Flexing your muscles does burn some calories, and there's actually a name for this type of exercise: isometrics. However, isometrics alone aren't enough to cause significant weight loss. If you're serious about shedding fat, create a multi-pronged plan that includes a reduced-calorie diet, regular aerobic activity and strength-training exercises such as isometrics, weight training or calisthenics -- which include body-weight exercises like pushups, pull-ups, squats and crunches.
When you hold an arm, leg or other body part still while flexing the muscle, you are performing an isometric exercise. For example, you can perform an isometric chest press by clasping your hands together in front of you and pushing your palms together to flex your chest muscles. When you hold a yoga pose, you are also performing isometrics because your muscles are flexed as you hold still. In contrast, a barbell chest press is an isotonic exercise because you lengthen and shorten muscles by bending your joints as you flex.
The number of calories you burn while flexing depends on the muscles used and the duration of the exercise; however, flexing is unlikely to burn a large number of calories. Thirty minutes of Hatha yoga, based heavily on isometrics, burns roughly 150 calories if you weigh 155 pounds. A pound of fat is approximately 3,500 calories -- so all other things equal, it would take nearly two dozen sessions to lose a single pound. Every little bit can help though, and "Shape" magazine recommends flexing muscles at your desk for five minutes each hour to boost daily calorie burn.
For maximum calorie burning, engage in aerobic exercises most days of the week. Running is a good choice; at 155 pounds you burn about 300 calories in 30 minutes at 5 mph, or roughly 370 calories at 6 mph. Swimming the crawl stroke for 30 minutes, you burn about 400 calories at the same weight. Other efficient calorie-burning activities include bicycling, performing karate, jumping rope and playing racquetball.
Flexing may not be an efficient calorie-burning activity, but it does build some muscle strength. Doctors often recommend isometric exercises for patients in physical rehabilitation, according to MayoClinic.com. However, they also note that isometrics are not as effective for strength-building as lifting weights or performing other isotonic exercises. Isometrics can also be dangerous if you have heart disease or high blood pressure because the drastic shift in muscle tension causes your blood pressure to spike. See your doctor before starting any new exercise routine.