It all depends on what else you're doing, really. "Some of the main benefits of doing cardio include weight loss, stress release, and stronger heart and lungs," explains Brian Ripka, founder of Ripped Fitness. It's also increasing your endurance, according to Adam Smith, assistant coach of the Reebok Boston Track Club. Because it involves that uptick in your heart rate, it's easy to think it'll burn a lot of calories and burn away your muscles. Alternatively, you have to use your muscles in order to do cardio—so some believe that can lead to gains in muscle.
So what's what when it comes to how running affects the body? "Performing too much cardio could burn muscle if you aren't adding any type of strength training into your workout regimen or complementing your training with enough calories," says Ripka. "If you lose too much muscle, or don't do any type of strength training to gain any, you will lose strength." That's true for any kind of cardio, whether you're jumping rope vs running.
When you're maintaining or gaining muscle, on the other hand, it's all about incorporating strength training. "The best way to do this is with HIIT-style training about three days a week," says Ripka. "And for those who love a long run or a brisk walk, you can still do that on the other days and you will actually see an increase in your endurance. The HIIT-style workouts consist of short bursts of all-out effort, and the beauty of it is, you only need 20-30 minutes." Not only does this help to maintain muscle quality, it also helps to build your endurance.
And if you just can't quit your long runs, rejoice! Running, one of the OG forms of cardio, is something that can help you build muscle. "You're using a lot of aspects of your body when you're running," says Smith. "You're using your core, your legs, and your arms, for instance. These muscles get more toned and built. On runners, you'll see it in the legs, the calves, and the quads."
You'll see it first in the legs, but runners also get the additional perk of ab work. "Over time you can see muscle gains in the oblique," says Smith. "You're constantly torquing your body when you're running. Hips, legs, and the whole core itself is involved in the running process, so over time you'll see results really anywhere, from your legs all the way up to your arms." In other words, running doesn't burn muscles but builds them.
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