Stance Phase / Swing Phase

The gait phase where a foot is on the ground bearing weight is stance; the phase where it swings through the air is swing.

Stance phase and swing phase are the basic building blocks for describing gaits, applicable to both people and legged robots: the phase where a leg's foot is in contact with the ground, bearing body weight and driving the body forward, is stance phase; the phase where the foot leaves the ground and swings forward toward its next landing spot is swing phase, and together the two make up one gait cycle for that leg. Normal adult walking spends roughly 60% of the cycle in stance and 40% in swing, with the period when both feet are down at once called double support; running adds a flight phase where both feet leave the ground together. The fraction of the cycle spent in stance is called the duty factor, and different gaits — such as a quadruped's diagonal trot — are, at bottom, just different ways of staggering each leg's stance phase relative to the others. Control needs differ sharply between the two: a stance leg has to generate ground reaction force, maintain balance, and avoid slipping, while a swing leg has to lift clear of obstacles and choose where to land — so both MPC and RL-based locomotion controllers need to know which phase each leg is currently in.

ExampleHumanoid-Gym generates each foot's stance mask with a sinusoidal phase clock: a positive sine value means the left foot is in stance, a negative value means the right foot is in stance, and a value near zero counts both feet as in stance (double support). The reward function then requires whichever foot should be down to be down, and whichever should be swinging to be off the ground.

Also called

Stance, Swing

Related

Gait · Gait Cycle and Duty Factor · Double Support Phase · Flight Phase (Aerial Phase) · Gait Planning · Swing Foot Trajectory Planning

Sources

Wikipedia: Gait (human)
Humanoid-Gym: humanoid_env.py (_get_gait_phase)