So, "everyone" gets shorter with age and this phenomenon is usually chalked up to compression over time. It is said that your height decreases because either the discs between your vertebrae have flattened out or, your bone density decreases with age, creating weak bones that can no longer stay upright so they curl forward. This seems to be the general understanding of why we get shorter as we get older. However, things are not always as they appear.
What I'm about to explain here is how someone can measure as shorter without any change in their mass -- only in how their mass is oriented. I've failed, many times, to get this point across to large crowds of clinicians and they always end up looking at me like I'm insane. How can someone possibly be shorter if some body part hasn't changed in length?
Let me show you.
#1: Here is a kindergarten-style model, complete with brads. (I said kindergarten-style, ok?) I've taped the foot down, drawn a posterior reference line for head positioning, and then measured my model's height (Ht. 1).
#2: I've add some pelvis thrust and did a remeasure (Ht. 2). (If you click on the picture it looks like the head is below the line, but that's because the top of the head is curling up toward the camera. Apologies, I did this while making--and burning--my kid's breakfast sausage.)
#3 I added a little knee flexion and remeasured the height (Ht.3)
#4: Then some post-tilt to the pelvis (Ht.4)...
Because this is totally sloppy, here's an up-close pic of the difference between the first and fourth measure.
Height loss, folks, without any change in the mass of the model.
A detail like this--that an objective measure like height can be totally subjective depending on how a person stands over their history of measurement-- is critical when reading research (how, specifically, was height measured, for example) and when trying to figure out how a disease works.
Many of you know my work on pelvic issues, but I'm really in love with bone and all of its processes -- including how it responds to load. There is a lot going on in bone disease research but the foundational tenets always seem to stem from an oversimplified model of forces--that the body is essentially a box where one end can be squished toward the other instead of many parts connected by many hinges that can deform in many directions. Simple models are great for kindergarten-style blog posts (!!!) but they shouldn't be the basis for academic theory--especially when a more complex model is so simple.
What I've presented here isn't rocket science or genius (the construction paper should have given that away); It's geometry. But if you aren't thinking about cellular health in terms of geometry, you'll miss the more biologically plausible explanation of bone loss, crumbling vertebrae, compressed and bulging discs. Maybe even worse, you'll be tempted to explain disease in terms of unavoidable gravity, making it seem like something that has nothing to do with your behavior beyond being on the planet. How debilitating is that message?