system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what is happening physiologically when a muscle knots up?

Are muscle fibers welding together, and/or literally contorting into a knot? Does heat soften it; if so, why would ice be a good idea? Would myofascial release therapy be helpful only if regularly treated, like massage?
gpt
A "muscle knotting up" is called a "tetanic contraction", which means the muscle contracted by a constant stream if nerve impulses telling it to do so. The causes can vary from phycological to chemical to pathological. Psychological reasons usually involve stresses and panic. The most common chemical reason it happens is because of a buildup of lactic acid inside of the muscle tissue. The act of contracting a muscle is mostly a chemical reaction started by a nerve impulse to where the muscle tissue uses glucose, then releases lactic acid into the blood to be carried away and (hopefully) neutralized. If your blood flow cannot move it away fast enough, the lactic acid builds up in the muscle tissue and makes the nerve impulses go haywire. "Warming up" before exercising is a way to get your blood flow to your skeletal muscles increased BEFORE the muscular exertion so that this doesn't happen. Pathological (relating to sickness or diseases)  can be infections like Tetanus ("Lockjaw") or diseases like Parkinson's.