Beginning in forward lunge position, front knee on top of ankle, back knee reaching straight down toward the ground.  Add a weighted bar for added resistance.  Be sure this bar is not placed directly on your spine, but rather on the muscle across your back.  Push off the ground from the rear leg, until that leg reaches a position parallel to the ground, while your upper body maintains a forward motion, chin and chest up, supporting the weighted bar.  Challenge yourself by lowering back to the ground directly into the lunge, and repeating on the same leg.  Switch sides after completing a set to fatigue.