Below is a great article that appeared in Science-Based Running

http://sciencebasedrunning.com/2011/07/the-basics-cadence/

Posted by Dave on July 5, 2011 | 19 Comments

Kenyan-born Bernard Lagat is one of the most-decorated American middle-distance runners in history, chalking up impressive victories in world championships and holding four American records. He’s also admired by coaches for his fluid running form. While it may not be possible for amateur runners to match Lagat’s graceful stride, there is one aspect of his form that anyone should be able to imitate: His cadence. Take a look at this video from a 5,000-meter race in London:

Lagat moves up four places in the final lap to get the victory, all the while maintaining good running form. But interestingly, although he speeds up dramatically for his final lap (in 51.94 seconds, a 400-meter time some college sprinters would be pleased with), his cadence—the number of steps he takes per minute—doesn’t change.

You can verify this for yourself by timing how long it takes for him to take 60 steps (the easiest way to do this is to count 30 motions of one of his arms). I get 60 steps about every 16.64 seconds, for a cadence of 216. As Jack Daniels observes in his book Daniels’ Running Formula, most elite distance runners share this trait: Their cadence doesn’t vary, whether they’re fresh at the start of a race, or struggling at the finish.

More importantly according to Daniels, most elite runners have a cadence that is much faster than beginning runners; he has rarely observed an elite runner with a cadence slower than about 180. I took a look at a couple more videos and confirmed Daniels’ observation:
Meb Keflezhigi has a cadence of about 203 in this video, and here in the dramatic finish of this year’s Boston Marathon, although Caroline Kilel’s form is suffering as she struggles to hold off Desiree Davila, her cadence is a consistent 180 steps per minute:

Davila’s cadence holds steady, too, at about 194 steps per minute.

Yet beginning and recreational runners typically have a cadence closer to 160, which Daniels says puts them at risk for injury because the longer strides necessitated by a slower cadence take runners higher off the ground. This in turn means that each footfall is harder, and many running injuries are associated with the shock of landing. While Daniels can’t cite a study associating slow cadence with running injuries, I put a lot of weight on his experience coaching thousands of runners.

So how can you improve your cadence? Daniels suggests running “as if you’re on eggshells,” and simply counting your steps as you run to track your progress. For the past week, I’ve been trying it myself in training runs. You can calculate your cadence by timing yourself. Count every step taken by your right foot for 30 steps. Then divide this time into 3600. The first time I tried this, about a week ago, 30 steps with my right foot took me 22 seconds. 3600 ÷ 22 = 163 steps per minute. I’m not much better than a typical beginning runner!

Fortunately you don’t need to do this kind of math in your head while you’re running; all you need to shoot for is 30 steps with your right foot every 20 seconds; this corresponds to a cadence of 180. After a few days, I was able to increase my cadence to 180 quite consistently. For the first couple days, I found it a bit harder than my normal cadence, but after that, I found it wasn’t any more difficult than what I had been doing before. Spot-checking a few times on each of my workouts confirmed that I had successfully elevated my cadence.

What this means for you: Anecdotal evidence suggests that if your cadence is slower than 180 steps per minute, you might be able to reduce your risk of injury associated with landing shock by increasing your cadence.

Update: For a more scientific perspective on cadence and injury, see this post.