In the case of our head and neck, when you change the responsibility of the supporting muscles and tendons, the structures struggle to keep up with their new tasks. For example, if you held even a small weight in your outstretched arm for just a few minutes, the muscles in your shoulders would start to burn. Now imagine asking that same muscle to hold your arm out ALL day. That is in essence what you are doing to your neck musculature when you have bad posture, poor work ergonomics or incorrect body movements. If you think that the small muscles that run along the back of your neck can hold that heavy head of yours you are sorely mistaken (no pun intended).