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?