Do you need to think about long ground contact in acceleration or does it occur naturally?
In both acceleration and top speed phases the thighs must scissor through a range of motion. The hip flexors and glutes must both be constantly active and countering each other. The maximum angle between the thighs does not differ very much during each phase. The main difference is the angle at the knee at ground contact. The knee is almost straight during top speed to maximize the length of the leg, and therefore the speed of the foot. The horizontal component of the foot’s force on the ground is limited by the long lever in exchange for the increased speed. By contacting the ground with a bent knee during acceleration, the length of the leg is decreased. The shorter length allows more horizontal force to be generated at the expense of a decreased foot speed. The decreased foot speed does not hinder acceleration due to the relatively slow ground speed (compared to the maximum ground speed during the top speed phase).
Long ground contact time occurs naturally due to the foot maintaining contact with the ground as the bent knee extends. The long ground contact time will occur if the thighs have a full range of motion. This will occur if the lead knee is rotated forward as far as possible. If the lead knee does not complete the forward rotation, perhaps by activating the glutes too early, then the trail leg hip flexors will be activated early, the range of leg rotation will decrease, and the foot will leave the ground early. This shortening may also be a result of a passive knee drive that contributes to less active glutes in the opposite leg, and less backward rotation of the thigh. By emphasizing active hip flexors and glutes, and achieving full range of rotation with the thighs, the long ground contact occurs automatically because the leg is bent at ground contact.