Comfort is the key. Try lowering the easel so your painting hand never has to go above shoulder height. Add a small block in front of the easel for your foot so one knee can be bent to relieve the pressure on one side of your body. Alternate the block between your feet as needed. Don’t forget to take breaks that involve stepping back, sitting down, and/or elevating your feet. Don’t wait until you feel pain to stop, because by then the damage may have already been done.