Some studies support a link between cigarette smoking and decreased bone density. Less bone puts the spine at risk for fracture and back pain. Exercise has clearly been shown to make a difference. You may not gain back bone mass lost, but you are more likely to keep what you have.
A new study shows smokers tend to have weaker back extensor muscles than nonsmokers. Smokers with back pain and without back pain were compared. They had about the same amount of muscle weakness. Even nonsmokers with back pain had greater strength than any smoker.
All in all it looks like exercise is the best medicine for many problems including osteoporosis. However, the benefit of exercise while still smoking has not been studied.