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In this particular case, I am talking about an extremely flexible human. In my story, this human catches another's head with his legs and manages to snap his neck. I know a human can't simply snap another's neck (They don't have enough strength), but could it be done if the attacker were able to pinch their opponent's neck (to weaken it), and then twist and snap it?

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    $\begingroup$ "I know a human can't simply snap another's neck" *Citation Needed $\endgroup$ – NPSF3000 May 19 '16 at 2:57
  • $\begingroup$ Falling out of a tall building onto their head should do it $\endgroup$ – Separatrix May 19 '16 at 7:05
  • $\begingroup$ I really don't see how this relates to the scientific field of physics, so I removed that tag. $\endgroup$ – a CVn May 19 '16 at 11:33
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    $\begingroup$ You could run an electric current through their body or neck somehow (a powerful charge lance or stun baton? Maybe a thrown device that attaches to the skin, if only briefly?) and then hit it with a mace or warhammer (or a club). While it is rigid it is easier to break. $\endgroup$ – TylerH May 20 '16 at 14:42
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It is possible in theory, but not the way it is shown in the movies. First of all, the human neck is both strong and flexible, and unless the person is taken unaware (like in a car accident), will automatically tense up in response to pressure, making it even stronger. Second, even if you could muster enough strength to break a neck, unless the rest of the body was restrained you would just end up twisting their entire body around.

The only way to pull off a neck snap realistically is by getting the victim in a full-body grapple, then stretching and twisting the neck until it breaks. You'd need to be very strong to do it though; flexibility alone isn't going to cut it.

It may, however, be possible to break someone's neck with an impact instead of a twist. A strong punch or kick may generate enough force, but only if the body was restrained somehow, so not really practical in a fight. It may be possible to get them in a hold and then throw yourself and them onto the floor with enough force to twist their neck to the breaking point, but this would involve more luck than anything. This may be the best option to use in a story though - make it basically a lucky fluke.

It won't be an instant-kill in any event; even if you fully paralyze the person from the neck down they'll still be alive until they suffocate. More likely than not they won't be fully paralyzed and you'll instead be left with a paraplegic who is perfectly capable of talking (or screaming). Think about the possible results of car accident neck injuries.

All in all, it's not a very effective killing maneuver.

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    $\begingroup$ The idea of the paralyzed victim of a car accident screaming for help on her seat while not being able to move anymore gave me chills. +1 $\endgroup$ – Nico May 19 '16 at 7:46
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I decided to do a little research into neck injuries in sports (especially contact sports) to get an idea of the ways in which people can (accidentally in these cases, we hope) break each other's necks. It looks like one common way is via whiplash, rather than twisting of the neck – in this video (warning, a wrestler dies) you can see that it's either the blow to the neck or the whiplash caused by the force of the blow which caused a broken neck in this case. As IndigoPhoenix wrote, if the person's body is immobilised, then their neck is much more likely to be broken – e.g. if a person is on the ground and someone steps/falls on their neck, as experienced by this person during a game of rugby.

The other means by which a fighter could break an opponent's neck with (relative) ease, which I left until last because it's so obvious, is via a "piledriver" or similar move that involves driving the opponent head-first into the ground. These moves incur a foul in professional Mixed Martial Arts, and are banned in professional wrestling, because the risk of a broken neck is simply far too high. Lifting an opponent off the ground so that their centre of balance is high enough to flip them head-first before they hit the ground is also an illegal tackle in rugby (not sure about gridiron, sorry, but it might be legal over there, the armour makes those guys crazy).

In the context of fighting narratives this seems to me to be a "power" move rather than a flexibility and dexterity move, so it might not be quite your character's style, but perhaps instead of "catching the opponent's head with their legs" and twisting to break, your character could instead grab with their legs and somehow flip/drive the opponent head first into the ground? Alternatively, maybe your character could wrap their legs around the opponents neck and then fall with them to the ground, so that when they both hit the ground the force of your characters fall is directed in a way that puts their full weight/force on the opponent's neck.

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It takes 1000-1250 foot-pounds (1350-1700 Newton-metres) of torque to break a human neck. That kind of force is generally achieved in a 5-9 foot drop. You can also work out exactly how far you need to drop someone of mass $x$ to break their neck:

$$ d = \frac{1}{g}\cdot\frac{T}{x} = \frac{T}{gx} $$

That is, the required drop distance (in metres) is equal to the torque required to break their neck (assume $1700 \text{ Nm}$) divided by the gravitational field strength (assume $9.81 \text{ N}\cdot\text{kg}^{-1}$ on Earth) multiplied by the person's mass, $x \text{ kg}$.

The higher the mass, the smaller the distance required. So that's one way a human can break another's neck: drop them out of a first-floor window onto their head. The torque from the impact will snap their neck.

The same can be said of any impact: if you hit hard enough, their neck will break. We can use the formula above to find a figure for force. Assuming an average person of around 85 kilograms, you need $\frac{T}{d} = gx$ Newtons of force:

$$ gx = 9.81 \times 85 = 833.85 \text{ N} $$

You can use this figure to construct an impact that will have the required force to break a human neck. Using the fact that in an impact, impulse $Ft$ is equal to the change in momentum $x\Delta v$:

$$ Ft = x\Delta v $$ $$ \frac{Ft}{x} = \Delta v $$

Assuming a stopping time of around 0.1 seconds (typical of many impacts), we can work out the required delta-V:

$$ \Delta v = \frac{833.85 \times 0.1}{85} = 0.981 $$

So, you need an impact with a deceleration of $9.81 \text{ms}^{-2}$ to break a neck. Not surprising, given that the dropping method is, essentially, exactly that impact.

You can, of course, use the force figure to work out any number of other creative ways to break your target's neck.


N.B.: All these figures require that the force is applied in a non-axial plane. Applying the force directly to the top of their head won't work, as it will all transfer down the spine and spread across the body like it's meant to. You need to effectively push sideways.

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