While each of our individual mechanics that makeup our body dictate the percentage of upper body each person uses when running, this all depicts whether or not your upper body weight will affect your running pattern.
As your body transfers weight from leg to leg with each stride, your upper body muscles in your abdominal and arm areas are being used to stabilise and coordinate the direction of your entire body throughout the run. Your arms will act as a counter balance to the leg movements in directing the momentum forward.
Where sprinters are involved, the upper body weight will be useful in propelling them out of the blocks quicker, their arms and core provide the torso stabilisation required to help transfer power through their center.
On the other hand, long distance runners will need a strong upper body to assist with good posture allowing your body to inhale and exhale more air efficiently. Strong arms will help to prevent wasted energy from excess rotation and help support good form.
Bad Upper Body Weight
Any upper body weight is still weight that you carry while running is extra weight that will start to matter when you break a race down to pace and miles vs. minutes. Shaving that little excess weight from up top can help to reduce your overall time.
While some muscle is necessary up top to counterbalance the legs, most experts will say that having a lighter body weight overall translates into faster race times. One of the reasons why many of the world class marathon runners are fast paced and good at what they do is down their extreme lightweight.
A second point to not building up to much upper body weight is the time your spend building the muscle. Any time spent in the gym lifting weights is time that could be spent running and training harder. Most runners will have their own mileage maximum, and the general rule of thumb is that if you can safely run more, do it and you will maximise your running and improve.
In conclusion, if you are a keen runner, your upper body and the percentage of body weight you hold will effect the way you run and the lengths you can go for. If you are into running longer distances, having a lower bodyweight up top will help to maintain a longer pace, whilst those who are sprinting and perform HIIT, having a bulkier upper body will help out much more.