What is the medical term for this?
moving one leg away from the other leg, as in getting up on a horse:

abduction

Explanation: Anytime the leg or any other limb of the human body moves in an outward motion, away from our body's midline, the abductor muscle is used to bring that body part out. This is also a physical human action referred to more generally as lateral movement. It is the opposite of the term "adduction," which is the movement that brings that limb back in close to the other leg or midline of the body.

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