system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is actually happening with a "knot" in your back?
gpt
Former massage therapist here.

A knot is a little bundle of tightly contracting muscle tissue. Sometimes during daily activity, small areas of muscle get slightly strained and hurt a little bit. Muscle fiber contracts when it hurts (because if you're over-stretching, for example, the overstretched muscle will want to contract to prevent further injury). When small areas contract a lot, this will cause more pain. Knots, cramps, & spasms happen when muscle fibers get stuck in a pain-spasm-pain loop.

When the tight bundles remain for awhile, blood flow to the area becomes limited. Blood brings oxygen and nutrients to cells while washing away cellular waste. With limited blood flow, cells are more likely to die. Cellular waste can be irritating to other cells if it builds up, and reduced oxygenation & hydration can hamper healing of the injured fibers.

Muscle fibers are surrounded by fascia, which is made of collagen and other proteins. If muscle spasms keep an area tight enough to restrict blood flow, fascia becomes less hydrated and more sticky. (You can see how sticky for yourself if you play with hide glue or warm gelatin dissolved in the bare minimum amount of water.) When fascia gets sticky, adhesions (or "stuck" areas) form.