The rig crew was picking up 4-1/2 inch casing and running into the hole. The Driller, Motorman and Tong Operator were working the floor, latching the elevators, hoisting the casing and spinning in the joints. The Floorman who was working on the catwalk hooked the winch to the casing sling and followed the joint as it was lifted to the rig floor. He removed the winch line and sling from the joint of casing and proceeded down the V-door to hook the next joint. The elevators were installed on the casing joint resting on the V-door and the safety pin was inserted. The Driller hoisted the blocks raising the joint of casing, which was approximately 40 feet long. As the pin end of the casing approached the bottom of the V-door and the joint neared the vertical position, it fell through the elevators. The casing hit the top of the V-door, slid down to the catwalk, and hit the Floorman who was bent over with his back towards the rig floor attaching the sling to the next joint of casing. The worker was struck in the lower back by the casing joint and was knocked face first into the pipe tub. The worker was transported to the hospital with a lower back injury.