The defense's version of George Zimmerman

George Zimmerman's defense team sent a parade of witnesses to the stand Monday, seeking to paint a humanizing portrait of their client as someone who tutored ch

George Zimmerman, left, speaks to defense counsel Don West during a recess in his trial in Seminole circuit court, in Sanford, Fla., Monday, July 8, 2013....
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George Zimmerman's defense team sent a parade of witnesses to the stand Monday, seeking to paint a humanizing portrait of their client as someone who tutored children, struggled with his weight, and needed help learning how to tie a Windsor knot.

Zimmerman has pleaded not guilty to charges of second-degree murder in the shooting death of 17-year-old Trayvon Martin, saying he was acting in self-defense. Key to the case is whether the jury believes Martin was accosted by Zimmerman after being followed by him, or whether Zimmerman shot Martin after the teen attacked him.

Most of the first full day of defense testimony focused on whether or not a voice heard screaming on the 911 call made by a witness to police belongs to Martin or Zimmerman. One by one, defense witnesses testified that the voice heard screaming for help on the call was Zimmerman. Among them were Mark and Sondra Osterman, who wrote a book defending Martin's killing as an act of self-defense and said they are donating the proceeds to help pay for Zimmerman's legal defense.

One witness, John Donnelly, who said he paid for suits for Zimmerman to wear in court and also donated to Zimmerman's legal defense, testified that as a former combat medic in Vietnam he could easily pick out the voice of someone he knew amongst chaos. "There is absolutely no doubt in my mind that is George Zimmerman, and I wish to God in my mind I didn't have the ability to understand that," Donnelly told the court. Last week, Trayvon Martin's mother and brother testified that the voice on the tape belonged to Martin.

It's unclear how important the identification of the voice on the tape will be.  NBC legal analyst Lisa Bloom said Monday, "I'd be very surprised if after the verdict [the jury] said we decided based on what the friends of George Zimmerman said about the 911 call."

The defense also called Sanford police detectives Chris Serino and Doris Singleton. Serino testified that Tracy Martin, Trayvon Martin's father, said "no" when he first heard the recording, a statement that Serino took as meaning that the voice screaming for help on the 911 call did not belong to his son. Singleton also testified that Tracy Martin said the voice on the tape did not sound like Trayvon Martin. Called to the stand later Monday afternoon, Tracy Martin disputed those accounts, testifying that he never told the detectives that the voice on the tape did not belong to Trayvon Martin. Pressed by prosecutor Bernie de la Rionda under cross examination, Serino acknowledged that Tracy Martin might have been uttering "no" in denial, while hearing the gunshot that ended his son's life.

The prosecution has noted that Zimmerman himself did not believe it was him on the tape. Serino stated Monday that he believed Zimmerman was simply reacting to the strangeness of hearing his own voice when he told Sanford police "that doesn't even sound like me" after listening to the call.

The defense also called Adam Pollock, owner of a gym where Zimmerman sought training in martial arts for what Pollack said was just under a year. Pollock testified that when he first met Zimmerman, the 29-year old was a "physically soft" man who wanted to lose weight and do fight training. Pollock told the court that Zimmerman was ultimately successful only at shedding pounds. Pollock testified that Zimmerman never even learned how to punch correctly.

During one dramatic moment, defense attorney Mark O'Mara got on the ground and asked Pollock to physically demonstrate what it would look like if one fighter were on top of the other, pinning him with his weight. The image evoked the testimony of Zimmerman neighbor John Good, who testified during the first week of testimony that Martin had pinned Zimmerman in a "ground and pound" position before Zimmerman shot Martin.

The prosecution has sought to portray Zimmerman as a law enforcement would-be who followed Martin against the advice of a police dispatcher, believing the teenager was "up to no good." Pollock's testimony that Zimmerman failed to properly learn to punch after almost a year of martial arts training could bolster the defense's assertions that Zimmerman was outmatched by Martin and wouldn't have started a fight, or it could support the prosecution's argument that Zimmerman was an aspiring crime-fighter who was out of control.

Shortly after testimony concluded and the jury left the chamber, the judge ruled that the defense could introduce in court the fact that Martin had marijuana in his system the night he was killed. The defense is expected to conclude its case later this week.

Note: George Zimmerman has sued NBC Universal for defamation. The company strongly denies the allegation.