Evolution of the Human Knee

For Young Learners

Function

Humans are unusual. Almost every other mammal walks on four legs. We walk on two, and that changed our legs completely.

A chimpanzee walking upright has to keep its knees bent, which is tiring. Our knees can lock straight, so standing costs us almost no muscle effort. That is why you can stand in a queue for ten minutes without your legs aching.

Clinical Relevance

Standing upright brought a trade. Our thigh bones angle inwards from the hip to the knee, which keeps our feet under our body when we walk. It also means the kneecap is pulled slightly outwards every time the thigh muscle contracts.

This angle is larger in people with wider hips. It is one reason kneecap problems and some knee injuries are more common in girls and women than in boys and men doing the same sport. There is more to it than the angle alone, and researchers are still working on the rest.

For Patients

Function

The human knee is adapted for upright walking on two legs. Compared with our closest relatives, our knees are larger relative to body size, they straighten fully and lock, and they sit under the body rather than out to the side.

Locking is the adaptation people notice least and use most. In full extension the joint reaches a position where the ligaments tighten and the surfaces fit closely, so standing requires very little muscular effort.

Clinical Relevance

Upright walking brought compromises that still shape the problems people have. The thigh bone runs inwards from hip to knee, which keeps the feet beneath the body during walking and gives the knee a slight natural angle. That angle creates a sideways pull on the kneecap whenever the thigh muscle contracts.

Our knees also carry our full body weight through two joints rather than four. The load per joint is higher than in a four legged animal of similar size.

None of this makes knee trouble inevitable. It explains why certain patterns recur: kneecap pain, wear in the inner compartment, and injuries from twisting on a planted foot.

For Medical Students

Structure and Location

Adaptation to habitual bipedalism distinguishes the human knee from that of other hominoids. The femoral condyles are enlarged relative to body mass, the lateral condyle is elongated anteriorly, and the distal femur carries a pronounced bicondylar angle so that the femoral shaft is oblique while the knee remains close to the midline.

The patella is larger and the trochlear groove deeper, consistent with the extensor demands of upright posture and of running.

Function

Full extension with the screw home mechanism permits prolonged standing with minimal quadriceps activity, an arrangement absent in facultative bipeds, which must maintain flexed knees and therefore continuous muscular work.

Comparative anatomy supports this reading. The chimpanzee knee is adapted to a flexed, abducted limb posture and lacks both the bicondylar angle and the degree of terminal locking seen in humans.

Clinical Relevance

The bicondylar angle produces a lateral vector on the patella during quadriceps contraction, which underlies the predisposition to patellofemoral maltracking and is larger where the pelvis is wider.

Bipedalism also concentrates body weight through two joints rather than four and shifts the resultant force medially in stance, which is consistent with the predominance of medial compartment change in the ageing knee.

These are explanatory rather than deterministic observations. They account for the distribution of common problems without predicting them in an individual.