If you're capable of doing these without cheating, they can be one of the most metabolically torturous leg builders in the gym. Do these in a non-alternating fashion and make them extra challenging. When you feel your quads start to spasm, when the sound of your heart pounding at maximal rate is muffling the sound of your breath, go up 5 pounds in each hand. Most Damaging: Repeated High-Impact Plyometrics The sheer impact being sustained by the knees, hips, and spine from burpees, box jumps, and the like is downright outrageous. Sure, box jumps can be a great way to activate some fast twitch muscle fibers before heavy squats or pulls, but don't get confused – this type of intelligent, goal-oriented programming isn't the problem. The problem is flailing up and down a 24-inch box for 15 consecutive minutes while sustaining contusions to your shins and thinking you've done something productive. Movements that involve a huge amount of external force production need to be programmed with caution, which is rarely, if ever, taken into account. Any time you're depending on your muscular system to generate high amounts of force and tension to complete a physical task, you'd better respect the process.