Last updated: March 20, 2020
One prominent yoga teacher said
One of the primary dangers of Yin Yoga practice, in general, is that it encourages extended periods of flexion of the spine—something that should be avoided by someone with low bone density, osteopenia or osteoporosis.
This quotation is from Margaret Martin’s 2012 blog post. She doesn’t dismiss Yin Yoga out of hand, and in fact lists several Yin Yoga postures that are “generally safe for individuals with osteoporosis, osteopenia or low bone density.” However, she also lists several postures that, in her view, are dangerous because they put undue flexion stresses along the spine, too much rotation stress along the neck of the femur, or too much stress into the knees. She may be right—for a few people—but she may also be overly protective for the vast majority of folks.
Margaret is very experienced in teaching students with osteoporosis. She believes that these students must not allow their spines to flex at all: all flexion must come from the hips. I believe that this is an over-reaction and an over-simplification. Outside the yoga studio, people with osteoporosis flex their spine through simple, daily living. Leaning over the bathroom sink to brush teeth, bending over to tie shoes, reaching for the kale in the fridge or for the salt on the table all cause some flexion in the spine. Yes, they can go too far: but to never stress the spine will lead to atrophy of the tissues you are trying to protect. (There is a term for this: disuse osteoporosis!) For people with mild osteoporosis, some flexion is necessary to maintain back health; for people with severe osteoporosis, caution is reasonable.
Opinions on this topic vary widely. Greg Lehman, a well-known physical therapy teacher and trainer, has said, “Spinal flexion under low loads (like stretching) has not been proven to be a risk factor for disc pathology.” Indeed he has written extensively on this topic and considers the concern to be unwarranted.
On the other hand, in his book Yoga for Osteoporosis, Dr. Loren Fishman, a yoga teacher and M.D., cites a 1984 study done by researchers at the Mayo Clinic that show flexion of the spine is very bad but extension is very good. However, the flexion exercises were yang exercises: they were basically sit-ups. People who cite this study often extrapolate and say any flexion of the spine is harmful. But this was not what the study claimed.
It is great that the Mayo Clinic study showed extension of the spine (basically Locust poses and seated scapular retractions, the kind we do in Eagle Arms) helps reduce the risk of fracture. This shows that we should keep doing extensions of the spine and build spine health. (In the Yin Yoga repertoire, keep offering Sphinx, Seal and Saddle pose.) But are all flexions contraindicated? I have found no studies that have investigated long-held, gentle stress of the spine in flexion and its impact on bone mineral density. At best we can say we don’t know, but maybe to avoid any risk we should avoid all flexions. However, is this really appropriate?
Avoiding all flexion may be harmful to your spine! All tissues need stress to become or stay healthy. Too much stress can be harmful, but so can too little. The Mayo Clinic study above showed that yang stresses on the spine are not good for women with osteoporosis, but they did not study women with osteopenia (a precursor condition to osteoporosis) nor the effect of yin stresses. We have known since the time of Julius Wolff that bones need some stress to remain strong: if we never allow any stress of the bones, they will atrophy.
Yang yoga can generate a large amount of transient stresses on the bones and joints due to the pulling power of the muscles. Indeed, a muscular stress on the bone generates several times more stress than the body’s own weight can generate. However, Yin Yoga theoretically generates smaller stresses because they are only due to gravity, not muscular effort. In Yin Yoga, time is the magic ingredient, not intensity. There is a big difference between yang stresses and yin stresses, but there has been no studies evaluating these differences. All we have to guide us is reasoning by hypothesis and individual experiences.
In 2015, Dr. Fishman and his fellow researchers completed a long-term study on over 700 people performing a daily 12-minute yoga practice using 12 postures. About 83% of participants had either osteopenia or osteoporosis. Bone scans to measure bone density were done at the beginning of the study and then about 4~5 years later. The poses were held on average for about 30 seconds per side, so these were “yang” stresses. The postures included several standing poses, twists and back extensions. No direct flexion postures were included. The results of the study showed significant improvements in bone mineral densities. The conclusion of the study was
The 12 yoga poses studied here appear to be a safe and effective means to reverse bone loss in the spine and the femur.
These 12 yang postures proved to be safe and beneficial for osteoporotic spines. Is Yin Yoga safe for students with osteoporosis? Maybe! Certainly, care should be taken, but some stress to the bones is healthy and Yin Yoga can be a safe way to obtain that stress. Dr. Fishman’s studies have shown that twists and extensions can be safe and helpful. However, since every body is different, whether you should do flexions is not something we cannot say a priori. Work with your medical team/therapist; experiment a little; check in with the results and adjust from there.
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 Greg Lehman, “If you want to stretch your hamstrings please continue to do so,” Reconciling Biomechanics with Pain Science (2012).
 A more recent, 2019 Mayo Clinic study on the same topic found that only extreme flexion or extension positions caused problems. “Yoga has many benefits. It improves balance, flexibility, strength and is a good social activity,” says Mehrsheed Sinaki, M.D., a Mayo Clinic physical medicine and rehabilitation specialist and the study’s senior author. “But if you have osteoporosis or osteopenia, you should modify the postures to accommodate your condition.”
 Again, I recommend Dr. Fishman’s book Yoga for Osteoporosis for a good overview of the cellular effects of exercise on our bone cells
 See Yi-Hsueh Lu, Bernard Rosner, Gregory Chang and Loren M. Fishman, “Twelve-Minute Daily Yoga Regimen Reverses Osteoporotic Bone Loss,” Topics in Geriatric Rehabilitation 00.0 (2015): 1–7.