go hit a punching bag full power for 3-5 min... you'll see those are the muscles that get fatigued first cause they take the brunt of the work but are smaller and more prone to injury.
Upper back, shoulders, and triceps mostly.
upper body conditioning and hypertrophy is grossly overlooked in mma.
look at the one's that dont get tired as easily and their upper body tends to be a quite a bit more hypertrophied than their lower body. (Fedor, wanderlei, kid yamamoto).