Maybe its genetics but my arms were 18 1/2 without juice, but then again when i was a freshman in HS i had 14" without even training. When I did start training i did straight bar, concentration and standing dumbell and threw in some hammers once and awhile. for tris i did skullcrushers, kickbacks and pulldowns. I always thought i grew well because i made sure my form was good and i never cheated by arching or jumping the weight up. I'm getting close to 19" now.