How did we determine which motion to call "flexion" and which to call "extension" with regard to the wrist and the neck?
Just to set the stage, I'm not asking for help on homework here. I'm a veterinarian and I teach human anatomy. Terms for joint movements make sense to me on a surface level, but the more I think about them, the more confused I get.
I have read some good comments and sources on biology stackExchange, such as this discussion.
But the more I think about it, the less it all makes sense.
My initial question was "why do we use dorsiflexion and plantar flexion" instead of simply saying "flexion" and "extension?" My confusion is not about plantar/dorsal-- coming from the veterinary side, that part is easy for me. Rather, my confusion is about why we don't simply refer to dorsiflexion as "flexion" and plantar flexion as "extension."
Someone mentioned in the link above that anatomists wanted to emphasize the connection between ankle and wrist in their terms. That made some sense... except that we end up using different terms in the ankle vs the wrist. And in any case when I think about this, I start to wonder-- how do we define flexion or extension of the wrist in the first place? I mean to some degree it's intuitive, thanks to everyday language use. I don't know exactly how that intuition arises-- perhaps by analogy to the elbow and knee, I generally think of "flexion" as being the more active movement for picking something up or moving something, while extension is usually for putting things back down, and flexion tends to make the bigger muscles bulk up. But if we want to be objective about it, how would we do that?
Either movement of the wrist reduces the angle between the hand and the arm. Either movement requires some muscles to relax and their counterparts to contract. Either movement can accomplish important physical functions.
I can understand how we arrive at objective terms for the knee and elbow. If we start in the anatomical position, we cannot significantly reduce the angle between the distal and proximal parts except by moving in one direction-- which we can call flexion. Extension is the opposite, and in the unfortunate event that some external force moves us beyond the normal range, we get hyperextension. So far so good.
But this doesn't apply to the wrist, or for that matter the head, so we're back to the drawing board. Starting from the anatomical position, any movement of the head or hand reduces the angle from 180 degrees to something less than that. (The foot is in fact the only one of these three where it makes much sense to simply use extension and flexion; since we start at 90 degrees from the shin, dorsiflexion reduces the angle, and so could to my mind be called flexion, while plantar flexion increase the angle up to 180 degrees, and so in my mind could simply be called extension. And yet for the foot, we don't use these terms!)
Others have mentioned it has to do with the coronal plane-- reducing the angle between the body part and the coronal plane is flexion. But this doesn't really hold up either; both flexing and extending the head increase the angle between the head and the coronal plane. Same with the wrist. This just doesn't seem universally applicable period.
When it comes to why we use plantar flexion and dorsiflexion of the foot instead of flexion and extension, I can appreciate that the foot is uniquely positioned at a 90-degree angle from the long axis of the body, so that the joint is not, in anatomical position, oriented along the coronal plane, while most other limb joints and the head are. So I am open to this being an explanation for why the standard terms don't apply, but to understand that, I need to understand how and why the other terms do apply more generally.
I am willing to accept it if the best possible answer is "we arrived at these terms through a history of common usage and there is no explanation other than the subjective opinions of early anatomists." But I'd love to know if there is something better.
To summarize my main question is: how did we determine which motion to call "flexion" and which to call "extension" with regard to the wrist and the neck?