Just my 2 cents worth on this....
If you are trying to throw hard by "muscling up" as you put it, then you are basically constricting your muscles thus decreasing your maximum range of motion, i.e. arm slot, stride, and even finish.
One of the things I have noticed when watching a lot of MLB pitchers is how some take a deep breath just before starting their mechanics to relax. Do you notice how flawless and relaxed they appear as they deliver the ball? It's almost effortless.
But more than relaxing to pitch with more velocity is finding a Pitching Coach or Clinic that will teach you the fundamentals of pitching mechanics. It doesn't matter how hard you throw the ball if you can't find the strike zone!
