Originally Posted by
AndreyT
When pedaling, the upper leg bones experience side loads mostly. The lower leg bones, on the other hand, experience significant length-wise loads. Under these circumstances, you should expect cycling to be beneficial to strengthening your lower leg bones (as much as these bones can be strengthened by periodic-loading type of exercise).
Another hogwash.
While the rest of this reply is spot on, the "side" loads theory can not pass muster.
Human bones are not suited for side loads whatsoever. They break upon decent bending. This is why leg muscles (upper and lower) are attached to the bones the way they are - to prevent side loads. Leg muscles exert loads mostly longitudinal to bones.
As posted elsewhere in this thread, what matters is impact loading, which only mountain biking can ensure to an extent. Road biking does not help bone strength/density.
Does not hurt it either.