Depends on the cycling. I use just about all of my muscles with the way I cycle. If you are talking road cycling than the muscles used (while still most of them) are limited more to the lower body.

http://www.kidport.com/RefLib/Scienc...odyMuscles.htm
http://www.ama-assn.org/ama/pub/category/7169.html
http://www.ama-assn.org/ama/pub/category/7167.html
http://www.ama-assn.org/ama/pub/category/7170.html

Enjoy.