Uphill running is a great way to increase running speed. However, the simple act of running uphill does not ensure the increase in speed. The run must be modified according to the action you want to improve in order to increase running speed. Keep in mind that running uphill also creates many calf (gastrocnemius) muscle injuries.
Also, there are negative consequences to uphill running. It can modify your running technique, usually to the negative side. There are several reasons for this. First, hill running, especially steep uphill running, forces you to drive your knees upward. As a result, you learn more to push straight up with the thighs rather than forward as should occur in effective speed running.
Second, uphill running on a steep slope forces you to fully extend the knee with greater power. This teaches you to use the knee extension for propelling you upward rather than relying on ankle extension as used by world-class runners.
Because of this, if you wish to run hills, the bulk of the hill running, especially if on steep slopes, should be done early, well before competition. In this way there will be plenty of time to eliminate any negative changes.
Even better, rather than running hills for strengthening the legs, you will find that strength exercises such as squats, heel raises, knee drive and pawback (the latter two with Active Cords) are much more effective. Not only will you gain more strength and power but you will also be improving your running technique.
In such cases it may even make you a better runner because of the strength developed. The squat will enable you to have less up and down motion which improves your economy. Heel raises will give you a stronger pushoff while the knee drive will give you increase speed and a longer stride length. The pawback will help diminish negative braking forces and improve speed.
Most effective for most runners is to run on hills that are not very steep so that running form and joint actions are basically the same as in running on level ground.. If there is a change in running technique as occurs on steep hills, then hill running should not be done.
Select a hill that has approximately a 3-5% incline and then you will be able to maintain effective running technique. When you use hills with only a slight incline, you can also run downhill on the same incline for over-speed running. Doing this can improve your speed even more.
Instead of just running, relatively steep hill running can be used to develop a more powerful pushoff. In this case, you emphasize and rely on strong ankle joint extension which is the key to propelling you forward in the run. In essence, you push off with ankle extension to move up the hill. The knee drive is weak and there is no pawback.
For more information on hill running and most effective running technique see Explosive Running.