Stand up straight, head erect, feet together. Lean the head backwards as far as possible. You will notice an immediate muscular effort in the back and lower parts of the body. Feeling loose, stand on your toes. If your posture is good there will be little or no readjustment of the head in relation to the body compared to the standing position.
Most adults have to re-adjust the head position, or fall over.
Flex a bicep. You will feel the tension centred on that specific muscle spreading well outside the intended area, into the lower arm and into the shoulder. This tension is not creating useful action, and is spreading its negative effect to neighbouring muscles. Breathing and Embouchure are the two subjects most open to interference and abuse in this way. Every player has had the experience of pulling in here or pushing out there with the breathing, or smiling here or puckering there with the embouchure.