There are two throttle backs during orbiter ascent, neither of which have to do with G forces. The first occurs at approximately 1:31 when the shuttle hits Max-Q, maximum aerodynamic pressure. Once through Max-Q the shuttle is throttled back up until after the 8 minute mark, when the final throttle back commences. This is to turn the engines off before all fuel in the external tank is exhausted, because an emergency shutdown, the one caused by the sensors in the ET is rather painful on the turbopumps, and while better than the explosively catastrophic failure that would occur should the turbopumps run dry, is still not a very happy option. The shuttle doesn't achieve excessive G's by design.