NATIONAL OSTEOPOROSIS FOUNDATION
Karen Kemmis, PT, DPT, MS responded:
Spinning is similar to riding a road bike; the spine is flexed over if the handlebars are set properly and the cyclist is leaning onto them. This is correct form for riding, but not considered a safe position for a spine with low bone density.
The handlebars of a road or spinning cycle can be adjusted to decrease spine flexion by raising them up, above what is considered normal. Another option is to carry out the spinning class without leaning over to the handle bars. Cycling can be performed with good posture in the spine by keeping the trunk more upright, lifting the breastbone, and keeping the chin gently tucked down to lengthen the neck.
Cycling is an excellent aerobic/cardiovascular exercise. However, since riding an outdoor or indoor bike is very low impact, it would be helpful to perform other exercises that can stimulate bone by providing some impact.
Lunges would generally be safe if done with correct form. When doing a lunge, the trunk should be in a neutral position as described above; the hip, knee and ankle should be in a straight line when looking from the front to back; and the knee should stay over the ankle when looking from the side. To determine if the additional weight is safe, a suggestion would be that you should be able to do 12-25 repetitions to the point of fatigue. If you fatigue prior to 12 repetitions, it would be considered high-intensity exercise which may be risky for a person with low bone density.
It is great that you are without fractures. If a person has lost more than 1 1/2 inches of height from their maximal recalled height, they should suspect silent fractures and speak to their healthcare provider, even if bone density is normal or in the low bone density (osteopenia) range.
Good luck with your continued efforts for bone, and general, health!