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Why Our Knees Are So Easy to Injure

Wow: "For example, in high school basketball, girls are 44% more likely than boys to suffer knee injuries, 165% more likely to have knee surgeries and 315% more likely to have ACL surgeries. In collegiate soccer, female players are 138% more likely than males to sustain ACL injuries and 79% more likely to have meniscus tears. Among national championship level volleyball players, women are 300% more likely to suffer knee ligament injuries."


That seems rather undemocratic?


Why Our Knees Are So Easy to Injure

Knee joints enable humans to walk upright on two legs, but the way they evolved makes them fundamentally unstable.


By Han Yu, WSJ

June 2, 2023 12:11 pm ET


The ability to move around on two legs, known as bipedalism, is a defining feature of modern humans. Yes, various other things make us human—a big brain, dexterous hands, language, material culture—but bipedalism is a turning point in human evolution. Just why it came about is debated, but experts generally agree that it gave our ancestors an evolutionary edge, helping them survive in a primitive world.


Yet becoming bipeds also doomed humans to a pair of knees that is one of the weakest links in our bodies. Anatomically speaking, the human knee “suffers from a rather precarious design,” anthropologist Owen Lovejoy has written. The knee is a hinge joint, working on the same principle as the hinges that attach a door to its frame while allowing it to swing forward or backward.


In a similar fashion, the knee connects the thigh bone (or femur) to the shinbone (or tibia). The end of the thigh bone has two round knobs called femoral condyles, while the end of the shinbone has two relatively flat sections called the plateau. The condyles fit into the plateau to create the hinge, while ligaments, tendons and connective tissues secure it in place so you can swing your lower leg.


But the human knee moves in ways that a door hinge couldn’t dream of. During dynamic activities, it exhibits six kinds of movement: It can swing forward and backward, rotate left and right, and rock a little from side to side. This multi-angle mobility allows us to move sideways as well as in a straight line, and also to turn, pivot, cut and make other movements needed in sports.


The knee is flexible because it is fundamentally unstable.


This dynamic mobility comes at a cost, however. The knee is flexible because it is fundamentally unstable—just a few pieces of rigid, ill-fitting bone bound up by rope-like soft tissues. The bigger “ropes,” known as the major ligaments, break easily because they take the brunt of any stress to the knee. Ligaments are stretchy, fibrous bands of tissue that connect bone to bone. There are four major ligaments in the knee. The most easily injured is the anterior cruciate ligament (ACL), which connects the thigh bone and shinbone diagonally and is situated toward the front of the knee.


There are 100,000-200,000 ACL injuries in the U.S. every year. Some occur as a result of external blows to the knee, but about 70% are caused by an unfortunate movement, such as a sudden change in direction, rapid deceleration, jumping or pivoting. Such movements are common in sports such as skiing, soccer and basketball.


Another commonly injured ligament is the medial collateral ligament (MCL), which connects the thigh bone and shinbone vertically on the inner side of the leg. When there is a blow to the outer side of the leg while a foot is planted, the MCL can rupture. Such injuries are common among football linemen, who are often hit on the side of their knees. Some 60% of skiing knee injuries also involve the MCL. When braking and stopping, skiers will bend their knees, push their heels out and turn the tips of the skis in, creating stress on the MCL.


In the U.S., about 1 million meniscus surgeries are performed every year.


Then there is the meniscus, the crescent-shaped cartilage that sits between the thigh bone and shinbone to absorb shock and reduce stress. Meniscus tears happen as a result of putting downward force on the knee while rotating it, as when we turn, cut or pivot at speed. In the U.S., about 1 million meniscus surgeries are performed every year. Yours truly has contributed to that statistic.


People who dodge traumatic injuries can suffer overuse injuries due to repetitive movements. The most common overuse injury is runner’s knee, formally known as patellofemoral pain syndrome (PFPS)—that is, pain at the front of the knee around the kneecap. It happens because running puts repetitive stress on the knee and causes irritation under the knee cap. PFPS can also affect people who jump, squat or cycle as part of their exercise routines.


Even non-sports activities can cause knee troubles. Frequent kneeling and squatting increase the risk of osteoarthritis in the knee. These postures put the knee in deep flexion and wear off articular cartilage, the protective tissue that covers the ends of the thighbone and shinbone. With this cartilage gone, osteoarthritis and the accompanying pain and stiffness set in.


Of course, physical activities have abundant health benefits. But they expose the human knee as the evolutionary compromise that it is. Standing and walking upright freed our hands to do wonderful things, but it also means that the entire weight of our upper body, and whatever we happen to carry, rests on our knees. When we run, the force on our lower limbs increases to several times our body weight. If we then proceed to do something fancy like rapidly changing directions or going up and down hills, the lower limbs lose their ability to absorb shock, further endangering the knees.


In collegiate soccer, female players are 138% more likely than males to sustain ACL injuries.


Women, in particular, may have drawn the short straw in knee evolution. Studies from a variety of sports and play levels show that females are far more likely to have knee injuries and surgeries than males. For example, in high school basketball, girls are 44% more likely than boys to suffer knee injuries, 165% more likely to have knee surgeries and 315% more likely to have ACL surgeries. In collegiate soccer, female players are 138% more likely than males to sustain ACL injuries and 79% more likely to have meniscus tears. Among national championship level volleyball players, women are 300% more likely to suffer knee ligament injuries.


Some blame these statistics on the fact that women have a wider pelvis, evolved for delivering the large-brained babies that our ancestors had become. With a wider pelvis, the quad muscles exert a stronger sideways pull on the knee, which can interfere with the smooth movement of the kneecap and cause excessive rotation of the lower leg. Some researchers also suspect that estrogen, the female sex hormone, weakens women’s ACLs by reducing production of Type 1 collagen, which provides the ligament’s mechanical strength. Once again, what ensures the reproductive success of the human species may render women’s knees more prone to injury.


Perhaps thousands or millions of years from now, if humans continue to exist, our knees will evolve into something stronger and better. Until then, we would all do well to look after this weak link in our bodies.



Han Yu is a professor of English at Kansas State University. This essay is adapted from her new book, “The Curious Human Knee,” published by Columbia University Press.

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