Ok, I admit, I actually made that diagnosis up, although it does make a whole lot more sense than a lot of other diagnoses out there...
Have you ever immobilized your body? If you've never had an arm in a sling or leg in a cast, your answer may initially be no, but without realizing it, you probably have subjected your body to far more immobilization that you've ever realized.
Our DNA needs the type of stimulation a more "primitive" lifestyle would create; movement and positions in varied terrains with varied loads, taking joints (not just shoulders and knees but even the small facet joints between your spinal vertebrae and the joints between the bones in your feet) through their range of motion on a regular basis.
We moderns, however, tend to walk on completely flat surfaces in a forward direction only and sit in the same position, in chairs, day after day. At night we sleep in beds and with pillows that are designed to keep all out joints in a neutral position. We wear shoes with rigid soles and arch supports (support sounds like such a nice word, who would ever suspect there was anything wrong with it?) that keep the many (over 30!) bones of the feet and ankles from moving. We even wear other types of clothing (bras, belts, tight pants, boxer briefs etc) that restrict the natural movement, expansion, jiggle, swing and bounce of our bodies.
I once treated a man that I only could diagnose with "Big Belt Buckle Syndrome". No, that's not an official diagnosis either, I must confess. But in reality, that's what he suffered from. He wore tight jeans, cowboy boots and a belt with a huge belt buckle every day, and his body had learned to move within the restrictions that his fashion sense created, with low back pain as one of the resulting problems. The moral of the story? Things (clothes, sleep surfaces, chairs, cars, walking surfaces etc) that look completely innocuous to us because we are so used to them do immobilize our bodies to varying degrees, and since we were not designed for it there is always some price to be paid (and, conversely and thankfully, many potential improvements at arm's reach!)
So does it really matter whether joints move or not as long as you're comfortable in the moment? It actually matters. A lot. You could put it this way: whatever evolution has prepared us for it also expects of us. What this means in terms of your musculoskeletal system is that all our bones and joints and ligaments and muscles need pressure and movement from and at various angles in order to stay fully healthy. When we remove a large percentage of the forces our bodies expect to be subjected to, our bodies become weak, unstable and rigid and this (not the number of birthdays you've had) over time results in degeneration of joints, osteoporosis, loss of muscle mass and stiff, malaligned bodies from our feet all the way up to our heads.
And here you thought you were taking good care of your body with that posturepedic sleep number bed and those custom-made orthotic shoe inserts! Sorry, but using such products, unless you absolutely have to due to an acute injury or advanced stages of degeneration, is doing your body a huge disservice. The bed that keeps you so comfortable is actually preventing your pelvic bones and spinal vertebrae from realigning themselves during sleep and muscles from stretching the way they would if your sleep surface was firmer. The way your chest sinks in in a soft bed deprives you from oxygen both during sleep itself and during the daytime due to a permanently collapsed ribcage, and that's just as bad as it sounds. The "supportive" shoe prevents the foot from acting as the shock-absorbing, mechanical force-conducting marvel that it was designed to be and instead over time makes it weak, stiff and painful.
But why do soft beds and supportive shoes feel so good to some people? Essentially, they are indulging your restrictions. A body made inflexible through years of positional deprivation can no longer stretch and yield enough to be comfortable on a harder surface. A hard surface now stretches the body's (already achy) tissues in a way that is perceived as discomfort. Similarily, a foot shod for decades may lack the stuctural integrity to hold up the body without collapsing, and hurts without the additional help of an arch support.
So what might another approach look like, one that would help our urbanized bodies stay healthy and happy? Here it helps to look at how non-industrialized folks do it. The ones that manage to get through the day wihout supportive shoes, chairs or beds and are walking around erect, comfortable and agile. They may walk around barefoot, or in minimal footwear, sleep without soft bedding and pillows and move three-dimensionally through their world exposing their bodies to forces by lifting, pushing, pulling, carrying, twisting and turning. At night they sleep in varying positions on much harder surfaces than we are used to, that stretch and mobilize joints in their bodies and even correct joint mal-alignments in their sleep. And speaking of joint mal-alignment, when you expose your body to a wide range of movement through most of your range of motion daily it is much more likely that mal-aligned joints correct themselves than if you keep your body relatively still or only move in the mid range of your joint range of motion the way most of us Westerners do.
When visiting west Africa many years ago I remember thinking that people sleeping on thin straw mats on the ground with only a thin piece of fabric covering them to protect against the terrorizing mosquitoes looked like they must be terribly uncomfortable But do you know what else I kept thinking throughout my stay? The people around me had the best posture I had ever seen anywhere in my whole life. Gradually, very gradually if you are stiff, moving towards a less supportive sleep surface and a flatter pillow is a move in the right direction.
Our feet are made for walking, barefoot. Wearing rigid shoes with thick soles and heels alters the alignment not only of the feet but of the entire body and prevents the foot from staying supple and strong. Gradually (again, very gradually if your feet are stiff and sore) move towards mostly using less "supportive" (there's that "nice" word again) footwear, removing the heels (that goes for guys too, just look under your shoe and you will find a heel) from your existing shoes or buying "neutral" footwear that doesn't have a heel. And no, you don't have to wear five-finger shoes to do that. Here's an example of a very formal-looking minimalist shoe Doing strengthening and corrective exercises along the way to prepare your feet for a more normal load will help make this in transition easier and smoother. And remember, this is not a binary issue. You don't have to take sides, choose only minimalist shoes and never wear heels. Juts wear them more, and see how you feel! Every little bit counts, and perfection and perfectionism doesn't have to enter into the equation. Our bodies are very forgiving, we just have to help them out a little bit.
Walk a bit more, and if possible walk in terrain and up and down hills, sideways etc. Walk barefoot whenever you can. If you sit or stand for work you can even place some pebbles in a flat pan and place your feet on them from time to time. It feels wonderful and invigorating, and gently wiggles the many bones in your feet and keeps the muscles and ligaments holding them together supple and healthy.
At home, don't use shoes, belts, bras or other restrictive types of clothing.
Sitting in is a position we find ourselves in way too often. The best thing to do about that is to try to stand and walk more. When you are sitting, vary the way which you sit and use props to keep your self from slouching and collapsing. Standing desks are becoming more and more popular, and you can even use a walking desk. If you're using a laptop, why sit at all? Sometimes you can choose lay on your stomach, for example.
Expand your positional and movement repertoire: find new ways of sitting, whether that means positioning yourself better in a chair or choosing one of the myriad ways you can sit on the ground. If ways of sitting other than straight up in a chair feel uncomfortable at first, you can use alternate sitting positions, such as crosslegged, kneeling on your heels, squatting etc as stretches that you only hold for 2-3 minutes at a time. As your body limbers up you may find yourself spontaneously choosing these positions over sitting upright in a chair. Use young children and images of people in more primitive cultures as a source of inspiration. There are literally hundreds of positions you can choose for sitting, lying down and moving about your day, and they all treat your body a variety of forces necessary to keep it healthy.
Once you get used to seeing movement and position as a necessary input in order to get the output of a healthy, well functioning body you will not only find yourself choosing a wider range of movements and positions, but actually craving them. You will develop a more intimate connection with your body due to the varying proprioceptive inputs and a greater confidence in your body through experiencing movements and positions you never experienced before. And hopefully you will also be ditching the outdated notion of "my body feels stiff - but I guess that's just because I'm getting older". No matter what your age or health status you can always nudge your body in a better direction by using it the way nature intended. And what better and easier way to do that than by doing what you already do: walk, sleep, sit - just in more varied and interesting ways!