Among other things 

Special protein constellations could consequently result in the optimal development of muscle strength, which is not achieved in other sprinters. “All top athletes probably have genes that enable them to achieve top performances,” says Raunser. In addition, given that skeletal muscle contains both fast muscle fibres capable of rapid bursts of power and slow ones that are suitable for enduring activity, Bolt’s musculature may be composed of a particularly effective combination of fibres.

1) Great Genetics

2) Great Nutrition

3) Great Training

Yesh, they all contribute to making excellent fibres.

I wonder if any other 100m gold medalist would be as fast if they were Bolt's age today. 

http://pandawhale.com/post/72904/one-race-every-medalist-ever-video-comparison-of-every-mens-100-meter-dash-medalist-since-the-1896-olympics