To work your core from a seated position, sit tall in your chair with your feet firmly planted on the ground. Lift your right foot slightly off the ground. Keep your hips parallel and avoid tilting to the side. Avoid holding your breath. Keep abdominals braced. There should be no stress in your back. Hold the leg up for 10 sec. or less. If that is too difficult, lift your heel and leave your toes on the ground. Repeat on other side. Work up to a set of 10 on each side.