You want to avoid compression force on the vertebrae. Especially on the anterior portions of the vertebrae. Things like crunches and sit ups are examples of these.
The key to exercise with the goal of slowing bone loss or improving bone density is to create tension on the attachment sites of the muscles and ligaments. It is very similar to any exercise progression. But the degree of osteoporosis is very important. You don’t need a lot of tension to get a change in bone density. But it does need to be greater than the person is used to placing on the connective tissues of the skeletal system. And it needs to be applied regularly. You can’t exercise once a week and get more bone density. And physician clearance is absolutely necessary.
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
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.