What could help decrease ground contact time in the top speed phase?
A short ground contact does not automatically translate into increased speed. Remember that you could have a very short contact time and not travel horizontally. A person could decrease ground contact time and run slower. If the backward force is not maintained and the foot is lifted off the ground to decrease contact time, the runner will travel a shorter distance in the air (decreasing stride length), and the running speed will decrease.
Short ground contact time is a characteristic of the fastest runners. It is a result of good mechanics: pulling with the glutes, contacting under the body, and curling the leg during recovery. If you contact the ground under the hips the foot will have a shorter distance to travel while on the ground. In contrast, contacting the ground in front of the body creates a longer distance for the foot to travel while on the ground. This also slows you down due to braking. Using the glutes to accelerate the foot in the air, then pulling on the ground will make the foot move faster giving it the potential to have a shorter ground contact time. It only has the potential for a shorter contact time because the foot must also leave the ground by using an active hamstring. If the hamstring is passive, the foot will probably stay in contact with the ground longer. So, rather than trying to directly decrease contact time, just improve your running mechanics and a shorter ground contact time should be the result.