Modifying run Animation to stop movement in arms

I’m trying to modify a running animation so that the character can hold a gun steady while the character is running. The base animation has the character’s hands moving at the side like a normal run. When I get keys for the hands and lower arms, the animation stops at those points, however the upper arms seem to want to continue to flail even after setting a key.

How can I get the arms to stop flailing? I would have though that the flat line after the first set key would mean that is what the animation should do, but I guess there is still the original data somewhere.

I have now tried to adjust keys throughout the animation, but that is leaving my character fidgeting, which does not look great. Any tips on how to accomplish what I’m trying to do?