I’ve been to masses offered by priests of various groups, including the FSSP and SSPX. My experience and observations are…if one truly has a legitimate reason that renders him incapable of genuflecting or kneeling, he would still be expected to do his best in making another pious gesture. For example, the elderly could make a generous bow instead of genuflecting.
Not sure if this makes sense, but to me it is comparable to the obligation… in that, if one truly has a valid reason for not being able to attend mass, divine law still obliges one to sanctify the sabbath. Similarly, if one cannot conform to the usual norm of genuflecting towards the blessed sacrament, for a truly justifiable reason, he should still practice a pious deed to the best of his ability.
There is a very aged senior at the local mission, who is blind and has problems walking (he needs a walker). Instead of genuflecting, he always does his best to make a low bow. Although he doesn’t have good knees, he still kneels throughout most of the mass, until his knees hurt too much, and he choose to sit but leans forward in his pew. This seems very evident that he is doing all he can, and I admire him greatly for it.