Simply put, stride rate or cadence is the number of footfalls taken by a runner, usually measured in minute increments. Its important to learn good form to minimize running injuries...and cadence is one of the primary elements to this!
First, let me state that generally speaking cadence is measured in "strides per minute" or "steps per minute" ...now, most runners will equate this to the number of individual footfalls/steps per minute, while garmin equates this to strides per minute (how many times the right foot lands, or left foot, but not both). Garmin is unique in this approach, and yet, this is how "cadence" is defined for cyclists, so to be consistent in the use of their equipment and therefore their definitions, SPM is strides per minute. So, if your Garmin is stating 90 spm, this is generally considered 170-190 spm in layman's terms....so shoot for 180!
Your cadence is not a result of how fast you are going. If you are running fast, you generally do not have a higher cadence than if you are running slow. If you are running fast, generally your stride length increases.
A cadence of 90 strides per minute is generally considered optimal for a runner. Why? According to Jack Daniels in Running Formula:
The main disadvantage of this slower turnover is that the slower you take steps, the longer you spend in the air, and the longer you’re in the air, the higher you displace your body mass and the harder you hit the ground on landing. When you consider that many running injuries are the result of landing shock, it’s not surprising that experienced runners tend to turn over faster than beginning runners do.
Simply put, the more steps you take in a minute, the less shock your body is undertaking. So, now that you know WHY you should be striving for foot turnover, how do you do it?
The best method is to select music for your target cadence. There are running metronomes, as well as garmin's footpod to help you better analyze your current running cadence. I found that just having the footpod has made me more aware of my cadence. I started in August with an average cadence of 82spm (garmin-speak) and now average 88spm. My speed has also greatly improved as a result! I have yet to focus on lengthening my stride, so I know that my speediness is primarily a result of foot turn-over. Of course, what my perfect cadence is may not be another's, but simply being aware of it and its impact in your overall running goals can certainly help you step in the right direction!