Does stiffness in the upper back make it harder to sing well? Does an issue with your knee interfere with vocal production? If your foot hurts, how can that make your singing different?
The body is a whole. Everything affects everything. You may not notice it, you may be used to whatever is going on, but it all makes a difference, whether you realize it or not.
If you have something happening in your body that causes you pain, or makes you stand in an “out of alignment” way, or you take medications that affect your entire system, sooner or later that affect can creep into your singing. Your larynx sits in the middle of your throat, on top of your trachea (windpipe), and those are both in front of your cervical spine. The cervical spine connects your skull with your thoracic spine (your torso) and that is supported by your legs and balanced by your arms. If your stance is skewed, your ability to inhale and exhale may be compromised, as the placement and movement of your ribs could be inhibited.
If the muscles in your body are very tight, unable to stretch and move, or very flabby and out of condition, responding very little, it may be difficult if not downright impossible to use them well. If they don’t move, you might find your inhalation shallow and your exhalation weak, if they don’t respond, you may not be able to engage your upper body to hold it erect and your lower body to stabilize your rib cage from below. If you can’t find a solid center of balance without locking your knees or perching on your legs as if they were stilts, you will find it hard to breath in deeply and freely, and that makes it hard to sing well. Your vocal folds work best when the larynx is sitting comfortably in your throat, but it can’t do that if you have tension in your body that shouldn’t be there (and who, in this crazy world, doesn’t have that somewhere or other?).
Being in touch with your foot might sound like noticing the thing that has the least possibility to affect your singing but this would be wrong. If your foot hurts, it can affect your entire body, as most of the nerves in your body end up there. Same for your hands. If you have a pain in your body anywhere, it saps your energy, draws your attention and generally makes it harder to function well.
Things that are far away on the edges, the periphery, of your body and your consciousness about your body, can and do make a difference in what your vocal folds can do. They are not directly the source of the sound and you can certainly sing when you are not in perfect condition. (If we had to be in perfect shape to sing, we wouldn’t have very many good singers in this world). If, however, you are to maximize the potential for your voice to respond and for it to be both strong and flexible, then you need your body to function as well as it can, and you need to attend to it for this, if no other, reason. Just because your feet and hands or your legs and arms or your pelvis or head are not causing your vocal folds to close and vibrate doesn’t mean that they don’t contribute to their ability to do so.
Learn to pay attention to everything in your body and to the messages those areas are sending you. If something is calling to you, asking you to notice it, listen! It could be that something seemingly unrelated is far more important than you realize.