CHECKING BELT UNDER THE ALLIS DUAL SCREEN. WAS UNDER LEFT SIDE OF ALLIS LOOKING UP AT ALLIS DIRT HAWG AND CHECKING CLIPS, WHEN COMING BACK OUT, A ROCK CAME OUT FROM TOP OF SCREEN AND HIT THORA CIC MUSCLE ON LEFT SIDE OF BACK. I FELL TO GROUND AND NOTICED SOFTBALL SIZED ROCK TO THE LEFT OF ME.