On December 17, 2017, about 2345 central standard time, SkyWest flight 5788, an Embraer ERJ700, N128SY, experienced a sudden pitch correction during descent into St. Louis International Airport (STL), St Louis, Missouri. Of the 57 passengers and crew onboard, one flight attendant was seriously injured. The airplane was not damaged. The flight was operating under 14 Code of Federal Regulations Part 121 as a regularly scheduled domestic passenger flight from San Francisco International Airport (SFO) to STL. According to the flight crew, the captain was the pilot monitoring and the first officer (FO) was the pilot flying and was on his second day of initial operating experience (IOE). The air was smooth with no turbulence. Air traffic control requested the flight give its "best forward speed" and cleared it direct to the airport. The crew discussed and planned the descent and after entering the data into the flight management system (FMS), noticed they were past the top of descent point for the 3-degree descent path they had entered. As a result, the captain had the FO select flight level change and the descent rate briefly exceeded 5,000 feet/minute and the airspeed increased. To prevent an overspeed condition, the captain instructed the FO to deploy the speed brakes, but the speed continued to increase, so the captain disconnected the autopilot and pitched the nose up. At the time of the unexpected pitch input, the rear flight attendant (FA) was preparing the galley for landing and was thrown off-balance onto the floor, causing her to twist her ankle. Medically qualified passengers assisted the FA until landing. After landing, the FA was transported to the hospital where she was diagnosed with an ankle fracture.