I get this question almost daily in the office. The answer? Both.
Hilton’s law says that the muscles that cross a joint AND the joint are supplied by the same nerve. This nerve supply provides proprioceptive (position and movement) and pain sensory information from the muscles and joints to your brain.
This means that if a joint is not moving normally, or is slightly out of normal position (there are types of receptors for both these kinds of movement information around your joints), your brain immediately knows. A common response is for the muscles around the joint to tighten in order to protect it.
The same is true if a muscle is at a different length or tone than normal. This has to affect the joint mechanics in some way. The joint can rotate or move into a slightly different position, be compressed by the tight muscles, be limited in its range of motion, or all of the above. None of these things are good.
As you can imagine, this can become a negative spiral with the joint moving less, the muscles getting tighter and both of them telling your brain things are not happy and healthy so you should lock this up.
In the early stages of these movement and sensory information changes, you probably won’t feel any pain. You can have a lot of what is called nociceptive (kind of like the opposite of proprioception, signaling noxious stimuli) information coming to your brain from a muscle or a joint without pain. Pain is a conscious, cerebral cortex experience that makes you aware, often when things get more severe or have been going on for longer.
Many people are scared to think that there is something wrong with their joint, they want it to be just a muscle problem. In my opinion, this is really not possible. However, just because the joint is involved, does not mean it has to be a serious pathology. You can have an unhappy, irritated, poorly moving joint that hurts and can be restored to normal function. Joint involvement does not necessarily mean more malignant things like osteoarthritis or serious pathology.
This is also why getting a muscle to loosen up without restoring proper joint mechanics will typically only work short term, and vice a versa. Until the unit as a whole is working properly, which is essentially controlled by your brain, you will not have a healthy, happy joint or muscle. This is why issues like this, even if they are not “serious” problems, require rehabilitation and treatment over time. This is why you can’t just put a joint that is “out” back “in” like it is a lego block, or why you can’t have one massage or stretch for 10 minutes and have muscles restored to their healthy length and tone.
Tyler Fix DC, CCWP