One of three Calgary police officers on trial for assaulting a man during an arrest last year says he believed he was in the fight of his life at the time and that his stress and anxiety levels were "at an all-time high."

"I didn't do anything excessive that day," said James Othen as he testified in his own defence.

Othen, 39, Kevin Humfrey and Mike Sandalack are on trial for assault causing bodily harm for their roles in the July 30, 2016 arrest during which Clayton Prince suffered broken ribs and a collapsed lung.

Othen's version of the arrest contradicts a short video depicting part of the incident. Othen said he believed Prince was resisting police while the video shows he was in the process of surrendering when the punches began.

After viewing the video, Othen told his lawyer Alain Hepner that he realized stress and anxiety likely caused him to improperly remember the details of the incident.

"I realize my notes aren't accurate," said Othen.

Video entered as evidence at the trial of three Calgary police officers accused of assault after a traffic stop. 0:07

Othen told provincial court Judge Margaret Keelaghan that over his decade in policing, he's been involved in about 10 foot chases.

"Without a doubt, a foot-chase is one of the most stressful situations you can be put in as a police officer," said Othen. "It's all because of the unknown."

On the day of Prince's arrest, Othen testified his adrenaline was pumping when he heard over his police radio that a fellow officer was involved in a chase which involved the suspect running through a crowded sushi restaurant.

Othen said he had no idea if the suspect was armed or on drugs.

When he and his then-partner Humfrey arrived at the scene, Prince was hiding in some trees and then began running. Othen said at one point Prince took a couple of steps towards the officers.

"My stress is at an all-time high; I think I will be in the fight of my life," Othen testified. "I was like, 'holy smokes' in my head, going 'here we go, this is going to be the fight."'

Then Othen saw a fellow officer pull his service weapon and point it at Prince.

Othen said as officers tried to arrest Prince, the suspect was positioning his hands in such a way that it would have been

difficult to handcuff him. He said he remembered kneeing Prince in the upper thigh and punching him four to six times in the head.

Othen used 'tremendous force': Crown

He said it wasn't until after he saw footage captured from a police vehicle dashcam that he realized his recollection contradicted what actually occurred. He said it was possible he started engaging with Prince just as the subject was in the process of surrendering and being subdued.

Photos show some of the injuries Clayton Prince sustained when he was arrested by Calgary police officers who now face criminal charges. (Clayton Prince)

"You've just interrupted his surrender with tremendous force," suggested prosecutor Jim Stewart during cross-examination of Othen.

Stewart replayed the video for Othen, which shows Prince on the ground with his hands on his head in the moments before Othen jumps on the complainant's back and begins delivering blows.

Other officers have testified the three accused constables delivered "extremely excessive force" to Prince during the arrest.

Stewart suggested to Othen he'd "cooked up a story" about Prince resisting arrest and then tailored his notes to reflect that story because the officers knew the Alberta Serious Incident Response Team would be looking into the violent arrest.

Othen also testified he did not see Hunfrey or Sandalack deliver any blows to Prince.

'Dynamic takedown'

Humfrey also testified in his own defence Monday, telling the judge he only struck Prince once and that it was "not with 100 per cent of my effort."

In his notes, Humfrey referred to the arrest as a "dynamic takedown." He wasn't sure how Prince was taken to the ground; in one moment he testified he saw the suspect standing, the next, Othen was on top of him.

As for his own participation, Humfrey says he got involved believing Prince was resisting arrest and used a "stunning technique" when the suspect pulled his hand away while officers were trying to cuff him.

Humfrey saw Sandalack "jab" Prince in the face once.

On Tuesday, Stewart will cross-examine Humfrey before Sandalack begins his testimony.

Defence lawyers have indicated they plan to call a German-Canadian psychologist who is researching false memories and their implications. She will testify by video conference from London.