Lunges work key muscles in the butt, hips, and thighs, which are the ones you rely on for support in the missionary, standing, and kneeling positions. Stand with your feet hip-width apart and, with your back straight, take one long step forward, bending your front leg until your thigh is parallel to the floor. Your other knee should almost touch the floor. Push back to the original position and repeat with the other leg for one rep. Hays suggests three sets of eight to 12 reps, four times a week.