Nearly every athletic movement originates in the core or has to transfer through it. Strength here delivers coordination, grace, and resistance to injury and back pain. Use slow, steady movements, focusing on form. Do the following routine circuit style, one exercise after the other, with no rest between sets. Do three sets of 20 reps each for crunches, pelvic tilts, and knee touches, one 15-rep set for swimmers, and one 30-second hold for each plank. Work through the list three times. As your conditioning improves, increase the reps by five and the planks by five seconds. Approximate time: 10 minutes.

Crunches
Lie flat on the floor with your legs bent and feet flat. Clasp your hands behind your head and roll forward from the waist, bringing your head toward your knees but keeping your lower back on the floor. To keep your abs engaged, concentrate on pressing your belly button toward the floor.

Pelvic Tilts
Start as if doing crunches, but place your arms on the floor, parallel to your torso. Press down with your hands, feet, and shoulders while lifting your hips up until your body is in a straight line from knees to shoulders.

Knee Touches
Start as if doing crunches, but place your hands on your thighs, so that your shoulders are off the floor. Slide your hands along your thighs by rolling your upper body forward, until your hands touch your knees. Don't let your shoulders touch the floor between reps.

Swimmers
Lie facedown and extend your hands in front of your head—think Superman pose. Arch your back so that your legs, chest, and arms are all off the floor. From this position, raise your left arm and your right leg simultaneously, as high as you can. Lower them and do the same with the opposite extremities.

Side Plank
Get on your side with your elbow below your shoulder, forearm perpendicular to your body. Place your top foot on or just in front of the other and keep your body completely straight. Lift your upper body so that only your feet and forearm are touching the floor, and hold. Do 30 seconds on each side.

Prone Plank
Place your forearms on the floor, parallel to your torso and with your elbows under your chest. Lift into plank position—body completely straight and only your feet and forearms touching the floor. Keep your head aligned by looking down at the floor. Hold for 30 seconds.