1. Start on the floor, on your hands and knees, with your hands firmly placed about shoulder width apart.
2. Tighten your stomach muscles.
3. Raise one leg off the floor and hold it straight out behind you. Be careful not to let one hip drop lower than the other, so you don't twist your trunk.
4. Once you feel stable with your leg raised, raise your opposite arm straight out in front of you at the same time (see photo).
5. Hold for 5 seconds, then lower your leg and arm.
6. Switch to the other leg and arm.
7. Repeat 8 to 12 times on each leg/arm, holding each rep for 5 seconds.
8. Build up to holding each rep for 10 to 30 seconds each time.