It's obviously related to the running from what you describe, but the question is what happens when she runs that doesn't happen at other times. Perhaps when she runs, the "bouncing" is irritation a nerve, or perhaps the way she holds the arm is pinching the nerve while running, then released when she stops. There are nerves that come from the spinal cord, through the shoulder and then down the arm and into the hand and fingers. The position of the head could be a factor with the cervical root of these nerves. If the chin protrudes too far out from the shoulders, she could be impinging on the cervical "facets." It would take a bit of time for the nerve to become inflamed once she starts running and as she runs farther, the chin may be coming out past the shoulders more and more. 12 year olds heal a lot faster than us old fogies, so maybe she just recovers quickly from this as well. I would suggest that she get a MRI of the neck and the shoulder, and see a pediatric orthopedist. This will determine if there is anything to be done to keep the area from getting worse.
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