Originally Posted by UrsAnn

Cycling "kills" muscle because a competitive cyclist (not your average person out for a recreational ride once or twice a week) is going anaerobic. Simply put, when you are producing the amount of energy we are discussing your body will pull from the easiest source of nutrition, fatty acids and glucose, once that is depleted it will start to pull nutrition form your muscles. Let's say you strength train 2 times per week for 30 minutes each time. You are building muscle which is the act of tearing down the tiny muscle fibers and then they heal and are bigger, now capable of a stronger load in strength. Well, if you go out a cycle for 1-2 hours 5 times per week and your body will eat your muscle once it has tapped out your fatty acids and glucose storages which it will quickly do with that much cardio. Again, we are not talking about the average person that is riding 10-15mph, but someone who is going all out for 1-2 hours at speeds of 16 at the slowest to 30 mph and watts of 150+ (lower watts for women).