The profile for blastoff is not as difficult as it used to be when John Glenn flew. Alan Shepard and John Glenn had to support 6 G's. That is six times their body weight. On the shuttle, when the solids are on, lifts to about 1.8 G's and when the solid rockets come off you slam into your straps a little bit and then ease back into your seats as it builds up to 3 g's as you are riding on the main engines. So, for the 1st 2 minutes you are at 1.8 -2 G's. Right after solid rockets come off its back at 1 G building up to 3 G. And last 30 seconds just before MECO Main Engine Cut off they throttle back so that you stay at 3G which is 3 times your body weight on your chest. IT's not that difficult to support, it's approx. - you are aware of your breathing but it's not that difficult to breathe.