Isometric abdominal flexion, contralateral shoulder extension supine - one leg flexed, one leg extended

Lie on your back with your knees bent and feet flat on the floor.
Straighten one leg out on the floor.
Position a stability ball on your bent thigh and place your opposite palm on the ball.
With your arm straight, press your hand into the ball.
You should feel both your abdominal muscles and the muscles around your chest and sides tense.
Hold this position.
Relax, and repeat.