Of all the joints in the human body, the knee is perhaps the most impressive. Not only is it the largest and most complex joint in the body, it’s also fundamental to our ability to walk upright. In contrast to the knees of other primate species, our knees are capable of extending fully, allowing us to walk comfortably on two limbs instead of four.
Whereas ball-and-socket joints like the hip consist of just a few components, the knee is made up of four major bones and 14 ligaments. It also relies on 12 distinct muscles to help us move and maintain our balance on two legs.
Despite their complexity, knees are also capable of supporting a surprising amount of weight. In fact, when evenly distributed over the kneecap, knees can support up to 5 times a person’s bodyweight. Powerlifters might like to attribute their remarkable strength to their arms and core muscles, but they wouldn’t be able to lift much without a sturdy pair of knees.
It’s also interesting to note that we’re born without fully-formed kneecaps, which act as the central points of articulation in our knees. Instead, our kneecaps form gradually in a process called ossification that can last anywhere from two to six years. If you look closely at a baby’s knees, you’ll notice that they look distinctly different from adult knees because their kneecaps haven’t finished developing yet.
This illustration gave me an opportunity to gain an even greater appreciation for the unique structure of the human knee. When viewed in a cross-section, it’s hard to believe that this strange bundle of bones, cartilage and ligaments has allowed us to become the only species of primate on the planet that walks on two legs. If our knees hadn’t evolved in this fashion, we might still be much more similar to our tree-dwelling ancestors than we are today.