The reason because we use, WITH ALL THE ATHLETES OF ENDURANCE (from 800m to Marathon), short sprint uphill, lasting about 10" / 12" (sometimes also 8" / 10" only), is a MECANICAL and NERVOUS REASON.
I don't care about the type of energy we use, that's not the matter. What I want is the athletes running at 100% of their speed, using explosive strength at the start, and a complete recovery between tests.
We want to recruit the higher percentage of fibres in the muscules interested.
The engine of a car works ALWAYS using all the components with the same percentage : if I have a car able to go at 200 km per hour of speed with 5,000 revolutions, when I go at 100 km of speed probably I use 2,000 revolutions only, but all the engine works in the same way.
Instead, since every muscle is made of several fibres, and the fibres have different composition (Fast Twitch, Slow Twitch, Fast type II or Intermediate) and, of course, different range of activity (the mythocondrial activity is mainly in the Slow Fibres, while the best opportunity for developing strength comes from the Fast Fibres), we can use ONLY a part of our fibres (that is always the same) depending on the intensity.
If, for example, I walk, I use, may be, only 20% of my fibres, of course slow. When I start jogging, the percentage raises to 30%. A marathon runner uses from 50% to 70% of his slow fibres, so there is still a percentage of slow fibres not used, plus, of course, all the fast fibres that, in small percentage, also the slowest runners in the World have.
Since the nervs bring the order of contraction from the brain to the muscle, that order arrives where in the muscle there is the "plug" (in Italian, PLACCA MOTRICE).
But everybody knows what happens when we have a long period of inactivity (for example, having plastic after a fracture) : the muscle loses consistancy and volume, the section becomes smaller, and we lose part of our strength.
This action happens FOR ALL OUR LIFE, about the fibres that normally don't work. They are not active normally, and are not able to react at maximal stimula, when we need.
The best and easier way FOR RECRUITING THE HIGHER PERCENTAGE OF FIBRES is to have some exercise using the max possible intensity. Jumpers, Throwers and Sprinters use to have short weight sessions very close their competition, with high percentage of the top possibility (90% - 100% of the maximal in some specific exercise), in explosive way, IN ORDER TO PREPARE THEIR FIBRES TO THE TOP REQUESTS OF EXPLOSIVITY and of TECHNICAL CONTROL, that are the base for a top performance in their events.
For a runner, we look at the same result, using something strictly connected with the dynamic and cyclic activity of running. That's the reason because I always prefer hills compared with weigths.
Speaking about marathon runners, there is also a second reason : also FAST FIBRES have a store where it's possible to find, in very little quantity, fuel useful for lasting longer. When, for example after 37 km, a marathon runner finishes the fuel that normally he uses (a mix of glycogen and fatty acids) from the tank in his Slow Fibres, he can still found some other fuel, allowing him to finish at the same pace, in his Fast Fibres. But, for using it, HE NEEDS TO HAVE THE KEY OF THE STORE. And, if you never worked for recruiting the fast fibres, are not able to arrive inside them, for using till last aerobic energy that you need.
So, the reason of short sprint is not connected, in Marathon, with the increasing of speed, BUT WITH THE POSSIBILITY TO LAST LONGER AT MARATHON PACE because also fast fibres are better connected with the nervous system (and, also, the technical coordination becomes better).