The hardest muscles to develope for ANYONE regardless of genetics are the forearms and the calfs. They are built up of the most dense muscle fibers, making them the hardest to tear apart (and grow thereafter). We use those 4 muscles the most during everyday life. Everytime you take a step, each calf muscle is shifting all of your bodyweight independently. Anytime you move your fingers, your forarms muscles are contracting and releasing. Which muscle is the strongest? The jaw. But I doubt my jaw could bench press 345 lbs. The brain is an organ not a muscle, even though it is very similar. The brain grows "stronger" by "excersize" (learning).
BTW, Im 18 and have been lifting now for five years. Im extremely cut, no body fat at all. But its incredibly hard for me to get bigger and gain weight. I'm 5'10" and weigh 155. Should I just increase my workout regimine and pile on the protein supplements?
The stronger you get your forearms, the more |337 you get at CS! >=)