There is a great article in the NY Times today that talks about running, cycling, and swimming efficiency.
Dr. Coyle finds that the most economical cyclists have an abundance of a particular type of muscle fiber, so-called slow twitch. It is not known whether other types of muscle can convert to slow twitch with training. But, he said, it may be that after years of training, nerves are directed to allow more leg muscles to participate in pedaling. The result might be greater riding economy.