Originally Posted by bhupaes
This is a crucial observation - that the tip of the racquet is actually moving to the left, while the hand is actually pulling the racquet to the right.
What is happening is that Nadal is rotating his upper arm, as in internal shoulder rotation. This causes the racquet that is pointing downwards to become more horizontal, and the tip moves outwards faster than the hand is moving in. This is the real effect of pulling across - it whips the racquet outwards, forwards and upwards, due to the leverage of the arm structure. This is the purpose of pulling in - it is not for adding sidespin.
In general, the racquet is pulled in using two movements - internal shoulder rotation, and the biceps. Those with bent arm forehands use the biceps more than those who use straight arms. Nadal clearly is an extreme sort of guy
, and seems to be using only ISR.
Sidespin can happen, however. If the grip is more western, the pulling in of the racquet causes the head to rise up more vertically, which is what this particular stroke sequence is showing. If the grip is more eastern, I think there will be more forward and sideways movement, as I suspect happens in Federer's stroke.
One more thing - the pulling in starts a little before contact. It is a deliberate move, and not something that happens because the arm is moving in a circle.
You can do shadow swings and experience what happens first hand... that's how I came to my conclusions.
The Oscar Wegner in video http://www.youtube.com/watch?v=MxUPDHegz98
around 2.50 demonstrates across motion and side spin. In our example Nadal create mostly topspin, see original video. You also can ask JY about sidespin in this particular Nadal FH video.
Wegner also said, around 6.30, that Federer slowly goes to the ball then all acceleration occurs sideway. IMO there is no correlation among his explanations and videos of Federer and Nadal FH.
Tell me please, in what frame exactly Nadal begins sideway acceleration.
When Nadal rotates the arm around shoulder, he creates centrifugal force which has normal component to the racquet string plane. This force normal component (motion dependent torque) automatically rotates the racquet in clockwise direction, see Rod Cross article http://www.racquetsportsindustry.com...the_serve.html
This normal component is function of angle (ϕ) between arm and racquet. If ϕ=0, this component is zero. This normal component is also function of arm angular acceleration. The more we accelerate angular rotation the less is motion dependent torque.
On other hand, Nadal applies extremely hard arm pronation/ISR. This is also angular rotation, which also creates its own centrifugal force and motion dependent torque. Rod Cross completely ignored this fact.
Moreover, this motion dependent torque pushes the hand to rotate the racquet in clockwise direction, opposite to the torque created by arm rotation. So, there is no automatic motion dependent whip effect. I believe, Nadal creates very fast wrist/hand swing, by using extremely actively handís (or whatever) muscles.
I think you still remember the physics, you can get it.