shoulders are the poorest designed feature of the human body

on monday i was doing basic yardwork and somehow tore my shoulder labrum, which i didn't know until wednesday. i wasn't convinced that i could've had an injury like this from only shoveling mulch so i started researching how shoulders work.

turns out they're pretty badly designed.

most joints in your body are built around stability. hip is a deep ball-and-socket. it sits inside a cup that's more than half enclosed in bone. knees are hinges reinforced by some of the strongest ligaments in the body. joints are typically engineered conservatively. they do a limited range of things and do them fairly reliably. except shoulders. they said no to that.

the shoulder joint connects your arm to your body also via a ball-and-socket but the cup its supposed to sit in is as shallow as a plate. imagine balancing a golf ball on a poker chip. that's your shoulder. the only thing that keeps your arms attached is a ring of fibrocartilage (labrum) and a handful of tendons (rotator cuff).

shoulders were designed this way because humans need an enormous range of motion in their arms. we evolved to throw things and reach overhead. the shoulder delivers on that, it's the most mobile joint in the body, but the problem is mobility and stability are fundamentally at odds. evolution looked at the tradeoff and chose mobility. shoulders are why we have orthopedists.

the labrum is just a rim of soft tissue that's sort of deep enough to hold things there. it's not bone and it has barely any blood supply so if you damage it, it heals slowly and badly. this is what the orthopedist explained to me after viewing my shoulder and then explaining the anatomy to me like i might find it interesting.

shoulders are apparently the most injury-prone joint in the body. evolution had millions of years to fix that problem and has not. orthopedic surgery is the highest paid speciality in medicine. i'm just saying.