Taijiquan encourages the student to exercise with intelligence, to 'listen' to what the body is telling you.
If something feels too heavy, too hard, too taxing... then it is a warning.
If a joint feels sore, then do not ignore it.
If the same injury occurs again and again, you risk long term damage that will eventually affect your quality of life.
Pain is not something to be fought or ignored.
It is something you must listen to and learn from. It is something you want to avoid experiencing altogether.