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Mar 2013
1:34pm, 31 Mar 2013
4,463 posts
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jonp
Who culled the thread? Why would anyone do that, hardly likely to take up much space even with x 1000s do posts it had!
Enjoying the discussion and input from Canute.
Fen, I think you're absolutely right, angle of trajectory is very important, you can think of it as angle of deviation of the COM as the body is released vertically, after that there is nothing you can do to alter your airborne time as you follow a parabola just as Canute describes (like firing a cannon ball). Efficient running at anything too much faster than walking pace it's pretty true to say that time spent airborne is pretty much your stridelength, or in other words the distance travelled of your CoM forward through the air will be your length of stride given that your landing (support point) is going to be pretty close to being under your body's CoM. You can of course artificially extend your stride length by reaching your lower leg forward ahead of the knee, but that just creates excess braking and certainly ain't good for the knee joint! Besides it then reduces cadence, elasticity of the body and good stuff like that.
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Mar 2013
8:57pm, 31 Mar 2013
15,660 posts
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FenlandRunner
Hiya, jonp, great to read your comments 
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Apr 2013
12:14pm, 1 Apr 2013
21,147 posts
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cabletow
Shame to reestablish what was a resource culled.
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Apr 2013
12:16pm, 1 Apr 2013
21,148 posts
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cabletow
Shame to see what was a resource culled.... I am sure that there are good reasons, but hey maybe the discussion will begin again
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Apr 2013
12:22pm, 1 Apr 2013
8,939 posts
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James1982a
It may have been because no one posted on it for a while
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Apr 2013
12:57pm, 1 Apr 2013
68,691 posts
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santababy
think it was a mistake according to something he said in another thread
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Apr 2013
12:44am, 2 Apr 2013
553 posts
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Canute
Well we have managed to re-live some of the old times with the issue of bouncing.
Can we resurrect what was one of the trickiest debates of all – ‘landing under the Centre of Mass’. There is a commonly stated but misguided belief that one should try to land under the COM. It is easy to show by simple application of the law of conservation of linear momentum that this is impossible at constant pace (in the absence of wind resistance). Furthermore, the law of conservation of angular momentum dictates that you will end up flat on your face if you try – and indeed there are a few amusing/hair-raising U-tube clips showing exactly this, as people emerge at speed from a slide travelling too fast to get their feet in front of their COM. There was one alarming clip, which I think has now been taken down, of a man holding a baby on his chest as he came off a slide. The additional mass of the infant relocated their combined COM further forwards. The man struggled desperately to get his feet forward far enough, but after about six strides with increasing rorational accelration, the pair went for a spectacular nose-dive. As far as I know neither suffered serious injury.
Videos of elite runners show that they land a very short distance in front of the COG, but the puzzle is that in most instances, the stance foot breaks contact further behind the COM than it had been in front of the COM at footfall. Why don’t they fall flat on their face as the law of conservation of angular momentum suggests they should? The reason is that the ground reaction forces are greater in the first half of stance, so in fact the net transfer of momentum before and after mid-stance is equal even though the time on stance is longer after mid-stance than before. Though of course, there must be at least some time on stance before the COM passes over the point of support.
OK, so that is pure nerd stuff, apparently of interest only to a physicist. Does it matter in practice?
Yes it does because there are two schools of thought about how quickly you should try to get off stance. One school, which includes Pose, says get-off stance quickly. Drills like Change-of-Stance promote this. At the Pose weekend in Loughborough a few years ago, one of Dr Romanov’s main themes when commenting on videos was about the need to avoid being too long on stance.
The other school of thought, advocated by Steve Magness (former assistant to Albert Salazar at the Oregon project, now cross-country coach at University of Houston) says leave your foot on the ground long enough to produce substantial hip extension, thereby preloading the hip flexors so that after lift-off, the swing leg recoils forwards efficiently. If Steve is right, drills life CoS might not be helpful. I am inclined to think Steve is wrong, or at least he places too much emphasis on extending the hip, but I think it is an important debate.
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Apr 2013
1:03am, 2 Apr 2013
623 posts
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richh
"There is a commonly stated but misguided belief that one should try to land under the COM."
This is not at all misguided. TRYING to land under the COM does not mean one actually does so, for the reasons you state. However, TRYING to do so helps prevent overstriding, in which one lands too far ahead of the COM.
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Apr 2013
2:08am, 2 Apr 2013
554 posts
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Canute
Fair enough, though in my opinion, some runners who try to land under the COM restrict the forward swing of the foot too much. I accept that over-striding is a worse error, as it increases the risk of injury in addition to wasting energy via excessive braking. However, runners who restrain the forward motion of the swinging leg too much are likely to run less efficiently.
In the old days of this thread several people argued strongly against paw-back. The combination of trying to avoid paw-back and trying to land beneath the COM results in an inefficient stilted swing.
While it might sometimes helpful to deliberately create an incorrect image of what you are trying to do, that can be a treacherous path to follow. I personally try to cultivate an image that matches what I wish to achieve, though I do not attempt to control every aspect of the movement consciously.
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Apr 2013
8:49am, 2 Apr 2013
2,601 posts
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Garfield
Good point...it's too easy to restrict one's stride length by trying not to over-stride...but finding that happy spot can be a challenge and I'm guessing trial and error as to what does and doesn't work.
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