The above pictures of Sam Stosur's kick serve and the following have been posted here on Talk Tennis by Anatoly Anopin. You might want to read the original threads at:
Originally Posted by mxmx
Thanks for the video and great pics. I will try and post a vid of my second serve one day...but i never really thought or realised as much that a angled wrist works differently with different serves.
Kick serve - How do you brush? http://tt.tennis-warehouse.com/showthread.php?p=6978051
Seriously About Pronation and Kick Serve #1 http://tt.tennis-warehouse.com/showthread.php?t=361610
The following photo sequence by Toly clearly shows that to get the rapid spin on a kick serve, the ulnar deviation has to be occuring very rapidly while the ball is being struck, and the ball is rolling along the surface of the raqcuet.
To get that most rapid phase of ulnar deviation to produce the maximum amount of spin, the racquet has to strike the ball lower than on a flat serve, with the "brushing up" on the ball occurring as a result of that fast ulnar deviation.
If you read the original thread Toly cogently points out that it is ulnar deviation that is responsible for almost all the spin, not the action of the arm swinging (although some might counter that the ulnar deviation is largely a passive result of the heavy racqhet being being thrown up).
So in pic one below, he points out the arm is moving forward, while it is the ulnar deviation that is doing the brushing in pic 2:
On the other hand, in a first serve the ball is struck more in front of you, and by that time the ulnar deviation has largely occurred, with any residual ulnar deviation slower than during a kick serve.
As for my knee...i would say its probably a little of everything you mentioned.
I once jumped and reached for a ball stuck in a fence, which made my knee more sensitive years back. But this may sound funny, but my car clutch combined with traffic, is placing too much strain on the knee. Combine it with this and that, and you sit with a overworked knee. I would not say i have weak legs in the sense that it needs muscle to protect it. But what is very strange is that my knee would very erratically "decide" for itsself it wants to be sore that day...not having done anything strange. My knee almost gets in a bad mood lol.
You really should get checked out and see what the problem is what that knee.
It may mean a big difference in whether you need to take some time off, or whether there is some strengthening regimen that could help you come back with less problems.
A worry is that articular cartilage does not contain pain sensors, so it isn't until the overlying joint space itself becomes inflammed that pain occurs.
And jumping and landing on that left knee, which you also pump the clutch with, could become a bigger problem if you continue the way you are.