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Understanding Blunt Force Trauma | Backface Signature

Written by: Robert Kaiser

What actually is ‘blunt force trauma’ or in military terms ‘backface signature’ and what type of injuries are the result?

PPSS backface signature injury image
The image of a ‘blunt force trauma’ or in military terms ‘backface signature’ injury was taken 48 hours after I was being shot testing our high performance bullet resistant vests by a Glock 17 firing a 9mm Para round from 9ft distance (the video can be viewed on YouTube). The black mark is a ‘burn mark’ resulting from the heat (due to the deformation of the impacting round)

Please let me answer this question in the shortest possible way, without throwing some hyper intellectual medical terminologies at you even I don’t understand myself. In more simple and understandable terms and in context of this blog, ‘blunt force trauma injuries’ and ‘backface signature injuries’ are the type of injuries that occur under your skin when a ballistic projectile is halted by your bullet resistant vest.  Even when the round is stopped, you can suffer an injury under your bullet resistant vest.

The severity of such injury is primarily determined by the speed, caliber, velocity and weight of the projectile, and can range in severity from a basic bruise to a ‘compression’ type injury and in the worst case scenario result in ruptured organs, internal bleeding and ultimate your death.

‘Blunt force trauma’ and ‘backface signature’ is the result of the energy exchange between the impacting projectile and the human body (shielded by a bullet resistant vest) which does not involve penetration of the skin.

The key task over every body amour is to a) stop the projectile, and b) protect you from internal injury resulting from the impact energy, and this is done by allowing the projectile to make contact, absorb the energy, deform or ‘mushroom’ the projectile, and then spread the impact of the projectile over a wide area of the torso, rather than leaving it concentrated in a particular spot.

Please let me make you aware of the fact that the thinner a bullet resistant vests is, the more ’blunt force trauma’ may occur when being hit by a round.

Of course… and unfortunately… any added layer of any ballistic material will of course increase the bulk of a body armour, hence body armour manufacturers are constantly fighting relentlessly to launch and produce the thinnest and lightest bullet resistant vests, sometimes willingly making sacrifices in regards to the protection from above highlighted injuries… just to make that so very special claim of being able to provide the thinnest vest and in order to get max level of media attention.

The key objective of high performance body armour must be to offer all required ballistic protection whilst using extremely effective, energy absorbing and impact reducing materials. Some ‘Ballistic Backing Materials’ or ‘trauma liner’ used for high performance bullet resistant vests are now less than 1mm thin, but I have seen many ‘ballistic packs’ without such important ‘ingredient’.

10 important facst about bullet resistant vests - robert kaiser 2

Now, trying to understand what is a good or bad body armour, ‘Backface Signature’ can be tricky because of the different test standards. In European, German and British standards, there is an allowable 20 to 25mm ‘Backface Signature’. The U.S. NIJ standard permits 44mm, which could allow for an internal injury. Ever since its introduction, the 44mm ‘Backface Signature’ was regarded as controversial. And, there is still discussion about the importance of ‘Backface Signature’ vs. penetration-resistance among the testing and medical groups.

To comply with ‘NIJ Standard 0101.06 for Ballistic Resistance of Body Armor’, the deformation (Backface Signature) a projectile creates upon impact cannot exceed 44mm (1.73 in.) in depth. To measure this depth, a vest is shot against a block of Roma Plastalina Clay. The vest is then removed, revealing a circular shape of deformation in the clay. A metal scraper is used to smooth the edge of the deformation, as an elevated ridge may have formed around it. A calliper is then used to measure the exact depth of the deformation.

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I urge law enforcement officers and security professionals to make a conscious decision when purchasing bullet resistant vests.  What is the most important criteria for YOU?  The concealability, the weight and the thickness of a bullet resistant vest, or the highest level of protection from ‘blunt force trauma’?

I would suggest this should be subject of your operational duties and requirements. If the concealability of a bullet resistant vest is key for you due to operational risks/threats/requirements, then you need to look for the thinnest and lightest body armour, and the ‘blunt force trauma’ protection may have to become secondary as most body armour manufacturers have now pushed technology to the very max.

Clearly the most important thing is to wear body armour that actually will stop the projectile… otherwise the risk of dying or a round entering you upper body and ricocheting in your rib cage and exiting via your skull is a real probability. Making sure you can go home to your family after the job is done is rather important.

The most in-depth research study on ‘blunt force trauma injuries’ or ‘backface signature injuries’ sutained while wearing such body armour can be found here (click here).

The 2008 article by Marianne Wilhelm “Injuries to law enforcement officers: The backface signature injury” is a really worth hile read for those who either have a serious need or ‘thirst’ for additional information/knowledge in this field.

I have mentioned this in my previous blog, but please allow me to repeat it… based on one of the most informative research projects on body armour worn by US Law Enforcement Officers ever, we can confirm that thousands of lives have been saved since the first officer was shot wearing a modern body armour in 1972.  Over 70% of law enforcement agencies report issuing body armour to all officers, and 53% made the wearing of bullet resistant vests compulsory when on duty. For further information on this survey please read: “Highlights of a truly informative police body armour survey

Any questions?

If so, please call my team at PPSS Group on +44 (0) 845 5193 953 or email or visit

Take good care of yourself.

Robert Kaiser

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