Hi i'm 14 years of age, and my metabolism is incredibly high. as i get taller i don't gain anything except muscle. i'm 6'0 and weight 138-145 average. before i started working out i was about 127. so i think its muscle. i'm really lanky, my arms are frickin long. which i think this is a good tennis physique. but i can't really gain anymore weight from muscle cause i have no body fat. any ideas to gain a little weight? maybe 10-20 pounds maybe? within a time where i work out so i don't get more fat than muscle.