If you see ads for equipment that promises to make you fit by moving your muscles for you, save your money. Exercise strengthens your heart only when you do it vigorously enough to increase your heart rate at least 20 beats a minute above resting. A study done in 1998 showed why passive exercise won’t make you fit. Men were asked to sit on a stationary bicycle with their feet on motor-driven pedals. When their feet were driven slowly, their hearts did not beat faster, but when their feet were driven at a high speed, their hearts did beat faster (Medicine and Science in Sports and Exercise, February 1998). However, this did not strengthen their hearts because the faster heart rate produced by the motor was due to hormones such as adrenalin, rather than increased circulation of blood. Only increased circulation of blood strengthens your heart. To become fit, you must move your own muscles. Machines that move your legs or arms for you can be helpful in physical therapy, but they will not make you fit.

Devices that send electric current to stimulate your muscles do not work, either. Several years ago the U.S. Federal Trade Commission charged marketers of widely advertised electronic abdominal exercise machinies with false advertising. The promoters claimed that that their devices would cause fat loss and inch loss, give users well-defined abdominal muscles, and were equivalent or superior to ordinary abdominal exercises, such as sit-ups or crunches. All of these statements were untrue. Your brain does send an electrical impulse along nerves that enter muscles to cause them to contract, but the electrical impulses generated by ab machines are so weak that they can't possibly cause contractions that strengthen muscles significantly. If they did give you enough electricity to strengthen your muscles, they would give you a very painful shock.

You CAN strengthen your abdominal muscles with devices that help you exercise them against resistance, but even this will not get rid of belly fat. When you take in more calories than your body burns, you store the extra calories as fat . Some people store fat primarily in their hips, while others store their fat primarily in their bellies. More than half of the fat in your body is stored underneath your skin and over your muscles. Exercising a specific muscle does not get rid of fat over that muscle. If it did, tennis players would have less fat in their tennis arms, but they don't. Ab exercises can strengthen weak belly muscles, but they will not remove extra fat from your belly. People who store their excess fat primarily in the belly are at increased risk for heart attacks and diabetes. You get rid of belly fat with any kind of vigorous exercise and a sensible weight loss diet.