Brutal hamstring exercise [video].

This will punch your hamstrings right in the mouth. Set up with both knees bent, feet flat on the ground with a slider under one of your feet. Lift the foot that is not on the slider off the floor. Perform a single-leg bridge with the other leg. Once at the top of the motion, straighten the knee by sliding the foot away from the body. When the knee is extended, keep the hips on the floor, bend the knee and start again with the single-leg bridge.