Here we see the action of a single leg walking. The upper leg swings as a pendulum. The knee joint allows the lower leg to bend and raise the foot so that the foot does not drag when it swings forward. The knee locks as the foot makes contact with the ground and the leg swings back. The hip moves gently up and down as a result of the foot making contact with the ground and the leg swinging back.

Walking speed is mostly determined by the pendulum motion of the leg swinging forward and back. The period of the swing is proportional to the square root of the length of the leg. A longer leg meansa longer period for the swing. I have also animated the hind leg of a walking cat for comparison purposes.