crobeck,

I share your thoughts, but with some creative interpretation we can
make some sense of the article contents. While I can't understand
'falling' or having the CG ahead of the support point on the ground
(unless there is significant wind resistance to counteract), I can
understand some of the intent about not expending energy needlessly.
If we had perfectly lubricated wheels instead of legs, running would
be very easy, with only wind resistance and uphills to worry about.
(Come to think of it, it would not much different from riding a bike!)
However, we don't have wheels, and that costs us energy in two ways:

1. Vertical bounce. If we expend muscular energy 'hopping' to our next
footstrike, we are hoisting ourselves into the air instead of simply
maintaining the trunk of our body at a steady height. It may be
possible to perform this bounce efficiently using elastic tissue (I
don't know); but if not, from a purely physical point of view,
steadier is better (up to a certain point). Since some of the time
running is spent in the air, it is not possible to avoid the hop
entirely. But increasing the rate of footfall will decrease the height
of the hop required. Off the top of my head, double the 'beat'
decreases the bounce by a factor of four, meaning energy required per
step is cut by four; but since the beat is doubled, the energy
required per unit distance will only be cut by half. Thus energy
requirement from the standpoint of hopping against gravity is
inversely proportional to footfall rate. This energy savings needs to
be weighed against the increased energy requirements of getting the
legs to drive forward and backward against their own inertia (and in
this case I'm quite certain elasticity can play a larger role, so I
won't presume to come up with a formula). Anyway, there is a sweet
spot somewhere in there, and it could well be at a higher cadence.

Incidentally, I don't know what to make of the response mechanism
desribed. It sounds a little fishy; although the advantage of a higher
cadence is that less control is required if the footstrike is very
short (but perhaps that would lead to injury?). Also, the timing calcs
are off: 180 steps per minute = 0.67 sec per foot, not 0.167 sec.


2. Horizontal speed modulation. I think the ice example was intending
to show that movement does not require much propulsive force. Once you
are in motion, you don't really need to push to keep yourself going
(excepting wind resistance and uphills). My brother and I used to
baffle people by making it look easy to walk (even quickly) on
slippery ice. The control was more important than the effort, because
you don't really need propulsive force to keep going, you only need
your feet to 'roll along' under you to keep you up. Running should
probably be the same. If someone has a heavy footstrike, it probably
means that he is decelerating his body. A telltale indicator would
probably be excessive movement of the foot in the shoe or black
toenails. In order to maintain speed, he has to push off forward to
re-accelerate back up to his original forward speed. That means energy
is being expended to slow down (at footstrike), and then to speed up
again (at pushoff). But does it make sense to slow down and speed up
the trunk of the body like this? Of course not. If the foot would
experience no forward or aft force (no sliding tendency; ie, no
friction on the sole), but only vertical force, the body would be held
up vertically while eliminating this deceleration/acceleration energy
waste. In theory, someone doing this perfectly would be able to run
onto ice and have virtually the same form on ice and continue at the
same pace. The unfortunate part is that there is a tradeoff: extra
muscular force and control is needed to achive this 'vertical force
only' footplant. There is probably a sweet spot here as well, and it
is probably with less force and higher footfall rate than what I
currently use.

It's interesting food for thought.

[reply]At one point, the article states that the "minimum force needed to move the 70kg runner is more than 700n." This is simply not true. 700 Newtons is just the (approximate) weight of a 70 kg runner, so this is the minimum force needed to move the runner vertically. Since generally we're trying to move horizontally when running, the forward component of the propulsive force does not need to exceed the weight. Any net force on an object will cause the object to accelerate. That's what F=ma means.
-Charlie[/reply]

Charlie, as a physics geek you need to look at the running motion more closely. We do not have wheels but rather two legs so there must be some bouncing, so there must be a verticle element so the minimum force is the minimum force necessary to cause verticle motion and any horizontal force is added to it.

Frank

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Frank,
An original Ironman and the Inventor of PowerCranks

The main problem I have with some of these types of articles is not that they don't work in practice...they obviously do work, at least for some people. But, I don't agree with statements to the effect that, if a baby instinctively begins walking in a certain fashion, doesn't it mean that we should mimic that fashion when we run? Well, perhaps we should drool all the time, too! It's interesting, but not germaine to convincing me of the arguement. Let's leave those statements out and stick to the science...those statements remind me of the kind you see in cosmetic ads. And, if we don't know the science yet, but have evidence that certain actions produce good results...let's talk about how to do those actions, so one day we may understand what does and doesn't work...that may help us understand the science.

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)

Fair is fair. I'll deduct partial credit from my answer for lack of clarity.

I'll agree that the total force needed for running must be greater than the runner's weight in order to lift the runner off the ground; my point was that the component of that force that is directly causing the forward motion (the horizontal component) need not be greater than the runner's weight.