Step off the elliptical and head outside to crawl under obstacles, walk across a fence balance-beam style, or climb a wall and jump off the top of it during a parkour or "free running" workout that uses the environment to inspire "natural" movement. "Most strength coaches and top-level trainers now identify exercises in terms of the movement as opposed to the muscles involved," McCall says. "Plus many people find isolated exercises for a single muscle or body part somewhat boring, so they're drawn to programs like this that challenge the entire body." And when you work everything from head to toe, you burn major calories. Some companies, such as MovNat, specialize in this type of training, and even larger chain gyms are launching more nature-inspired workouts held indoors.