I have for you another blast for the heart rate, another HIIT exercise to put in the middle of a circuit for that full-body fat blast! Welcome to the one-legged mountain climber. The mountain climber performed with two legs is a fantastic little exercise to kick the heart rate up, to warm up all the muscles in the body and to activate the core. This one literally gives the mountain climber a leg up! Lift a leg, and it places far more demands on the upper body, core, and the leg and butt of the "climbing" leg.