> In which situations a normal mammal find its bones squished (but not crushed)?

Running, exercising, etc.

Pure speculation on a few possible explanations (noting that TFA describes ordinary exercise as generating enough squishing force, any other actions the researchers did being to isolate the causal factor):

1. Mammals don't naturally exist in environments where exercise isn't mandatory. _Some_ system had to generate those compounds, and there was never any pressure to keep it from being tied to the other stress-related changes bones undergo (like density changes). Once it evolved once somewhere in some critter once via that machinery, the advantage was large enough that it was locked in.

2. Exercise directly contributes inflammatory factors. Tying the cure to those to a measure of exercise intensity (bone squishing) helps calibrate the dosage. Even if an imperfect measure, the proxy of bone loading might confer some advantage in, e.g., not wasting energy on unnecessary proteins.

3. Exercise indirectly suggests a resource-poor environment, leading to lower chance of offspring succeeding and needing longer lives to produce enough viable offspring. The compounds in question aren't free to produce, so there's an advantage in only providing them to organisms who will meaningfully benefit. Mind you, resource-poor environments often promote shorter lifespans and faster growth cycles as an evolutionary trait, _but_ without a mechanism to force your offspring to mature more quickly the osteocyte-mediated lifespan could allow for resource-dependent lifetime offspring counts.

4. The mechanism could have evolved for unrelated reasons, perhaps directly contributing to the bone density calibration osteocytes already mediate. Any impact on lifespan is incidental or otherwise produces less evolutionary pressure for or against the feature than the direct purpose of the compounds.

5. More than having evolved for unrelated reasons, the fact that the compounds have any effect at all might not be evolutionarily selected for. We produce plenty of unimportant metabolic byproducts that we don't particularly care about, and it could be that some more important osteocyte function incidentally produces these "waste" proteins.

I don't know if any bodily phenomena are so easily explained. The insides of a single cell are insanely complex, and to try to apply a singular causal explanation to macro phenomena seems implausible.

What do people even mean by "curing cancer", "curing aging", it's not like a single thing in the first place

The leg bone connected to the aging bone.

Was kind of expecting/hoping for this bone song: https://www.youtube.com/watch?v=pDJKgi2e-Aw

It's not quite vice grips; I had to dig into the paper to understand it. Their specific DTAL protocol provided "moderate mechanical loading" of "2 N at 2 Hz for 300 cycles, twice/week, [for] 3 months". For reference, a 200g stationary object exerts that much force on a surface due to gravity. With 300 cycles at 2Hz, the exercises lasted for 150 seconds, or 2.5 minutes, and the exercises occurred twice a week.

Note, however, that their experimental model section says they applied axial force (think columns, not joists) to the tibia using an ElectroForce 3300 (https://www.tainstruments.com/3330-system/) while the mice were under anesthesia. From this, it seems like they knocked out the mice and placed their legs underneath a mechanical ram to apply axial compressive forces.

Because people are typically awake when exercising, given that muscles are designed to carry forces around the skeleton, I imagine, even after scaling from a mice to a human purely by sizing, the human force equivalent will be many times higher.

If you're looking to try this out for yourself, the seated leg press will most closely simulate the study's method. Figure out what "moderate" loading is for you, and shoot for 3 sets x 10 reps at least twice a week. Overall, choose skeletally-compressive exercises where you're "pushing," such as weighted squats (e.g. barbell squats) and any type of "press" exercise like the leg press, chest press, military press, and even cycling (you push your pedals). For the purposes of this study, avoid exercises with any pulling motion, including pull-ups, pull-downs, rows, and swimming.

From a force perspective, running has some compressive elements, but the load comes from gravity alone, and good runners minimize compressive forces. Since running therefore is primarily a pulling exercise, I can sort of see what they're getting at.

TFA explicitly calls out exercise as something which would generate those compounds. Exercise has many other impacts though, so if you're trying to isolate osteocytes as a causal factor you want an experiment which avoids interacting with those other systems as much as possible.

Lots of kinds of interactions with the physical world could potentially compress or jostle bones