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to the OP, I would guess that your thumb just isn't used to the position and motions you are making to manipulate the circle pad. Thus, after a half hour or more, the muscles in your thumb are starting to get sore. This will go away over time as you muscles get used to it. You can help speed up the process and cut down on the soreness by eating high protein for the next few days. Fish or Chicken are good choices for that.