In 100 meter sprinting start acceleration is about 64% of the final result and maintaining peak speed about 18%. The actual start is about 5%of the results and reaction time 1%. This is not to discount the importance of any aspect of sprinting as percentages include the musculoskeletal system, mechanics and more.
Elite athletes tend to have a longer acceleration phase than novice that suggests they have better coordination of their body segments or better movement skill. The most important contributing factor when all aspects of sprinting are put together, body strength, technical skill, reaction time, acceleration, etc... is speed endurance. Speed endurance is the number of explosive contractions that can be made over a given distance. Speed endurance allows for maintaining the coordination of movement necessary for acceleration and less chance of losing speed during the distance covered.
With all the wonderful technological and physiological advancements that have been made in athletics it still boils down to getting in shape to excel.