The Walk Designer sets every single frame separately. If you look at the keygraph, you'll see that each frame has a key-mark on it. This is different from the usual situation you were expecting, in which you set a couple of frames and let the interpolation fill in between. So, if you want to move the arms in a more loose way, you'll either have to (1) alter each frame separately, or (2) draw a rectangle around the relevant arm joints, across the frames you want to change, and hit the minus sign at lower right to open up the keyframes for this joint. Then you can move them with the usual interpolation. This will, of course, get rid of the motions that Walk Designer gave to these joints.

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