Extension: Inside Out vs. Cross Court

[lukman41985]

In general, it seems to me, that the pros seem to extend and straighten their arm to a greater extent on inside-out shots than on crosscourt shots. Here are a couple clips of Nadal:
Inside-out:

Cross-court:

Do you guys see the difference? An inside-out shot is produced by a racquet path that goes away from the body. As such, the arm needs to staighten as it moves away from the body as the torso and hips rotate through. On cross-court shots, the arm does not move away from the body as much, and as a result, the arm tends to straighten less. Another interesting finding, and this, I remember, was John's finding, is that the wrist tends to stay laid back longer on inside-out shots than on crosscourt shots. The reasoning behind this, I think, is analogous to the reasoning behind the role of arm extension. The operating principle is that inside-out shots are produced with a flatter arc (a less curved swing plane) than crosscourt shots. The variations we find in arm extension and wrist lay-back are explained by this principle.

[johnyandell]

Not sure we know for sure that the inside out and inside in are actually flatter, (they can hit different trajectories from anywhere on the court depending) but I agree that you tend to see the straight arm hitting arm position more frequently with these two players from the inside position.

[lukman41985]

Yes. I'm not saying that the trajectory of the ball or the low-to-high motion is flat, I'm saying the curve of the swing plane is flatter on inside balls. That is, for Federer, he goes less from his right-to-left, and for Nadal, less from his left-to-right, on inside-out shots. The curve coming from inside of the ball, square to the ball at contact, and back inside in the follow-through is flatter on inside-out shots compared to cross-court shots. This movement is independent from any sort of low/high vertical movement. I'm sure the 3-D study will clear all this up.