WASHINGTON (Reuters Life!) - Torsion, flexibility, velocity, momentum.
All have contributed to Tiger Woods' success as the world number one golfer -- and all will put his newly reconstructed knee to the test as he tees off on Wednesday in his first tournament since surgery eight months ago.
Woods' swing has been the envy of golfers around the world ever since he burst onto the professional scene in 1996.
His action is pure efficiency, combining hip, shoulder and wrist motion to exert the greatest possible force on the ball.
The applied physics of his swing propels the club head at an estimated 125 mph at the point of impact with the ball but it also concentrates intense and repeated kinetic energy on his left knee.
The Anterior Cruciate Ligament (ACL) surgery that put the 33-year-old American out of action for eight months is commonplace among athletes, although less so among golfers.
The ACL is the ligament that threads between the tibia and femur in the knee. When the ACL is torn, as it was in Woods' case, the leg rotates in abnormal fashion, causing pain and a sense of instability.
Most damaged ACLs are replaced by ligament tissues from elsewhere in the body such as the kneecap or hamstring, although other choices include tissue from cadavers. Immediately after his own surgery, Woods' doctors reported a successful procedure.
Eight months later, and a day before the start of the Accenture Match Play Championships in Marana, Arizona, Woods said he's looking forward to playing professionally again.
"I feel a lot stronger in my leg, you know both legs are stronger than they've ever been. Stability's something I haven't had in years so it's nice to make a swing and not have my -- and I've said this before -- my bones move," he told a news conference at the Ritz-Carlton Golf Club in Marana.
For sports medicine specialists like orthopedic surgeon Dr. Stephen Haas, Woods' preparation for a return to professional golf after eight months is not surprising -- nor is the fact that he needed ACL replacement at all.
"The main issue with Tiger is that this is his left knee which is planted with spikes on his shoes to prevent any twist. The huge amount of torsion that he generates as he swings and as he's twisting over his left knee puts all the pressure on the ligaments in the knee and he's totally dependent on the knee being stable.
"Since the foot can't move all of the twisting forces are dependent on his knee being stable and solid."
Woods may be feeling stronger but the Tiger of old may not be quite ready to return at full force.
Haas says it's more than likely that his doctors have advised caution at this stage of his recovery.
"He's not just going to go out there full-blast for the first time in a tournament, so I think that he should be able to return to his previous level of excellence as far as his knee is concerned."
According to Haas, who has 30 years experience treating athletes, there is often a psychological component that hampers a return to full form.
Some athletes, he says, are reluctant to extend themselves to their previous level for fear of re-injuring themselves.
Woods, he says, has proven himself mentally strong enough to overcome any such fears.
"As long as he's able to get back to playing at his level without having to think about his knee or without having to compensate for it he'll be ok," he said.
Editing by Belinda Goldsmith