Five hundred is a lot of pushups, enough that even elite athletes would consider it a brag-worthy accomplishment. Exactly what you gain from it, and what risks go along with those benefits, is more complex than the simple, macho sense of achievement you would get from the practice.
To do one pushup, start by lying flat on the ground, face down. Place you palms at shoulder level, just outside shoulder width. Push up on your palms with only the pads of your toes also touching ground. Keep your torso straight and your eyes pointed toward the ground. Keep pressing until your arms are straight, but not locked, then return until your body is just an inch off the ground.
A pushup is a body-weight resistance exercise, meaning it builds muscle strength and tone by using your own body weight to work your muscles. Although mostly associated with arm and chest strength, proper form in a pushup works many other muscle groups. Oregon-based fitness coach Ben Cohn says it works the buttocks, calves, lower back, abdomen and even the neck. This is especially true if you do 500 pushups, as you will engage the peripheral muscles more as you fatigue.
As a resistance exercise, pushups don't burn calories as efficiently as cardiovascular exercise. However, 500 push-ups can accumulate an impressive amount of caloric burn. At 50 pushups per minute, a reasonable pace for people in capable of doing 500, a 150-pound person would burn about 100 calories per session. Because heavier people burn more calories with pushups, a 200-pound person would burn 110 doing 500 pushups.
According to Walter Willett in "Eat, Drink and Be Healthy," the more muscular a body is, the more calories it burns in every activity. This means that if you do 500 pushups regularly, the change in your muscle mass will burn more calories all the time, even while you're sleeping or watching television.
The benefits of doing extreme amounts of pushups are not without risks. You're not in much danger of serious injury; even if your muscles fail completely, you won't fall that far. However, people who do more pushups than their body is ready for are at risk of repetitive stress injuries to their elbows and shoulders, or muscle strains in the back and abdomen. If you feel pain in either area, scale back your training.