Here is from GIRM 43, all I found on this issue. Someone tell me if there’s something else.
In the dioceses of the United States of America, they should kneel beginning after the singing or recitation of the Sanctus until after the Amen of the Eucharistic Prayer, except when prevented on occasion by reasons of health, lack of space, the large number of people present, or some other good reason. Those who do not kneel ought to make a profound bow when the priest genuflects after the consecration.
While I don’t see it here (but if someone knows where it is, please post it), common sense tells us that:
*]It says, if you can’t kneel…[don’t kneel]. That probably means stay in the same position as you were already in. (standing)
*]In order to make a profound bow, you’re probably standing
*]If everyone else is kneeling, and you can’t for medical reasons (or whatever) you should probably sit, unless you’re in the back row or choir loft or something similar.
My interpretation of the document is:
If everyone can’t not to kneel, then they should stand, and should make a profound bow when the celebrant genuflects after the consecration.
If a few can’t kneel, they should sit, or stand out of the way. Bow if you can.