

What Happens To Your Body When You Only Do Cardio

Time to step off the treadmill and in to the weight room.

Cardio training
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We've all been there: You just finished your 30-minute sweat sesh on the treadmill and you know you should probably break up the pavement crushing with some free weights, but meh, you're just not in the mood. Not only can cardio be more fun than other types of exercise, you get that rush of endorphins that makes you feel uber accomplished.

Plus, cardio can give you loads of benefits: The Mayo Clinic found aerobic exercise can slow down the aging of certain cells in the body, and years of research highlights how cardio is good for weight loss and heart health. "Cardio is any exercise that works your heart and gets your heart rate up. The goal of cardio is to challenge your heart to work hard and get strong," says Michele Gordon, creator of Cardio Sweat Party in New York. 

But cardio-only movements mean you miss out on your body's other muscular needs. While you might not need to add variety to your routine each and every time you log your daily workout, sticking to cardio only can have serious effects on your bod. (The Slim, Sexy, Strong Workout DVD is the fast, flexible workout you've been waiting for!) Here's what happens:

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If you want to be strong enough to pull off a yoga headstand or pick up your niece and nephew as many times as they ask, you can't build your muscles with only cardio, according to a study published in the Journal of Applied Physiology. As Gordon says, "Creating a healthy workout plan that incorporates cardio, muscular, and flexibility training is important. If you skimp out on resistance training, then you will not build muscular strength and you will not boost your metabolism as high as it could be."

FYI: The Nationl Institute of Health recommends adults do at least two hours and 30 minutes of moderate-intensity cardio or an hour and fifteen minutes of vigorous-intensity cardio every week while adding in strength training at least two or more days a week.  

Related: I Got Rhabdo and It Nearly Destroyed My Body

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Over time, your body will get used to your routine and you'll likely burn fewer calories, according to a study conducted at the Department of Anthropology at Hunter College in New York. 

Gordon adds that by varying your routine with other types of movements you'll get closer to your goal. "The more muscle you have, the more energy your body expends. The more muscle you build from resistance training, the more fat you'll burn all day long," she notes. "Cardio alone will not have the same impact as it would if coupled with resistance training."

That's why pairing strength training (we're talking pushups, burpees, you name it) with short bursts of cardio will build a tougher body, according to a study conducted at the Department of Kinesiology at McMaster University. 

Related: The 18-Minute Fitness Routine That Will Totally Change Your Body

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Talk to any personal trainer (or ahem, your mom) and they'll remind you that even if you have some pretty serious fitness goals, taking time to recover and rest your body is essential for your health and stamina. Since cardio requires a lot out of your body, if you don't break up the constant motion with some rejuvenating yoga, you might be in pain... all the time. "When you overdo your cardio, [and] don't make the time to do other forms of exercise/recover, your body gets tired and stressed," Gordon says. She adds that if you're doing lots of running or dancing, you may start to develop overuse injuries, according to a study published in The Journal of Athletic Training. To rectify this, Gordon advises creating a strong cross-training program where you alternate cardio with other forms of exercise. 

David Jou, a physical therapist with MOTIVNY, suggests switching up your cardio routine with metabolic conditioning, which is like cardio but less repetitive. So on the days you’re eager to hit the pavement for a run but know your muscles needs something else, opt for a workout that incorporates movements like box jumps or burpees so you get the benefits of cardio and strength training without overdoing it in either direction. "Always give yourself rest days during the week, though," he says. "Train hard but recover harder." 

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Yikes—right? Jericho McMatthews, a N.A.S.M. and A.C.E.-certified personal trainer and co-creator of Core de Force says that losing those lean muscles that come from strength training puts more pressure on your body and over time, could cause some serious damage. "Cardio-only workouts will put you at higher risk for joint pain, breakdown, and injury since strength training is necessary for increasing and maintaining bone density as well as strong tendons and ligaments," she explains.

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