Check out the photo on he left. This is a great example of the difference muscle fibers can make on the human body. On the left is Maurice Greene, a short distance sprinter, and on the right is Robert Cheruiyot, a long distance marathon runner. Greene the sprinter has thick muscular legs because of his training resulting in fast twitch muscle fiber dominacne while Cheruiyot, the marathon runner shows very thin legs because of his focus on endurance, and thus slow twitch fiber training. If you want big visible muscle, you have to train your fast twitch muscle fibers, and the best way to do this is with isometrics.