looking like a rockstaar!! I can feel the energyy in it.
For the advice you are looking for in the underarm, I would sugest give a quick check to some anatomy basis about the pectoral, braquialis,coraco braquialis and deltoid muscles that are the ones intersecting into the ribcage, the humerus and the scapula. giving it a quick review will give you the right hints of why and how the armpt and arm has the forms it has.