I'm still not clear what "lower arm actuators" are. Consider the Warhawk, or the Summoner, or the Hellbringer---they don't have lower arm actuators but they have that elbow joint. The Mad Dog and Timber Wolf do have lower arm actuators.
For the longest time, I assumed that the shoulder actuator is what allowed the shoulder to move like you're doing the chicken dance, the upper arm actuator was also in the shoulder and allowed the arm to move like you were making the "blow your horn" sign to a Big Rig, and the lower arm actuator was the elbow.
Then I reckoned that the shoulder actuator was the whole shoulder, the upper arm actuator was the elbow, and the lower arm actuator was the wrist.
Now I'm convinced that the art and the stats have nothing to do with each other---which I think I read Herb mention in a thread about TROs. It doesn't really make any sense to me, though---don't these mechs have space for lower arm actuators if you wanted them?