Back in the 80′s, when I was runner at Northfield Mount Hermon, I learned something that has stayed with me since. Relaxing your jaw helps relax your whole body.
It is hard to tense up your shoulders and other muscles, if your jaw is relaxed. This works with cycling as well as running….and I am guessing with other activities as well as work situations.
Try it: if your jaw is relaxed, it is hard to tense up your shoulders. And tense shoulders can make you tight all over. Shoulder tightness, if left untreated, can spread up (into the jaw) and/or down (to the low back and pelvis) the body. (This is not based on scientific studies, but on my own massage experience and knowledge of how the body is connected…this said just to cover my butt.)
So, instead of wasting your energy clenching your jaw (and hence tightening your shoulders….etc), you could spend it actually running or riding faster. All this in the spirit of relaxing all those tight shoulders out there.