DS only needed a few minutes of shh/pat and then would drift off the rest of the way on his own. So my back could handle that. However, I did httj for every nap for ~40 minutes, so that's when I needed the help. I brought a bench/vanity seat over to DS's crib. I could lean over the railing while sitting up, use one hand/arm to shield his eyes and prop him up against and the other could pat him on the back. My arms would get tired and sometimes fall asleep, but at least my back got a break.
Also, if need be, I would kneel by the crib and reach over that way.
HTH a bit...