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I got an interesting answer to one of my questions and I would like to know some additional info.

A myoneme (or spasmoneme) is a contractile structure found in some eukaryotic single-celled organisms, particularly Vorticella. It consists of a series of protein filaments that shorten rapidly upon exposure to calcium. Although the shortening can be up to 100 lengths per second, faster than any muscle, the relaxation time is several seconds (compared to approximately one tenth of a second for muscle). The myonemes of Acantharea also display slow contraction and undulation movements.

Sounds nice, but I'm unsure if it could be of any use for an animal. Now, let's assume we have a hypothetical creature that looks like a tiger; but with bones, skin, tendons, and ligaments that have tensile and/or compressive strengths of 4-7 GPa, plus equally high stiffness where that's desirable.

This creature has muscles that contain another contractile filament (namely, myoneme) beside myosin in a certain percentage.

Would myoneme muscles be of any use (locomotion, lifting, striking, etc...) in this configuration?

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    $\begingroup$ Every contraction of the nearby myosin actuated cells will cause calcium to be released. So, unless the fibers are isolated from the rest of the muscle, each movement will cause sorta tetanic contraction $\endgroup$ – Adrian Colomitchi Mar 20 at 13:14
  • $\begingroup$ Rather than an answer I have a question that could answer it: what does "relaxation time" mean in this instance? If it is the speed at which the fibers expand to their original size again then you might be able to actively stretch them by using antagonist muscles. If "relaxation time" stands for the time it takes before the muscle can be reactivated with Ca2+ you might still stretch them passively. If the relaxation time is about the time it takes for the Ca2+ to leave the filaments so it can stretch again its mostly useless. $\endgroup$ – Demigan Mar 20 at 17:32
  • $\begingroup$ Semi-non-topical note: might be useful for small hive-hunters (think large numbers of small predatory worms that hunt as a pack). $\endgroup$ – Élektra Mar 20 at 23:36
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It might be useful for ambush-predators which have a hunting strategy of striking exactly once and either catching their prey or staying hungry.

It would not be so useful for any predators which attack their prey head-on, because if the prey manages to dodge the first attack, the predator will be disabled for a couple seconds, giving the prey chance to escape or strike back.

In any case, it would only be useful for some very specialized muscles. Most muscles of an animal have multiple purposes which require them to stretch and contract repeatedly. A tiger needs to jump and bite in order to hunt, but it also needs to be able to walk and chew with reasonable efficiency in order to survive.

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    $\begingroup$ Super Bobbit Worms. $\endgroup$ – Joe Bloggs Mar 20 at 16:23
  • $\begingroup$ Sooooo.... Using both muscle types would translate to hysterical strengthx10 on-demand and with a few secs of cooldown? $\endgroup$ – Mephistopheles Mar 20 at 16:43
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    $\begingroup$ @Mephistopheles - Probably not. That extra muscle would taking up cross-section that could be used for more normal muscles. You'd be weakening your existing muscles for it. $\endgroup$ – ltmauve Mar 20 at 19:21
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Well, if your creature looks like a tiger, it's fair to assume it hunts similarly. Under such assumptions, how about a croco-tiger?

In other words, I'd say the best place for such fibers to exist would be at the creature's jaw muscles. Most tigers hunt by catching prey and sinking their Jaws around the windpipe, strangling prey. In your creature then, it could use normal musculature to get the initial grip and immediately after a successful latch, it would activate these special proteins, increasing the pressure and thus, reducing the time for prey to be strangled, while using the several second cooldown to ensure that even if the normal muscles give out, it will not let go. By using that strategy, your predator would be pretty deadly. It only needs to get you once and there's no longer hope of escape before you die. However, besides the jaw, I don't think there would be any other place where such fibers would be advantageous, especially not the limbs, unless it uses its limbs to fix itself to the ground once it gets a hold of prey, then maybe on the paws, but that's it.

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