I don’t have a 3D muscular arm handy (heh), so I grabbed a random image off Google Images instead. 
That’s a pretty muscular arm!
Then I went ahead and approximately mapped it with capsules in Paint.NET.
Looks pretty good, but the forearm could use some work. The actual arm mapped quite well.
So I then went and did the same with ellipsoids. This took me longer to get results that I’m happy with, but I don’t think it looks better. I think it looks significantly worse. 
Now the forearm still isn’t quite right but I feel it did map a little better than the capsule. But the arm itself is way worse now.
As far as speeding up calculations, capsules should be way faster than ellipsoids, not just because they’re officially supported in PhysX, but because they’re essentially two spheres on either end of a cylinder. Both of those shapes are dead simple to test collisions against, relatively speaking of course.
EDIT: Looking back I should have pulled the forearm’s collider back to the actual elbow. Still, it was meant to be a quick mock-up, being a perfectionist about things like that isn’t helpful. 