No matter their background in biology or medicine, sports fans everywhere are familiar with the anterior cruciate ligament, more commonly called the ACL. This is because injuries to the ACL are among the most common sports injuries. Probably most common in football, torn ACLs also occur in baseball, soccer, basketball, track and field, tennis… basically all high-impact, fast-paced sports.
Why are ACL injuries so common? Because Homo sapiens never fully completed our adaptation to bipedalism.
[Update: This article is now included as a section in my new book, Human Errors, go check it out!]
The anterior cruciate ligament is located in the middle of the knee joint. This ligament connects the femur (thigh bone) to the tibia (shin bone) and is located underneath the patella (knee cap), deep inside the joint. It does most of the work of holding the upper and lower legs together.
(The kneecap, patella, is not shown here, to provide view into the interior.)
The reason the ACL is so vulnerable to tearing is that it is forced to endure much more strain than it is really designed for.
In quadrupeds, the strain of running and jumping is spread among four limbs and the muscles absorb most of it. When our ancestors transitioned to walking on just two feet, however, the strain placed on the legs doubled, since it was being spread over half as many limbs. This was too much for the muscles by themselves, so our bodies recruited the bones to help with the strain.
As a result, the legs became straightened so that the bones could bear most of the impact and strain. Compare a stranding human to a standing ape: our legs are fairly straight, while apes’ legs are bow-legged and usually bent. For bearing weight, they use their muscles more, and we use our bones more.
This straight-leg arrangement works out okay for normal walking and running. The problem comes in when we try to make sudden shifts in the weight strain on our knees. When we stop suddenly while running, or when we make a sharp turn at high speed, our knees must bear the force of this sudden, intense strain.
Sometimes, the ACL is simply not strong enough to hold our leg bones together as they twist or pull away from each other under the force of our momentum, and so it frequently tears.
Making matters much worse, we as a species are getting heavier, making our momentum even greater during those sudden shifts. This is especially true for athletes, who now weigh more than ever before, and who also make lots of sudden high-speed weight shifts.
It is not just your imagination if you have noticed that ACL injuries have been getting steadily more common in the world of professional sports as the athletes get ever larger.
Even worse still, there aren’t any proven preventative measures that can be taken. It doesn’t appear possible to isolate the ACL with exercise in order to make it stronger. It is what it is. Repeated strain doesn’t make it stronger, it makes it weaker.
Tucked deep inside the knee, when the ACL is torn, it can only be repaired surgically. This is followed by a very long recovery and rehabilitation period because ligaments are not very vascular: there are few cells and few blood vessels. It heals very slowly. This is why ACL tears are among the most feared injury in professional sports. It usually means a full season lost.
Bottom line: the knee is an example of human anatomy that is less than perfectly adapted for a bipedal posture and lifestyle, and the anterior cruciate ligament frequently tears as a result.
-NHL (@nathanlents)
The Human Evolution Blog 
Thank you for the feedback and information you have presented in the above article. It is always fascinating to learn about various philosophies when it involves the human body and moving. When I look at the occurrence of ACL injuries over the years I believe that it is more related to modern day adaptations rather than how humans have evolved. The increased weight of athletes is just part of the picture. Along with the increased weight they usually carry more muscle mass, run faster, jump higher, and hit harder than ever before. These actions within themselves do not lead to the ACL injuries. However, bad/faulty movement mechanics are what cause an increase in the prevalence of the aforementioned injury. I have watched countless videos and time and time again ACL related injuries occur as a result of valgus stress at the knee joint. I propose that in most instances, the ACL isn’t torn as a result of that single action. The ligament has been under cumulative stress as a result of those same bad/faulty movement mechanics. For most of the athletes who suffer from such injury I can trace back how they move, squat, run, etc …. and predict an increased probability of an ACL injury.
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Is the weak ACL due to not fully finishing evolution for bipedalism though, or rather just a result of our particular form of bipedalism not requiring a strong ACL? For example, Homo Sapiens (ourselves) are evolved to be a distance runner, of which we are among the best in the animal kingdom, so our leg anatomy with the weak ACL is adapted for that and can handle it fine. But we are not designed for sprinting or quick side-to-side movement like Neanderthal was for example. Now I have read that even though we walk upright, we do so with a knuckle-walker’s spine, and that this could be said to be due to our basically being a “transitional species” as far as bipedalism goes.
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Great questions that I’m not sure we have full answers to. So true that human evolution was marked by a selection for endurance running, probably because of the hunting strategy known as persistence hunting. The “turning on a dime” and huge bodies that lead to torn ACLs just weren’t a part of the selective environment. Also probably true that herniated disks are the result of incomplete adaptation and poor design. Evolution doesn’t produce perfect, that much we know!
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. I have watched countless videos and time and time again ACL related injuries occur as a result of valgus stress at the knee joint. I propose that in most instances, the ACL isn’t torn as a result of that single action. The ligament has been under cumulative stress as a result of those same bad/faulty movement mechanics. For most of the athletes who suffer from such injury I can trace back how they move, squat, run, etc …. and predict an increased probability of an ACL injury.
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ACL is adapted for that and can handle it fine. But we are not designed for sprinting or quick side-to-side movement like Neanderthal was for example. Now I have read that even though we walk upright, we do so with a knuckle-walker’s spine, and that this could be said to be due to our basically being a “transitional species” as far as bipedalism goes.
Read more at hoc cham soc da
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