Is it permissible to pray behind an imam who does not fully straighten up from rising from prostration and remains bent over, and stays like that during the Tashahhud, knowing that he is overweight and suffers from knee pain?

If a worshipper does not straighten up completely after rising from prostration, but remains bent without reaching the point of prostration, their prayer is valid and sufficient. This is because the aforementioned bending does not cause the worshipper to cease being considered as sitting after rising from prostration and during the Tashahhud (definition). Ibn Taymiyyah, in Majmoo‘ al-Fatawa, explained that straightening and leveling differ in standing compared to bowing, prostrating, and sitting. A standing person is one who straightens and levels, whereas a sitting person may have a bend without losing the description of sitting.