SAN FRANCISCO (AP) ― Barry Bonds’ former mistress testified Monday that he blamed a 1999 elbow injury on steroid use, and that the body and behavior of baseball’s home run king changed during their nine-year relationship.

Called by prosecutors to the witness stand, Kimberly Bell choked up as she recalled Bonds once threatening “to cut my head off and leave me in a ditch,” an outburst prosecutors attribute to steroid use. The defense portrayed Bell as an unreliable witness, hungry to capitalize on her affair with Bonds, and Bell acknowledged that the relationship benefited her financially.

Bonds, who holds the major league record for home runs in a career, is accused of four counts of making false statements and one of obstruction for telling a federal grand jury in 2003 ― months after his relationship with Bell ended ― that he never knowingly used performance-enhancing drugs.

Bell took the stand Monday morning after San Francisco Giants clubhouse manager Mike Murphy nervously testified that Bonds needed a bigger hat for the 2002 season. Prosecutors say that testimony is important because an enlarged head is a side effect of human growth hormone use.

Under questioning from Assistant U.S. Attorney Jeff Nedrow, Bell said she asked Bonds about the problem with his left elbow, which she described as “a big lump … it looked awful.” She testified that Bonds responded by saying his steroid use caused the injury, because the muscle and tendons grew too fast for the joint to handle.

“It blew out,” she said.

Bell also said that Bonds talked about the widespread use of steroids among baseball players, including his suspicion that Mark McGwire was juicing during his assault on the single-season home run record in 1998 ― a mark that Bonds later broke.

“He mentioned that other players do it and that’s how they got ahead, that’s how they achieved,” Bell said. Dressed in a dark blue suit, Bonds alternately watched Bell on the stand, scribbled notes and whispered to one of his defense attorneys, Allen Ruby.

Bell, wearing a gray pantsuit and white shirt, said she and Bonds met briefly on July 3, 1994, and attended a barbecue the next day. From there, they shared a romantic relationship that continued even after Bonds married another woman in 1999.

Bell said that Bonds’ sexual performance declined in the later years of their relationship. She said that his testicles changed shape and shrank. She also testified that Bonds grew ― and shaved ― chest hair and developed acne on his back.

A visibly uncomfortable Bell testified that Bonds’ behavior also changed over time. “He was increasingly aggressive, irritable, agitated and very impatient,” she said.

Bell became emotional as she testified that Bonds verbally abused her starting in 1999, saying that ― in addition to threatening to decapitate her ― Bonds said “he would cut out my breast implants because he paid for them.”

The second half of the Bonds-Bell relationship was the same period when Bonds noticeably bulked up and started posting unprecedented power numbers for the Giants. The seven-time NL MVP hit a season-record 73 homers in 2001 en route to a career total of 762 by the time of his last season in 2007, not long before he was indicted for his grand jury testimony.

In anticipation of defense attempts to portray Bell as a gold digger, Nedrow asked Bell about an interview and photograph shoot she did with Playboy that appeared in 2007. She posed nude and discussed Bonds sexual performance in the magazine.

“I was trying to put my life together,” she testified. “Maybe it wasn’t the best decision.”

Bell testified that Playboy agreed to pay her $100,000, but sent the money to her agent, David Hans Schmidt. Schmidt committed suicide in 2007 while under investigation for allegedly attempting to extort the actor Tom Cruise and Bell said she saw little of the Playboy payment ― “about $17,000 or $18,000.”

At times combative, sorrowful and composed, Bell spent most of Monday trying to deflect defense attorney Cristina Arguedas’ vengeful portrayal of her. It was the first time anyone other than Ruby had questioned a witness for Bonds.

Arguedas spent long stretches discussing Bell’s attempts to write a book about Bonds and steroids after questioning her about the radio tour she went on to promote her Playboy appearance.

Bell said she was a guest on a “few” radio shows, and Arguedas shot back “More than 20?” It turned out that she appeared on about 20 radio shows, including the popular Howard Stern talk show. Bell said that the radio appearances were required by Playboy and that she never published the book Arguedas questioned her about.