I can tell you are careful not to overextend because it must be painful. I’ve seen parts of the recovery process due to trauma. In order for you to heal, you underuse those muscles. Then they shorten from lack of use and it can be painful getting back to where you were before the trauma. It is amazing what muscles are attached indirectly to your arm – I bet it hurts to begin using it to the extent you need to for a full bow stroke. Talk to your physical and or occupational therapist about your goals and maybe even consider bringing your violin into a session so he/she can see exactly what mechanics are involved to better help you .