We speculated that, during the acceleration phase, the most favorable magnitude of relative vertical impulse is one that creates a flight time just long enough to allow repositioning of the lower limbs; all other strength reserves should be directed horizontally.
-Auckland University of Technology
Acceleration is very horizontally influenced because of posture, not muscle recruitment. During initial acceleration athletes are similar in top speed mechanics except some elements are more horizontally influenced. During maximal speed, vertical forces are needed earlier and earlier in order to get more hip extension. The key is the timing and sequence of events, where hip extension is highly dependent on stiffness qualities of the early stance phase. Most general and balanced programs can support this sequence, but often times interventions of specific work are necessary.