Speed play (aka Fartlek) — Fartlek, commonly known in the U.S. as interval training, is the Swedish word for “speed play.” Interval training or speed play is a great way to pump up your metabolism and make your workouts more fun. Interval training alternates short bursts of intense activity with lower intensity activity. For example; alternating sprints with a slow jog, or powering up a hill followed by an easy downhill lope. Interval training teaches the heart and muscles to use oxygen more efficiently. A few studies have found that interval training alters the mitochondria (the enginelike organelles that produce energy in cells) to burn more fat. You will likely burn more calories, increase your metabolic rate and your level of fitness at a faster rate than with your old familiar workout.