Lie on your back on the floor with your knees bent and your back in a neutral position. Cough to activate your transversus abdominis, and keep it contracted throughout the exercise.

Keeping your shoulders on the floor, let your knees fall slowly to the left (A). Go only as far as your body will comfortably allow you to go — you should feel no pain, only a stretch. Use your trunk muscles to pull your legs back up to the starting position. Repeat the same movement, letting your knees fall to the right (B).

Remember to breathe freely and deeply during these exercises — don't hold your breath.

Start on your hands and knees (A). Make sure your hands are directly below your shoulders, not out in front of your body. Keep your head and neck in alignment with your back. Think about your core muscles — cough, contract and hold them tight.

Raise one arm off the floor and reach with it (B). Hold for three deep breaths and return it to the floor. Now try it using your other arm. Do the same thing with each leg (C).

Challenge yourself by raising one arm and the opposite leg at the same time (D). Do this on both sides. When raising your leg, be careful not to let your pelvis roll to the side. Keep your hips centered and focus on tightening your trunk muscles to hold you in place.

Lie on your stomach on the floor. Raise yourself up so that you're resting on your forearms and your knees. Keep your head parallel with your back — imagine your back being flat as a tabletop. Align your shoulders so that they're directly above your elbows.

Squeeze your trunk muscles. Create resistance by pressing your elbows in toward your knees and your knees toward your elbows. Neither your elbows nor your knees should move from their position on the floor. Breathe freely and deeply — hold for three deep breaths before releasing the contraction. Return to the starting position and repeat.

From the modified plank position, squeeze your trunk muscles and focus on keeping them contracted. Raise your right arm off the floor (A). Hold for three deep breaths. Drop your arm down to the floor. Repeat this exercise using your left arm.

Now try lifting each of your legs, in turn, from the modified plank starting position (B). Hold for three breaths before returning to the starting position.

For a challenge, lift both an arm and the opposite leg at the same time. This is an advanced move — one for you to try only after you've mastered the single-arm and single-leg lifts.

Lie facedown on the floor. Place a rolled towel or a small pillow under your hips to support your back. You might also use a folded towel to support your head.

Tighten your trunk muscles. Raise one arm a few inches off the floor (A). Hold for three deep breaths, and return your arm to its starting position. Repeat with your other arm.

Now try the exercise lifting first one leg then the other. (B)

Be careful not to raise your arms or your legs more than a few inches off the ground. That's all you need for this exercise to be effective in strengthening your lower back.

Beginning on your left side, raise yourself up onto your left forearm. Tighten your trunk muscles to keep your shoulders, hips and knees in alignment (A). Make sure your left shoulder is directly over your left elbow. Rest your right arm along the side of your body. Hold this position for three deep breaths before relaxing. Repeat this exercise on your right side.

Try an advanced version of this exercise. Balancing on your left hand, raise your hips off the floor and extend your right hand toward the ceiling (B). Hold for three deep breaths before relaxing. Switch sides.

Side planks challenge your stability and work the muscles along the side of your body. Keep proper form in mind — try not to let your hips sag out of alignment.

Work today for better core fitness — your body will thank you tomorrow.