I'm not 100% sure of any of this, just from years of observation & video analyses.
But I see the Kenyans & Ethiopians as running with a anterior pelvic tilt most of the time, which is why Bekele & Ereng have such high & 'skyward-facing' back kicks.
Maybe its because they were taught to run like this, or maybe its precisely because they WEREN'T TAUGHT ANYTHING regarding technique, that they just run according to what enables them to maximise race performances. I'm not sure.
Even when Bekele is sprinting at 53 sec final lap of a 5k or 10k, he's still primarily keeping his hips "behind" him, maintaining that anterior tilt. That's why I believe he has such back kicks.
But American sprinters are taught to run with a posterior pelvic tilt, hips "tucked underneath" the torso, so that they will NOT HAVE SUCH HIGH REAR KICKS. This will also maximise the forward knee drive. I've seen many distance coaches teach the same drills that sprinters use to distance runners.
I wonder if the drills many distance coaches in America have been teaching their athletes actually train them to run more like sprinters than middle/long distance runners? Maybe these drills are appropriate for unleashing that final sprint in a race, but not as the abiding mechanics suited to maintain race pace at distance events?
Imagine if u have already been running with hips under your trunk for most of an 800m race, only to watch as other runners suddenly "sit in" & pull away at the end, but you've got no other postural position to change into except for the 'slower'"hips behind" position, which does not allow u to drive forward as effectively. Wouldn't you start to think you have 'no kick' or 'no speed'?
From my own observation alone, most Caucasian distance athletes tend to run with a relatively posteriorily tilted pelvis for an entire distance race. I think Matt Tegenkamp seems to be a recent example. They're already utilising too much hip flexor/glutes/quads right from the start.
Also compare Caucasian distance runners from the 1970s/80s to those of the 21st century. The former tend to have thinner quads, skinny long legs, not so muscled overall. The latter seem to have HUGE quadriceps and calves, esp the halfmilers.
Compare also the build of African runners from that same bygone era to those of today. Can you really see anything too different between the two?
Western athletes today seem to rely more on the quads & hip flexors compared to their predecessors. They weight-train more and do more speed drills which seem to foster short distance mechanics rather than what distance events require. The recent thinking seems to be that "everyone knows how to run slow, but we need to get faster to beat the Africans so let's run like sprinters!"
A possible reason why I believe the Africans tend towards an anterior pelvic tilt is due to the physiology of their lower legs. They tend to have smaller gastrocnemius and longer soleus, and rely as such on the latter to run. In order to activate the soleus more than the gastroc, I think maybe they need to keep their hips behind their trunks. Their back kicks could be dictated by the structure of their lower legs. Whether this calf structure is a function of nature or nurture I can't tell for sure.
Look at westerners and at their lower legs. I think you can find many elite distance runners today who have very developed gastrocnemius relative to the soleus. I wonder if its due to the way they are born, or is it because westerners (of recent years at least) adopt sprint-like or semi-sprint-like running mechanics, which emphasises the gastrocnemius (upper calf)?
Look at Steve Scott, Dave Wottle, Jim Spivey, Craig Masback. Do these fellas from the 70s/80s have huge quads and huge upper calves? Compare with Alan Webb, Nick Symmonds, Khadevis Robinson, etc.
Both eras have produced sub 1:50 and sub 3:40 runners, but we cannot deny that the going seems to be getting tougher for Caucasian athletes of today than those of three or four decades ago. Why is this so? Can part of this be that we have become confused as to how to run like distance athletes?