Why do we have kneecaps on our legs but no “armcaps” on our arms?
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The olecrenon process (elbow point) serves the same purpose – extending the moment arm about the joint to maximize torque.
The forward extremities bear a far lower amount force and torque, even among quadrupedal species, than the hind limbs. The forelimbs are more for balance and agility than locomotive propulsion. The mammalian class of chordates are, generally speaking, like a rear wheel drive vehicle. All of the power is in the back tires, and the steering wheel is in the front. Birds of flight, conversely, rely on their fore limbs to propel them, while steering, in some part, with their tail feathers.
Now there are certain higher primates that exploit primarily arboreal habitats for whom brachiation is the preferred mode of locomotion. Even if this exerted a similar type of force on the limbs, which it doesn’t but let’s just say it does, it would take eons of mutation and natural selection to produce a skeletal analog to the patella.
Short answer, at no point along the evolutionary path, at not a single decisive node of branching on the tree of life, has there been sufficient environmental pressure of genetic survival to warrant or reward the growth of an elbow cap. It’s just not necessary. If there has ever been a mutation that presented this particular allele, it was of no significant biological advantage.
The knee cap is a sesamoid bone, your body has many of them, they form in tendons that are under high stress. Most people also have them in their thumb, big toe or back of the knee. So to answer your question, the tendons that play a role in the elbow aren’t stressed enough to generate a sesamoid bone, but other parts of the body are.
Human limbs are modified (evolved) from their quadrupedal terrestrial antecedents, most of which also have patellas but no equivalent sesamoid on the elbow.
These quadrupeds bear most of their body weight on their forelimbs, while rear limbs are more for propulsion.
Pronation/ supination in thoracic limbs becomes more important in primates, but power remains critical in pelvic limbs for bipedal primates (humans)
Without a patella, the ligaments of the quadriceps group would not tolerate the tension across the knee joint required to propel us and other species in ordinary locomotion.
Since I’ve seen a lot of ELI10 answers, I’ll have a go myself.
First, we have to understand that arms and legs have different purposes.
Arms are generally for picking up things and reaching. Arms benefit more from extra range than they would from extra power.
Legs are the other way around, since their main purpose is supporting your body weight. Legs benefit more from extra power than they would from extra mobility, in general.
the kneecap, or the Patella, came into being through evolution to let our legs function more efficiently, in two ways.
The first way, is by providing something less prone to breaking tearing, which coincidentally also slides easier. This way, knees last way longer than they would without a kneecap, as the tendon that goes over top of the knee would wear out way faster because of all the friction.
The second way is a bit more physics based. The kneecap puts distance between the knee joint, and the part where the power is relayed through. By putting distance between the rotating point, and the point where force is applied, it becomes easier on the muscles to uphold your body weight, even though you didn’t have to increase your muscle mass to do so.
Throughout evolution, a kneecap was deemed favourable, so it was passed down and evolved into the kneecap we know and love to bump into things today.
In retrospect, it also says a bit more about the preferred characteristics of an arm. The absence of an “elbow cap” reduces power output, but allows the arm to reach a higher speed (speed and power are usually a trade-off pair within biomechanics).
Apparently, the ability to move arms swiftly was more favourable throughout evolution than it was to generate more power.
TL;DR: Arms have different functions than legs, and therefore different preferred characteristics. Evolution shaped the respective limbs for lighter and heavier tasks.
I assume it’s because the patella (the disk-ish, flat bone that sits atop the two main bones of the upper and lower leg) is sort of like a cap, while the elbow has no such bone.
how many times you fell over directly using both elbows to hit the ground?
Why is there a puppy dog, but not a puppy cat?
It would be elbowcaps, and we don’t have them because we didn’t evolve that way, simply put. The elbow is a better joint, but it doesn’t have to support as much pressure, so it doesn’t need to be so complicated. Kneecaps are certainly overcomplicated, but that’s evolution for you.
Knees go back and forth. Elbows go back and forth, side to side, and twist.
The kneecap provides stability in the back and forth motion, but you need more versatility in your elbows.
Less ELI5: It’s the same principle as your hips and shoulders. Unlike the knees and elbows, hips and shoulders are the same type of joint (ball and socket) yet you have far greater range of motion in the shoulders over the hips. This is governed by structures outside the actual joint itself (huge muscles around the hips, more tendons puppeteering the shoulder). The downside is that the more versatile joint is also less stable. This is why you hear about rotator cuff injuries more often than you do hip sprains.
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