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I am busy working on a race of humanoids, to defend themselves against their planets large predators I gave them a pair of retractable keratin blades which are housed just below the Ulna in the forearm, they extend some 10cm beyond the elbow when extended. My problem is this:
How could the blade be extended and retracted but whilst still being rigid enough to stab an attacker without the blade simply retracting?

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    $\begingroup$ There are mechanisms to lock in place retractable umbrellas, some modern police clubs and switch knives; The last two are used to slash at things. Can you explain why these methods do not work in your case? $\endgroup$
    – Tortliena
    Nov 22 at 10:53
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    $\begingroup$ Hm. If you're a male you kind of already know the answer... $\endgroup$
    – elemtilas
    Nov 22 at 11:46
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    $\begingroup$ These are biological not mechanical, Tortliena, the blade is similar to a cats claw. $\endgroup$ Nov 22 at 12:40
  • $\begingroup$ Quick question: do they have anything that could be considered an exoskeleton? $\endgroup$ Nov 22 at 13:51
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    $\begingroup$ @elemtilas all the more reason to have blood available for other activities ;) $\endgroup$
    – Trioxidane
    Nov 22 at 17:00
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I'm a talker not an artist, so I'm not going to match the beauty of ProjectApex's very good answer. Words will have to suffice I'm afraid.

What you're looking for is a locking mechanism that engages the rear of the blade, holding it relatively motionless and padding any impact or shear forces. While this might be a little tough to justify through evolution alone, weirder things do exist in nature.

Stashing a blade between the radius and ulna sounds interesting, but you'll need to make some fairly radical changes to the structure of the elbow to allow the blade to pass. Rather than the human elbow structure with the ulna basically cupped around the elbow joint, rotate the radius and ulna such that they both cup the 'side' of the join while rotating. This gives us a clear channel along the back of the joint for the blade to pass through. Unfortunately this will restrict the rotation of the forearm a little, but we're getting a built-in weapon here.

Now on to the locking mechanism. If the faces of the elbow joint on the humerus are angled a little, the radius and ulna can be set to rotate around their long axis slightly when the elbow is fully bent. Each of the forearm bones can then have half of a locking mechnism that closes on the shaft of the blade, locking it in place. The interior end of the blade would have a shaped end that engages with the locking mechanism, with fibrocartilaginous padding and bursae to provide a little shock absorption. For bonus points a similar clamp structure at the wrist end of the arm could hold the blade in place when not deployed.

The blade itself needs to be housed in a synovial sheathe to provide protection to the surrounding tissue and lubrication of the blade while moving. This might lead to a little leakage of synovial fluid - the stuff that lubricates your joints - around the blade's exit sphincter in the elbow. It might be a good idea to have a slow, steady flow of synovial fluid along the blade to help reduce the chance of infection, so maybe your humanoids have a constant slow emission from their elbows. Sounds a little gross, but I imagine bactieral infection of the blade sheathe is an unpleasant experience.

Putting it all together now... the blade is locked in place inside the arm by static arrangement of the bones of the forearm, backed up by dedicated musculature to prevent the blade from deploying accidentally. Bending the elbow past a certain point rotates the forearm bones apart enough to release the blade, allowing blade entender tendons to push the blade across the elbow joint. At full extension the elbow is rotated a little more, further rotating the forearm bones and clamping the locking flanges onto the end of the blade, locking it into place.

As awesome as a bone blade might seem though, these creatures will really come into their power once they hit the a technological level high enough to augment these blades. Ceramic blades with implanted enhancements. Powered blades able to slice through body armor like a craft knife through tissue paper.

(OK, now I'm just having fun. You can ignore that last bit.)

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If they have something like an exoskeleton? Literal Gear-like protrusions and locking mechanisms.

I'm not kidding. An exoskeleton opens the door to more mechanical-esque approaches. Leafhoppers have a unique gear system in their hind legs. This is because their jumps are so powerful that if they don't make sure their legs are synchronized they could miss the desired spot they wanted to reach by a lot (these little things can jump faster than your eyes can process they're gone).

enter image description here

As for a locking mechanism, look no further than the mantis shrimp, whose powerful strikes are due to a spring mechanism. Their claws stay locked in place while their muscles contract, building up elastic energy that's released in one blow once the lock is released (kinda like a more extreme version of a flick)

With that in mind, it's not impossible that, having something like an exoskeleton in their forearms, your humanoids could rely on gear-like protrusions and locking mechanisms for their blades. Additionally, you could take a page from the spearing mantis shrimp and add in 2 locking mechanisms, one to keep the blade extended and one to allow you to build elastic energy.

With that done, add in some extra bits to make the thing able to extend longer and you could feasibly end up with something not too unlike an organic version of Cyberpunk's mantis blades. Here's a sketch for reference.

enter image description here

without something like an exoskeleton? Probably a stay apparatus.

Now, if the thing doesn't have an exoskeleton or something similar, you could likely still pull it off, although you'd instead rely on special joint locking mechanisms (you know how your legs can "lock" in a straightened standing position? Something like that). It would most likely require something like the stay apparatus found in horses so they can nap while standing, but I'm pretty sure you'd still require a joint system for this retraction.

enter image description here

As for a wolverine style retraction and extension? I'm not sure if it's possible or reliable in the scale you want it to happen, as in organic life things like loose parts sliding around aren't common, if they are present at all. The easiest way to ensure that the muscles will be enough is to have the blade mounted on a joint system, as you'll have actual rigid components backing up the bladed portion rather than relying purely on muscle contractions/extensions, with the potential added benefit or granting some extra reach.

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  • $\begingroup$ Maybe compare it to an elongated and more straightened velociraptor claw being held back by tendons against the forearm and snapping forward when needed sounds more deadly? $\endgroup$
    – Gillgamesh
    Nov 22 at 15:48
  • $\begingroup$ Asking because I have a similar mechanism in my story (with an exoskeleton). Would needing the hand’s wrist to rotate cause complications with this? $\endgroup$
    – Mark Price
    Nov 22 at 20:58
  • $\begingroup$ @MarkPrice I'd say it probably depends on how the muscles are anchored. In the sketch I originally pictured the wrist being linked to a section of the joint, as if the blade part of the appendage was the main limb, with the muscles of the wrist and hand itself being centered in the region. An arrangement where the muscles to the hand and wrist are linked to the "forearm" portion could result in some weird stretching, but I wouldn't be able to tell you with certainty. Honestly I pictured it as a way to keep the hand out of the way and the whole thing more streamlined that still made sense. $\endgroup$ Nov 22 at 22:06
  • $\begingroup$ @MarkPrice my sketch, despite more focused on an organic approach, is also based on a mechanical arm blade concept originally depicted in the Manga "Eden, it's an endless world!" and in the joint systems of spearing mantis shrimps, so if you want to draw inspiration from the original source, feel free to do so. $\endgroup$ Nov 22 at 22:08
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You are basically looking at an upscaled version of an insect stinger, stripped of any possible venom gland

A stinger (or sting) is a sharp organ found in various animals (typically insects and other arthropods) capable of injecting venom, usually by piercing the epidermis of another animal.

enter image description here

An insect sting is complicated by its introduction of venom, although not all stings are venomous.

It's normally hidden, it is extracted on demand and it is rigid enough to allow piercing through the target skin.

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