This is one of the most successful, yet simple mobility exercises for healing hunched shoulders, neck pain and lower back pain all at once. I can’t tell you how many people it has helped, including me.
The lower back will bend backwards in order to move your arms to the side or up if you lack shoulder mobility. Tightening the core can prevent this, while at the same time strengthening the transverse abdominal and correcting lower back compression. This also gradually stretches the shoulders for optimal alignment.
Also, if you feel pain while doing this correctly, use a massage ball to role out those areas against the wall or on the floor.
Here is the video: