What makes your muscles turn into knots in your upper back? Well, they are not really “knots.” They are a type of muscle spasm. Here’s one example.
See this little guy? His head is way in front of his body. The dotted line indicates the midline of his body. The line should be passing through the center of the top of his upper leg bones (instead of behind them.) It should also pass closer to the opening for his ear.
This kind of posture puts a lot of strain on the muscles of the upper back. Why? Because your head is heavy!
Your bones are supposed to support you. You muscles were designed to help you move. When your heavy head is in front of your body like this, your muscles are working way too hard. Your muscles are trying to hold your head to keep it from falling even farther forward.
Over time your upper back muscles will become tighter and tighter and you will have more and more discomfort. They become tighter because they are (1) trying to hold up your head and (2) trying to prevent damage to themselves. That might seem like a good plan but it really doesn’t work so well.
The end result is the poor posture that you see on people who have muscles that are “too tight” in the front of their body. Collapsing-forward posture is one of the things that turns muscles into knots.