To become stronger and faster, athletes use a technique called interval training, in which they exercise very intensely, rest and then alternate intense bursts of exercise and rest until their muscles start to feel heavy. You can apply this principle to your own exercise program, but the rest period between intervals is not the same for every sport.

Rest periods are different for runners and weightlifters. Runners run intervals as fast as they can and recover enough to run the same fast pace several times. Research shows that runners need very short recoveries between intervals, usually only about 30 seconds. So a runner may run a quarter mile 12 times, averaging 1 minute, with a 110-yard slow jog between each. However, weight lifters need much longer recoveries between hard sets, at least two and a half minutes.

Runners become short of breath and feel a burning in their muscles when lactic acid starts to accumulate in muscles. It takes only a few seconds for a trained athlete to recover from hard running and clear lactic acid, so runners usually need short 30 second recoveries between each hard run. On the other hand, weight lifters feel a burning caused by a tearing of the muscle fibers and it can take a much longer time for their pain to disappear so they can lift very heavy weights again.