Usain Bolt Was Not the Fastest Starter in His Record Races

In the 2009 World Championship 100m final — where Usain Bolt set the 9.58 second world record — he did not have the fastest reaction time off the blocks. Tyson Gay and several others reacted faster. Bolt's record was built on his acceleration phase and his extraordinary top speed, not his start. His stride length (~2.44m) and stride frequency combination were the real anomaly.

Source: Journal of Biomechanics — various sports science analyses of 2009 100m final