Keeping your torso stable, slowly reach your left arm out to your side. Brace through the abs by pulling your navel to your spine. Hold this position for five seconds. If you feel too unstable, try moving your right hand so it is under the centre of your chest rather than under your right shoulder.
Keeping your torso stable, bring your arm back to the plank position. Do not round your back or twist your spine.
Repeat this same motion on the other side, extending your right arm to your side. This counts as one rep.