One feature of the MP4/4 was the driver's position. Due to the car's low-slung aerodynamics and the FIA safety rule which stated that the top of a driver's helmet had to be below an imaginary straight line drawn from the top of the roll bar to the top of the cowling, the drivers were required to be in a reclining position rather than in the conventional upright seating position of rival contemporary Grand Prix cars. Senna had absolutely no problems with the seating position. Prost, being smaller, preferred to be slightly more upright, and because of his smaller physical size, the team was able to accommodate this without modification.