20-Minute Fitness for Intermediate Exercisers: Workout 2 Cardio Ride a stationary bike for 10 minutes, either at the gym or at home, or simply hop on your bike and cycle a few quick laps around the block.

Strength Training If you have an exercise mat or yoga mat, lay it on the floor. If not, find a carpeted area for the next 10 minutes of your workout. Be sure you have enough room around you to extend your arms behind your head.

All of the exercises in this circuit engage your core muscles. As you do each exercise, concentrate on breathing. Breathe in as you relax your muscles and exhale as you contract your muscles.

This circuit should take about 5 minutes to complete. When you've finished the first circuit, repeat it a second time, for a total of 10 minutes of strength training.

MAKE A DIFFERENCE IN 20 MINUTES.

Take a Dip The next time you're at the beach with your family, don't just watch your kids from the sh.o.r.eline-dive in! Swimming is a huge calorie burner. Just 20 minutes of swimming with your kids can burn up to 135 calories-the equivalent of nine sugar-free popsicles.

PRESS-OUT.

Lie on your back with your feet off the floor, knees bent at 90 degrees and shins parallel to the floor. With your hands by your sides, slowly extend one leg to a 45-degree diagonal, keeping your foot flexed. Hold for about 5 seconds, then bend your knee and return to the starting position. Switch legs and repeat. Continue, alternating legs, for a total of 12 repet.i.tions with each leg, or 24 repet.i.tions total.

Tips Keep your shoulders down and your upper torso still.

Beginners can press the leg on a slight diagonal. Challengers should aim for the leg to be parallel to the floor.

Keep breathing throughout the exercise.

FEEL THE BURN.

Increase the intensity of your press-outs by extending your leg so that it is parallel to the floor and just a few inches above it, instead of at a 45-degree angle.

CRUNCH.

Lie on your back with your knees bent, feet flat on the floor and toes pointing forward. Place your hands lightly behind your head, elbows bent. Engaging your core muscles, draw your navel toward your spine and slowly curl your head, neck, and shoulders off the floor toward your thighs. Pause at the top of the movement and squeeze your abs, then slowly lower back to the starting position. Continue for 1 minute, or about 12 to 15 repet.i.tions.

Tips Come up off the floor only a few inches and keep your hips and lower back firmly pressed against the floor.

Keep your head aligned with your spine and look past your knees.

Try to release any tension in your neck and avoid pulling with your hands.

Keep your core muscles engaged throughout the exercise.

FEEL THE BURN.

To make your crunches more challenging, take your feet off the floor and bend your knees at a 90-degree angle so that your shins are parallel to the floor. Be careful to keep your hips firmly pressed to the floor as you do your crunches.

BICYCLE.

Lie on your back with your feet off the floor, knees bent at a 90-degree angle and shins parallel to the floor. Place your hands lightly behind your head, elbows bent. Engaging your core muscles, draw your navel toward your spine and slowly move your right knee toward your left shoulder as you rotate your torso toward the knee. At the same time, extend your left leg outward. Pause for a moment and squeeze your abs, then switch sides. Continue alternating legs for 1 minute, or a total of about 15 to 20 repet.i.tions.

Tips Keep your hips firmly planted on the floor to avoid rocking.

Keep your elbows spread wide and avoid pulling on your neck.

FEEL THE BURN.

To create a more intense workout, bring your lower leg parallel to the floor and, engaging your core, keep it just a few inches above the floor.

SUPERMAN.

Lie facedown with your legs straight and your arms extended past your head. Gently contract the muscles in your lower back and mid-back to lift your torso off the floor, about 3 to 5 inches, while simultaneously lifting both legs off the floor-as if you're flying like Superman. Hold for several seconds, then return to the starting position. Continue for 1 minute, or about 12 to 15 repet.i.tions.

Tips Think of reaching out and up with your spine, rather than up and back. This will take compression off your lumbar spine.

Keep your shoulder blades down and back throughout the movement.

Keep your breathing consistent throughout the movement.

FEEL THE BURN.

Increase the burn by holding the Superman position for about 10 seconds and quickly paddling your legs, using small movements like you're swimming. Lower and return to starting position.

20-Minute Fitness for Advanced Exercisers: Workout 1 (All Cardio) Run for 10 minutes on a treadmill at home or at the gym, or simply run around your neighborhood. For increased intensity, add speed work: Try running faster in two 1-minute bursts, one in the first 5 minutes of your run, and one in the last 5 minutes of your run.

JUMPING JACK.

Begin with your feet together and your arms by your sides. Jump and spread your feet out to the sides about 2 to 2 feet as you lift your arms to the side and over your head. Keep your knees soft and your core engaged as you land gently, with your heels on the floor. Immediately jump and pull your feet back in as you bring your arms to your sides. Repeat quickly for 2 minutes.

Tips Be sure to land gently, with your knees soft, to minimize impact on your spine and knees.

Don't allow your knees to collapse inward. If you find your knees are doing this, don't jump out as wide.

Keep your abdominals engaged and avoid arching your back.