Crew was on a liner change. EE was unbolting the rec. hopper while standing on the motor stand. His wrench slipped off the bolt he lost his footing and fell backward approx. 42"" down onto the stand deck landing on his left shoulder blade and head. EE went to the ER and examined and released with bruising. Dr. inst.him to stay home a few days, if the pain persisted to get an MRI.