One common criticism used by atheists against the existence of God is the so-called “poor design” of the human body. They criticize many areas, with the knee joint often being a focal point. In reality, a healthy knee joint has an impressive range of motion and can last for decades without significant wear. No human-engineered prosthetic knee can match this longevity. Even with the best available materials, all prosthetic knees tend to show signs of wear after just a few years.
One of the leading critics of the human body’s design is Nathan H. Lents, a scientist, author, and professor affiliated with the City University of New York (CUNY). He is especially critical of the knee joint. As an advocate for human evolution, he claims, “The problem is due to incomplete adaptation.” He states, “The anatomical adaptation to upright walking never quite finished in humans. We have several defects that are the result of the failure to complete the process… The ACL is vulnerable to tearing in humans because our upright bipedal posture forces it to endure much more strain than it is designed to.” It’s interesting that he used the word “designed,” even though he does not believe it was designed. He believes it simply evolved through natural selection acting on random mutations.
The truth is, the human knee functions as what engineers call a floating joint because it has no fixed center of rotation — it is free to rotate and roll. It is stabilized by ligaments, including the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). These ligaments work together with the femur and tibia to form a four-bar linkage mechanism. Professor Lents refers to people tearing the ACLs. However, the ACL is not a bad design. Injuries to it are uncommon in everyday life but are more frequent in high-impact sports such as soccer and ski jumping. Sports like tennis or skating generally do not pose a significant risk to the ACL.
Being overweight is another common factor contributing to knee problems. In the United States, more than two-thirds of adults are overweight, and one in three is obese. Just ten extra pounds of weight can add approximately sixty pounds of force to each knee during running. Obese individuals are twenty times more likely to require a knee replacement than those who are not overweight.
In reality, the knee joint is an ingenious design, as you can see in the illustration. Engineer Stuart Burgess explores this in detail in his excellent book, Ultimate Engineering. Many who view the human body as a product of evolution assume it is poorly designed without understanding the facts. God has created the human body with many remarkable engineering features, including our knee joints. Truly, ultimate engineering requires an Ultimate Engineer.
— Roland Earnst © 2026
Reference: Ultimate Engineering by Stuart Burgess, Discovery Institute Press, © 2026, chapter 2.
