Woods' knee will hurt; his mind will need to compensate

Tiger Woods will play the final round of the U.S. Open on Sunday on a wounded knee, one that was operated on in April for the third time in 14 years.

How wounded? Nobody knows for sure.

Woods' latest procedure, according to his official Web site, was an
arthroscopic surgery (often referred to as a "scope") to address pain
he'd experienced in his left knee since mid-2007.
Mark Steinberg, Woods' agent, told the Web site that "cartilage damage was found during the procedure, which Dr. [Thomas] Rosenberg was able to

The agent's description suggests there was some flaking or fraying of cartilage, which certainly can occur in a knee joint as a result of wear
and tear. It also suggests a debridement, or clean-up procedure, was
performed to smooth out the rough edges.

As Woods began the U.S. Open, the knee seemed to be a nonfactor. But as the weekend has worn on, the effects of the recent surgery have become more apparent.

Woods grimaced noticeably on a drive on the 15th hole in Saturday's
third round. His left knee, the one on which he had surgery, is the
front knee when he drives, meaning he has to transfer his body weight
through to that leg as he swings.

"I just keep telling myself that if it grabs me and if I get that shooting pain, I get it, but it's always after impact," Woods said. "So go ahead and just make the proper swing if I can."

This is yet another example of Woods' amazing mental toughness, which certainly factors into how he processes pain. The body's tendency once it learns that a particular movement or position is pain-provoking is to avoid it or, at the very least, tense up the muscles around the painful area to protect against further injury. All this happens involuntarily and could effectively shut down Tiger's long game.

Woods' overriding of that internal protective mechanism on Saturday, particularly on the final five holes, was an amazing display of intensity and focus. Woods made a conscious decision to continue to play with his hard-driving style and simply deal with the consequences, knowing the sharp pain would come at the tail end of his swing.

Video of Woods as he moves through his swing shows his left knee
undergoing a great deal of torsion, or twisting force, as he
completes his follow-through. The end result is a combined rotation and
extension, which significantly increases the compressive force through
the knee joint, particularly on the medial (inner) aspect.

A close look at the direction Woods' knee is pointing relative to his foot position during follow-through gives an undeniable look at the degree of the twisting force through the joint. Plus, the speed with which Woods' knee snaps back towards the final rotation/extension position increases the force through the joint, potentially worsening his discomfort. It is thus no surprise that Woods felt the brunt of his pain late in his swing.

Despite his best efforts to the contrary, Woods appeared to favor the knee slightly on shots from the fairway late in the round. Through three rounds, he's hit the fairway exactly 50 percent of the time at the Open (21 of 42 -- slightly less than his PGA Tour average of nearly 60 percent).

When Woods hits out of the rough, those swings take an additional toll
on the body -- especially the lead leg -- as even more force is required
to move the ball forward. The more Woods stays in the fairway and out of
the rough, the better his knee likely will feel in the long term.

Woods also is at the end of a long week at the U.S. Open (with the
possibility lurking of a Monday 18-hole playoff should he tie for the lead after
72 holes Sunday night).

Torrey Pines has played about 7,500 yards this week, meaning Woods
has walked more than 40,000 yards during rounds and practice (despite
playing only nine holes at a time). That's about 24 miles. The effects of walking the course several days in a row cannot be underestimated. (Plus, Woods had not walked a full course before Thursday.) During regular walking, each step places roughly three times one's body weight through the knee (which is a combination of muscle activation around the knee and reaction forces produced by the ground when the heel strikes it).

With Woods weighing roughly 185 pounds, that translates to approximately 555 pounds of force per step for a near-marathon distance. And he still has one more day to go.

The combination of the highly repetitive but (relatively) low-load stress of walking with the less-frequent but higher-load strain of driving the ball serves to increase the overall toll on Woods' knee.

The fact that he has to take his longest walks after his most vigorous swings adds to his challenges. Near the end of Saturday's round, Woods occasionally used his club as a pseudo-cane as he walked from the tee to his next shot.

By the end of the third day, even Woods had to acknowledge the pain had gotten progressively worse throughout the tournament.

"Is it getting worse? Yes, it is," said Woods after an eagle on the 18th
hole Saturday gave him the 54-hole lead at Torrey Pines. "Certain shots,
I'll feel it. I can't say it's a drive, can't say it's a wedge. I'm not
sure what shot it's going to happen on."

Not that Woods has let any of this knee business get in the way of his taking the lead at the end of three rounds. Nor should anyone expect it to stop him on Sunday. Will it hurt? Undoubtedly. Will it affect elements of his game? Perhaps. Will he find a way to compensate? Most certainly. After all, this is Tiger Woods we're talking about here. Traditional rules don't apply.

Remember, Woods has won before in his first tournament since having surgery. After
the December 2002 procedure, Woods won the Buick Invitational the following
February ironically at Torrey Pines.

He's 18 holes -- or maybe 36 -- away from possibly doing it again.

Stephania Bell is the injury expert for ESPN.com fantasy.