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

Quote Originally Posted by pudding_bowl View Post
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.