The body angle should change progressively. The 45 degree recommendation is somewhat arbitrary and doesn't have any real foundation in biomechanics. It certainly isn't effective to stay at a static position considering the athlete needs to be upright at some point. Therein lies one of the primary issues of acceleration- Lower departure angles from the blocks are advantageous for acceleration and in most cases can only be pulled off by the best sprinters. Regardless though they still need to stand up tall and run with proper top-end speed mechanics if they want to be effective beyond 40m.