I distinctly remember saying that it has not got a lot to do with sprint technique drills.
Usain Bolt has the most lumber lordosis of any world class sprinter. This creates a very fast hip hyper-extension when you run. And with the length of his legs, he’s going to push off with a very fast velocity.
It is a combination of many things, and the above is just one of those things.
The width of his shoulders compared to the width of his hips is only matched by Asafa Powell. There are some other sprinters who’s hips are as narrow as Usains, but their skeletal shoulder width is not as great as his. There-for the ratio in Usain, is quite striking. Also, in ratio to his hieght, he has the narrowest hips of any sprinter. Andre Cason had narrower hips, but Andre Cason was not 6 foot 5 inches tall.
He has the best skeletal proportions in terms of the ratio. Now, if a 5 foot 7 inch sprinterhad similar ratio’s as Usain, they wouldn’t be far behind in their speed (assuming all other elements were in place - such as a great training method.)
Yes, he has muscular deltoid muscles, and does not show much cross-sectional mass in other places. This might suggest that the deltoid muscles may be the most important upper body muscles on a sprinter.
Whatever core strength he displays in the gym, is probably quite high, when you take in to account that it is more difficult for a tall person to give high scores in core strength tests in the gym.
Also, I bet he has larger psoas muscles than allmost any sprinter in the world. If not, all of them.
His arm swing helps to increase vertical forces because his arms are long, and has the shoulder muscularity to deal with it.