The muscles at the rear of your thighs have a key strength-providing role in almost every movement you make, from jumping to simply bending your knees. They also work in tandem with the quads, contracting as they lengthen to steady your forward momentum. Because of this, they need to be strong and supple, which means training them dynamically from different angles with enough intensity to build new muscle. This hamstring-specific workout on the right will do exactly that.