Bending backwards for babies and ballet

I currently have three pregnant students, all at slightly different stages, so the following item captured my attention. I think this might also explain why women dancers – not to mention gymnasts – can bend backwards so much further and easier than men.

Nature has a new article detailing the recent discovery that women’s spines have evolved to be more flexible and supportive than those of men to increase comfort and mobility while bearing the weight of a developing child. The adaptations can be traced back as far as Australopithecus, more than two million years ago.

A female australopithecine, like today’s moms, used her spine to support baby’s weight.
John Gurche

Katherine Whitcome and Daniel Lieberman from Harvard University in Cambridge, together with their colleague Liza Shapiro of the University of Texas at Austin, measured the centre of mass of 19 pregnant women and found that they leaned back by as much as 28º beyond the normal curve of the spine, they report in Nature 1. The researchers found this lowers the torque around the hip created by the baby’s weight by roughly eight times.

Exaggerating the curve in the lower back can place more stress on the spine: vertebrae are more likely to slip against each other, leading to back pain or fractures. Whitcome and her colleagues found that a woman’s spine has several features that help to prevent that damage. In women, the curve in the lower back spans three vertebrae; in men, it encompasses just two. The added vertebra helps distribute the strain over a wider area.

In addition, specialized joints located behind the spinal cord, called zygapophyseal joints, are 14% larger relative to vertebrae size in women than in men, suggesting that the joints are well adapted to resist the higher force. The joints are also oriented at a slightly different angle in women, allowing them to better brace the vertebrae against slipping.

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