Never say never but I'm having a hard time figuring out what you'd have to do to "knock" something loose. Any knocking would likely put you way, way out of the game, mewing in a tight ball in the corner.
Your prostate is well away from any straining muscles from normal activity. Your body is set up to handle the stray cell that doesn't belong. If it wasn't, we'd all dead years ago, long before we could worry about
PCa.
Getting physically fit before treatment is more likely to help your recovery. I exercised regularly and lost seven pounds before I went in. I suspect others will tell you the same (or wish they could).
Oh, and you have every right to be scared. I don't remember any of this being in the handbook when I signed up at birth.