Something to be very carefull of- if you don't crosstrain with a weight bearing type activity you'll make a training adaptation and get the bird bones. It's really creepy feeling too, been there for a short while. Got mine back but you don't want to go there.
Literally, you'll ditch bone mass and get thinner and sometimes hollowing in the bones just like a bird. This is not only a pre osteoporosis problem, but if you biff you break really easily. You can tell the people who have this, they're usually really scrawny gaunt men who ride and ride but don't get that healthy buff look like the tri guys or racers who go to the gym. There's often muscle mass loss with it too.
I know a guy who got this, then got caught in a pile up and broke a leg in three places. Wouldn't heal for a couple years! Estrogen protects us to a point, but why take a risk?

Lizzy

I've been doing these Pilates exercises for core strength- they're amazing! Don't have a link, got them from PT but my SI joints were wobbly and I've gotten really stable lately. Six pack abs are back! Get a book, not a tape- The tapes don't explain it very well.