Pain is weird - just ask the dude over at painscience.com. But understanding pain helps us explain pain and explain the sensations many people often feel. Understanding pain can both be a desensitizer in its own right and help facilitate behavioural changes that lead to good rehabilitation.
One complaint we often here is the subjective feeling that a knee, a shoulder, a joint, the pelvis just FEELS OFF. It just feels weird. Its not their shoulder anymore its some other thing. The pelvis feels like its "out of place" or "off". The knee can feel "swollen". Balance gets perturbed. A patient might feel like they just can't push off or they are weak. But if you objectively measure strength the strength can be comparable to the other side. These are really common feelings when pain persists or when you have had an injury in the past. Its like the memory of pain is lingering and you just don't "confidence" in that body part anymore.
So how do can we explain this?
Traditionally, we would resort to the biomedical model and say "Ya, totally, that SI joint is out of place and now let me crack it back into position". Well, we know that is just poppycock. Even though the crack might feel better it was most likely not ever out of position. That's so 1976 and 1976 sucked. What might be a better way to explain this. Perhaps, a less wrong way to explain because I am sure there are even faults in this explanation and with time it should evolve too.
This is where we turn to the concept of cortical body maps. I wrote a little before about those in this post here. Another great explanation on why people feel that their pelvis is out of alignment is here. Below is how I would describe the issue to a patient explaining that their left knee just feels "off" and "different".
You've described your knee as feeling weird and just different from your other one. You note it feels weaker and like you can't control it. It just feels "off". This is not uncommon. And many people feel this. You might be weaker and you might have a difference in how you control that body part. And some of that comes down to how your brain controls movement and how it "feels" and perceives your body in space. Its pretty amazing actually and not at all weird even thought it seems like it. Athletes have known this for years. They know that it is the brain that really controls movement and so sometimes the best way to control movement is to practice training the brain. That's why you see downhill skiers visualizing the course before they do their run. Or a diver imagining their dive before. Movement starts in the brain so its a good idea to train it.
But, pain does odd things. One thing it can do is influence how the brain controls movement and how you perceive your body. All of us have a map of our body in our brain. You know where your body parts are and you know how to control them. This is called a representation. With persisting pain that representation or that map can become distorted. Less precise. Its like spilling coffee on a real map. If you are out in the wilderness you want as much information on your map as possible to know how to navigate around. You want borders, elevation changes, rivers, paths etc. With pain, that coffee spill blurs the borders between Belgium and Luxembourg (I think they are near each other). With that blurring it makes it harder to get around and to navigate where you are going.
Pain does the same thing with the maps in your brain. They get smudged. You feel weird. Your pain can travel. You can feel off balance. Your body part can feel distorted. It can even be harder to do imagined movements. ALL OF THIS NORMAL...and you can train to improve this.
Exercise, visualization, focused practice or fancy stuff like "Graded Motor Imagery" can all help change those maps.
You also want to remember that you don't have to change those maps to be out of pain. You can always have that weird sensation. It is not unusual and if you ask your friends who had injuries in the past they will often report that the body part still feels weird or different from the other side. Its OK. These are normal changes and aren't necessarily a sign of anything nefarious.
...and that is how I explain those sensations. There are plenty of other ways and I will probably be doing it different years from now.