Well I've been starting to work out a few weeks now and I've been focusing on my abs, arms, shoulders, back, the upper body area. I'm looking pretty cut and I eat pretty well ( I eat 3 to 4 bowls of MiniWheats every day... I'm obsessed with those things man ) and I wanted to add a part to my workout in the picture below. I want to strengthen and make this area bigger and I was wondering if anybody had any strategies and workouts to help me do so.
The area I'm focused on is this area-
Anyone with some good advice on how to workout this area behind your under arms on the back area but flanges out when you lift your arms up please help.
The area I'm focused on is this area-

Anyone with some good advice on how to workout this area behind your under arms on the back area but flanges out when you lift your arms up please help.