What are motion axes?

Before figuring out how your knees move, you'll first need to understand the concept of motion axes. Motion axes can be used to describe any two objects moving relative to one another. TakeImagine for example, two objects commonly seen moving relative to one another: a cat and a mouse.

A mouse moving around a cat along a circular path (indicated in red).

If the mouse is moving a circle around the cat, you could say that the mouse is rotating around the cat (you could also describe the mouse as orbiting around the cat like a planet around a sun, but with a circular orbit). Below is the same motion in video form.

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