There should be three steps taken per second. Interestingly, this applies at fairly slow jogging speed, up to fairly fast running (but not sprinting). If you watch elite runners as they tire in the final stages of a race, they increase stride rate, not length. New runners will do the exact opposite, lunging each leg forward. As I pointed out earlier, this does nothing but create a braking action each time contact is made, and disrupts the foot strike under the centre of gravity. Employ little, short and quick steps when you are at the end of your next race.
So, once a runner has mastered the proper cadence of 180 individual steps or 90 pairs of steps a minute, and has very good form, what else can be done (aside from more training) to improve? Now the runner is ready to go back to what he/she avoided earlier, which is stride length.
Since stride rate generally should stay the same through a wide range of running speeds, at that point the only way to speed up is to increase stride length, which must be done without violating the rules of proper form, which dictate that everything happens behind the runner, and that overstriding is inefficient. When you see an elite Kenyan gracefully running sub-5 minute miles, he appears to be almost jogging. This is because his stride length is tremendous compared to an average runner, yet he does this while keeping the same form as if he were running much slower. He will not overstride, as he knows this is futile. Again, remember, everything happens behind the runner. All the power is not in lunging forward, but in pushing, or toeing off. As an efficient runner speeds up, his push off becomes more powerful, and another thing happens - the degree of knee bend when the leg is behind him increases, in many cases, dramatically. The secret here is similar to the degree of arm bend, and the weightlifting analogy that went with it. The lower leg is a weighted lever, and a runner lifts this lever thousands of times in a run. The higher the degree of bend after the push off when the leg is behind the runner, the less that the weight of the lower leg is stressing him over time. The higher bend also assists in continuing the explosive power of the push off.
(This article is reproduced here with the permission of the author, David H.)