In the last couple of days I have been focusing on my arms and shoulders.
With these bitterly cold winds its so easy to tighten around the arms and shoulders which creates tension in the whole run.
Yes arms really are so important in your run. Try running with your arms hanging loose by your side and you will soon find out how difficult running becomes. Its the same if there is excess tension, everything becomes a little fixed and doesn't move as efficiently as it could.
A good indicator of excess tension in the arms I find is if you run like you are thumbing a lift with thumbs sticking upwards. The chances are that there will be tension right through to the shoulder and even across the front of the chest. Which will also be likely to be restricting your breathing.
Arms therefore need to be able to move freely as you run, particularly at the arm/shoulder joint and really be part of your whole running body. Include them in your awareness and see what you notice.