During his first interrogation immediately after his arrest, Breivik had first refused to allow himself to be photographed but had then posed for the camera in his underwear showing off his muscles "sort of like a body-builder," a police interrogator testified last month.
"They asked me to take off my clothes… I flexed my muscles a little at that time as a joke to lighten the mood," the 33-year-old self confessed killer told the court on the 35th day of his trial.
"I was trying to be funny, but it wasn't funny. I am critical of myself on that," he added.
The far-right extremist is on trial for the killing of 77 people, mostly teenagers, in a shooting rampage on the island of Utøya last July and the bombing of a government building in Oslo.
While Breivik's guilt is not in doubt, his sanity is at the heart of the trial at the Oslo district court, where he is charged with committing acts of terrorism.
Experts have starkly different opinions about Breivik's sanity, with two court-ordered psychiatric evaluations reaching opposite conclusions.
The first conducted last year found him to be suffering from "paranoid schizophrenia" and therefore not responsible for his actions, while the second carried out shortly before his trial began in April concluded that he was sane.
Other experts testifying before the court have offered varying diagnoses for Breivik, but have all agreed that he is criminally sane.
If Breivik is found sane, he faces a 21-year jail term which could be extended indefinitely if he is still considered a threat to society. If he is found insane he would receive closed psychiatric care, possibly for life.
The five judges are due to deliver their verdict either on July 2th or August 24th.