Start by sitting in an upright position with your arms down by your sides.
Extend one arm back behind you, parallel to the floor. Bring the other arm forward in front of your body, parallel to the floor. Keep your arms as straight as possible.
Alternate moving your arms simultaneously in front of and behind the body, tensing the arm muscles at the top of each movement.
Repeat for 20 seconds total, then rest for 10 seconds.