This is a very good question.
Many robots have big servomotors in their feet.
When I saw them walking, I thought they are like old-time prisoners dragging a heavy cannonball.
So I wanted to have lightweight legs.
Besides, this prototype cannot bind its knees. The servo in the knee is used to make the robot turn.
Therefore, the legs of my robot are like two stilts. And if you see people on stilts, you see them tilting from side to side.
Now, I understand why robots have servos in their feet. They are used to tilt the whole body.
In both picture, the robots are about to move and the angle is almost the same. But this is less beautiful with my robot who cannot move his ankles.Experience is a hard teacher because she gives the test first, the lesson afterwards (Vernon Sanders Law).
But it is funny anyway