In an earlier article, I summarized bioengineer Stuart Burgess’s recent paper “How Multifunctioning Joints Produce Highly Agile Limbs in Animals with Lessons for Robotics.” I also interviewed Burgess on ID the Future on how his paper highlights human limbs’ exquisite design. Here, I will explain how his research overturns the claim that the ACL in the human knee appears poorly designed. 

Criticism of the ACL

The ACL is a major ligament that connects the femur and tibia bones. Design critics have argued that the ACL appears poorly designed since it often tears. They further claim that this evidence demonstrates that the knee resulted from an unguided evolutionary process. Atheist biologist Nathan Lents criticized the engineering of the ACL on The Human Evolution Blog:

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.

Biologist Alex Bezzerides critiques the ACL in his book Evolution Gone Wrong: The Curious Reasons Why Our Bodies Work (Or Don’t) in nearly identical terms. He labels what he considers the poor design of the knee as “evolutionary baggage” (p. 122). Neither biologist presents a substantive analysis of the knee’s bioengineering to support their claims. They simply played the role of armchair critics. 

Biomechanical Analyses

In contrast, Burgess performed detailed biomechanical analyses of human limbs that included performance maps, kinetic and structural evaluations, and multivariable trade-off maps. He demonstrated that every component of the knee, including the ACL, is ideally designed for the broad range of human activities. In his article “A Bio-Inspired Condylar Hinge for Robotic Limbs,” Burgess describes many of the features of the knee that he copied in the bio-inspired robotic leg that he designed:

The bio-inspired design has the same desirable features of a human knee joint including a moving center of rotation, high strength, high stiffness, compactness, and locking in the upright position.

One key error committed by Lents and Bezzerides is only considering one variable; they concluded that the ACL is too thin since they only focused on its durability. Burgess, as an experienced engineer, considered all relevant variables including the knee’s range of motion, mechanical advantage, and compactness. 

He describes the importance of compactness in his article on multifunctioning joints:

The parts of the knee joint are highly integrated, especially the meniscus in the way it surrounds the condyles. Joint locking and unlocking represents reconfiguration. To unlock the joint, the tibia is made to rotate internally by flexion muscles, especially the popliteus muscle behind the knee, as shown in Table 2. Like the wrist joint, there is miniaturisation in the sensors, nerves, blood vessels and lubrication system that helps to achieve compactness. The layout of the knee joint represents a unique solution where the layout performs multiple functions with multiple aspects of fine-tuning and integration.

If the thickness of an ACL were suddenly increased, the performance of the knee would degrade in terms of reduced flexibility and range of motion, decreased energy efficiency, and increased stress on multiple tissues.

ACL Injuries

The explanation for ACL injuries is not poor design. Burgess noted in my interview with him that ACL tears were far less frequent in past centuries. The increased rate of damage today is due to unhealthy modern lifestyles and misuse, particularly in high-impact sports. He also described how the body responds to heightened knee activity by increasing the size of the ACL and other tissues proportionally to always maintain optimal performance. The claim of poor design represents yet another imperfection-of-the-gaps argument where the conclusion resulted from critics’ limited understanding of the biology and lack of training in engineering.