Phases of a 100m race.

At what point do the face, shoulders, arms etc relax during a 100m race? After 40m?

From what I know, which is not much compared to the rest here, your sapost to be relaxed from 0-100 meters.

Good point Talus.
At the first of the race, coming out of the blocks, it is a natural reaction to be “tense” with the forceful actions out into acceleration. You do not think to yourself to be tense and then relax at a certian time. You should be relaxed and in control of all your actions. Remember about the story of Ben recognizing his pressing during races he was losing to Carl early on.

in the start, about how many steps (or for how long) should you be “pushing”

or how long should it take you to get “up and running”

I have trouble b/c I lead forward to force myself to get more push at the beginning but I often am bent at the waist and don’t get to triple extension

I want to get max. push at the start but I don’t want to make it harder for myself to transition to full speed

You do not need to think about forcing yourself out of acceleration phase or into transition. It is a natural progression that happens. It also goes back to your start. If you apply a method as described in earlier posts (arm mechanics) on the new thread, it will help you in this matter.