PERSON STEPPED UP ONTO A LOWBOY WHICH HAD A 4W SHEILD ON IT TO UNLOCK CHAIN, WHEN HE STEPPED BACK DOWN FROM THE LOWBOY TO START UNLOADING 4W SHEILD HE FELT PAIN IN RT. KNEE. PERSON SAID HE FELT SOMETHING WHEN HE STEPPED UP ONTO LOWBOY & THEN IT GOT WORSE WHEN HE STEPPED OFF.