Start on all fours. Keep arms extended, hands on the ground and push feet back, into a straight arm plank (see Photo 1).  Tighten core and bring stomach in, while also maintaining a straight back. Lift your right arm and reach over to the left side, then open up to the right side, stretching your right arm up to the sky (see Photo 2). Shift weight in feet from a forward plank to a side plank, but only for a moment. Bring arm down, and shift weight back to a straight arm plank. Repeat with your left arm. First stretch it across to the right side, then open up to the left side of the body in a side plank.