Maybe Chad Fox threw too many sliders.

With every trip to the disabled list — nine in 12 seasons — the pitcher couldn't help but wonder what he might've done wrong, what he could do differently next time.

Finally he concluded: "I just think it's the makeup of my arm."

For every Cal Ripken and his 2,632-game streak or Brett Favre with his 291 consecutive starts, sports has many more Chad Foxes — a 38-year-old trying to make a comeback with the Chicago Cubs after yet another elbow problem. As much as stats and schemes, maneuvers and matchups, injuries sway the fates of pro teams.

Unraveling the mystery of why some players prove durable and others fragile is worth millions of dollars to front offices. Is it genetics? Luck? Conditioning? Pain tolerance?

Nowhere is the pursuit of those elusive answers more evident than at the annual NFL scouting combine, going on this weekend. Sure, clubs measure prospective draft picks' speed, strength and smarts through drills and interviews.

But they also make players undergo extensive evaluations by team physicians. Doctors request MRIs, pore over medical records, examine old injuries.

Team physicians concede that the process is more adept at identifying previous problems than predicting future ones.

"It's very much an art and not a science," said Dr. Patrick Connor, who works with the Carolina Panthers.

There are plenty of players who were completely healthy at the high school, college and minor league levels — then constantly go down once they reach the highest realms of professional sports.

Maybe their genes didn't bless them with a body capable of withstanding the rigors of major-league sports.

A football player who stays healthy in college may find that the increased number of games in an NFL season is his undoing, said Dr. Matthew Matava, a team physician for the St. Louis Rams and Blues.

"He may have been at the threshold of the break point at 12," said Matava, an associate professor of orthopedic surgery at Washington University. "Now he passes that once he hits 16."

Not only does the frequency of games increase but so do the travel and the quality of the competition.

"A 19-year-old, 20-year-old guy — forget the banging and the size of the players," said New York Knicks president Donnie Walsh. "Just coming out and playing every day hard, you don't do that in college. You put a 19-year-old guy in that, how is it going to affect him?"

Still, looking to the past is a valuable indicator. Research and experience have backed up common sense, that somebody who has been injured before is more likely to be injured again.

And not just to the same body part. A right knee injury, for instance, can increase the chances of an athlete hurting his right hamstring or ankle.

"The leg is a linked chain," said Timothy Hewett, a professor who serves as director at the Sports Medicine Biodynamics Center at Cincinnati Children's Hospital Medical Center.

If one part is weakened by injury, others have to compensate — and that leaves them more vulnerable to injury themselves.

Hewett's research on athletes with torn ACLs has shown that long after they complete rehabilitation and return to competition, tests can still make out differences between the two legs.

It's called an asymmetry, when one half of the body is stronger, or more flexible, or more responsive than the other. He has found that asymmetries help explain why some athletes are more likely to get hurt regardless of their injury history.

"A symmetrical body is a safer body," Hewett said.

Hewett, who works with Bengals team physicians and consults with European soccer clubs, believes tests can be designed to measure how vulnerable to injury an athlete is — and that proper drills can help lessen that.

"I think these people think they're snakebitten in some way," he said, "but there's much more to it than that."

No doubt luck plays a role: Some hits are going to break any football player's leg. But it's also true that some athletes seem to take fewer of those hits.

"An athletic sixth sense" is what Connor, the Panthers team physician, calls it. He recalled Barry Sanders as an example of a running back who avoided the blows that pounded others at his position.

"Right before he gets clobbered by three or four guys, he just ducks under," Connor said.

Getting tackled with a little less force on each play adds up over the course of a career.

"Injury-prone — is there such a thing?" mused Ken Locker, an athletic trainer for the Dallas Cowboys from 1973-89. "I always said a player was one step slow. He couldn't get through the hole, so he got crushed."

Whenever a new season starts, no matter the sport, it seems there's always a player coming off an injury trumpeting his offseason workout program, which he's confident will keep him healthy.

The role of strength and conditioning in injury prevention isn't clear, though. Players can increase their vulnerability by over-training. Certain drills help some athletes more than others, Hewett said. Flexibility and balance matter — not just stamina and brute strength.

"We know that old-fashioned strength — who can bench press the most, who can run the fastest straight ahead, who squats the most — doesn't equate to the best athlete on the field and doesn't equate to who avoids injury," Connor said.

San Antonio Spurs forward Bruce Bowen had the NBA's longest active consecutive games streak (500) before being suspended last March. He credits his offseason conditioning for his durability.

"I've used the analogy of a bank account," he said. "You get paid to a certain point and you deposit your checks so that you can take care of your bills later. You're depositing everything for this 82-game season in the summer."

Fairly or not, everybody from fans to commentators to athletes often equate durability with toughness. The flip side of that, of course, is that the guys who are always on the sidelines must be soft.

Whether pain tolerance is a factor probably depends on the type of problem. Coaches and players like to make the distinction between "hurt" and "injured."

"If you're hurt, I would think you can play through being hurt. Everybody's probably hurting in some way," said Washington Nationals outfielder Adam Dunn, who has been on the disabled list only once in a major league career that began in 2001.

"If you're injured, that's another story."

Detroit Tigers third baseman Brandon Inge once played with a broken big toe on his left foot. It was "excruciating pain," he said, but he didn't miss any games.

"There's a fine line between sucking it up and being stupid," he said. "If you have a tweaked muscle and you play, it can turn into a torn muscle and you can't play. I had a strained oblique muscle and I played through it, and it tore and I had to take 15 days off."