A medical expert testified Thursday that George Floyd died from a lack of oxygen as he was being pinned face down on the pavement, in the ninth day of the U.S. trial of Derek Chauvin. the former Minneapolis police officer charged in Floyd's death.

Dr. Martin Tobin, a lung and critical care specialist at the Edward Hines Jr. VA Hospital and Loyola University's medical school in Chicago, took the stand for the prosecution Thursday.

He told the jury that the combination of Floyd's prone position on the street, one of Chauvin's knees on Floyd's neck and the other on his back, and his hands constrained behind his back had the effect of putting his body in a vise and interfered with "central features of how we breathe."

The testimony undercuts the defense contention that Floyd died from his use of illegal drugs and underlying medical conditions, not from the Chauvin's actions.

Los Angeles Police Department Sergeant Jody Stiger is cross-examined by defense attorney Eric Nelson in a Minneapolis courtroom on Apr. 7, 2021.

Wednesday, Los Angeles Police Department Sergeant Jody Stiger, an expert on the use of force by police who testified as a prosecution witness, said that Chauvin should not have pinned his knee against the back of Floyd's neck for nine-and-a-half-minutes while Floyd was handcuffed and in a prone position.

Stiger testified that Floyd did not present an immediate threat or resist arrest as Chauvin bore down on Floyd with most of his weight while Floyd pleaded he could not breathe.

"My opinion was that no force was reasonable in that position," Stiger said. "The pressure... caused by the body weight could cause positional asphyxia and could cause death."

Chauvin, who is white, was a 19-year police veteran until he was fired. He has pleaded not guilty to murder and manslaughter charges in the case being heard by a racially diverse 12-member jury. Chauvin's lawyer contends that Floyd, an African American, died from underlying health issues and that Chauvin followed his police training in the way in which the 46-year-old Floyd was arrested.

Floyd's May 25, 2020, arrest sparked widespread protests in the United States and around the world against police brutality and systemic racism.