How should the nodding be for someone praying while sitting on the ground or on a chair—should it be with the head only, or with the torso?

One who prays a fard (obligatory) prayer while sitting, due to inability to stand, must perform the bowing (ruku') and prostration (sujud) as long as he is able to do so. This applies even if it means bending the back slightly for ruku', and placing the hands without the forehead for sujud if he is unable to place the forehead. If he is completely unable to perform ruku' and sujud, he should gesture with his head, making his gesture for sujud lower than his gesture for ruku'.