So I just rigged up my first model ever, and i’m starting to do an animation. However, when I pose, say, the whole left arm (including left hand, left arm, left shoulder) There is a sort of valley on the underside of the arm mesh where the middle vertexes get sucked in (i think - see picture). Is this a problem with how i rigged the bones? Please help, I’m eager to start animating. 
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