Related question >>

A person has decided to mechanize themselves. Their spine was severed at the height of the L-1 vertebrae, legs and guts where removed.

The leg prostheses come in many forms adapted for different kind of terrains and even for swimming or climbing. There's nothing electric or requiring battery power. When a prosthesis is not being used, it can be turned inside out and opened like an unfolding box. Example here >> Once opened, the prosthesis can be worn like a piece of armor or stored away.

when the legs are ''boxed'' or closed again into an usable shape, they can be activated with a crank and start running or walking (there's a gear system to decide the pace) or swimming until they are manually stopped. Direction is changed by turning the torso and the legs will follow due to a mechanism that connects a the torso to the legs. In emergency cases only the spine can be bent backwards to block the limbs, making the legs able to stop faster and safer than human legs.

Can a mechanism like this function? I'm not talking about the box and unboxing thing but about the legs needing just an initial input and then they maintain motion until stopped. I know that perpetual motion machines can't really exist, but could the weight of the torso or maybe the swinging of the arms mitigate this problem, or maybe the solution could be anything else that I'm not able to imagine at the moment?

  • $\begingroup$ does the explosion box link count as spam or advertising? In case I could link just the image or delete it and find another example $\endgroup$
    – Green
    Jul 8, 2021 at 9:18
  • $\begingroup$ (a) I'm OK with the link because you're only using it as an example of what you're talking about (unless you're affiliated with the link such that you get paid if anyone follows and/or buys something... but I'd have to give you credit for quality shameless advertising). (b) We occasionally get "can X exist?" questions, and they surprise me. It's your world, does it matter if they can exist in real life? Aren't you simply looking for suspension-of-disbelief? I'd assume it can exist and ask us to help work out the details. After all, you don't need our permission. $\endgroup$
    – JBH
    Jul 8, 2021 at 16:27

1 Answer 1


It would be very complicated, and break down a lot, and take a lot of muscular strength.

You'd need to regular crank the legs with your arms, build up some degree of movement in springs and elastic bands, and charge up a complex mess of gears. You couldn't maintain it with swaying arms and such, you'd need to have some sort of internal thing to store energy.

You'd need to regularly oil and replace the many gears, it wouldn't be very agile, you'd lose a huge amount of energy, and you'd be much slower and less agile than a normal human.

Purely mechanical robots and such are possible so you could do something like this, just, they're not very efficient and easy to use.

Bending your spine backwards to stop your legs sounds like a good way to break your back and die, but yes, it would slow you faster.

  • $\begingroup$ ehm to stop the legs there's another lever but if this can't be done, one can just bend the back and they will instantly go into paralysis. Falling on your back is not such a dangerous thing I recon hitting your butt and neck are more dangerous, But in combat sports I often got thrown on my back or shoulders on solid floor, If you are a dead piece of meat waiting to fall, then it hurts, but if you brace and adjust yourself then falling on concrete is not different than falling on a mattress. Falling on your back is even less dangerous when your whole body weights less than it used to. $\endgroup$
    – Green
    Jul 8, 2021 at 9:38
  • 2
    $\begingroup$ Are you a person who has cut their legs off to replace them with cybernetics? This person isn't gonna have great balance from their randomly twisting geared cybernetic legs. Falling on your back is normally done with good balance so you can move in the right way. $\endgroup$
    – Nepene Nep
    Jul 8, 2021 at 9:48

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .