Straight-Knee Walking

A humanoid walks with its supporting leg nearly straight, rather than staying bent-kneed in a crouch the whole time.

Straight-knee walking means a biped keeps its supporting knee nearly extended during stance, combined with heel-strike and toe-off, more closely resembling how humans walk; the alternative, common in many humanoid robots, is bent-knee walking, where the robot stays in a half-crouch the entire time. Griffin and colleagues at IHMC summarized three reasons robots default to bent knees in a 2017 paper: the commonly used linear inverted pendulum model assumes constant center-of-mass height, which requires the knee to bend to absorb height changes; once a leg is straight, the knee joint can barely adjust the ground-reaction force any further; and a fully extended leg sits exactly at a kinematic singularity, where the Jacobian matrix loses rank, causing problems for controllers based on inverse kinematics or inverse dynamics. The cost of bent-knee walking is that the knee joint carries large torque continuously, consuming more power, and ground clearance is also smaller. Achieving straight-knee walking requires a controller that allows the center of mass to rise and fall and that handles the singularity properly; IHMC has verified this on two humanoids, Atlas and Nadia.

ExampleWatching video of a humanoid walking: if the knees stay bent throughout and the body height barely changes, that's typical bent-knee walking; if the supporting leg is nearly straight, the torso rises and falls slightly with each step, and the heel strikes first and rolls through to the toe, that's closer to straight-knee walking.

Also called

Straight-Leg Walking, Extended-Knee Gait

Related

Bipedal Locomotion · Linear Inverted Pendulum Model (LIPM) · Singular Configuration (Kinematic Singularity) · Zero Moment Point · Whole-Body Control · Human-like Gait (Straight-knee, Heel-to-toe Walking)

Sources

Griffin et al., Straight-Leg Walking Through Underconstrained Whole-Body Control (arXiv:1709.03660)
Fasano et al., Efficient, Dynamic Locomotion through Step Placement with Straight Legs and Rolling Contacts (arXiv:2310.13134)