“Lack of oxygen flow means that cells must use anaerobic metabolism to create energy. When a muscle is held tight, this process happens constantly, even during sleep. Lactic acid is continuously produced and broken down into lactate and hydrogen ions. The constant contraction has the same effect as a strenuous workout; the waste recycling process can’t keep up and hydrogen ions build up in the muscle, activating nociceptors, and causing chronic muscle soreness and pain. So if muscle pain resulting from constant contraction is what you’re feeling—which it most likely is, since this is the cause of most chronic muscle pain—then your pain should go away when your muscles release and oxygen flow is restored.”