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It seems like a common thing in television and movies to have a bad guy in a stand down with the cops but they won't shoot him even though they have a clear shot because he has a gun on a hostage or his finger is on a detonator. They always give the excuse that he could have muscle spasm causing the gun to go off/detonator to detonate.

I had heard that this was a myth but as I've tried to verify the truth, all I see are debates going both ways to explain why this is or isn't a myth. So what is the truth? Can a sniper headshot cause muscle spasm?

As an example, here is the show I watched last night that brought up the question in my mind. It is from season one of the show Millennium, an episode titled "522666" dealing with a bomber in London. Near the end of the episode, the main character Frank Black is stuck in his car with the bomber in another car in a standoff as the bomber has a detonator in his hand. Frank alerts the police who arrive and the following plays out -- (ironically as I'm reading the transcript, I am realizing that a possible answer to my question was right there in the episode and I missed it)

[The first sniper moves into position and reports in to Pierson by radio.]

[We see through the scope of his rifle which has a night-vision lens. We can see Dees' head and left hand holding the cell phone to his ear.]

SNIPER #1: Got a clean profile.

[He then moves his rifle over and spots the remote switch in Dees' right hand on the steering wheel which we see through the scope.]

SNIPER #1: Wait. He's holding a transmitter in his hand. A muscle spasm might set it off.

PIERSON: Then go for the medulla oblongata shot. Take him out.

[The sniper moves to find a new position.]

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A Spasm or Reflex action can be caused by the PNS interaction, instead of the CNS (Brain). And it depends on where in the head they are shot. en.wikipedia.org/wiki/Sniper#Shot_placement or with what caliber bullet. – cde Feb 1 at 18:26
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an example or two of this would add to the question. – cde Feb 1 at 18:50
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Is this really a movie question? Seems maybe better suited for biology.se – DA. Feb 2 at 2:06
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I already pointed out that it might belong elsewhere but others disagreed and my disclaimer was edited out. – sanpaco Feb 2 at 2:07
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@DA. truth or realism in TV, that would make it on topic. – cde Feb 2 at 2:27
up vote 17 down vote accepted

Yes. Because Death is not Instant, even in a clean "apricot" headshot. The human body doesn't just die immediately, even when heart and head are destroyed. Cadaveric Spasms are one problem.

Cadaveric spasm, also known as postmortem spasm, instantaneous rigor, cataleptic rigidity, or instantaneous rigidity, is a rare form of muscular stiffening that occurs at the moment of death, persists into the period of rigor mortis2 and can be mistaken for rigor mortis. The cause is unknown, but is usually associated with violent deaths happening under extremely physical circumstances with intense emotion.

Post-Mortum Movement or Twitching is another. The stored energy in our muscles doesn't just disappear at "death". And misfiring neurons or nerves can trigger muscles that have enough ATP stored up. You will see people that are dead twitch, move and spasm for hours after "death".

For some science, here's a dancing squid:

The Squid and Frogs muscles are twitching from autonomous reflex movement, triggered by sodium in the soy sauce, but this same movement can happen randomly from decaying cell misfires. Imagine the squid had a gun in its hand. All it takes is a finger to spasm.

Even certified diagnosed brain death doesn't stop it:

Many brain-dead patients have spontaneous movements such as jerking of fingers or bending of toes that can be disturbing to family members and health care professionals and even cause them to question the brain-death diagnosis. These movements occur in 39 percent of brain-dead patients, according to a study published in the January 11, 2000 issue of Neurology, the scientific journal of the American Academy of Neurology.

The doctors conducted additional tests and confirmed that there was no brain activity. “The living cells that were ordering these muscles to move were not brain cells or brain stem cells, but cells located in the spinal cord,” he said. “It‘s important for family members and health care professionals to be aware of this possibility.”

There's even a reflex named after it, Lazarus Sign:

Like the knee jerk reflex, the Lazarus sign is an example of a reflex mediated by a reflex arc – a neural pathway which passes via the spinal column but not through the brain. As a consequence the movement is possible in brain-dead patients whose organs have been kept functioning by life-support machines, precluding the use of complex involuntary motions as a test for brain activity. It has been suggested by neurologists studying the phenomenon that increased awareness of this and similar reflexes "may prevent delays in brain-dead diagnosis and misinterpretations."

In regards to sniping specifically, ideally they aim for the medulla oblongata to prevent Voluntary Muscle Movement. Essentially attempting to instantly turn the person into a paraplegic. Involuntary movement is still a problem.:

In a high-risk or instant-death hostage situation, police snipers may take head shots to ensure an instant kill. The snipers aim for the "apricot", or the medulla oblongata, located inside the head, a part of the brain that controls involuntary movement that lies at the base of the skull. Some ballistics and neurological researchers have argued that severing the spinal cord at an area near the second cervical vertebra is actually achieved, usually having the same effect of preventing voluntary motor activity, but the debate on the matter remains largely academic at present.

But the trope can be justified.

About half the time these the shows (always cop procedurals) introduce the hostage taker as having a Dead Man's Switch, a widely common trope. Unlike a trigger that needs to be pushed, a Dead Man's Switch needs to be released. Someone going limp from death is a problem when a Dead Man's Switch is used. Suicide Bombers have this often. It's a small scale version of Fail-Deadly. A gun could have a release trigger for example.

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And Very Graphic Video of a headshot victim still moving arms and fingers liveleak.com/view?i=4a293f822b – cde Feb 1 at 18:49
    
Yeah, but I would argue that the muscle really wouldn't spasm enough to pull the trigger of a gun, especially a pistol. It also depends on the gun. If you have a lever action, it takes more force to squeeze the trigger when firing your first round. – steelershark Feb 1 at 18:54
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@steelerfan idk, the squid and the guy with the headshot in the comment above seem to be pretty lively and have fast, strong movement, not just like a minor twitch. – cde Feb 1 at 19:02
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I'm not eating that. – LarsTech Feb 1 at 20:38
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I humbly concede to your answer. I asked a surgeon that I work with about this. He said that while it necessarily wouldn't be called a spasm, the muscles could definitely constrict with enough power to pull a trigger even on a semi-automatic. +1 :) – steelershark Feb 2 at 2:21

I couldn't find a great source, but judging by some of the forums that I saw,* the reflex trigger finger is unlikely, but still somewhat possible. However, other factors are more likely to be an influence - e.g. how good your aim is if you're attempting to shoot the hostage-taker; whether the hostage-taker will get nervous and pull the trigger on the hostage (by accident or on purpose) before somebody else shoots; where the hostage-taker ends up getting shot and how that affects his/her ability to function; and so on.

*the two I found most useful were actually from the same site: this one about the realism/usefulness of holding a hostage with a gun pointed to their head; and this one about traumatic brain injury

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