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In case you didn't see it, in the Geoff Baker writeup of the game:
"I have been putting in a lot of work in the weight room," Fister said. "I've spent a lot of time lifting, conditioning and throwing. So, yeah, I feel a lot stronger this year. I'm in good shape. We'll see where it takes us."
So maybe that's where the durability and increased velocity come from. Roger Clemes comes to mind as a guy who benefitted from a rigorous training regimen (in addition to a little magic pixie dust).