Jim Litke AP columnist

Common sense and science have been warning for some time that we’re pushing athletes toward the limits of size, speed and toughness without regard for how they get there, or stay there. Even so, there remains no shortage of kids willing to risk everything for the opportunity.

By most accounts, 23-year-old Thomas Herrion was one of those.

He hung on with the Dallas Cowboys until the final cuts at training camp last fall, played in NFL Europe earlier this year, spent much of the summer working out in the sweltering East Texas heat and was chasing a spot on San Francisco’s roster when he collapsed and died just a few minutes after walking off the field after a preseason game in Denver late Saturday night. The reason Herrion worked so hard to stick with the 49ers, he told pals, was so he could buy a house for his mother.

The cause of Herrion’s death won’t be determined until toxicology tests are completed, usually about three to six weeks. He was listed as a 6-foot-3, 310-pound guard, but estimates of his playing weight by teammates and coaches at some of Herrion’s stops often added between 10 and 30 pounds.

That sounds big — too big to be healthy, according to some medical experts — but it’s just about average for NFL lineman these days. The story of how that came to be could haunt the league for years to come.

Twenty years ago, some of those same experts were warning that super-sizing pro football was a recipe for disaster, and explaining how so many NFL players got so big was easier. Before baseball was outed by Jose Canseco, football had Lyle Alzado. He played a different sport in an earlier era, but Alzado, who similarly admitted steroid use after his career was over, was just as provocative and just as certain that players on every side of him played juiced, too.

“There are freaks of nature,” he liked to say, “but not enough to fill an NFL roster.”

It’s even more true today. By every measure, steroid use is down, there still aren’t enough “freaks of nature” to go around and yet players are bigger than ever.

When Alzado ran riot with the Broncos, Browns and Raiders in the late 1970s and early 1980s, the NFL didn’t test for steroids and there were no more than two dozen 300-pounders. Two seasons ago, the offensive linemen on all but three teams averaged 300 pounds. According to this season’s rosters, as many as 350 players have already tipped the scale at that weight.

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When Vikings All-Pro lineman Korey Stringer died four years ago of complications from heat stroke, it forced the NFL to rethink the strategy of brutal practices in brutal weather. But left unexamined were the underlying dangers — how a heart set up to support someone who should weigh 220 pounds would hold up in someone at 320.

“Pick any of the body’s systems — skeletal, muscular, circulatory — the same is true across the board,” Bob Goldman, a prominent steroids researcher and sports medicine expert, said at the time.

A few years earlier, Goldman finished a study on the evolution of linemen in college from 1950 through 1990. Over that time, they added, on average, 50 pounds of bulk. Goldman did not consider steroid use, other than to say he suspected it was higher than what drug tests turned up.

But he also believed most of the new generation came by their bulk honestly.

“Money is a powerful incentive. If you can develop a lineman who’s 6-8 and 330 with the same speed and agility of guy who’s 250, who’s more dangerous?”

The NFL began answering the question with a rule change in the mid-1970s. Stuck with a spate of low-scoring games, the league’s competition committee decided to allow offensive lineman to extend their arms to block, and stopped cornerbacks from jamming receivers at the line of scrimmage. Those changes resulted in smaller, quicker, even lighter cornerbacks and receivers. Lineman, on the other hand, just got bigger and bigger.

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All those warnings from experts like Goldman went largely unheeded. Lineman didn’t grow to 300-plus pounds in the NFL, they began arriving that way. Not only that; many of them were not just big, they were agile — despite having 25 to 30 percent body fat, meaning they were carrying as much as 90 extra pounds.

But agility was not the only thing that increased with size.

So did the risk factor for strokes, high blood pressure, traumatic joint injuries and cardiovascular problems. As unsettled as we should be by what happened to Herrion — “a sad thing,” Cowboys coach Bill Parcells called it, “He kind of came in as one of those underdog kind of kids and hung in there,” — it’s a little late in the game to be surprised.