I agree with the material on positioning the muscles and joints - keep the position as 'neutral' as possible - forcing the joints to stay in positions near the ends of natural movement cause stress, fatigue, and often long term problems.

Being able to rest the weight helps too - I have seen 'neck straps' which are attached to each end and slung behind the neck.

With the piano accordion, the main work in moving the bellows comes from the left shoulder joint - which will really let you know you have done too much, especially on 'bellows shake' until you have strengthened the joint, and increased the ability for endurance.