Question about how the elbow works - I hope it's the right forum

[PierreC]

Hello everybody,

My name is Pierre, from Belgium. I am not a physician, and I don't know much about the human body, but I need a special kind of information that my family doctor couldn't answer too well. I hope this isn't off-topic, otherwise feel free to tell me off.

Here's my problem: I am a pool (billiard) instructor. As such, I try to explain to my students how - and more importantly why - they should get a proper stance to shoot straight, in order to avoid years of frustration and progress quickly.

To make a long story short, what a pool player needs to make his cue travel straight to and fro, and ultimately hit the ball without swerving, is to make their shooting arm elbow work in the plane of the joint.

I explain to my students that the elbow is the only joint that should work when addressing the ball, that it's a joint that is a simple rotation joint, and that it swings the forearm in a plane defined by the humerus bone and the wrist point. Therefore, their stance must be such that their rear arm is parallel to the cue, and their forearm drops down vertically (and loosely) from the elbow to the cue, and that, in order to bring the cue under their chin to aim, they must twist their entire upper body and "wrap" around the cue while maintaining their shooting arm configuration. If they fail to do this, then they'll need to involve shoulder muscles and tense up everywhere to move the cue somewhat straight, which won't fail to impact their accuracy. Then I show them the routine to address the cue ball and get into position properly.

All this isn't easy to explain, and it's even less easy to demonstrate at the pool table. Many students just don't "get" the mechanics of the shooting arm. So, being an engineer, I'm trying to devise something to help beginners get into position. Either a video analysis software, or a piece of electronics that straps onto the shooting arm and forearm and gives audio feedback on the arm's positioning while the player gets into position.

But before doing this, I'd like to ensure that my idea of how an elbow works is correct. I looked at a diagram of an elbow in the dictionary and it looks very complicated. So my questions are these:

- Can I really assume the elbow is a simple joint with only 1 degree of freedom (to a certain extent of course, I realize it's wetware, not steel)

- Is the elbow's plane always parallel to the humerus, or does this vary from person to person?

- The diagram in the dictionary shows that 3 bones meet at the elbow (humerus, radius and ulna), and two of them can move relative to each other (radius and ulna) when the forearm rotates: does this mean that the elbow's configuration, and thus the joint's plane changes when the wrist is rotated?

- Finally, I was talking about the joint when the arm is limp. But when the biceps is called into action, does this also change the plane of the elbow? I'm asking because I'm assuming biceps surely doesn't pulls exactly in the plane of the elbow, so it should modify the joint's travel too.

If the anatomy of the elbow isn't too complicated, then a training aid would be worth making, otherwise it'll just end up screwing up my student's game even more, which is obviously not what I want.

Thanks for any information you may have, and sorry for the long-winded post.

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[pudding_bowl]

The ulnar humeral joint is a hinge joint and only has 1 plane of motion "forwards and backwards". Hold your arm infront of you with palm up - you'll see that the forearm bends away from the elbow - this is known as the carrying angle and is there so that when we walk we don't hit our hips - hense why it's larger in females.
You can assume that you have 1 plane of motion for the ulnar/humeral articulation - however as you noticed there is a second joint which helps with wrist rotation. This doesn't per say change the plane of motion of the elbow but certainly contributes to why it is hard to keep that plane steady while using the pool cue.
The biceps attaches to the radius - contributing to supination (palm up) of the wrist. Provided other muscles are working in synergy then the wrist will stay steady and the bicep will contract without moving the wrist.

I think making a training aid isn't a terrible idea - be a bit ignorant on the finer details of the anatomy, your students don't have to know about the extra planes of motion.
I say this because the action of synergistic muscles (from the legs to back to shoulder) will stabilise the body and joint in a way that you can get the elbow to move in 1 smooth plane. It will just take practice for the students to get it.

I suggest you even dumb it down, make it really simple. say the elbow only moves forwards and backwards so they just need to stabilise at the shoulder and then let the bicep do the work. The description you gave sounded very good.

All comes down to practice.

[PierreC]

Pudding Bowl,

What a fantastic answer, right on the money and worded so even ignorant me can understand. it's exactly the sort of explanation I was looking for. Thanks a lot!

I had read about the carrying angle but didn't really understand it until now. As for the biceps, I didn't realize it had a dual function (flexing the arm and helping rotate the wrist) by pulling on a secundary bone, and not directly on the ulna. What a strange arrangement from a mechanical standpoint, but what a wonder of nature that humans can do precise things by the clever compensation of other muscles. And it's all subconscious, that's just amazing. It also explains other techniques I teach my advanced students, such as fine-tuning the cue's travel path by rotating the wrist ever so slightly, or putting the biceps under constant tension and releasing the shot by letting go of the triceps muscle instead, for hard but precise shots.

As for the teaching aid device I have in mind, the idea is to cut the time it takes for a student to train and become aware of what their own body does by proprioception alone. It takes many years to develop the feedbacks necessary to play really straight, and it pains me to see people spend time and money playing wrong. I want to spare them the years it took me to finally understand what I was doing wrong and why.

From your explanation, it's pretty clear that the elbow joint isn't a simple, isolated joint one can measure with a simple device, ignoring the rest of the body. What's more, it occurs to me that any external device will not really measure anything very precise because of the soft tissues surrounding the bones. The portlyness of the player will have an impact on accuracy.

So I think I'll go with your idea of keeping things simple: I'll try to make a simple armband-like device that tells the player if their rear forearm deviates too much from the vertical plane (because if the forearm is close to vertical, the player isn't very far from the correct position) coupled with an accelerometer at the butt end of the cue stick, to inform the player of any sideway movement, both information being delivered in real-time by sound tones. That will keep the player's position roughly in check during training, and they can work out the rest with a few explanations on how to play relaxed and how to grip the cue. The device should at least prevent them from adopting a really bad position over time, without an instructor around to correct them, and straying too far from the right stance.

Again, thanks a lot. I hope this little project of mine will come to fruition with your help.

-- Pierre

The ulnar humeral joint is a hinge joint and only has 1 plane of motion "forwards and backwards". Hold your arm infront of you with palm up - you'll see that the forearm bends away from the elbow - this is known as the carrying angle and is there so that when we walk we don't hit our hips - hense why it's larger in females.
You can assume that you have 1 plane of motion for the ulnar/humeral articulation - however as you noticed there is a second joint which helps with wrist rotation. This doesn't per say change the plane of motion of the elbow but certainly contributes to why it is hard to keep that plane steady while using the pool cue.
The biceps attaches to the radius - contributing to supination (palm up) of the wrist. Provided other muscles are working in synergy then the wrist will stay steady and the bicep will contract without moving the wrist.

I think making a training aid isn't a terrible idea - be a bit ignorant on the finer details of the anatomy, your students don't have to know about the extra planes of motion.
I say this because the action of synergistic muscles (from the legs to back to shoulder) will stabilise the body and joint in a way that you can get the elbow to move in 1 smooth plane. It will just take practice for the students to get it.

I suggest you even dumb it down, make it really simple. say the elbow only moves forwards and backwards so they just need to stabilise at the shoulder and then let the bicep do the work. The description you gave sounded very good.

All comes down to practice.

[pudding_bowl]

Sounds excellant! Keep in touch as I'd love to hear how it works out for you!
I'm a keen pool player at times - but certainly no training or anything like that, just the odd game at the pub with a mate. Glad I could help.

[vam44]

Hey I really like your question as analysing a sport action that requires the body parts to move in a very precise pathway is in tune with the way physiotherapist think!


THere are some quite cheap motion analysis software available over the internet that you can teach yourself to use. eg do a search for
'motion analysis software"

I am learning to use one called V1 ( i dont know wheter it is the best one available but I have downloaded a free trial version.) You use video of the persons sport action and I could imagine that you could then give your student feedback on the correct position of the shoulder and forearm. These are often used to analyse golf swings.

Having said that Its not just the mechanics of the joints and muscles that are relevant here.....you would have to consider the process of learning motor skills. This is a fascinating area of neuroscience that defy a simple explanation!