The only conditions being you must use any type of sword, unarmoured neck and take the head clean off.*
Let’s say you’ve been bitten by a zombie and the best idea you have to prevent you from joining the ranks of living dead is autodecapitation. Fortunately you have a long bladed weapon for the task. The tricky part I think would be generating enough force and momentum in a limited and awkward (backwards and across) space to sever the spine and keep going, since once the spine’s out any motor control goes too and you’re reliant on momentum. How much resistance would the spinal column give? What would be the best sword for such an event? Razor sharp katana? Big, heavy zweihander?
Disclaimer: I am not planning on this nor do I encourage the practice in any way, shape or form.
The question of momentum is absurd… in order to achieve a good swinging slice, you need to get a good rotational motion that uses arm, chest, trunk, and leg muscles. If you were doing it to yourself, you could only engage your wrists, biceps, and upper chest, which would limit you more to a sawing type of affair.
Assuming you did attempt to saw your own head off, the limiting step would be very early in the process, after you’ve severed the jugular vein and carotid artery. This would result immediately in the blood pressure of your brain dropping immediately to zero, causing loss of consciousness within a few seconds, and death in a few seconds more if not treated immediately with heroic intervention.
In that case it’s simple. Wrap the ends of the blade with padding. Folded newspaper works just fine. Tie rope to each end of the blade to produce a crude sling. Tie the sling to a longer rope. Place the sling behind you neck and the blade under your jaw.
Now hang yourself.
Guaranteed decapitation. Assuming you have the rope, this would take about 2 minutes.
I was thinking more swinging it above your head a few times to build up the speed then lowering the blade for one clean stroke, rather than hacking at yourself which is rather flawed as you pointed out.
That’s your best bet, not sure it would work though. Usain Bolt would have the best chance. An Olympic class diver could try fixing it horizontally with the blade facing upwards and hurl themselves onto it. Biomechanically, the neck is in too awkward a place to get a good swing at it, no chance of decapitation that way. Professional headsmen often failed to score a head in one, despite ideal circumstances and specialised swords. If imminent zombification is a problem, I suggest cutting off one of your feet. Then you’ll be the zombie that walks in small circles, and little threat to anyone.
If your heart is set on auto-decapitation, the easiest way to achieve it is with a sturdy rope around your neck and a long drop. The hangman’s job was to ensure the drop was long enough to break the neck, causing instant death, without accidentally removing the head.
set the (sharp) blade between two rocks edge up. make it stable. rig your self with weights and make like a stiff plank ten feet above it and then drop yourself flat with your neck over the sword edge.
The simplest option, and one that they always seem to overlook in movies, it to tie yourself to a tree with a dog chain. Since zombies lack the co-ordination and intelligence to undo a chain, and the can’t bit through chain, the threat is effectively ended.
Your rotator cuff is not a flywheel, and besides, when you reposition for the cutting stroke, you have to change the center of rotation and therefore lose all the angular momentum.
What you’ve proposed is like standing on the 50 yard line of a football field and saying “I need to reach the opponent’s goal as quickly as possible, so I’ll build up speed by running to my own goal first, then turn around and come back.”
Torsion. You need rotational speed. Wedge the hilt into something that resists twisting. Twist. Secure the blade with a slipknot. Place neck in the way of the blade, and tug on the knot. The rotational speed of the blade will be much higher than you just running at it, or letting it fall, even from a great height.
Affix the sword at the height of your neck while sitting on a motorcycle. Make sure the blade is parallel with the ground and it is firmly affixed to withstand pressure of 3x your weight. Get on the motorcycle and drive very fast with your head held high meeting the sword at your top speed. Say goodbye right before impact.
Seriously, using only your hand(s) and arm(s) to swing the sword, there is no way on earth. I’m sure we could rig up a mechanical contraption to propel the sword through your neck or your neck through the sword, but otherwise forget it.
So, no chance at all of beheading yourself if it was just you and the sword? What about holding the blade behind your head with both hands, the edge towards your neck, then pulling it forward hard while throwing your head back? Then you’d sever the spine…but have to hope the weight of the sword and forward motion will carve through the oesophagus. This information will no doubt be useful in the event of undead uprising.