to add to what has already been said, people have a tendency to flex 1 side, so start flexing the other bodypart (bicep/tricep...whatever) because it'll teach you to recruit that muscle better as you continue to do it--EX i tend to flex my right arm and so my right arm is bigger than my right and i get better contraction in my right arm than my left. so I've started flexing my left arm instad of right. Also i started using my left hand more to carry stuff and pick things up, push doors open...etc, The whole point it to learn to recruit that lagging bodypart muscle fibers better.