The flight was at 4,000 feet when Air Traffic Control gave the crew a traffic report. The crew could not locate the traffic, because at the time they were flying in instrument meteorological conditions. About 3 minutes later the crew received a TCAS (traffic collision avoidance system) alert to climb. The flight climbed from 4,000 feet and leveled off at 5,300 feet. During the climb both flight attendants fell. One flight attendant broke her leg, the other was not injured. The flight attendants were standing completing their pre-landing duties when the event occurred. The pilot radioed ATC that he needed to have an ambulance meet the flight when it landed. The flight landed without further incident, and the flight attendant was taken to a local hospital. Probable Cause: an abrupt maneuver in response to a traffic avoidance system alert, resulting in a flight attendant falling and breaking a leg.