For me, the whole issue of having back-kicks (the 'good' kind, where u land on ball of foot under centre of gravity & your sole of swing foot faces directly behind u & not vertically sky-ward) has to do with the muscles u r activating in relation to the pace u r running.
If u r running predominantly aerobically running with shorter strides & faster turnover will be more economical. To do this I think many Africans opt for more hamstring activity to recover the leg rather than hip flexors, because I figure many of them used to run barefooted when younger, and also because they usually have relatively short torsos relative to their legs (which r really long for elite African runners).
If u have really long legs & a light smaller trunk it is very easy to overstride by emphasising push-off & by thrusting the hips forward with hip flexors. But what u get is a relatively slower stride rate & too much bounce, which wastes energy & does not turnover fast enough to train u aerobically. Thus this pattern of movement is usually difficult to maintain for long distance races.
For 400/800m types & even a number of pure 800m types, my observation is that they tend to run more from glutes & hip flexors for most of the race because they rely a lot on power & raw speed. For them the 800m is run more as an extended sprint or "long 400m" race. Usually such runners will run the first lap way faster than the second & they like to lead. There is usually no kick at the end, just hanging on. Besides the lactic build-up factor, it is also due to the fast fatiguing hip flexors & glutes.
Such athletes usually have huge strong trunks that appear almost perfectly proportionate to their legs. I figure Khadevis Robinson & Jonathan Johnson come to mind as such athletes.
There are other athletes who seem to have way longer trunks compared to their leg length. For me its people like Nick Symmonds & Alan Webb. These guys r more endurance based but also possess great power & speed. However they rely on shorter choppier strides with moderate back-kicks & high turnover. These guys can run even splits on both laps, or near even splits, and rely slightly more on a fast finish rather than building a huge lead at the start.
But the Africans are built in a totally different way, as mentioned earlier. They seem to be naturally light framed & skinny and thus endurance work seem come more easily for them, even the 800m runners. There are exceptions like Nixon Kiprotich who runs with high knee lift and forward hip thrust, with the feet touching down slightly in front of his body, and his trunk somewhat 'sitting' on his hips. Also Bily Konchellah who has an obvious bounce and float on each stride.
Watch the 1988 Seoul Oly men 800m final & see Paul Ereng (eventual winner)with his humongous back-kicks! Thank God he opted to run from behind for most of the race or he would have spiked many of the finalists. Also observe Joaquim Cruz & Jose Luis Barbosa, the 2 Brazilians. Huge back-kicks with low knee lift and lots of power from the upper body. Compare them with Ereng who relies less on upper body power & more on the leg turnover. Similar lower body mechanics but different emphasis. The funny thing is Ereng was a former 400m runner (if I recall correctly....) & yet he doesn't run with reliance on hip flexors & glutes, even for the final finish!
If u r Bekele and u run the 5000m & 10 000m, u want to keep your hips loose most of the way, so that they can rotate faster around the joint. To do this u rely on foreward lean & low knee lift, with lots of hamstring firing. This relaxes hip flexors (thereby giving loose hips) & results in high back lifts.
There are many findings that show how significantly aerobic the 800m event is. If u r running in full sprint mode right from the start to the end, chances are your mechanics will fall apart very early on and u will truly die all the way to the finish.
This is where tempo/threshold traing coupled with VO2max work come into play. You need to fully develop these areas, and that means u need to adopt the appropriate mechanics for race paces that coincide with these energy systems, or u will not be able to put in the training effectively. That's why I see some sprinter type 800m runners preferring cruise intervals to sustained tempo runs for 20-40 minutes. They just find it inconvenient to change their mechanics to run 'slow.'So they stick with shorter bouts of work (much like pure sprinters do) so that they can still run 'sitting'.
In essence this creates a 'one gear' runner who may find it difficult to race the Kenyans, who run with their pace all over the place. The recent Beijing 5000m caught Matt Tegenkamp off guard because he couldn't cope with the changing paces within the race itself. But Bekele & 3 Kenyans could.
I believe this is because they have more developed aerobic capacity, yet without neglecting their top-end speed & power. To do this I think u need to be flexible enough to change gears by altering your mechanics, and to be sensitive enough to know what to alter when u want a faster or slower pace so that u r most economical at that altered pace. Its not just how long your stride is or how quick is your cadence. Its also about postural positioning which results in changes of muscle firing.
From a slower pace when u want to accelerate, u need to lean more and recover your support leg faster. This will result in shorter ground contact time and therefore the back-kick isn't that pronounced anymore. With quicker leg recovery the forward knee lift will naturally be higher due to the momentum generated by faster hamstring firing. The hip will by then be swinging up to more than 45 deg and this means hip flexors will be recruited. Foot touch down will be on a more bent knee & lower trunk position & this will recruit the glutes more as stabilizers.
If u run like this all the way for 5000m u will die off. But for the final 400m to 500m this is still viable provided u are well conditioned.
The same for the 800m. If u can remain aerobic for most of the race and yet stay with the leaders, then u will be in a much better position to finish well. But this is only possible if u can adopt the right mechanics which allow u to run fast but not exhausting the power generating hip flexors and glutes too early on.
That's why I think u can't really place training the energy systems above biomechanics, or vice versa. They're both important on their own right and in relation to each other.
But I tend to agree with Oztrack's Steve Bennett who feels that "nice movers" tend to last longer with fewer breakdowns, and that training purely for the energy systems will only elicit short term gains in performance. Its the improvement of one's efficiency at the various race paces which allows for "ease of speed" at any chosen event.
Wilson Kipketer has been reported to cease his workouts whenever he felt he wasn't able to hold good form any more. I think that's a safe way to train well and effectively.
