EE WAS USING IMPACT WRENCH TO REMOVE STUDS FROM BOTTOM FLANGE ON #4 FLASH TANK LEVEL CONTROL VALVE. IMPACT WRENCH WAS BEING UTILIZED ABOVE HEAD LEVEL IN THE VERTICAL POSITION. AS EE WAS REMOVI NG STUDS HE STATED HE FELT SOMETHING ""GIV"" IN HIS ELBOW. HE HAD TO RELEASE THE WRENCH IMMEDIATELY. EE STATES HIS ELBOW, ARM AND HAND HAVE A LOSS OF STRENGTH. EE WAS WEARING ALL REQUIRED PE AT