Chapter 6 — Use It or Lose It: The Law of Load

What does the bone teach? Load is answered by load. Give it nothing? It gives you less. Give it a day worth answering? It answers back.

A healed bone is not finished. No bone is. Bone doesn't have a shelf life. It has a subscription.

Almost every bone in your body is being torn down and rebuilt, a little at a time, all the time. More where you use it. Less where you don't. There are no deadlines. Only cycles.

This is the chapter of the law. Here's the story of the men who wrote it down, told once, and the cells that actually run it.

The Diary

Bone keeps a diary of what you asked of it. Not in words. In architecture.

The hard outer wall, the cortex, thickens where force keeps showing up. The struts inside line up along the lines of strain. A tennis player's racket arm grows a thicker wall: in one study the cortex of the upper arm bone was about 35 percent thicker in men and 28 percent in women than on the other side (Jones 1977). Our reading: a different load history likely sends the brain a different body map too. Not shown. Disuse is an ask too: mineral withdrawn, lattice thinned, because the bone was told the day had gotten small.

A life that only uses one sentence of the diary will get a bone that looks like that sentence. One motion can be the fastest route on the map. It is still one sentence. Life brings traffic. Keep the other routes.

The Men Who Wrote It Down

People noticed the diary a long time ago. In 1867, the anatomist Hermann von Meyer and the engineer Karl Culmann saw that the struts inside the top of a thighbone follow the stress lines an engineer would draw for a crane (Skedros & Brand 2011).

Julius Wolff, a German surgeon, turned that into a law in 1892: bone takes the shape of its load (Wolff 1892). The name stuck to medicine like a good nickname. "Wolff's law." Some of his math didn't hold up. The big idea did. Today's careful scientists prefer the phrase bone functional adaptation: loading shapes structure, disuse sheds it, habit is recorded in geometry (Ruff, Holt & Trinkaus 2006). Wolff as a historical label, yes. Wolff as an exact equation stamped on every femur, no.

Then came Harold Frost, an orthopedic surgeon working out of Pueblo, Colorado. Frost gave the idea a machine, and he called it the mechanostat (Frost 1987; Frost 2003).

Think of a thermostat. A thermostat checks the temperature and turns the heat up or down to keep the room where it should be. Frost proposed that bone does the same with load. Enough strain, in the right places, and bone builds or keeps what it has. Too little for too long, and the rebuilding crews remove what the day no longer justifies. Push past a healthy overload and tiny damage can outrun repair, and the story tips toward trouble.

Frost himself called it a hypothesis. A good one. It predicted a long list of things, and he kept updating it (Frost 2003). His exact numbers for the set points are debated. They differ by bone, by age, by hormones. So we won't carve them into stone here. The idea is what you need in your teeth.

Frost's Lesson

You met Frost's lesson in Chapter 1. It bears repeating.

Bone gives back what it gets. Treat it to a life of real, varied load, and it builds itself into a frame that can carry that life. Treat it to nothing, and it quietly pulls out. Not as punishment. As bookkeeping. It's the Golden Rule written in mineral: what you give is what comes back.

And it runs both ways. You load your skeleton. Your skeleton holds you up. Hold up your end, and it holds up its end. Praise the load.

What did Frost teach? Use it or lose it. What does bone give back? What you give it.

What Kind of Load?

Not all load teaches the same lesson. Experiments in animals sharpened the grammar (Meakin 2014):

• Changing load beats steady load. Bone answers motion, not just weight. A load that never varies stops telling bone anything new.
• Big peaks matter. So does how fast the load arrives.
• Unusual patterns get noticed. A new direction of strain gets more attention than the same old one.
• A few cycles can be enough, especially when rest breaks them up so the bone doesn't get bored.

That's not a workout brochure. It's a description of what bone listens for. And age, sex, hormones, drugs, and the particular bone all change the answer. One mechanostat setting for every cubic millimeter of every person is too neat. Local mechanostats, with modifiers, is closer to real life.

Take the Load Away

You met the astronauts in Chapter 1. Here's the rest of their story.

When gravity stops pulling down, the body's fluids shift toward the head. In a review of 148 space travelers, the lower limbs lost about 5.4 percent of their bone density, and the lower spine and pelvis about 6.2 percent. The skull gained about 2.2 percent (Stavnichuk 2020). Bed rest on Earth does something similar: in people kept in bed for 17 weeks, skull mineral rose while weight-bearing sites fell (LeBlanc 1990). Even people sealed for 520 days in a pretend Mars mission gained skull mineral (Novikov 2016).

Take the load away from the legs, and the legs give bone back. Change what the skull feels, and the skull answers too. Bone follows the load it actually gets.

Our reading: A life spent sitting and lying more is a fluid shift too. We'd expect a small version of the same effect. That hasn't been measured.

The Cells That Run the Place

Deep inside every bone, sealed in tiny chambers of mineral, live the cells that run the place. They are osteocytes.

Each one started life as a builder on the bone's surface and got buried in the very bone it made. It didn't die. It grew dozens of thin arms that reach through tunnels a few hundred nanometers wide to touch its neighbors. Researchers estimate the adult skeleton holds about 42 billion of them, linked by about 23 trillion connections. Laid end to end, their arms would run about 175,000 kilometers, more than four times around the Earth (Buenzli & Sims 2015). By one calculated estimate, about 9 million new osteocytes take their places every day (Buenzli & Sims 2015). The tunnels they live in are about 300 times thinner than a hair (Pazzaglia & Congiu 2013; Franbourg 2003).

That's a network on the scale of the brain's. Different tech: fluid and chemistry instead of electric spikes. Same basic job. Feel. Decide. Give orders.

The builder stays behind, buried, and becomes the sensor. Same cell. New address. New job.

How the Sensor Feels the Load

When you walk, bone bends a tiny amount. That bending likely pushes fluid through the tunnels, a model worked out on paper (Weinbaum 1994). In a lab dish, chicken osteocytes, and not the builder cells, sent out a chemical signal when fluid pulsed past them (Klein-Nulend 1995).

What kind of flow matters too. In one lab test, steady or pulsing flow in one direction raised a calcium signal in bone-like cells much more than back-and-forth flow (Jacobs 1998). Those weren't osteocytes, and the test measured a signal, not new bone. So the exact message is still being worked out.

Here's one thing that's clear. When osteocytes sense use, they lift a brake. The brake is a protein called sclerostin. Load turns sclerostin down, and bone gets built (Robling 2008). Take the load away and the brake goes back on. Osteocytes also help call in the cells that clear bone, so building and clearing stay in the same system, not two separate hobbies (Dallas 2013; Qin 2020).

When a small crack runs through bone, it snaps the osteocytes' arms, and cells beside it die. That silence, or that damage, is a signal too. The cells around it call in the crews to tear down and rebuild that patch. Some osteocytes live for decades (Dallas 2013), buried sensors keeping your skeleton matched to the life you actually live.

Current, or a Current of Fluid?

Squeeze dry bone and it makes a tiny voltage. Collagen does that on its own: it's piezoelectric (Fukada & Yasuda 1957). For decades, people hoped that current was the message telling bone where to build.

But living bone is wet. In wet bone, most of the voltage comes from fluid pushed through the tiny canals, carrying charge as it moves (Gross & Williams 1982; Otter 1985). Most researchers now think the fluid flow itself is the main signal, tugging on osteocytes that send chemical orders. The electricity is real. It's just more likely the messenger's footprints than the messenger.

Current, or a current of fluid. Let's not get our wires crossed.

The Crack Draws the Map

Here's the part I love.

Bone doesn't just survive tiny cracks. It may use them. Where load lands harder, more cracks form. More cracks mean more bone dug out and rebuilt. Every rebuild lays fresh bone along the line the force travels. More turnover, straighter lines.

Our reading: The cracks draw the map, and the rebuilding follows it.

Prove us wrong: Find bone that's heavily loaded, cracking, and turning over fast, where the new rebuilt tubes don't line up with the load.

What Adult Bone Can and Can't Do

Grown-up bone is a busy editor. It is not a sculptor.

Once the growth plates close (Chapter 5), adult bone can thicken or thin the layers it already has. It can make the cortex denser where load keeps coming, and thinner where it stopped. In a bowed knee, the inner side carries more load, and the bone there answers by getting denser. Adult bone mostly keeps the big shapes it was given. It doesn't regrow the round shape of a ball. What it adds comes at the edges: bone spurs and flattened rims, usually where a joint is wearing out.

An X-ray shows the mineral that's left. Its outline was set mostly during growth. Later loading changes the thickness and the density.

So the adult lesson is simple: you can't redraw the house, but you can keep it lived in.

Cartilage Plays by a Different Rule

Cartilage is a different machine with a similar rule (Chapter 3). Cartilage has no blood vessels. Small nutrients reach it by diffusion; cycling load helps move the large molecules (O'Hara 1990). It has no rebuilding crews either. Bone builds where it's used. Cartilage stays healthy where it keeps meeting. Take the meeting away and that patch starves, quietly.

Close

Bone is not a stick that happens to bleed. It's a living mixture that edits itself under every step, lift, twist, and bed-bound week you hand it. Genes matter. Hormones matter. Food and age and disease all change the answer. But load always belongs in the sentence.

What does the bone teach? Load is answered by load. What's the commandment? Use it, or it leaves. And the Golden Rule? Do unto your bones as you would have them do unto you.

Next: where the calcium goes when you need it, and who's doing all that rebuilding.

— Dr. Blakley, D.O., with M. Chad Mahan, M.D.

Sources for this chapter (full references in SOURCES.md): Wolff 1892; Ruff, Holt & Trinkaus 2006; Frost 1987; Frost 2003; Meakin 2014; Stavnichuk 2020; LeBlanc 1990; Novikov 2016; Lang 2004; Buenzli & Sims 2015; Pazzaglia & Congiu 2013; Franbourg 2003; Weinbaum 1994; Klein-Nulend 1995; Jacobs 1998; Robling 2008; Dallas 2013; Qin 2020; Fukada & Yasuda 1957; Gross & Williams 1982; Otter 1985; Ma 2022; Jones 1977; Skedros & Brand 2011; O'Hara 1990; Sansalone 2013.