What Does It Really Take To Kill Someone With “Compression Asphyxia”?

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Over the years, medical examiners and plaintiff’s attorneys have advanced a variety of speculations to explain the arrest-related deaths of suspects for which no medical cause is readily apparent.

This parade of horribles has included hog-tying, positional (prone) asphyxia, neck restraint, pepper-spraying, and cardiac disruption (electrocution) caused by conducted electrical weapons. And one by one, impartial scientific investigation has largely discredited these assertions.

Now a highly credentialed international research team offers fresh evidence that another alleged cause of ARD is a myth as well. That’s “compression asphyxia,” a fatal interference with breathing supposedly caused by officers kneeling, sitting, or lying on a resistant suspect’s upper body to establish and maintain control for handcuffing.

While compression asphyxia is a commonly documented phenomenon in certain real-world circumstances, it’s “crazy” to consider it a realistic possibility in the context of police street practices, according to the researchers’ leader, Dr. Mark Kroll, an adjunct professor of biomedical engineering at the University of Minnesota and California Polytechnic University.

“Weight on the chest as a cause of arrest-related death?” Kroll says. “No way!”

Details of his group’s findings are newly published in the journal Medicine, Science and the Law. That report is a dense thicket of technical language, but in an exclusive interview with Force Science News, Kroll recently translated its essence into laymen’s terms.


With more than 370 patents to his name, Kroll is one of the most prolific inventors of medical devices in the world. He has won the highest international award in biomedical engineering, and in the law enforcement realm his scientific input was critical to the development of the TASER CEW.

He appears frequently as a consulting witness in police litigation, and it was in court that he first encountered the plaintiff claim that compression asphyxia could explain ARDs that seem otherwise baffling. In light of the truth behind other faddish but specious “causes” of ARD, he decided expert investigation was warranted.

He recruited four other PhDs or MDs from British and American universities to join him, including a forensic pathologist, a specialist in the mechanical strength of human bones, an authority on compression deaths from “crowd crush,” and a physician trained in emergency medicine who has studied ARD injuries.


First, the team established the known basics of death from compressive pressure on the human body.

“A fully loaded soft-drink vending machine weighing 1,100 pounds falling on you will kill you,” Kroll says. “So can a car crushing you when a jack collapses, or the impact of a steering wheel slamming against your chest in a collision. But important experiments at the University of California-San Diego showed that a prone, hog-tied subject could withstand 225 pounds of weight on his back and still breathe just fine.”

The team located official records from centuries back when lawbreakers and religious martyrs were subjected to “pressing” under weights piled on their chests for purposes of interrogation or execution. One highway robber bore 350 pounds for half an hour, while a woman was pressed to death in 15 minutes by an estimated minimum of 700 pounds.

“The pressing data shows that about 400 pounds on the chest was survivable because of diaphragmatic breathing, with communication still possible,” Kroll says. “But over 626 pounds was fatal.

“A major mechanism of compression deaths” is the generation of what’s known as “flail chest,” he explains. This involves the fracturing of enough adjacent ribs in two or more places each to cause a segment of the rib cage to break free and move independent of the chest wall.

Such major damage “prevents effective breathing and can cause death even after the source of compression is removed,” Kroll says. “With five to seven ribs involved, you’re shot. Even breathing via your diaphragm (located below the chest) won’t help at that point.”

What he and his colleagues sought to objectively identify for the first time was the specific amount of traumatic force necessary to break enough ribs to cause flail chest. In effect, how much weight on the upper body is required to kill someone via compression asphyxia.


Obviously, testing live volunteers until they died was not an option. But based on the known strength of rib bones, the researchers were able to design a “biomechanical model” of the thorax that allowed for mathematical computations that approximate human experimentation.

From that, they predict a practical rule of thumb, Kroll reports: roughly 570 pounds (they calculated 573 plus or minus 57 pounds) of pressure on the front or back of the torso of a male subject in his 20s or 30s is required to break six ribs sufficiently to cause fatal flail chest.

“In other words,” Kroll says, “it would take two 285-pound cops standing and balancing on the back or chest of a suspect to produce compression asphyxia. And that’s simply not going to happen in the real world.”

Even if an officer were to drop forcefully on his knees onto a suspect, “it wouldn’t significantly change things,” Kroll says. “The weight of dynamic force from dropping needed to cause death is actually higher than the amount of necessary static weight piled on a subject, because the rib cage has an impressive built-in ability to absorb the physical shock of sudden impact.”

Kroll acknowledges limitations to the study. The team’s calculations did not extend to female subjects, children, the elderly, or those with bone disease. He believes, though, that the findings are relevant for most subjects who resist arrest and might end up in a compression situation.


“There are about 800 ARDs a year in the US, but some simply remain a mystery at this point,” Kroll says. “Sometimes when people fight the police the human body seems just to run out of gas for reasons that aren’t understood. These aren’t murders, yet the explanation is not clear and it’s not right to tie them to junk causations with no basis in science.

“I believe we need a new cause-of-death category: Arrest-Related Death Syndrome. We can name it first and understand it later as we learn more about it.”

[Note: More information on this and other related in-custody death topics are covered in the Force Science Certification Course. For more information e-mail: training@forcescience.org or call Scott Buhrmaster at: (312) 690-6213.]

Dr. Kroll can be reached at:mark@kroll.name. His team consisted of G. Keith Still, PhD, of Manchester University in England; Tom Neuman, MD, of the University of California-San Diego; Michael Graham, MD, of St. Louis University; and Lanny Griffin, PhD, of California Polytechnic University. Their study is titled: “Acute forces required for fatal compression asphyxia: A biomechanical model and historical comparisons.”

4 Responses
  1. Thomas L. Allen

    Why weren’t these experts consulted in the George Floyd trial? There’s no way a knee to the neck could obstruct the trachea. Only one person was on his back, and he didn’t have his full weight on the back. It seems to me that this could be newly discovered evidence that could acquit officer Chauvin. The general public needs to know this in order to have a better perspective of how George Floyd died. The coroner’s report is closer to the truth, except that it rules the cause of death homicide. The experts hired by the Floyd family, had an obvious bias and weren’t credible. Officer Chauvin didn’t have a fair trial and the people were not sufficiently informed about this information

  2. Rachel

    This study fails to factor in the potential added pressure resulting from an officer (or multiple officers),(whose adrenaline levels are surging), actively exerting downward physical pressure in their attempts to restrain the subject! If an officer has his knee on your chest or your neck in an attempt to restrain you, I just about guarantee that officer is also flexing all their muscles and straining to exert as much physical psi against the restrained individuals body as humanly possible! What the actual quantitative result of such effort would be is not within my capacity or expertise to calculate, but this contributing phenomenon should be explored. Also, it appears that this study has failed to address the effects of physical restraint methods on restrained individuals who suffer from breathing impairments, such as asthma. I suspect that the fatal pressure thresholds for these individuals would be much lower than that of an healthy, able-bodied 20-or 30-year old! Again, this particular phenomenon, or subject subgroup rather, should be scientifically explored. I think that, while the data presented is definitely interesting as well as mildly enlightening, this sturdy, as described in the above article, does not contain sufficient data to make a final determination for nor against the probability of ARD resulting from “compression asphyxia”! To state that such deaths are simply not possible, is ludicrous because you are essentially saying that one full-grown able-bodied male is incapable of asphyxiation another without incorporating a weapon of some sort (such as a ligature, etc.)! Quite simply, we know this to be an factually untrue statement!

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