my stride length is 1.39m at 17kmh, which is 200mm shorter than elite Eritrean and European athletes running at the same speed, so my frequency is much higher than theirs to hold same speed. My lack of flexibility means that my push off is much closer to my body due to lack of range of motion. The faster I go the worse it gets, What I have been told is that for an increase in efficiency and a better use of energy I need to reduce the frequency and increase the stride length, which means core work and flexibility, as Lit says, I bet we all have insanely tight hip flexors.
That's the theory behind it, I intend to do some work over the next 6 weeks and go back and get it measured again to see if there is a change. Watch this space!