The exercise that you should never do if you have osteoporosis is the crunch or sit-up. Dr Sinaki of the Mayo Clinic has done numerous research studies on exercise and the health implications for people with osteoporosis. In this classic study, she demonstrated that if a person with osteoporosis did crunches on a regular basis, the risk of developing a spinal compression fracture would be increased even when compared to a person with osteoporosis who did not exercise regularly at all.
As people age and as our society continues to become more sedentary, the risk for developing osteoporosis increases. With exercise it is important to pick exercises that build you up and do not tear you down. Be smart and avoid flexion of the spine if you have osteoporosis!