Maybe it's just me, but I always find its my legs that are tired first, mostly my quads. Other than some additional upper leg muscles if I'm working on ground tricks, it's always my quads that are tired. Maybe they are just the most out-of-shape in my case?
I do deadlifts, squats, and toe raises as part of my regular workouts, but I'm not sure how much those help with endurance, which seems to be my problem. I think Donutz or someone once mentioned holding a horse stance as good practice, which strikes me as a logical approach. I keep meaning to do that more even though I'm no longer doing martial arts.
I'm curious, what muscles fatigue for everyone else?