Your Child Is Part Bear

You may be expecting some bear-related baby humor here. But this is actually a post about the amazing SI joint. I was just trying to tempt you into reading something that might sound dry.

But it’s not! The SI joint is a fantastically complex and unusual joint. Scientists struggled over the years to correctly understand it. Here, let me present you with some of their findings.

Quick review: the sacroiliac (SI) joints link your torso with your legs at the pelvis. Read more about it here.

A Phrase I Liked in High School
Ninth grade biology in the 90s. From his perch at the overhead projector, the cheerful, heron-like Mr. Gresens calls out in delight at every opportunity, “ontogony recapitulates phylogeny.”

The 19th century phrase stuck with me, and it means that the development of an individual creature, particularly in its embryonic state, mirrors the evolution of the species. Well, our awareness of genes means the embryonic development is more complex than this pithy phrase indicates, but the current accepted perspective is that “embryos do reflect the course of evolution, but that course is far more intricate and quirky,” says Berkeley’s Understanding Evolution site, than this cool rhyme.

The phrase came back to me recently when I was reading about the SI joint. In a long, detailed study, full of its own catchy phrases like “intra-articular bony tubercle,” something jumped out at me. (Not a baby bear.)

Not Weird
You had a baby–you know–it’s not weird for me to say this–brand-new babies are kind of animal-like. And science confirms it! At least when it comes to their hips. In infants, the SI joint is oriented the same way it’s oriented in quadrupeds (Solonen, 1957). Like baby bears. Or cubs, as some call them.

“052 Ambrosia” by KellyELambertPhotography is licensed under CC BY-NC-ND 2.0 

As babies begin walking. the SI joint reacts to this new skill and begins to morph into its adult form, with a propeller shape and a complex curvature. Back in 1957, a researcher named Solonen deduced that things like increasing body weight, load on the femur, lying supine, and pressure on the public symphisis informed this process. The biggest factor, though, was the twisting movement between the pelvic bones and the sacrum.

In other words, the SI joint, which is controlled by the muscles around it, develops in response to the body’s growth and movements.

Why It Matters:

  1. During pregnancy, the body grows and changes its movements quite a bit, which leads to changes in the SI joint.
  2. Since the SI joint changes in response to movements, therapeutic movement can improve its function.
  3. The muscles around it control the SI joint. So, improving the strength, firing, and balance of those muscles can return the SI joint to pain-free functioning.

If your appetite is whetted, and you gotta know more about the SI joint, you can look at this comprehensive paper, which is where I pulled the factual information for the first three sections of this post from.

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