Cycling is an aerobic sport so (lack of) muscle mass is not terribly relevant to success in racing. Just look at the pro peloton and you'll see guys with, and without, significant muscle mass. It's largely driven by genetics. The only place added weight from muscle mass is going to hurt you is climbing where it will impose a certain penalty in your watts/Kg. Weight training also won't help your cycling performance (outside of say track sprinters) so it's not something many hard core cyclists bother to do. That's not to say you shouldn't incorporate weight training into your exercise plan if you want to. Cycling is not a weight bearing exercise so cross-training with some walking/running or weight training are necessary to maintain bone density as you age.
Last edited by Dunbar; 06-29-14 at 11:32 PM.