eli5: Why is it that bone growth hurts (growing pains), but skin stretching doesn’t hurt (stretch marks)?

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We aren’t actually sure that that’s what causes growing pains, as an aside. That said, I always assumed it was the muscle strains and tendon stresses as anchor points grew away from each other faster than the muscle and tendon could keep up with.

Stargate525

Skin is supposed to be stretchy (so we can do things like bend and breath) while bones are not

1cyWind

Skin stretching hurts. I was in a wheelchair for 4 months + 3 more months on crutches before I could walk again. The months of not using them plus swelling made the skin super tight on my feet, so I had to work on stretching my skin back out. Not just by moving my ankles, but moving around the skin on top of my feet too. I lost all of the extra folds of skin on the knuckles of my toes, so I had to stretch the skin on each toe back out to be able to bend them again. Shit hurt. Skin is pretty stretchy though. Before long it had stretched back out and doesn’t hurt anymore. I’m sure moving around for the first time as a baby hurts too, folding our knees and elbows for the first time, we just don’t remember it.

itsthebab

Not all bones grow at the same time or same rate. So you will go through periods where your knees literally don’t fit together or your shoulder clicks slightly out of place, etc… there’s not a ton of flexibility there. I will add however, that acne on a stretch mark does hurt really really bad though.

VirtuallyRealized

Growing pains happen over a much faster time frame than stretch marks do. Growing pains also involve the growth of bones, while stretch marks (usually) do not. Put these two known facts together and it’s easy to see why you hear about one more than the other. That being said, stretching, even for a moment, can hurt if pushed too far. Stretching over time can also result in pain, but it’s not a commonly reported or talked about occurrence. As said in the comments, pregnancy can result in a painful itch due to skin stretching over time, and I can personally attest to knowing a person who snapped a tendon in their foot when they stretched it to far because they over extended it during a stretch.

ESGD13