8 Cardio Myths That Can Cause Weight Gain

All cardio and no strength isn't just boring, it may cause you to burn fewer calories overall. "Strength training builds lean muscle mass, which both increases your metabolism and decreases fat," says celebrity trainer Elizabeth Hendrix Burwell, co-owner of High Performance Gym. "So the more muscle you build, the more calories you burn on a day-to-day basis." (Here's all the science you need to know about burning fat and building muscle.)

Some strength-training workouts can even double as cardio: A recent study by the American Council on Exercise found that kettlebell exercises can burn up to 20 calories a minute—the equivalent of running at a 6-minute mile pace! Maximize weight-loss benefits by incorporating up to four nonconsecutive days a week of resistance-based exercise such as kettlebells, TRX, and weightlifting. (Try this weekly strength-training workout plan for beginners.)

Should you start with cardio or strength training? "If you're hitting the treadmill for an intense cardio session and then plan to hit the weights afterward, you'll have little left in your tank to make your resistance training count," says Lindsay Vastola, a certified trainer and founder of Body Project Fitness and Lifestyle. When it comes to doing a full, high-intensity cardio session and an entire resistance training workout, perform each on separate days, Vastola says, so you can give each one your all and burn more calories in the process. (Here's more on how to structure your workout with cardio or strength first.)

Slogging away on the treadmill to hit some magic number is a waste of time and energy since machines can only roughly estimate your metabolic rate, Vastola says. Ignore the red digits on the console and focus on intensity instead. If you work harder in shorter bursts, you'll burn more calories even after your workout is over. Use a heart-rate monitor (aim to stay between 75 and 85 percent of your max heart rate) or the rate of perceived exertion scale of 1 to 10 (strive for an 8 or 9 on high-intensity intervals) to determine if you're working hard enough. (Try these strength-training moves that boost your metabolism, according to science.)

Your body does burn fat as fuel during lower-intensity workouts (when you're in the "fat-burning zone" of about 65 percent of your max). However, that's not necessarily what you need to focus on for weight loss. What counts the most is your overall calorie expenditure, not the fuel source. "The higher the intensity of your workout, the more total calories you will burn," says Marta Montenegro, a certified strength and conditioning coach and adjunct professor of exercise and sports sciences at Florida International University. That burn lasts up to 24 hours after your last rep or step, and studies show you'll shrink your belly fat faster, she adds.

But before you go switching all of your cardio sessions to high-intensity, maximum-effort training, remember that this type of exercise isn't without its risks, such as injury and overtraining fatigue. Montenegro recommends alternating between low- and high-intensity workouts to give your body proper time to recover and build consistency. For example, do your high-intensity interval training on Mondays and Thursdays, low- to moderate-intensity on Wednesdays and Saturdays, and mix in some yoga or strength on Tuesdays and Fridays. (Here's how to structure a perfect week of workouts.)

You can't drive a car without gas, so why expect something different from your body? The trouble with this theory is that the large muscles that power you through your cardio exercise rely heavily on a combination of carbs and fats for energy. When you run or bike on an empty stomach, your body will turn to the carb and fat fragments in your bloodstream and muscle stores, not to the fat in your fat cells to energize your workout, says Michele Olson, Ph.D., an adjunct professor of sports science at Huntingdon College in Montgomery, AL. This strategy could completely backfire, she adds, as you may become hyperglycemic and low on hydration, which can cause you to cut back on the intensity or stop before you've put in the 40 to 50 minutes research has shown is necessary for your body to burn fat.

Skip the pre-gym fast and show up ready to rock (and torch major calories) by fueling up about 90 minutes before your workout. Olson suggests something light and easy to digest, such as a small piece of fruit and half a cup of low-fat yogurt sprinkled with a couple tablespoons of granola, or try one of these pre-workout snacks. And be sure to wash it down with one or two full glasses of water.

There a ton of benefits to running a 5K or marathon—improved cardiovascular fitness, more stamina, working out for a good cause if you run for charity—but seeing the number on the scale go down isn't necessarily one of them. All the training you do to cross the finish line makes your body efficient at conserving energy so you can go the distance. And as you increase endurance, you'll gradually start burning fewer calories during your runs, says Jon-Erik Kawamoto, a certified personal trainer, strength coach, and former competitive runner. Great for your race, but the exact opposite of what you need to lose fat. Couple that with the common increase in appetite—and subsequent increased calorie intake—and some runners may in fact gain weight. (This is exactly why you might gain weight during marathon training.)

To meet your race goals and shed a few pounds in the process, supplement your running program with resistance training up to three times a week, focusing on equally working opposing muscle groups (such as your back and chest) and improving joint mobility and function to build strength and burn additional calories, Kawamoto says. He also suggests switching out one day of running for a cross-training cardio workout to help prevent injury and offer a new challenge to your cardiovascular system. And make sure your eating plan provides the nutrients your body needs without adding unnecessary calories.

Now to totally confuse you: While often it's beneficial to split up your sessions if both are at a killer intensity, there are times when fusing strength and cardio can be both efficient and effective. In one study, people who cycled for 20 minutes in the middle of a resistance workout saw a greater metabolic impact post-exercise than those who hopped on the bikes before or after lifting weights. "This means your calorie-burning metabolism will stay on fire after the exercise session has ended," Montenegro says. So next time you can't decide between cardio or strength, why not perform both? An easy way to do it is to use the treadmill as active rest between strength sets.

We wish! Not only do most of us (and the machines we work out on) overestimate how many calories we burn during our workouts, we underestimate how many calories we're eating too.

Exercise alone just isn’t effective enough to burn fat, says Bret Contreras, a certified strength and conditioning specialist. "A recent study suggests that the average obese person loses approximately 5 pounds of fat over the course of eight months through cardio or resistance training alone," he says. That's an awful lot of work for very minimal results, so don't forget the "calories in" side of the equation and follow a healthy diet that delivers the calories you need to eat to lose weight.

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