Imagine 3 lines ___ ___ ___ but in a vertical position.
First line = thigh. 2nd line = hips. 3rd line = torso.
The abs are on the 3rd line. They pull at the middle line so that is does not tilt backwards.
However, most (?), if not all, the hipflexors relax so as to allow the
first line to tilt backwards to contribute to stride length.
If the abs do not prevent the 2nd line from tilting back, it would decrease the knee lift of other leg/swing leg which would come down to the ground from a lower point with decreased ground reaction force.
From my experiances, the single best thing a novice sprinter can do is strengthen there torso muscles. Fast sprinters allready have strong mid-sections.
People with lordosis may need to give very speciall attention to there abdomen strengthening.