Short-fast, slow-easy, short-fast, slow-easy. In a nutshell, that's interval training.
It's one of the cornerstones of seven-time Tour de France winner Lance Armstrong's training regimen. You don't have to be a superstar for it to work wonders for you, too.
Interval training, originally called "fartlek" (Swedish for "speed play"), has evolved from bursts of unstructured speed punctuated by slow, rest intervals into a measured anaerobic and aerobic system that builds endurance, speed and strength.
Anaerobic bursts use glycogen stored in muscles. Additionally, the anaerobic interval deprives the muscles of oxygen and builds up lactic acid. The aerobic interval repays the oxygen debt and breaks down the lactic acid. During the payback stage, carbohydrates are converted into energy.
Over time, interval training leads the body to build new capillaries, so it's better able to deliver oxygen to muscles. Moreover, muscles can handle higher and higher levels of lactic acid and the heart grows stronger.
Additional benefits of interval training are increased endurance, fewer injuries and ability to increase intensity without burn out.
"Speedwork or short sprints can replace long slow distance workouts," Tom Seabourne, Better Bodz trainer, said. "Spend five minutes warming up at an easy pace, then gradually increase your intensity until you are moving at about seventy percent of your maximum speed. You may feel a slight burn in your legs. Your lungs may open up for the first time in years. Hold this pace for about a minute, then slow down to your normal tempo for two minutes. Increase your speed again to 70 percent for another leg exploding, lung expanding, minute. Cool down to a relaxed pace for another five."
According to the Mayo Clinic, interval training improves aerobic capacity.
"As your cardiovascular fitness improves, you'll be able to exercise longer or with more intensity," the Mayo Clinic said.
Interval training can be done on any aerobic apparatus, like a treadmill, jump rope, elliptical trainer, stairmaster or cycle. Always warm up thoroughly prior to interval training.