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.
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.