Rob Johnson had hip surgeries in the offseason. We know that, and we know they are the reason he has been held back in spring training. However, despite Johnson's limitations, we have heard many positives about him coming out of spring training. Particularly, Wak has said Johnson's swing looks much freer, and he is able to drive the ball much better. He attributes the changes to Rob's healthy hips.
Biomechanically (I hope that is a word), what Wak is claiming makes sense. A good swing is like a whip. It starts from the core, and a batter's arms/hands twist around their pivoting core before they get involved in the swing. Fundamental to pivoting the core is twisting the hips. Limited range of motion in the hips would limit the twisting action that begins a swing, which should in turn limit bat speed and range of motion. Consequently, bad hips could keep a player from turning on pitches, and limit their power in general.
If Rob Johnson takes a step forward offensively this year, many people will probably point to his healthy hips. I don't want to say they don't make a difference. However, I'm not going to be as quick to credit them for any offensive progress.
Johnson spent the better part of three years in succession in AAA. His wOBA in those three years climbed dramatically - from .258, to .311, to .351. Those numbers were accumulated in 359, 465, and 459 plate appearances, respectively, so not terribly limited sample sizes. Clearly, Rob Johnson made significant progress as a hitter in AAA, without any hip surgeries.
Johnson's wOBA with the M's last year was .274, and none of the major projection systems have him topping .300 this year. If he makes a leap forward like he did in the minors, he will easily cross that threshold, and people will probably be surprised by his sudden improvement. Reasons for his sudden improvement will be sought, and undoubtedly, his hips will become the predominant explanation.
If/when Rob Johnson hits better in 2010 than he did in 2009, I don't think there is a way to discern what portion of it is natural progress, and what can be attributed to his surgeries. I find it interesting that he was clearly a better hitter in the second half of last season, when intuitively he should have gotten worse as his hips deteriorated. However, the obvious benefits of healthy hips can't be ignored.
I just hope Rob Johnson is a better hitter, one way or another. His hip surgeries should only make him a stronger candidate to improve, especially since there were reasons to think he would improve before the surgeries.