Here is an article from the National Osteoporosis Foundation http://www.nof.org/articles/18
and here is one from the International Osteoporosis Foundation
No flexion is a pretty standard reccomendation that has been well documented for many years. A quick acedemic search should also yield many studies to back this reccomendation.
The third edition of ACSM’s Exercise Management For Persons With Chronic Diseases and Disabilities. Specifically Chapter 36.
ACSM’s Resource Manual for Guidelines For Exercise Testing and Prescription (Fifth Edition)
ACSM’s Guidelines For Exercise Testing and Prescription (Eights Edition)
I hope this give you some direction.
Joanne’s & Janet’s answers are right on. Both the National and International Osteoporosis Foundation’s are excellent resources for information on osteoporosis research and application. Janet’s given you their website info.
ACSM is always a great resource for information regarding research and application across the field of exercise, including exercise and osteoporosis.
Take care, Daniel