The aid exercises the subject in prono-supine positioning hand movements. Specifically, the subject is asked to keep the ball between the first three fingers on the hand and guide it over the sinusoidal path.

This exercise means that the large prono-supine positioning joints in the forearm are used to the maximum.

It is therefore useful to monitor the patient’s behaviour while executing this task.

Any incapacity to explore the maximum levels of supine positioning could be compensated for by bending the trunk on one side, while limitation of prone positioning will cause early and excessive movement of the arm away from the trunk.