On August 7, 2007, about 1740 central daylight time, an Embraer EMB-145LR, N810HK, operated by Trans States Airlines, Inc., and piloted by an airline transport pilot, encountered turbulence while descending near Knox, Indiana. The 14 CFR Part 121 passenger flight was operating on an instrument flight rules flight plan. One flight attendant received serious injuries during the turbulence encounter. No other injuries were reported. The flight originated from the Roanoke Regional Airport, Raonoke, Virginia, at 1655 and was en route to the O'Hare International Airport, Chicago, Illinois. According to the operator's report, the airplane encountered an area of severe turbulence. The report further states that the flight attendant was injured as she was making her way to her jumpseat. The flight attendant suffered a broken leg during the upset. Plots of data obtained from the airplane's flight data recorder showed that the airplane experienced an vertical upset registering about 2.5 times the force of gravity.