You don't always have to jump around doing cardio to burn major calories. Strength training builds metabolism-boosting muscle and burns even more calories after your workout your body recovers and repairs itself. This workout, from trainer Autumn Calabrese, builds on this principal by adding isometric holds to strength training moves. And it's challenging but quiet. So grab a pair of weights and get ready to build muscles in a whole new way (without disturbing your downstairs neighbors).