1. Begin in a half-kneeling position with your right foot forward and left leg back, and bend both elbows.
2. Keeping your abs engaged, draw your left arm forward and your right arm back, like you're about to take off in a race.
3. Exhale as you drive your left knee and right arm forward and up.
4. Hold this position briefly before returning to your starting position.
5. Repeat on the same side for 15 seconds, then switch to the other arm and leg.
6. Not hard enough for you? Matthews suggests kicking up the intensity by rising up onto the toes of the standing leg.