Taking the stand at Chauvin's murder trial, former California police officer Barry Brodd said officers don't have to wait for something bad to happen; they need only to have a reasonable fear that there's a threat and then adjust their actions accordingly.

"It's easy to sit and judge... an officer's conduct. It's more of a challenge to, again, put yourself in the officer's shoes to try to make an evaluation through what they're feeling, what they're sensing, the fear they have, and then make a determination," Brodd said.

He also said he didn't believe Chauvin and the other officers used deadly force when they pinned Floyd on his stomach, with his hands cuffed behind his back and Chauvin's knee on his neck or neck area for what prosecutors say was nine and a half minutes.

Brodd likened it to a situation in which officers used a Taser on someone fighting with officers, and the suspect fell, hit his head and died: "That isn't an incident of deadly force. That's an incident of an accidental death."

Several top Minneapolis police officials have testified that Chauvin used excessive force and violated his training while medical experts called by prosecutors have testified that Floyd died from a lack of oxygen because of the way he was restrained.

But Brodd said: "I felt that officer Chauvin's interactions with Mr Floyd were following his training, following current practises in policing and were objectively reasonable."

Brodd also testified that the bystanders yelling at the officers to get off Floyd complicated the situation for Chauvin and the others.

"The crowds started to grow in size, start to become more vocal. So now officers are always trained to deal with right, so what threat is the biggest threat?" he said.

"Is it the suspect on the ground in front of me in handcuffs that we have relatively controlled? Or is it the unknown threat posed by the crowd that could go from verbal to trying to interfere with my arrest process in a matter of seconds?"

Brodd also appeared to endorse what prosecution witnesses have said is a common misconception: that if someone can talk, he or she can breathe.

"I certainly don't have medical degrees, but I was always trained and feel it's a reasonable assumption that if somebody's, 'I'm choking, I'm choking,' well, you're not choking because you can breathe," he said.

Chauvin, a 45-year-old white man, is on trial on charges of murder and manslaughter in Floyd's death last May after his arrest of suspicion of passing a counterfeit $US20 note at a neighbourhood market.

Chauvin lawyer Eric Nelson has argued that the 19-year Minneapolis police veteran did what he was trained to do and and that Floyd died because of his illegal drug use and underlying health problems, including high blood pressure and heart disease.

Fentanyl and methamphetamine were discovered in his system.

The defence began presenting its case on Tuesday after the prosecution rested following 11 days of testimony and a mountain of video evidence.

Australian Associated Press