Up to age 45, every person who gave me a massage commented on how much tightness and tension was in my back and shoulders, and I usually had to ask them to ease up and use light strokes and stay away from the spine. People who would occasionally slap me on the back or shoulders would be surprised at how much I would jump, and I sometimes had to restrain myself from hitting them back or yelling.
At 45, my shoulder started to lock up, and I learned about myofascial therapy, a type of do-it-yourself massage based on the idea that knots or 'trigger points' can develop, and they can cause all kinds of problems. I not only restored normal function to my shoulder, but also have also reduce much of the constant tension in the back and neck.
Trigger points develop when a muscle fiber and the sheath around the fiber get glitched. These knots become tender at the glitch, and can also referred pain to nearby areas, some points can even cause anxiety, muscle weakness, shallow breathing, or heart arrhythmias (mostly in certain muscles around the sternum or neck). Pressing on trigger points not only causes intense pain locally, but it usually reproduces or aggravates the other symptoms. And massaging them myself improves things.
When one muscle glitches up, other muscles take up the load. The muscle in pain works less, and the body shifts the work to nearby muscles to protect the place of pain or discomfort. And soon those muscles that are working more than they were designed to get stiff, maybe even glitching up into knots themselves. Which can create syndromes of pain and stiffness.