Its the strength of the erector spinae and latissmus dorsi that is crucial in aiding an athlete to maintain the lean during acceleration. The stronger the dorsal and erector muscles the longer you can hold the position and so prolong the acceleration phase.
I aim for a straight back, aligned head, low foot carry & triple extension. When your trying to stay low in no way are you looking to round the back/spine. The muscle responsible for keeping the body and spine straight is the erector spinae (although the core is extremely important too).