Finally willing to talk about the past, Mark McGwire sobbed and sniffled, giving the missing — and unsurprising — answer to the steroids question.
Ending more than a decade of denials and evasion, McGwire admitted Monday that steroids and human growth hormone helped make him a home run king.
Tufts University physicist Roger Tobin wrote in a 2007 study that even moderate increases in muscle mass from steroid use can lead to 4 percent increase in batted ball speed appear to increase the number of home runs hit by a batter dramatically.
Tobin, a specialist in condensed matter physics with a longtime interest in the physics of baseball, published his paper, "On the Potential of a Chemical Bonds: Possible Effects of Steroids on Home Run Production in Baseball," in the American Journal of Physics.
Tobin’s paper notes that Babe Ruth's record of 60 home runs in a single season stood for 34 years until Roger Maris hit 61 homers in 1961. For the next 35 years, no player hit more than 52 home runs in one season. But between 1998 and 2006, players hit more than 60 home runs in a season, six times. McGwire set the record for most home runs in a season, hitting 70 in 1998. That was beaten by Barry Bond, who hit 73 home runs in 2001 — topping Maris' mark by 20 percent. Bonds has denied steroid use, despite speculation that he did in fact use performance-enhancing drugs.
Tobin said the explosion in home runs coincides with a mid-1990s "steroid era" in professional sports. Use dropped to historic levels in 2003 when Major League Baseball instituted steroid testing, the article offers as background.
"A change of only a few percent in the average speed of the batted ball, which can reasonably be expected from steroid use, is enough to increase home run production by at least 50 percent," Tobin said.
This disproportionate effect arises because home runs are relatively rare events that occur on the "tail of the range distribution" of batted balls, Tobin explained.
Tobin applied a similar, though less extensive, mechanical analysis to pitching and found a smaller impact. He calculated that a 10 percent increase in muscle mass should increase the speed of a thrown ball by about 5 percent, or 4 to 5 miles per hour for a pitcher with a 90-mile-per-hour fastball. That translates to a reduction in earned run average of about 0.5 runs per game.
"That is enough to have a meaningful effect on the success of a pitcher, but it is not nearly as dramatic as the effects on home run production," Tobin said.
"I wish I had never touched steroids," McGwire said. "It was foolish and it was a mistake. I did this for health purposes. There's no way I did this for any type of strength purposes."
Behavioral side effects of regularly taking large quantities of steroids include fury, paranoia, delusions, depression and mania.
"They lead to psychotic episodes of aggression," said FOXNews.com's managing health editor Dr. Manny Alvarez.
After being confronted by the AP during the home run streak in 1998, McGwire admitted using androstenedione, a steroid precursor that was then legally available and didn't become a controlled substance until 2004. Baseball and its players didn't agree to ban steroids until a year after his retirement.
McGwire's 70 homers in 1998 came in a compelling race with Sammy Sosa, who finished with 66. More than anything else, the home run spree revitalized baseball following the crippling strike that wiped out the 1994 World Series.
The Associated Press contributed to this story.