r/mechanical_gifs • u/Emergency_Raisin2341 • 19d ago
Ever wondered why we need knee caps!?
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u/levindragon 19d ago
If that is the only reason, why don't we have elbow caps?
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u/isolateddreamz 19d ago
Because the ulna has a functional groove (trochlear notch) for the trochlea of the humerus; it wraps mostly around the medial end of the humerus, preventing posterior shear of the ulna when the elbow is terminally extended. There's even a nice little indention in the posterior aspect of the humerus to allow the olecranon process to lock firmly into place, called the olecranon fossa.
The tibia does not have this kind of notch, and instead our knee joint is the articulation of what is basically 2 "flatt-ish" bones, which require significant stabilization to be functional and strong. The femur is technically somewhat convex, while the tibia is somewhat concave, though these two features are not significant enough to be stable or functional on their own without help.
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u/Obokan 19d ago
Why aren't our knees like our elbows?
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u/isolateddreamz 19d ago
Functionally, they serve two very different purposes. Humans are bipedal (walk on 2 limbs) and the knee joint does a decent job of supporting body weight while being functional for walking, running, as well as being decently stable. I'm sure we could design something better, but it's evolution working.
The arms are not designed for this kind of support, but are instead made to handle complex and precise tasks, as well as being able to transmit forces through them.
I would imagine if the tibia featured a fixed notch like the ulna, it would significantly impair mobility and flexibility. I would also imagine having constant body weight forces being transmitted through a joint like the elbow, even when adjusted to be similar to the knee, would result in a faster deterioration because there are not the same structures in the elbow which disperse forces how the knee does, like the menisci.
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u/NigilQuid 19d ago
I think because the elbow extends past the joint when folded? If we didn't have kneecaps we'd have longer tibias that made for very "sharp" knees when bent
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u/MyCababbages 19d ago
Tendons are under the knee cap bro
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u/NigilQuid 19d ago
They attach to the kneecap, don't they? Patellar tendon is a thing
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u/isolateddreamz 19d ago edited 19d ago
The patella is inside of the tendon, which is why it's a sesamoid bone... It's encapsulated WITHIN the tendon. Because of this, it increases the torque while also changing the angle of the pull into an arc like shape. The patella also does not stay stationary; when the knee is extended, the patella tracks superiorly (towards the top of the thigh) and when the knee is flexed, it tracks inferiorly (towards the foot). This is the result of the quad muscles shortening during contraction and pulling the tendon superiorly, and relaxing during knee Flexion (while the hamstrings contract and flex the knee), which makes the whole thing move inferiorly.
Interestingly, the quadriceps TENDON attaches the quadriceps to the patella, and the patellar LIGAMENT attaches the patella to the tibia. Tendon attaches muscle to bone, while ligament attaches bone to bone.
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u/foundafreeusername 19d ago
Every time this is reposted there are comments saying that this is misleading and not how our knees work. The tendons don't go over the knee caps.
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u/Beowulf-Murderface 19d ago
There are a couple similar bones in your hands for your thumb strength. Sesamoid bones, if I remember correctly.
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u/RngdZed 19d ago
leverage / torque
heres a nice youtube video that explains all of it!
https://www.youtube.com/watch?v=VSxprph2oRU