Biceps
Flex your biceps. How much space is left between your biceps and the crease in your arm? No space=long muscle belly. 1 finger space= average muscle belly. 2 finger space or more = short muscle belly. That should tell you your potential for growth. If you have long muscle bellies you can look forward to big biceps that may not peak real high, unless youre Arnold or something. If you have average muscle bellies, then you should have a decent peak and a biceps about the size of an average russet potato. If you have a short biceps, you may get a high peak, but small bicepslike a tomato maybe. Its important to know what your genetic potential is before you diagnose yourself as having a training problem. If its truly a training problem its usually not an exercise selection problem as biceps are pretty easy to work. Its often a weight problem. My thoughts? Most people lift too much weight when curling. Look, a muscle the size of a potato doesnt curl 70lbs dumbbells without mucho assistance from Mr. Momentum. Not saying thats you, but if it is, try cutting your weight down, maybe even in half, and go for the feel with absolutely text book perfect form. Make it look good when you curl.