I'm not the tallest but I am rather tall for a gymnast. I am 5'8 and a half. I am 16 years old, competing as a 15 year old. All my muscles are pretty dense, but my arms aren't very big. My back is descent sized, and my core is good. I work out around 25 hours a week but I feel like whatever I do my arms do not get any bigger, and i have specifically been having trouble with holding planche position. As well if any exercises for shoulder muscles please list. Thanks!