Your arms are not attached to your ribs, they are suspended above your ribs.
Why is this important? Because if your arms were attached to your ribs, they would inhibit the rib movement which is necessary for good breath support.
To breathe well, keep your elbows away from your ribs. Allow your ribs to move independent from your arms; with each inhale your ribs swing up and out and with each exhale they swing back down and in to thier neutral resting position. This motion must happen regardless of the position of your arms.