Magnitude and Duration: Measuring Police Use of Force from Bodycam Video

by | March 4, 2020

Originally published on the Force Science Institute website. Republished here with permission.

In response to a domestic violence call, officers met with the victim at her residence. The suspect had reportedly left the area but soon returned. After a knock at the door, the officers greeted the suspect and, with their body-worn cameras engaged, instructed him to stay in the building’s hallway. The suspect immediately became verbally abusive, failed to follow instructions, and began to fight with the police.

One officer used his expandable baton in an attempt to subdue the suspect. Video from the body-worn camera shows an officer striking the suspect eight times. While the limbs were assumed to be the primary targets during the confrontation, the third strike contacted the back of the suspect’s head. Following the sixth strike, the suspect said he would comply and appeared to stop fighting. He was subsequently restrained by the officers and provided medical aid.

Even using the earliest possible change stimulus for our analysis, the officer stopped his baton strikes within 1.5 seconds of the “change in threat level.”

In the months following the incident, the suspect complained of incident-related medical complications and sought to sue the officer and his department. At that point, GTD Scientific Inc. was retained to examine the body-worn camera footage and apply injury biomechanics methodologies to help explain the magnitude and duration of the officer’s use of force.

Physics and Injuries

As is often the case, a biomechanical explanation of reported injuries is a critical component of use of force investigations. By identifying the magnitude of force necessary to create injuries generally, we can start to identify and explain the cause of specific case-related injuries.

In this case, multiple cuts and bruises were identified on the suspect’s body. From the video we identified two cuts; one on the back of the head from the third strike and one on the shin from the fourth strike. In addition to the cuts on his shin and head, bruises were located on the suspect’s arms and leg.

Using previously published research, including that from GTD Scientific Inc., we estimated the magnitude of force needed for a baton to create the injuries seen on the video. The injuries then allowed us to identify the range of force involved with each baton strike delivered by the officer.

GTD-NYPD Baton Testing

Among the research which helped identify the range of force used in this case was our own independent baton testing data and experience. Working with active duty officers from the New York Police Department (NYPD) a few years prior, we identified the range of forces that an average officer could generate with batons of different sizes, including a baton like that used in this incident.

From these previous studies, we could identify how the magnitude of force (load) generated by a baton affects the body. In other words, we identified the type of injuries we would expect to see when compared to how hard an officer hit someone with a baton. We then looked at the injuries in our specific case and identified the magnitude of force as a percentage of peak load. This percentage would give us an estimate of how hard an officer hit the suspect in relation to how hard he could have hit the suspect at “full force.”

Figure 1: Correlation between baton strike magnitude and injuries showing the likely strike load region in green.

The baton data shown in the graph above shows the force generated by officers using maximum effort. From the data we compared the range of strike efforts to known injury tolerances. Since the head strike did not result in a skull fracture, it was determined with reasonable certainty that the officer never struck the suspect with more than 40% of maximal effort in his intervention.

Continued Beyond “Compliance”

The magnitude of the use of force was not the only question posed in this case. We were also asked to look at the duration of the force. Specifically, we needed to explain why the officer might continue to strike the suspect even after the suspect could be heard giving up. To answer this question, we looked to Force Science research and considered their time to stop studies.

Asking how hard an officer struck somebody with a baton or wanting to know why they didn’t stop immediately are reasonable questions in any use of force case.

In our case, while the suspect was struggling with an officer, other officers can be heard instructing the suspect to “get down on the ground.” Although the suspect did not initially comply, he eventually announced his submission, after which the officer delivering baton strikes stopped and the suspect was subdued.

The duration of force was questioned after a careful review of the video showed that the officer delivered two additional strikes after the suspect exclaimed his submission. Given the dynamic nature of the incident, it is unclear whether the officer heard the suspect or interpreted his words as a definitive sign of submission.

The lack of clarity in use of force encounters highlights one of the greatest challenges in measuring and assessing an officer’s response to dissipating threats. Dr. Bill Lewinski, the executive director of the Force Science Research Center, has explained, “An officer perceives what they pay attention to, and in use-of-force encounters there can be a lot of equally important stimulus competing for their attention. In this chaos, officers must first perceive and recognize that the suspect has ceased to be a threat before the officer will change their response to the threat.”

Without knowing exactly when the officer perceived the suspect’s submission, we chose to identify the suspect’s vocal submission as the earliest objective stimulus, which could have theoretically prompted the officer to change his response. This allowed us to presume the most conservative time estimate and give the greatest benefit to the suspect.

Using the suspect’s verbal submission as the “change stimulus,” we identified the timing of each baton strike in relation to this change and other observable events. As illustrated in Figure 2, the officer delivered a total of two strikes after the suspect showed the first possible sign of submission. These two strikes occurred within 1 and 1.5 seconds of the “change in threat level.”

We compared our findings to independent Force Science research, which detailed the factors that affect an officer’s speed of response when they detect a change in threatening behavior.  After considering theoretical research, applied research, and clinical research, Force Science observed that, in dynamic, “real-world” encounters, the total amount of time for an average officer to respond to a “change in threat” was as high as 1 to 1.5 seconds.

Even using the earliest possible change stimulus for our analysis (the suspect’s verbal submission), the officer in our case stopped his baton strikes within 1.5 seconds of the “change in threat level.”  This finding was consistent with time it can take an officer to stop the use of force in response to a perceived change in threatening behavior.

Figure 2: Timeline of the intervention showing the time necessary for the officer to change his response after submission of the suspect.

Drawing Conclusions from Video

Asking how hard an officer struck somebody with a baton or wanting to know why they didn’t stop immediately are reasonable questions in any use of force case. In our case, we were able to use video from the body-worn cameras to capture two quantifiable metrics. First, strike magnitude studies compared with observable injuries allowed us to show that the force employed by the officer was submaximal—he didn’t hit the suspect as hard as he could. This evidence strongly supported the position that the officer wanted to control the subject rather than harm him.

Second, time measurements considered in light of response studies, allowed us to show that the time the officer took to react to the earliest possible indication of surrender, fell within the range expected during dynamic use of force encounters.

As a final note, the findings in this case were made without direct measurements from the scene. Instead, the key to understanding the magnitude and duration of force was the timely use of the officer’s body-worn camera.


  1. Lee, R.H., Gamble, W.B., Mayer, M.H. and Manson, P.N., 1997. Patterns of facial laceration from blunt trauma. Plastic and Reconstructive Surgery, 99(6), pp.1,544 – 1,554.
  2. Allsop, D.L., Perl, T.R. and Warner, C.Y., 1991. Force/deflection and fracture characteristics of the temporo-parietal region of the human head. SAE Transactions, pp. 2009 – 2018.
  3. Desmoulin, G.T. and Anderson, G.S., 2011. Method to investigate contusion mechanics in living humans. Journal of Forensic Biomechanics, 2.
  4. MacIntosh, A. R., & Desmoulin, G. T. (2019). Police Officer performance and perception using light, medium and heavy weight tactical batons. Applied Ergonomics, 75, 178 – 183.
  5. Lewinski, W.J. and Redmann, C., 2009. New developments in understanding the behavioral science factors in the “stop shooting” response. Law Enforcement Executive Forum (Vol. 9, No. 4, pp. 35 – 54).

Geoff Desmoulin, PhD, RKin., EngL., is a Certified Force Science Analyst and the Principal of GTD Scientific Inc. GTD offers biomechanical consulting services for clients throughout North America and abroad and has been retained in complex injury litigation cases involving municipal police department use of force, violent encounters and TASER device use. Dr. Desmoulin developed landmark testing and shooting reconstruction methodology recently upheld as reliable and admissible by the U.S. Federal District Court for the 9th District of California. He was also the science and engineering host for Viacom Networks hit television show “Deadliest Warrior.”

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