How is walking and jumping possible?

Let's start with walking. I'm getting a traction of 100N from the floor, but reciprocity means the floor must also push back on me with 100N. How do I ever get moving?

Then we have jumping. How does it work? I squat, contract my muscles and somehow I gain momentum upwards. What's the physical + anatomical reason that I can jump? Like, if I didn't intuitively know how to jump, how would I explain it to a robot or an alien?