I've been thinking about this a bit. I get most of my exercise from cycling. I keep telling myself to also add some upper body exercise, but haven't gotten motivated for that. Plus I can ride my bike for hours, but can only motivate myself to do weights for a few minutes.

And, I think my cycling muscles are getting stronger. Maybe.

So, could it be not really losing muscle as much as moving muscle from where it isn't being used for cycling to where it is being used for cycling.

I.E. One becomes a faster cyclist, but slower runner.