Susan has Rett Syndrome. Rett syndrome is a rare genetic disorder, occurring mostly in girls. It affects mostly eyesight,  movement and speech.  The disease progresses very slowly.  Part of the treatment for Rett syndrome is to stimulate movement.

 

This photo shows Susan using the exerciser with the bucket seat, gently swinging.  After a few minutes of doing this, her arms and hands have relaxed from their usual clenched position.