... though iirc the Flash accelerates to vision-blurring speed in well-under a second ... from a standing start. Working out what this means in terms of stresses on his body would be an interesting essay.
Though Flash's power could include some cushioning to the brain while he's accelerating that is not normally in force, i.e. when surprised. A bit like how a boxer can withstand strong blows to the stomach when braced.
There's a lot there though - i.e. I recall the TV show regularly had his shred his clothes running but his skin didn't even turn red. They also had him super-speed through doors so fast the motion wasn't visible (also didn't displace any air or damage the door).