Get down on your hands and knees, wrists directly under shoulders and knees under hips. (If you have any wrist discomfort, place a rolled towel under your hands.) Inhale as you arch your back and tilt your tailbone up, then exhale as you tuck your tailbone under and round your back. Draw your abs in and contract your pelvic-floor muscles [shown], doing a Kegel by squeezing the vaginal muscles as if to stop the flow of urine. Repeat for 8 to 10 reps. Sit back on your heels between reps to rest if needed.