Reverse-Knee (Digitigrade, Bird-Like) Leg

A leg that looks like its knee bends backward, but is actually walking on its toes, like a bird.

A reverse-knee leg looks like its “knee” bends the wrong way, but it's actually mimicking a bird's digitigrade structure: the joint that appears to bend backward corresponds to a human ankle, while the true knee sits higher on the thigh, close to the body. The best-known examples are Agility Robotics' Cassie and Digit. This leg layout concentrates heavy parts like motors near the hip, keeping the shin and foot light, which lowers the leg's swing inertia and helps with fast stepping and energy efficiency. Its downside is that it doesn't match human leg structure, making it harder to map human motion-capture data onto the robot's joints through motion retargeting, and it looks less human — which is why most humanoid robots aiming for whole-body imitation instead use a human-like, forward-bending knee.

ExampleAgility's Cassie biped uses reverse-knee legs, and its light, distal leg design let it complete an outdoor 5-kilometer test run.

Also called

Digitigrade Leg, Bird Leg

Related

Bipedal Robot · Agility Robotics Cassie · Agility Robotics Digit (incl. Digit 5) · Point Foot vs. Flat Foot · Motion Retargeting

Sources

Digitigrade - Wikipedia
Agility Robotics