Stiff and painful shoulder when sitting at your desk? Grab something behind your back and let it hang. Take your time. After a minute, you will feel that your muscles will loosen up. Retract your shoulder, so the joint is as much as possible centred. Go a few times forth and back. You can play with different angles and small pendulum movements to find your sweet spot.
Here I used a clubbell, but you can easily swap it for a kettlebell, dumbbell or simply a bigger water bottle.
The heavier your object is, the more carefully you have to be. Be mindful of the exercise duration so that your arm doesn’t become numb.