sports Exerc. 2008 Jul;40(7):1336-43. doi: 10.1249/MSS.0b013e31816c4839.
J Strength Cond Res. 2012 Aug;26(8):2293-307. doi: 10.1519/JSC.0b013e31823a3e2d.
Dis Model Mech. 2015 Sep 1; 8(9): 1059–1069.
J Biol Chem (2004) 279: 12005-12008.
FitnessRX for Men. “High-Intensity vs. Low-Intensity Cardio”
Scandinavian Journal of Medicine & Science in Sports. 20(2): 39-47, 2010.
Med Sci Sports Exerc, 1990. 22(3): p. 348-56.
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