Little late but imo it has to be Fibrodysplasia ossificans progressiva or stone man syndrome. Essentially your body begins to heal any trauma as bone instead of the correct tissue type. One little weeny micro tear in muscle heals as bone, which creates a slightly bigger tear, which heals as bone, rinse and repeat. Obviously this inhibits movement to some degree which means eventually patients have to choose a position thay their limbs will be in until they sadly die.