If your legs aren't sore a day or two after this workout, you're doing something wrong. Use a box, stair step or sturdy chair if a park bench is not convenient. This workout targets your glutes, ...

(a) Lie face up on a bench (or across a few chairs if you're at home) with your back and bum supported, legs extended off the end and hands gripping the bench behind your head. (b) Lift your legs up ...