http://web.archive.org/web/200102150...n/recipes.html
That shows 4 poisons with Muscle Lock. The lowest triv is Muscle Lock I. The highest is Muscle Lock IV. The others fill inbetween as you might imagine.
I'm guessing Sloth is supposed to trigger Muscle Lock IV. If it doesn't I'm not sure what benefit there would be to getting the higher tier poison. More procs? longer duration? I'm not entirely sure.