Some such systems exist, but they mostly use normal muscle fibres that can also be found in vertebrates.

Their high relative strength is due to the cube-square-law: when you scale a creature up, its weight increases faster than its strength. This is because the volume (and therefore the weight) of a 3-dimensional object scales in a cubic fashion, whereas muscle strength scales with the crosssection, i.e. in 2 dimensions and thus with a square function.

This means very big animals like elephants have a lower power/bodyweight ratio, while very small ones like mice and insects a very high one.