A “muscle knot” is a trigger point: a small patch of muscle tissue in spasm.
When you say that you have “muscle knots,” you are talking about myofascial trigger points.
There are no actual knots involved, of course — it just feels like it. Although their true nature is uncertain, the main theory is that a trigger point (TrP) is a small patch of tightly contracted muscle, an isolated spasm affecting just a tiny patch of muscle tissue (not a whole-muscle spasm like a “charlie horse”9). In theory, that small patch of muscle chokes off its own blood supply, which irritates it even more — a vicious cycle called a “metabolic crisis.” The swampy metabolic situation is why I like to think of it as sick muscle syndrome.
The painful symptoms of a bulging disc may often be relieved by treating trigger points in the piriformis gluteus maximus, and rectus abdominis muscles. The spine is formed in such a way as to allow movement whilst also protecting the spinal cord (the nervous tissue that runs from the brain and down the length of the spinal column)
A few trigger points here and there is usually just an annoyance. Many bad ones is a syndrome: myofascial pain syndrome (MPS).
TrPs can be vicious. They can cause far more discomfort than most people believe is possible. Its bark is much louder than its bite, but the bark can be extremely loud. It can also be a surprisingly weird bark (trigger points can generate some odd sensations).