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As written here,

Flexion refers to a movement that decreases the angle between two body parts. Flexion at the elbow is decreasing the angle between the ulna and the humerus. When the knee flexes, the ankle moves closer to the buttock, and the angle between the femur and tibia gets smaller.

and

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.

The examples provided in such a website are perfect, as the angle between the mentioned bones decreases with flexion. That because in such cases extension is = straight and flexion is rounded. But let's consider the back or, for instance, the wrist:

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The end parts of the fingers are closer to the forearm both in flexion and extension. And the angle entity looks like the same, but only with opposite direction.

And the same about the back. Extension puts the end parts of the back close as much as flexion.

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There are commonly established and accepted definitions of motion for ambiguous joints that are related to the planes that the body is separated into. These planes are the sagittal, frontal and transverse.

The sagittal plane basically divides the body into right and left halves, and any motion that goes forward and back is motion in that plane. So as you stand normally, and lift your hand to shake someone else's hand, that motion is in the sagittal plane.

For flexion, any anterior motion (or towards the front) is a flexion. Any posterior motion (towards the back) is an extension. The reason the wrist is flexion towards the palm is that in the anatomical position, the palms face forward, so motion towards the palm is flexion.

There is a pretty good demonstration of this at Lumen Learning, as well as abduction, adduction, circumduction and so on.

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