This video demonstrates an exercise to strengthen extension of the trunk.
In case you cannot see the video, here is a still sequence -
|Lie on your back on a firm surface. Bend the knees to 90 degrees and firmly hold a soccer ball between the knees.|
|In an 'unwinding' movement, slowly lift the buttocks and lower back.|
|Hold this position for a few seconds.|
|Slowly allow the lower back and then the buttocks to 'unwind' to the resting position.|
|Relax, and then repeat ten times.|