This video demonstrates an exercise to strengthen extension of the trunk.




In case you cannot see the video, here is a still sequence -

trunk extensions Lie on your back on a firm surface. Bend the knees to 90 degrees and firmly hold a soccer ball between the knees.
trunk extensions In an 'unwinding' movement, slowly lift the buttocks and lower back.
trunk extensions Hold this position for a few seconds.
trunk extensions Slowly allow the lower back and then the buttocks to 'unwind' to the resting position.
trunk extensions Relax, and then repeat ten times.