What is fartlek training, and why is it such a great way to condition for athletes? Fartlek is a Swedish word meaning "speed play," and that's essentially what this type of workout is built upon. (See alsoÂ STACK Science: Developing Slow- and Fast-Twitch Muscle Fibers.)

Sound familiar? That's because fartlek training is more commonly known as interval training, in which you alternate periods of high-intensity exercise with shorter periods of low-intensity work. This mimics the start-and-stop rhythm needed to dominate in most sports. Plus, the formula is key for losing fat, because it keeps the body burning calories long after the workout ends.

In general, fartlek training should be performed once or twice a week, alternating with other types of activity during the rest of the week. A fartlek training session can be made easier by reducing the number of intervals or the total time of the workoutâor by reducing the duration of the high intensity periods and increasing the low intensity periods.