I wish I could find this article on Bicycling Magazine's web site:
http://www.beezodogsplace.com/Pages/...s-Cycling.html

See #6 on particular:
RUN, HIKE, SKIP ROPE OR JUMP AROUND with your 6 kids for 20-30 minutes, two to three times a week, to promote bone growth with weight-bearing vibration. "The impact must be significant." says Christine Snow, director of OSU's Bone Research Lab. "The vibrations of cycling aren't enough." A Johns Hopkins study found that light-intensity activities such as walking did not strengthen bones. Other studies hint that as few as two minutes of significant impact or vibration per day bestow some benefit.