I’ve been explaining why one set of shoulder muscles hurt more than the other today.
The reason your right* shoulder muscles are usually worse than your left is not because it’s your mouse arm. It is because it’s your dominant arm. It’s the arm you would use to drive a screw into the wall. It will be your stronger arm and this means it has more muscle bulk in it.
This means that over your lifetime it will pull down your right shoulder. Need proof? Have a look in the mirror, your dominant arm should be lower than your non-dominant one. In reality, no one should have level shoulders.
Both of your trapezius muscles act like the wires of a crane. They hold your heavy head up with your neck acting like the arm of the crane. Your traps have to work hard to hold your head up. This is particularly true when you peer into the computer, down at your laptop, ipad or phone. And, the more you do this the worse your shoulders feel – mainly on the dominant arm side.
*(And as the adverts say: “Other arms are available” if you are a leftie)