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.
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.
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.
Latest Answers