Is a straightening movement in which the relative angle between two adjacent segments increases.

Extension is a straightening movement in which the relative angle between two adjacent segments increases.

Explanation:

In common, motion is divided according to the anatomical plane it occurs in. Anatomical terms of movement are used to define the effects of muscles on the skeleton.  

Extension refers to a movement that increases the angle between two body parts. Extension at the elbow is increasing the angle between the ulna and the humerus. Extension of the knee straightens the lower limb. For example, when standing up, the knees are extended.

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