When you exercise, your heart pumps faster to deliver more oxygen-rich blood throughout your body. As workouts intensify, bodies require more oxygen. Maximum heart rate is the absolute fastest your heart can beat during exercise without harming your body. Subtract your age from 220 to pinpoint your max, or total number of times your heart can safely beat per minute. Still, while max heart rate represents your body's limit during exercise, keep your pulse lower than this, within your target heart-rate zone, or 60 to 80 percent of max, found by multiplying max rate by 0.6 and 0.8. Not because you'll burn muscle -- your heart rate has nothing to do with muscle wasting -- but to prevent heart and body injury.
When exercising, what you want to burn is fat. Unless you are an elite athlete, such as a cyclist who needs slighter upper-body muscles to decrease wind resistance, you do not want to burn muscle. Doing so slows metabolism and can negatively affect the body. "The more muscle you have, the more calories you burn, because skeletal muscles require more oxygen than body fat," says Anja Garcia, a registered nurse and an AFAA fitness instructor at DailyBurn.com. The healthiest way to slim body parts, she adds, is through high-intensity exercise, sufficient sleep and a high-protein, balanced diet. Pushing beyond 85 percent of maximum is not recommended. Beyond that, risk of heart and body injury rises while benefits only mildly increase.
There are a few situations in which the body burns muscle. "If you overtrain and for several consecutive workouts, your body isn't given time to recover through rest and proper nutrition, muscle burn can occur, or if you don't get enough protein or sleep," says Garcia. Bodies burn available fuel in a certain order. First, your body torches body fat and glycogen, or carbohydrates, stored in the liver and muscles simultaneously. When it is the only available fuel source left, the body will burn muscle. Weeks of energy are stored in our bodies from fat, though, so this is highly unlikely.
Every time you lift a weight, muscle breaks down. Your body repairs it with dietary protein and rest, says Garcia, so be sure to take off at least one day between weight-training sessions. You can enter a dangerous body state where your body breaks down too much muscle during high-intensity exercise, she adds, but this is extremely rare. The term rhabdomyolysis refers to the breakdown of muscle fibers that leads to the release of muscle fiber contents, or myoglobin, into the bloodstream, harming the kidney. Cardio does not burn muscle except very rarely and under extreme conditions, think marathon running, involving long-term strenuous exercise without rest or replenishment.
Stay on Target
To ensure your heart is beating safely within the target zone, of between 60 and 80 percent of max heart rate, the Cleveland Clinic recommends stopping exercise to do a 10-second pulse check. If your pulse is below target range, you can safely accelerate your workout. If your heart is beating beyond your target zone, slow it down a few notches. You'll gain the most exercise benefits and sustain the lowest risk of injury to your heart and body when exercising bull's-eye within your target zone. No need to overdo it. Build up gradually to exercising within target and never push your body beyond 85 percent of maximum heart rate.
Julie D. Andrews is a writer and editor living in New York City. Her articles have appeared in print or on the websites of "Prevention," "Glamour," "Fitness," "Shape," "Cosmopolitan Latina," "Elle" and "New York Magazine."