Pronation, Internal shoulder rotation, ulnar deviation, leverage & racquet head speed

[spacediver]

Ok there was another thread discussing this but I figure this post deserves its own thread.

NOTE: this series of images doesn't depict what actually happens during the service motion. It is intended to break down the different rotations into simple components. I have also not yet included images that show what happens when the wrist deviates and flexes, even though these motions do make significant contributions

Part One




Pronation is a technical term that refers to the counterclockwise rotation of the forearm around its own axis.

See figure below:



Important thing to note is that this movement contributes nothing to useful racquet head speed, in this particular anatomical configuration. In fact, if the ball is struck in the centre of the racquet, pronation in this position contributes absolutely zero to racquet head speed. This is because the racquet is simply rotating around its own axis. It is twisting, and this twisting is useless for the purpose of increasing racquet head speed.


Next point:

Consider the exact same rotation of the forearm, but with the wrist cocked to the side (radially deviated).



Now something important happens. Because the racquet is no longer colinear (i.e. no longer in a straight line) with the forearm, it no longer only twists. It undergoes a rotation about a different axis. This allows something rather magical to happen. Consider the image below:



The image depicts a rotating stick. The start position is black and the end position is red. The angle that the stick traverses is about 45 degrees. Now the key thing to understand here is that the right end of the stick travels at a much faster speed than the portions of the stick closer to the left end. This is a form of leverage, where we can generate high velocities by increasing the length of our lever. It's one of the reasons Del Potro is capable of such devastating forehands - his arms function as a very long lever, and even though he is rotating into the stroke at the same angular velocity as someone shorter than him, the end result is a faster forehand (though perhaps with less torque, but we need not worry about that).

Now in the previous image that shows forearm pronation with a cocked wrist, you can see how the racquet is moving in a way that exemplifies this form of leverage. Try it at home with your own racquet, and it should become clear.

[spacediver]

Part Two

Now for the final point:

Forearm pronation actually contributes relatively little to racquet head speed - rather, it is internal shoulder roation that achieves this (this is counterclockwise rotation of the upper arm). Because the forearm is colinear with the upper arm, and because the forearm tends to adopt the same motions that the upper arm generates, internal rotation of the shoulder achieves a very similar effect upon the racquet as pronating the forearm does.

This is true only if the forearm and upper arm are colinear, as in the below images. In both these pairs of images, the forearm is NOT pronating - the only rotation is the shoulder internally rotating.









Note, however, what happens when the forearm and upper arm are NOT colinear, such as when the elbow is flexed as shown below:



In the above image, the only rotation involved is internal shoulder rotation, but notice how much racquet head speed is achieved due to the extra long lever provided by the flexed elbow.

In reality, things are not this simple - the serve is a dynamic motion where joint rotations are evolving over time, changing the relative angles between joints. These images represent abstract components of the motion.


In sum, pronation, strictly speaking, does contribute to racquet head speed, but internal rotation of the shoulder provides much more. These days, people in the know are starting to dissociate technical pronation from internal rotation. Often in the past, they were lumped together. In this lumped state, "pronation" certainly contributes to a great deal of racquet head speed.

[salsainglesa]

thats what i said but the naked guys makes it cristal clear ... also, your torso is at an angle, not straight like this, this will hurt your shoulder.

http://www.youtube.com/watch?v=1t6bLABbebc&NR=1

since this is a new thread i'll repost the nice video of this guy showing this thing

[toly]

It is very good idea about bend elbow!

[salsainglesa]

good luck with that, the force generated by the shoulder will need to be countered by the biceps... i sense danger!

the idea would be to let this force straighten your elbow, not actively doing so with your triceps...

but tell me how it went, maybe you find out something

[spacediver]

Are you referring to my post salsa? If so, I think you've misunderstood my meaning.

[toly]

Originally Posted by salsainglesa

good luck with that, the force generated by the shoulder will need to be countered by the biceps... i sense danger!

the idea would be to let this force straighten your elbow, not actively doing so with your triceps...

but tell me how it went, maybe you find out something

Roddick bends elbow during pronation phase and impact of the flat serve.

[spacediver]

Salsa, don't look at the images I posted thinking they represent how the service motion actually occurs - I explained in my post that the joint angles evolve over the motion.

The elbow starts in a flexed position and reaches extension by the end of the motion. This happens while the shoulder is internally rotating.

Think of the images as abstract components of the motions.

[bhupaes]

Good post, spacediver. Thanks.

[WildVolley]

I'm confused. No one holds a racket as you show in the first diagram. The racket is naturally cocked to the side if you grasp the handle with your fingers. In such a motion, the racket acts as a lever and the motion DOES contribute to racket speed.

Was that your point?

[larry10s]

Originally Posted by WildVolley

I'm confused. No one holds a racket as you show in the first diagram. The racket is naturally cocked to the side if you grasp the handle with your fingers. In such a motion, the racket acts as a lever and the motion DOES contribute to racket speed.

Was that your point?

.........yes

[spacediver]

yep - I'm trying to break down how pronation affects the motion of the racquet, starting with the simplest case, and then illustrating what happens when you change the anatomical conditions. Also, in the other thread where people werre discussing this, one of the posters claimed that pronation does nothing for racquet head speed, since it only twists the racquet. I thought this series of images would shed light on the discussion.

[WildVolley]

Originally Posted by spacediver

yep - I'm trying to break down how pronation affects the motion of the racquet, starting with the simplest case, and then illustrating what happens when you change the anatomical conditions. Also, in the other thread where people werre discussing this, one of the posters claimed that pronation does nothing for racquet head speed, since it only twists the racquet. I thought this series of images would shed light on the discussion.

Then I did read it correctly. Your diagram does a very good job of showing the difference a little twisting of the forearm, arm, shoulder (I really don't know which is most important) does for racket speed.

[LeeD]

You can say whatever you want, but if you don't pronate on the flat serve, you cannot even hit the ball.

[salsainglesa]

Hey hello you people... well i did understand but i was thinking it was important to note the angle of the torso.
Its a good analysis nevertheless, i do like it very much.

about the bent elbow, you should look better at it as not straightening actively the arm articulation, it does seem slightly bent but not at contact, maybe a little bit before and a little bit afte. its more like not not completely straighten, it is loose, its a better image, and a better feeling, since you wont be contracting muscles here and there and hurting yourself.

[gzhpcu]

Originally Posted by LeeD

You can say whatever you want, but if you don't pronate on the flat serve, you cannot even hit the ball.

If you are referring to pure rotation of the forearm, yes. Pronation is the biomechanical consequence of a loose arm on the correct service motion.

[sennoc]

Originally Posted by gzhpcu

If you are referring to pure rotation of the forearm, yes. Pronation is the biomechanical consequence of a loose arm on the correct service motion.

Not exactly. It's not a biomechanical reaction of your body, it is a conscious but effortless action of muscles.

[spacediver]

Originally Posted by salsainglesa

Hey hello you people... well i did understand but i was thinking it was important to note the angle of the torso.
Its a good analysis nevertheless, i do like it very much.

about the bent elbow, you should look better at it as not straightening actively the arm articulation, it does seem slightly bent but not at contact, maybe a little bit before and a little bit afte. its more like not not completely straighten, it is loose, its a better image, and a better feeling, since you wont be contracting muscles here and there and hurting yourself.

Yea I think I'll re-render them with the bent torso to make it look more realistic.

As for the bent elbow thing, the elbow is supposed to remain bent in the before and after. Again, this isn't to illustrate what happens on the serve, but to show the biomechanical consequences of certain anatomical configurations.

The important point was that with a bent elbow, internal shoulder rotation doesn't have the same effect upon racquet movement as it does with a straight elbow. Also, with a bent elbow, there is increased leverage.

In reality, the elbow extends over time (and reaches just about full extension before contact), but these images are to illustrate the components, not reality.

Not sure if I understood your point about the elbow though - perhaps you meant something different.

[gzhpcu]

Originally Posted by sennoc

Not exactly. It's not a biomechanical reaction of your body, it is a conscious but effortless action of muscles.

It is a natural, low-stress movement, which helps avoid injuries. You do not consciously pronate. Set your movement up correctly and the pronation occurs without your being conscious of it.

[sennoc]

I almost agree

Yes, pronation is very natural at serves. But if you use it as a tool, things become more complicated.

Take your racquet, do some tests. Close your eyes, listen carefully to your body. You will find that natural pronation is much longer and happens later than during good serve sequence.

If you want to gain all benefits of upper arm internal rotation, you have to pronate as late as possible before the contact - and as fast as possible. And this motion is not natural. It's almost natural, but definitely not natural.