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New study: More evidence against the myth of ‘restraint asphyxia’

There is a long “hangover legacy” of court decisions that are not in harmony with the latest human-factors research


Overwhelming scientific evidence has found that restraining an arrestee in the prone position does not create an exceptional risk of serious injury or death.


Article reprinted from Force Science News #380

“Prone restraint is needed for officer safety, and the stake needs to be driven into the heart of the stubborn myth that this procedure is inherently excessive and dangerous.”

– Mark Kroll, PhD

Overwhelming scientific evidence has found that restraining an arrestee in the prone position does not create an exceptional risk of serious injury or death.

Yet thanks to allegations leveled by plaintiffs’ attorneys and police critics, the myth of potential harm persists, including the claim that the weight of an officer placing a knee on a suspect’s back to aid in stabilizing and handcuffing can cause “restraint asphyxia,” a supposed fatal impairment of the subject’s ability to breathe.

Now the latest study of prone positioning has debunked that assertion.

This research measured the amount of downward pressure (“weight force”) that’s transferred from an officer to a suspect when the officer temporarily applies one or both knees to a suspect’s back to help maintain control until the cuffed subject can safely be rolled to his side or raised up.

The conclusion: none of four knee-on-back techniques commonly taught and used in law enforcement transfers any amount of weight even close to being dangerous, regardless of how heavy the officer applying the force is.

Believed to be the first of its kind, the study is authored by a six-person research team, headed by Dr. Mark Kroll, an internationally renowned biomedical scientist with the U. of Minnesota and California Polytechnical U. who testifies frequently as an expert witness in police litigation.

“Our findings are important,” Kroll told Force Science News, “because North American officers control and restrain agitated and resistant subjects in the prone position over half a million times each year. Subjects end up being proned out in about 60 per cent of physical force encounters – without a death or serious injury resulting.”


Six academy recruits and 35 active-duty male and female officers in Minnesota were the volunteers for Kroll’s testing. Their weight ranged from under 150 lbs. to over 260 lbs., with most weighing between 175 and 200 lbs.

One at a time they applied four standard restraint-and-stabilization handcuffing techniques involving knee placement to the back of a training mannequin that was lying prone on a sophisticated electronic scale.

They were “instructed to do their best to hold each position with consistent normal downward pressure for 30 seconds,” Kroll explains. The scale was programmed to record multiple readings during this period and to average “the actual weight force transferred” to the “arrestee” through each knee contact.

One technique required an officer to face the subject’s side and apply both knees to the subject’s back. The other three maneuvers were single-knee restraints, with the officer variously positioned at the side or up by the suspect’s head and one knee applied to the back at different angles, avoiding direct contact with the neck or spine. The unengaged knee was kept on the floor or butted against the suspect’s side or shoulder for support and principal weight bearing.


In prior scientific studies, weights of up to 225 lbs. have been placed on a prone subject’s back without causing any clinically significant respiratory impairment, Kroll points out. For fatal consequences to be likely “would take two or more LEOs, weighing 287 lbs., each standing on the back of a prone subject,” he writes.

The measurements from his study were nowhere near those levels.

The average force weight delivered by the single-knee techniques ranged from merely 55 to 73 lbs., regardless of the officer’s body weight, Kroll reports. The weight from the double-knee technique was “slightly” greater, but even then the average weight force for a 200-lb. officer was about 99 lbs., still well below a consequential level in terms of injury potential.

“In force-involved litigation,” Kroll writes, “the weight of the LEO, including all equipment, is often stressed” as presenting a risk of great physical damage. The underlying “presumption – that a large fraction of the LEO’s weight is transferred” to the subject – “stands refuted by these data.” In short, Kroll states: “Our data do not support a risk of restraint asphyxia occurring from standard knee-on-back techniques.”

However, Kroll cautions against “stretching” the findings to far different circumstances; for example, two or more officers lying or sitting on a suspect’s back with their full weight, so that both the suspect’s upper torso and lower back are under heavy pressure simultaneously. “This extreme positioning could hypothetically impair both chest breathing and belly breathing at the same time, producing fatal results in a matter of minutes,” he says.

He also warns: “Regardless of any scientific studies, officers need to be aware that there is a long ‘hangover legacy’ of court decisions that are not in harmony with the latest human-factors research. These can be used to blame officers unjustly for deaths following prone restraint, when, in fact, other matters like the suspect’s health or drug intoxication may be the causative elements. It’s important to document precisely how restraint was applied and to be certain that any attorney defending police actions be current with scientific findings.”

Kroll’s report on the knee research appears in the current issue of the quarterly American Journal of Forensic Medicine & Pathology, under the title “Applied Force During Prone Restraint: Is Officer Weight a Factor?” A free abstract, plus a link where the full study can be purchased is available here.

Joining Kroll in the research were Atty. Michael Brave, Sgt. Scott Kleist of the Plymouth (MN) PD, medical consultant Mollie Ritter, Dr. Darrell Ross of Valdosta State U., and cardiac pathologist Dr. Steven Karch.

Our thanks for certified Force Science Analyst Gary Klugiewicz for his assistance with this report.

The FSRC was launched in 2004 by Executive Director Bill Lewinski, PhD. -- a specialist in police psychology -- to conduct unique lethal-force experiments. The non-profit FSRC, based at Minnesota State University-Mankato, uses sophisticated time-and-motion measurements to document-for the first time-critical hidden truths about the physical and mental dynamics of life-threatening events, particularly officer-involved shootings. Its startling findings profoundly impact on officer training and safety and on the public’s naive perceptions.