The appearance of a joint bending in the opposite direction of a human knee is a common source of confusion when observing the anatomy of many animals, including mammals like dogs and horses, and various bird species. The question of what these “backwards knees” are called is a popular inquiry into animal movement. The answer requires a deeper look into comparative anatomy, revealing that the joint in question is almost never the knee. The visual deception comes from fundamental differences in how humans and other animals position their feet and legs.
Defining the Misconception: Ankle vs. Knee
The joint in quadrupeds often mistaken for a backward-bending knee is actually the animal’s ankle, known anatomically as the tarsus or the hock. The distinct shape and location of the hock result from the animal’s specialized posture and gait.
Humans exhibit a plantigrade posture, meaning the entire sole of the foot rests on the ground. Most mammals are either digitigrade (walking on their toes, like dogs and cats) or unguligrade (walking on the tips of their digits, encased in hooves).
In digitigrade or unguligrade animals, the metatarsals (foot bones) are greatly elongated, elevating the hock joint high off the ground. This makes the hock look like a second knee joint. Because the hock functions as the ankle, it bends in the same direction as a human ankle, but its elevated position creates the optical illusion of a knee bending the wrong way.
Avian Leg Structure
The “backwards knee” confusion is pronounced when observing birds such as flamingos or chickens. Similar to quadrupeds, the prominent, high joint that flexes backward in a bird’s leg is its ankle. The avian leg has a specialized bone structure distinct from mammals.
The bird’s leg contains two unique, fused bones. The upper bone is the tibiotarsus, a fusion of the tibia and some upper ankle bones. Below this is the tarsometatarsus, formed by the fusion of lower ankle bones and the metatarsals, which creates the elongated foot.
The visible, backward-bending joint is the intertarsal joint, connecting the tibiotarsus and the tarsometatarsus. This joint is functionally the ankle, flexing to pull the foot closer to the body. This anatomical arrangement provides the stability and leverage necessary for perching and takeoff during flight.
Where is the Real Knee Located?
The true knee joint in both quadrupeds and birds is called the stifle joint. This joint is the anatomical equivalent of the human knee, connecting the femur (thigh bone) to the tibia (shin bone). The stifle joint always bends forward, or cranially, just as the human knee does.
In many animals, especially those with long legs like horses and birds, the stifle is located high up on the leg, close to the body. It is often obscured by muscle, skin, and fur or feathers, making it difficult to see. This hidden position means the true knee is not the joint that catches the eye, leading to the misconception.
The stifle joint includes the patella (kneecap) and is stabilized by ligaments such as the cruciates. Its function is to permit flexion and extension of the hindlimb, generating power for running and jumping. The forward-bending nature of the stifle is consistent across almost all land vertebrates.
When Knees Are Backwards: Pathological Conditions
While the natural anatomy of animals does not feature a backwards-bending knee, a medical condition in humans can cause the knee to hyperextend excessively backward past the straight position. This condition is formally known as Genu Recurvatum, or “back knee.”
Genu Recurvatum is defined as the knee joint extending more than 5 to 10 degrees beyond its normal range of motion. This excessive backward bending can be caused by inherent ligament laxity (often hereditary) or weakness in surrounding muscles, such as the quadriceps.
The condition may also result from a congenital defect, previous knee injury, or neuromuscular diseases like cerebral palsy. Similar pathological hyperextension is occasionally observed in domestic animals due to trauma. The unnatural hyperextension places stress on the joint, potentially leading to pain and accelerated wear on the cartilage.