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. Intervals are a fixed number of repeats of a fixed distance at a fixed pace with a fixed recovery time. There are two types of intervals: long and short. A short interval takes less than 30 seconds and does not build up significant amounts of lactic acid in the bloodstream, so an athlete can do lots of repeat short intervals in a single workout.

Long intervals take two minutes or more and are very tiring. In interval training, a runner may run a quarter mile 12 times, averaging 1 minute, with a 110-yard slow jog between each. A weightlifter may lift a heavy weight ten times in a row and then repeat another set of ten. Runners run intervals as fast as they can and recover enough to run the same fast pace several times. Runners need very short recoveries between intervals, usually only about 30 seconds; but weight lifters need much longer recoveries, 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, but it takes only a few seconds for a trained athlete to recover between each hard run. On the other hand, weight lifters feel burning caused by tearing of the muscle fibers and it takes a much longer time for the pain to disappear so they can lift very heavy weights again.

You can apply the concept of interval training to your program at any level of fitness. When you start a new exercise program, exercise for 30 seconds, stop for 30-60 seconds, longer if you need it. Alternate exercising and resting until you feel tired or your muscles feel heavy. Then stop for the day. The stronger you get in your sport, the more intense your intervals can become. You work at your maximum capacity for 30-60 seconds, then take 60-90 seconds to recover, then go very hard for another 30-60 seconds. Do this vigorous interval workout once a week until you get tired. At first you may only be able to do two or three intervals, but your muscles get stronger and you build up the number of intervals you can complete. Go easy the next day or take a day off if you feel any discomfort.