Questions about running theory; cadence vs stride length?

In my opinion there is no magic number. Everyone is different, though almost all amateur runners I see would benefit from increasing their cadence.
If your increase your cadence and shorten your stride (which I read in some study can reduce the impact by 10-20%) your are not really adding much stress, but neither are you adding speed. So if you want to run faster you have to increase at least one of the two while the other one stays constant or only decreases slightly. Whichever one you change (and should change without risking injury etc.) depends on you and your body type. But change slowly, do not suddenly try do double your cadence just because you read about some magic number somewhere. Your body has probably already found a running style which fits you, but which can be improved by slowly pushing yourself beyond the limits of your current comfort zone. Lastly, to improve your running you should not solely focus on quicker leg turnover or lenghtening your stride. Those two and faster and more efficient running in general are also natural outcomes from other changes you make, such as posture, how much you lean forward, head, arms etc etc.....

Disclaimer: I am not a certified running coach, but I am talking from my own experiences and studies I have read

This is not true; 180 steps/minute is an average that has been observed several times in different studies but one should bear in mind that it is an average.



This depends on what is leading to a runner's injuries; if they are due to overstriding, there is a good chance that increasing cadence (and therefore decreasing stride length) will help solve this problem but it is still quite possible to get injured with a cadence of 90. Most running injuries (IME) are from trying to do too much too soon, not due to cadence (although that may be a contributing factor).



As you've said, speed is due to stride length and cadence; to go faster you increase one or the other (or both). Generally it will be a combination of increased stride length and cadence that leads to an athlete going faster although for some, cadence will stay within a fairly small range while stride length changes more dramatically.

Shane