My mom has told me about Isometric exercises and she was told that regarding them you hold the position for several minutes instead of constant movement. I would recommend calling your physical therapist back to ask for further clarification of the instructions on these exercises.

I’m a Physical Therapist and will answer your question about isometric exercises. First of all, these isometric exercises should not cause any pain at all. There shouldn’t be any exerted force – think of just holding the position, meaning just contract the muscle. There should be no actual movement in the muscle. Isometric exercises for your shoulder will help strengthen without any pain. This will give time for your shoulder to heal but will keep and build muscle strength as well as endurance.