Bridges work your butt and lower back and stretch your abdominal muscles. as an exercise they are a great way to improve posture and stretch the abs a little.
What's involved: Glutes, quads, lower back.
Powered by: Hips and glutes (primarily)
Tip: When pressed for time go for version 2. It pushes your hips higher and works your glutes harder so you get a better workout in less time plus it stretches your abs more.