A respiratory physiology expert testified during the murder trial of Derek Chauvin on Thursday that the former Minneapolis police officer at one point had half of his body weight on George Floyd’s neck during his arrest last summer. 

Floyd was killed during the arrest, a moment that led to nationwide demonstrations against police violence and for racial justice.

“What we’re seeing is that half of his body weight, plus half his gear weight, is coming down," Martin Tobin, a witness for the prosecution, testified Thursday as he looked at a photo captured from the arrest. 

In the photo, the tip of Chauvin’s foot could also be seen slightly off the ground as he kneeled on Floyd, who was pinned to the street.

"That’s 91.5 pounds … coming down directly on Mr. Floyd’s neck,” Tobin, a renowned pulmonologist, testified. “Everything is directed down on his knee.”

The prosecution is arguing that Chauvin directly caused Floyd's death by putting his knee to Floyd's neck for nearly nine minutes. Chauvin faces murder charges over Floyd's death.

Chauvin's defense lawyer argued in his opening statement that Floyd died from a mixture of causes, including preexisting conditions and drugs that were in his system.

“The reason we're seeing that is because the toe is off the ground, and there is no body weight sitting back, he's not hunkering back on his heels, so everything is directed down on his knee,” Tobin continued, adding that in that position, his “shin and his toe and his boot is playing no contribution.”

The image was one of a number Tobin, who said he thoroughly reviewed footage of the arrest ahead of his testimony, was presented by prosecution when he was on the stand.

Another image Tobin zeroed in on during his testimony was a still from police body camera footage that showed Floyd appearing to push his knuckles and fingers against the street and the tire of a nearby police vehicle as he was pinned to the ground during the arrest. 

“To most people, this doesn't look terribly significant. But to a physiologist, this is extraordinarily significant,” Tobin said.

“This tells you that he has used up his resources and he's now literally trying to breathe with his fingers and knuckles,” the doctor testified.

“When you begin to breathe,” Tobin said, “you begin to breathe with your rib cage and your diaphragm. Then the next thing you recruit after that is your sternum muscle which is the big muscle in your neck.” 

“When those are wasted up, then you're relying on these types of muscles like your fingers to try and stabilize your whole right side. Because he's totally dependent on getting air into the right side,” he continued. 

Floyd used “his fingers and his knuckles against the street to try and crank up the right side of his chest,” Tobin said, adding that he believes Floyd had also attempted to use his shoulder to breathe during the arrest.

“It's a very poor way of breathing. But it's what you have to do when everything else is failing,” he said.

He also said that had a healthy person been subjected to what Floyd had been during the arrest, they “would have died as a result.”