What is the best exercise?

Fartlek (speed-play) training and interval training take advantage of the body being able to temporarily exceed the anaerobic threshold, and then recover (reduce blood-lactate) while operating at below the threshold, but still doing physical activity. Fartlek and interval training are similar, the main difference being the relative intensities of the exercise, best illustrated in a real-world example: Fartlek training would involve constantly running, for a period time running just above the anaerobic threshold, and then running at just below it, while interval training would be running quite high above the anaerobic threshold, but then slowing to a walk during the rest periods.