Raise your hand if you believe that yoga promotes bone health and helps prevent osteoporosis. Keep your hand raised if you think research has shown this to be true. Hold that hand high if you’ve read this research. Look Ma, no hands. In fact, there are no randomized, controlled trials to support claims of many yoga teachers, including me, that practicing yoga promotes healthy bones and prevents osteoporosis. We don’t actually know yogasana’s effect on bone mineral density nor has anyone studied how much force specific yoga postures generate, which is thought to be important to bone density – until now.
Exercise benefits bones in two ways. 1) Ground reaction force (GRF) is the ground pushing back back into a body with equal force. For example, walking has a GRF of 1 to 1.5 times your body weight. Running is 3-4 X body weight. When you are standing or walking in alignment, gravity compresses bones of your spine, pelvis, and lower extremities, which respond to these loads by laying down more minerals, thus increasing bone density 2) Muscle contractions load bones, again signaling them to lay down more minerals, thus increasing bone density. This bone mineral density (BMD) is used as a measurement of bone health. Research shows that both high and low impact exercise increases BMD in the spine and femoral neck, which are the areas most studied. In non-seated, non-supine yoga postures, one to four of your limbs support your body against gravity. Force is generated as you move into, hold, and move out of postures; and as you shift weight between your extremities. The nature of yoga suggests that it is a low-impact exercise that uses the body’s own resistance to generate force, but very few studies even measure GRF in exercise and none in yoga – until now.
In the first study of its kind, Sylvia Wilcox, a yoga teacher and lead researcher, measured GRF for 28 weight bearing Hatha yoga poses in a study published in 2012 in the International Journal of Exercise. Her research team used parameters from the only known previous study that divided exercises into either high-impact or low-impact GRF, where high impact was equal to or greater than 2 times body weight and low impact was less than two times body weight.
My favorite part of the study is a table that lists GRF measurements for a sample of the 28 poses studied. But, I wanted to see them all, so I located the lead author’s Master’s thesis that did indeed report GRF for all 28 asanas. The intention of this study was not to compare asanas and rank/recommend those with the greatest potential for bone building, as tempting as that may be for someone like me scanning the list. It’s main purpose was to obtain ground reaction force data from common yoga postures to see how their generated forces compare to activities like running, walking, dancing, jumping jacks, etc. As expected, Wilcox’s study was able to define yoga as a low-impact practice in terms of measured GRF. In fact, yoga measures lower forces than any activities measured in previous studies.
Age, Weight, & Gender and Force Generation
Analysis showed no significant differences between test participants due to weight or age. For five of the 28 postures, there were significant differences between men and women explained by differences in centers of mass between the sexes. For postures connecting upper & lower body to the ground (think plank) force through the arms is greater for men than women because men’s center of mass is concentrated on the upper-body; conversely, force through the feet is greater for women than for men, because women’s center of mass is in the pelvis. In virabhadrasana/crescent, as part of Surya Namaskar (Sun Salutation), as subjects transitioned from Adho Mukha Svanasana (downdog) men generated more force in the front leg than women. It was theorized, based on researchers’ observations that because many of the males used momentum to swing the back leg forward into the lunge that they landed with greater force as opposed to the slower, more steady transition by female participants.
Upper Extremities and Force Generation
Almost nothing is written about low-impacts generated through the upper extremities. Where as handsprings done by gymnasts measure forces at 3 x body weight, yoga’s chataranga, updog, and pincha myurasana generate forces less than 1 x body weight, with only plank and crow generating maximum vertical forces of 1.08 and 1.05 body weight respectively. The researchers designed a small six month intervention with one non-yoga exercise that produced a similar range of forces in the upper extremities as those in the yoga study. In the intervention, changes in bone mineral content were recorded. It is entirely possible to replicate the magnitude, rate, and frequency of such impact forces using yoga postures instead of the non-yoga exercise as performed in the intervention study, thus showing that yoga may influence bone mineral density.
Can You Optimize Your Yoga Practice for Increased Bone Mineral Density?
Wilcox observes that in prior animal studies, low impact exercise with rest intervals produced similar bone building results as high-impact forces. If these findings apply to humans, could Hatha yoga, which is an excellent specimen of low impact applied loads with rest intervals between postures, be sufficient to stimulate bone cells in practitioners? Remember, no study, including this one, has attempted to show that yoga does or does not increase bone mineral density. However, it is known that resistance training in the form of concentric and eccentric loading of muscles, not only increases tendon stiffness that makes them stronger and able to withstand greater loads, but is also osteogenic, or bone mineral producing. You can make your own yoga practice potentially more osteogenic by finding opportunities to turn static, passive stretches into dynamic postures that explore active mobility, use your own body as resistance, and include isometric, concentric, and eccentric muscle work so that you are generating force at all ranges of motion. I go into more detail about these methods in an earlier blog post, called Strong at Any Length.
If you want to play now, check out these free video shorts (less than 3 min each) that demonstrate how to potentially bone up your asanas.