posted May-21-2007 12:58 PM               

http://www.ingentaconnect.com/content/tandf/rjsp/2005/00000023/00000007/art00013

Consideration to how popular the Pose method is, doesn't this link raise questions to why so many follow it?

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posted May-21-2007 08:08 PM               

However, it does seem to refute what the Pose people say since it looks like the oxygen cost increased using the Pose method, opposite of what is claimed by Pose-rs. Maybe it takes longer than 12 weeks for the athletes to get adapted to the Pose method such that it is more efficient?

Note: I have no axe to grind pro or con with respect to Pose. Whatever floats your boat!!!

Edited to add: I just looked at it again, and funny thing is the guy who created the Pose method, Romanov, is one of the authors on the article!

[This message has been edited by milkbaby (edited May-21-2007).]

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posted May-21-2007 10:57 PM               

It is totally reasonable that the stride length would decrease with the POSE method. In fact, I thought that the POSE form requires a higher step frequency compared to the heel or mid-foot striker form? A shorter stride length is the same as a higher step frequency, so that’s nothing new. (Most researchers deal with the step-frequency, not stride length because you can use it directly in the spring-mass model.)

The model also predicts that the CM displacement would decrease with higher step frequencies but it unfortunately fails to predict a minimal O2 consumption at the preferred step frequency (PSF). That’s a tough phenomenon to understand, at the moment. It’s related to the optimal swing frequency for the legs. A few people have tried to tackle this problem, but the results haven’t been encouraging. There may be a breakthrough in a while, but it’s not here now.

What has been found is that the O2 consumption does increase as the subject moves away from their PSF. The study’s finding that the O2 consumption by the experimental “POSE” trained group is greater is probably due to the fact that they are not running at their PSF at that speed. BTW, the phrase “increase in submaximal absolute oxygen cost” seems to contradict their final assertion regarding ” a decrease of running economy”.

I find it hard to believe that this got through peer-review.

Ted

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posted May-22-2007 02:48 AM               

quote:

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Originally posted by TedAndresen:
I read lots of abstracts in running biomechanics and I had a good deal of trouble understanding their results. I assume that “After the treatment period” and “a post-treatment difference” mean the same thing.

It is totally reasonable that the stride length would decrease with the POSE method. In fact, I thought that the POSE form requires a higher step frequency compared to the heel or mid-foot striker form? A shorter stride length is the same as a higher step frequency, so that’s nothing new. (Most researchers deal with the step-frequency, not stride length because you can use it directly in the spring-mass model.)

The model also predicts that the CM displacement would decrease with higher step frequencies but it unfortunately fails to predict a minimal O2 consumption at the preferred step frequency (PSF). That’s a tough phenomenon to understand, at the moment. It’s related to the optimal swing frequency for the legs. A few people have tried to tackle this problem, but the results haven’t been encouraging. There may be a breakthrough in a while, but it’s not here now.

What has been found is that the O2 consumption does increase as the subject moves away from their PSF. The study’s finding that the O2 consumption by the experimental “POSE” trained group is greater is probably due to the fact that they are not running at their PSF at that speed. BTW, the phrase “increase in submaximal absolute oxygen cost” seems to contradict their final assertion regarding ” a decrease of running economy”.

I find it hard to believe that this got through peer-review.

Ted

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Hi Teb

I am trying to understand your objections.....

I can understand an "increase in submaximal oxygen cost" (presumably at the same running speed) as being equivalent to a "descrease in running efficiency". What is the problem with this?

I don't have a problem in seeing "after" as being equivalent to "post." Is not this just done for the sake of readability? They do mean the same thing don't they?

You say that the study 'fails to predict' minimum oxygen consumption at the PSF. Surely this is the very thing they are testing arent they? Trying to see if Pose is more efficient (less oxygen consumption).

Though the abstract does not say this, one can guess that the testers knew that going off the PSF would be less efficient short term. This is perhaps why they did the test for 12 weeks, to give time for adaption to a higher frequency.

-b

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posted May-22-2007 03:51 AM               

Considering his reputation at stake, I'm assuming that he worked very closely with his test group.

I fully believe the popularity of the pose method is due to what was proven. Through decreasing vertical oscillation as found, means the runner has a lessened impact force for absorption compared to the control group for the study. I interpret that finding to prove that it’s a more comfortable way to run, not a faster one.

I describe the human body as a battery. One can derive battery power and apply it to increased firing intensity (strength), or it can burn energy in multiple firing. However it can’t do both for a very long period of time.

Track shoes follow the same running style of forefoot landing, but that represents running in a very controlled environment, not for a street mapped marathon.

From what I’ve learned, pursuing one’s stride length increase is a more optimum goal rather than turnover rate if the quest is the most efficient model for long distance running.

Landing on the forefoot requires the plantar fascia to absorb an increased tension, as well as the impact to the calf muscles upon step. The calves also fire to absorb the impact of landing. And upon initial landing, the calves also have to remain tight until the heel down force has stopped. That time of contraction requires energy. And then the calf muscle has to fire for a third time again on push off. That’s how I explain the increase in oxygen use.

That to me is rather unique in that a well balanced heel strike runner is more efficient simply because the calf muscles don’t have to fire for impact weight absorption. Striking on the heel means the calf muscles only fire once and that’s on pushoff.

Fire three times using your calf muscles for a mid or forefoot landing, and once with a heel strike demonstrates to me why the Pose Method requires higher oxygen and energy use to accomplish. I can only describe why it’s popular, yet no advantage in philosophy to follow…

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posted May-22-2007 05:09 AM               

quote:

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Originally posted by brianfie:
Hi Teb

I am trying to understand your objections.....

I can understand an "increase in submaximal oxygen cost" (presumably at the same running speed) as being equivalent to a "descrease in running efficiency". What is the problem with this?

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Brian,

Maybe I am misreading this:

“a mean increase in submaximal absolute oxygen cost (from 3.28?±?0.36?l?·?min -1 to 3.53?±?0.43?l?·?min -1 ; P ?<?0.01).”

mean that the experimental group is burning up more energy per minute than the control group while running at the same speed? (1 liter of O2 = 4.82 Kcal of metabolic energy consumption)

Then the last sentence:

“a reduced vertical oscillation in comparison with the control group and a decrease of running economy in triathletes.”

Running economy is in units of energy/(kg-min), so if the oxygen cost increased wouldn’t the energy per (kg-min) also increase and not decrease?

Ted

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posted May-22-2007 07:30 AM               

I seems clear to me. They say the treatment group had average:

-decrease in stride length (from 137.25cm to 129.19cm)
-a lower vertical oscillation (6.92cm. vs 8.44cm)
-a increase in submaximal absolute oxygen cost (from 3.28l/min -1 to 3.53l/min -1).
-a decrease in running economy.

-b

[This message has been edited by brianfie (edited May-22-2007).]

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posted May-22-2007 09:34 AM               

quote:

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Originally posted by sport jester:

That to me is rather unique in that a well balanced heel strike runner is more efficient simply because the calf muscles don’t have to fire for impact weight absorption. Striking on the heel means the calf muscles only fire once and that’s on pushoff.

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Using a semi-theoretical approach to predict better running form is folly in my view. I do not think there are any accurate theoretical models of the mechanics of the running body. Therefore, any speculation is not a lot of practical use unless it can be shown to yield results.

Still, it is hard to resist joining in the fun for a few posts, so here goes:

Sport Jester seems to overlook the importance of 'springiness' in the body. We land, the muscles absorb the impact, store elastic energy, and this gets used to improve the efficiency of the toe off.

If you run in place or skip with a jump rope, it is much more efficient to land on the forefoot. It is not more efficient to land on the heels - even though you save a 'firing'. Why? Because this way you can make best use of the natural elasticity of the leg. This is not an argument for forefoot striking during running, because the foot can roll forward and load up the calf muscles with a heel strike. It is, however, and argument against what SJ is saying - that is, you should not use the calf muscles during the first phase of the stride. I think the reverse is correct.

The human body is a not that well optimised for running. Perhaps that is why there is so much controversy about how to do it properly. We have the big, heavy muscles all down the leg. Compare with specialised running animals. They have thin legs with the muscles all at the top. Note how the part of the leg below last joint (fetlock on a horse) flexes under the impact of landing. Here we have the familiar elastic thing going on again...

On another tack, muscles do not 'fire once' neurologically speaking. Neurons are firing all the time, even when muscles are not contracting. Really, the default state of a muscle is contracted, not relaxed. If by 'firing' SJ means contracting, then, even with forefoot landing, there is one contraction per leg per stride.

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posted May-22-2007 11:02 AM               

quote:

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Originally posted by brianfie:
Ted

-a increase in submaximal absolute oxygen cost (from 3.28l/min -1 to 3.53l/min -1).
-a decrease in running economy.

---

Sorry, I still see the last two as contradictory. Running economy is measured in Kcal/(kg-m) (1.3 being a typical value) or liters of O2/(kg-min) or liters of O2/(kg-m). If O2 consumption per minute increases it would seem to me that economy would also increase.

What am I missing?

Ted

[This message has been edited by TedAndresen (edited May-22-2007).]

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posted May-22-2007 01:30 PM               

I am merely thinking empirically here. If O2 consumption goes up for the same speed, the runner is burning more oxygen for the same result(speed). More O2 consumed is equivalent to more calories consumed - for the same speed. Hence less efficiency.

-b

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posted May-22-2007 03:32 PM               

Yes a forefoot landing is more efficient when pushing straight upwards. However like running, jump rope as exercise works up a quick sweat. The higher you push yourself up, jumping rope as no different in running, the more energy you consume.

Biomechanics is simple. Stability in balance is that you can't move forward if you push straight up. And you can't push up if you push straight forward.That's the efficiency in balance between the two forces in motion.

Pure vertical motion and forward motion however are apples and oranges in comparison. The difference is adding momentum to the biomechanic process and required measurement. And that formula is called weight transfer efficiency.

The less energy you need to transition from one foot to the other regardless of walking or running, determines your transfer efficiency. Personally I prefer to use your heart rate to gauge efficiency improvement, simply because it’s a passive measurement to how much energy you waste in forward motion.

Statistically to your comment of our natural poor running biomechanic 60% of your forward motion is simply bodily momentum and 40% of your speed is developed through muscle firing. In testing, I walk and run at with 85% momentum efficiency with 15% of forward motion replaced with muscular energy consumption. Therefore, if we both consume the same amount of caloric energy and theoretically if we’re the same size, then with more efficient forward motion, then I can either apply the energy saving toward higher speed, or longer distance in comparison.

Animals with large torsos and small legs as you note have that ratio difference in size because they’re much more efficient in forward motion, and therefore can shift much higher weight loads with much smaller muscle mass necessary.

And the biomechanic advantage is that lighter limbs require less energy after push off to return it to its most forward position in preparation for their next step. It proves how inefficient we truly move when comparing ourselves with them.

Since you have a natural speed limit through your biomechanic efficiency, increasing your turnover rate merely increases the energy you expend to accomplish the same speed.

[This message has been edited by sport jester (edited May-22-2007).]

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posted May-23-2007 01:33 AM               

quote:

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Originally posted by brianfie:
Ted

I am merely thinking empirically here. If O2 consumption goes up for the same speed, the runner is burning more oxygen for the same result(speed). More O2 consumed is equivalent to more calories consumed - for the same speed. Hence less efficiency.

-b

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Brian,

Running efficiency would be mechanical energy or power output divided by the energy or power consumed through O2. Unfortunately we cannot be measure the mechanical energy or power output of a runner. It can only be estimated, and then only very roughly.

Instead, researcher us running economy. It is measured in units of liters of O2 consumption per meter. To standardize it to each runner, it is normally divided by the runner’s mass so it comes out as liters of O2 consumption per kg per meter. I think that typical values are about 0.16 milliliters of O2 per kg per m.

Economy does not depend on the speed. It is pretty constant at all speeds. For an average person it is approximated as 100 calories per mile in the American system of measurements. (The British abandoned miles and pounds decades ago.)

Ted

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posted May-23-2007 01:53 AM               

quote:

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Originally posted by sport jester:

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I don't know what you are describing. You are not familiar with the spring mass model. You can find out more about it through Google.

Yes a forefoot landing is more efficient when pushing straight upwards.

I don't know that to be found in any peer reviewed research. Efficiency is NEVER mentioned.

Biomechanics is simple. Stability in balance is that you can't move forward if you push straight up. And you can't push up if you push straight forward.That's the efficiency in balance between the two forces in motion.

No, efficiency is defined as mechanical energy in joules or power (watts) output over energy or power consumed (liters of O2 consumption, Kilocalories, joules.). It is a percentage. It has zero to do with stability.

Pure vertical motion and forward motion however are apples and oranges in comparison.

No, vertical and horizontal motions are linked because of a cross term relating to the leg-angle from touch-down to toe-off. They are interrelated

The difference is adding momentum to the biomechanic process and required measurement. And that formula is called weight transfer efficiency.

I have never encountered that concept or formula. Did you make that up. If not, could you furnish a reference, link or citation to a credible source?

Ted

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posted May-23-2007 04:41 AM               

quote:

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Originally posted by TedAndresen:
Brian,

Running efficiency would be mechanical energy or power output divided by the energy or power consumed through O2. Unfortunately we cannot be measure the mechanical energy or power output of a runner. It can only be estimated, and then only very roughly.

Instead, researcher us running economy. It is measured in units of liters of O2 consumption per meter. To standardize it to each runner, it is normally divided by the runner’s mass so it comes out as liters of O2 consumption per kg per meter. I think that typical values are about 0.16 milliliters of O2 per kg per m.

Economy does not depend on the speed. It is pretty constant at all speeds. For an average person it is approximated as 100 calories per mile in the American system of measurements.

Ted

---

Ted

Perhaps the problem is definition of terms. I take an increase in economy to mean greater (more) efficiency. So an increase in economy means an improvement (higher) efficiency. So with greater ecomomy, (more efficiency) either you go faster on a Kg of O2, or you use less Kgs of O2 to run at the same speed.

The runners who trained with the Pose system used more O2 (from 3.28l/min to 3.53l/min ) to run at the same speed. So, in a lay-persons terms, their efficiency, or economy, deterioated.

If 'economy' means the reverse to an exercise physiologist I can understand the confusion

-b

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posted May-23-2007 12:31 PM            

-mds

quote:

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Originally posted by brianfie:
Ted

Perhaps the problem is definition of terms. I take an increase in economy to mean greater (more) efficiency. So an increase in economy means an improvement (higher) efficiency. So with greater ecomomy, (more efficiency) either you go faster on a Kg of O2, or you use less Kgs of O2 to run at the same speed.

The runners who trained with the Pose system used more O2 (from 3.28l/min to 3.53l/min ) to run at the same speed. So, in a lay-persons terms, their efficiency, or economy, deterioated.

If 'economy' means the reverse to an exercise physiologist I can understand the confusion

-b

---

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posted May-23-2007 02:36 PM               

Given your criticisms of my post, I’ll offer you this link from the NY Times to Weight Transfer Efficiency in runners(You may have to sign up to get the article).

Everything else I wrote is based upon the principle.

http://www.ottopohl.com/Stories/2002_Stories/NYTheads2.htm

The idea of your spring mass model requires energy consumption to be factored into any question. If a ball could reach the height of its release altitude after bouncing, then you’re arguing perpetual motion, which is impossible. A ball loses energy (see Otto Pohl article explaination).

If any movement could be without any additional energy output than our natural tendon function, then we wouldn’t need to eat…

The following link and quote speaks to possible define the parameters of this thread.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=15233599&qu ery_hl=4&itool=pubmed_docsum

"Running economy (RE) is typically defined as the energy demand for a given velocity of submaximal running, and is determined by measuring the steady-state consumption of oxygen (VO2) and the respiratory exchange ratio. Taking body mass (BM) into consideration, runners with good RE use less energy and therefore less oxygen than runners with poor RE at the same velocity. There is a strong association between RE and distance running performance, with RE being a better predictor of performance than maximal oxygen uptake (VO2max) in elite runners who have a similar VO2max)."

The idea of RE to me is a simple question; given any speed, if your oxygen consumption level is higher with one technique, and lower in oxygen consumption with another, then the less oxygen your body needs, the more efficient and therefore better runner you are towards any racing goal of distance or speed.

The question I posted was in asking why a less efficient (higher oxygen consumption) is a favored way to run?

[This message has been edited by sport jester (edited May-23-2007).]

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posted May-24-2007 01:14 AM               

quote:

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Originally posted by sport jester:

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The Times article deals with the walking model, the inverted pendulum model, and not the running model, the spring mass model. The gait cycles are different.

The kinetic and potential energy considerations are reversed between the two models. The walking (inverted pendulum) model operates on rigid incompressible legs, similar to wooden stilts. The running (spring-mass) model operates on compressible springy legs. If you like, the body moves up and down differently in the two models.

While walking one foot is always in contact with the ground. While running the body is air-borne during the aerial phase and one foot is touching the ground during the contact phase.

In walking the potential and kinetic energies are out of phase or "out of step" with one anopther.

When walking the body (center of mass) is at its maximum height (maximum potential energy) when the speed (kinetic energy) is at its minimum. During the walking cycle one energy can fed into the other one, so the energy consumption per mile or “cost of transport” (liters of O2/(kg-m)) is very low.

The situation is reversed in the running cycle. The body is at its maximum horizontal speed when it is at its maximum height in the middle of the aerial phase, so the potential (height) and kinetic (speed) energies are at their maximums at the same time. As the body comes down and the foot contacts the ground, it looses height so the potential (height) energy decreases. At the same time the forces directed up along the contacting leg slow the body down, so the kinetic (speed) energy also decreases.

At the mid-stance the body is at its lowest height so the potential energy is at its minimum at the same moment that the body is going at its slowest speed, so the kinetic energy is also at a minimum. Both energies are at their maximums and minimums at the same time in the running gait cycle so the body cannot feed one form of energy into the other and the energy required to run per mile is much higher.

You can also find more information on this by using Google to search: "running walking gait cycle potential kinetic energy”

The question I posted was in asking why a less efficient (higher oxygen consumption) is a favored way to run?

I think that the idea of "favored" has more to do with limiting the compressive or impact load on the joints rather than the O2 consumption. I think that is the big selling point with POSE, low impact forces.

There was a discussion of this on the biomechanics list (BIOMCH-L) a few weeks ago regarding the occurrence of hip fractures among people who did not engage in activities that involved higher g compressive loading of the bones. The general conclusion was that greater compressive loading of the bones led to greater bone strength and a lower incidence of hip fractures. I think that a few contributors felt that the compressive loads had to have maximum values between 2 and 3 g. That’s the range of compressive loads that one encounters while running, not while walking.

I would imagine that 2 to 3 g's is the kind of compressive loads that the astronauts in the station need to maintain on a daily basis or they will have trouble with fractures when they get back to earth. They do that by "running" on a tread in the station with strong elastic chord pully them down onto the tread. I think a Russian astronaut had a serious problem with that a few years ago. He had to go through extensive rehab.

Ted

[This message has been edited by TedAndresen (edited May-24-2007).]

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posted May-24-2007 01:09 PM               

A very nice reply…

And you hit the nail on the head for me…
The pose method in summary isn’t a faster way to run, but a more comfortable way to run.

But for me the question stands; I don’t want to run with greater comfort, I seek better speed.

The Pose Method is the full application to your differential description. If we do achieve peak speed at float, then Pose logic argues one would want to have as many peaks per mile possible. But that goal slows you down and costs energy.

I’m in full appreciation to your reading the article. The factors I view differently stem from the idea that walking and running biomechanics should be no different in gait cycle.

One’s goal in running through increased stride distance is that through increasing arc radius size of the pendulum, the runner spends more time at peak aerial speed.

And to find one’s peak efficiency of stride length is easily measured walking. So how fast can you walk? It's that speed of transition to running from walking which determines how efficient you are in stride length achievement.

As impact forces double in transition from walking to running, the faster one can walk, the less energy they consume for achieving the same speed. And less energy spent is longer distance or quicker speed.

The faster I can teach you to walk, the faster I can teach you to run.

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posted May-25-2007 11:39 AM               

Thanks for the good clear explanation. The bit about energy transfer was useful. I expect it would be correct in thinking that, in running, there is also a transfer going. Rather than between potential and kenetic as in walking, it's between kenetic and elastic. The elastic energy being stored as muscles and tendons get loaded up on landing.

Walking is very efficient when the limbs are moving around the natural frequency (as in the upside-down penpulum). As they are pushed faster and faster walking rapidly gets less efficient. There is always a speed where running become just as efficient as walking - even if special techniques of efficient fast walking are developed there is always a point where a gait change is beneficial.

Running is a very different strategy biomechanically.

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posted May-25-2007 12:40 PM               

Very cool post. Yes, walking gets less efficient as speed increases, which is why I stated the importance to transition speed efficiency from walking to running. What you overlook in my opinion is that running doesn’t become as efficient as walking, it’s that your increasing walking speed at some point should allow running not to be a demand, but a choice. And as walking speed increases, it eventually becomes the most efficient choice for higher speed.

It’s then no longer a fixed speed of walking to running, it becomes a window of choosing based upon the environment. For example, it’s much more efficient at a 10% incline to walk at 8 MPH than it does to run at 8 MPH at the same incline rate. Which would you prefer to choose? Do you have that choice, because I do…

The lower your transition speed, the less efficient of walking technique you have that you’re bringing into your running biomechanics and thus the slower you run. Increasing your walking speed translates into proving better biomechanics for running and higher speed.

If you can walk 20% faster, then running 20% faster is easy.

Muscular elasticity plays a role in running biomechanics. In running it’s an energy demand in landing absorption rates. In efficient walking, it’s a function of maximum range of motion in stride length. Which is a more useful goal?

Because if your elasticity goal becomes ROM, your running heart rate drops for achieving the same speed, which is how the women of Kenya walk.

In taking that biomechanic philosophy into running proves to average a 20 Beat per minute decrease in heart rate for same speeds measured.

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posted May-25-2007 01:23 PM               

quote:

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Originally posted by sport jester:

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And to find one’s peak efficiency of stride length is easily measured walking. So how fast can you walk? It's that speed of transition to running from walking which determines how efficient you are in stride length achievement.

I’m sorry, you are just outside my area. All I have seen has indicated that the preferred step length and the resulting preferred step frequency (PSF) is not constant but changes with speed. I’ve only tested a few runners myself, but the general consensus is that the PSF or step rate increases with running speed.

quote:

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Originally posted by brianfie:

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I expect it would be correct in thinking that, in running, there is also a transfer going. Rather than between potential and kinetic as in walking, it's between kinetic and elastic. The elastic energy being stored as muscles and tendons get loaded up on landing.

Brian,

Lots of people share this view. I have a problem it:

If you drop a steel ball on a heavy steel plate, it will bounce to almost the same height as the initial elevation. If you take a 2’ 2x4 and attach a spring to the end of it so it bounces like a pogo stick and drop it on a concrete floor, it will bounce back to about 60% of its release height. If you drop a basketball on the floor, it too will bounce up to a height that is about 80% of its initial height.

The bounce height compared to the initial height is called the “coefficient of elasticity”. It represents the amount of energy that is elastically recovered from the compressed steel ball, coiled spring or basketball.

I don’t see that happening with people. Even if you bend you knees so that your muscles and tendons can exhibit spring like behavior, you cannot bounce your way down the street. You have to consciously and repeatedly add energy to the “system” from one bounce to the next.

I bet that cadaver studies have indicated that the tendons may have some elastic energy storage, but I don’t think there is that much energy recovery in muscle tissue. I actually think that the energy recovery is very tiny.

Walking is very efficient when the limbs are moving around the natural frequency (as in the upside-down pendulum). As they are pushed faster and faster walking rapidly gets less efficient. There is always a speed where running become just as efficient as walking - even if special techniques of efficient fast walking are developed there is always a point where a gait change is beneficial.

Yes, that is called the “walk run transition speed”. Walking gaits are not in my interest area, but I think that a typical transition speed is approximately 2.3 m/s or 26.82/2.3 = 11:40 min/mile or 5.14 min/mile. I recall that this limit is encountered because -- if someone tries to walk faster than their transition speed --(walking on rigid legs or stilt-like legs) their upward velocity at each step is so great that it makes them fly up a tiny amount and leave the ground. That’s what I recall. Of course, Google.com or scholar.google.com would lead you to additional information.

Ted

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posted May-25-2007 02:48 PM               

You may well be right that the spring effect is tiny. I am not sure.

I agree you can't bounce down the street 'for free'. Even if there was a huge amount of elastic energy wound up in the leg, it would not be for free because the 'springiness has to be sustained by muscle tension. That means work and costs energy. You could think of it as bouncing down the street on a pogo stick where you have to hold one end of the spring. Still, you get something from the spring. The muscle does not have to move (extend) much. It can get a lot of power just from resisting motion.

This could explain why, when you jump with a jump rope you prefer to land on the toes, rather than the heels. Its not just about shock absorbing, it is actually a lot easier.

I think even your mathematical models have springs. Perhaps these models approximate well to a reall runner because the leg is springy - albeit a powered spring with a fair amount of damping. Could this explain the Preferred Step Frequency? That is the PSF is the natural frequency of the leg modeled as a damped spring-mass system?

-b

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posted May-25-2007 02:58 PM               

I don't doubt that there may be a lot of territory to explore in the transition, between running and walking. If you are discovering something productive here, good luck to you.

I often run at speeds where race walkers in full cry could easily overtake me - not that I have ever encountered any. However, I practice normal running because some of the time I want to run a lot faster than my easy pace.

I could learn to walk at my easy pace and try to run only when I wanted to race, but I doubt that would work well for me. I prefer to leave that experiment to others

-b

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posted May-26-2007 11:46 AM               

quote:

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Originally posted by brianfie:

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You may well be right that the spring effect is tiny. I am not sure.

It’s a tough issue that has been clouded by the hype in the media where “sources” make statements like: “Kangaroos recover 90% of the energy from each bounce”, etc. It may be true. I just don’t see it in humans.

Still, you get something from the spring. The muscle does not have to move (extend) much. It can get a lot of power just from resisting motion.

This could explain why, when you jump with a jump rope you prefer to land on the toes, rather than the heels. Its not just about shock absorbing, it is actually a lot easier.

Wow, that is very interesting! I had no idea. I think you have something there. Bravo!

I think even your mathematical models have springs. Perhaps these models approximate well to a reall runner because the leg is springy - albeit a powered spring with a fair amount of damping.

It seems to compress like a spring and then expand like a powered air ram. The energy per gait cycle is the energy required to launch the body. I would think that the leg is almost totally damped, so that the runner must replace all the energy that the leg absorbs and discards during compression with muscle work to re-launch the body.

Could this explain the Preferred Step Frequency? That is the PSF is the natural frequency of the leg modeled as a damped spring-mass system?

That’s been tried, but the numbers don’t seem to work out. The leg has a springiness factor. It is called the leg-stiffness. It is a quantity that states how much force (Newtons) is required to compress the leg a standard distance (millimeters). Typically it takes about 10 to 20 Newtons (2.2 to 4.4 lbs) to compress the leg 1 millimeter.

A leg stiffness of 15 N/mm is a typical value for a runner. In the metric system, the millimeter is a “minor” unit. Most researchers state their units in force per meter (N/m), instead of force per millimeter (N/mm). If it takes 15 Newtons to compress the leg 1 millimeter and there are 1000 millimeters in a meter, then it would take 15,000 Newtons to compress the leg one whole meter. To abbreviate, a researcher would state that the leg stiffness as 15 KN/m. If you do a Google search on “leg stiffness” and “running”, you will see lots of publication where they state leg stiffness values in the range of 10 KN/m to 25 KN/m.

Stiffness values for other things are also of interest. I think that a soft treadmill might have a stiffness of 70 KN/m. (You can measure that by just standing on the treadmill and having someone measure how much the board goes up and down when you get on and off the treadmill.) The stiffness would be weight (lbs) * 9.8 / (2.2*deflection(mm)). That should give you a stiffness value in the area of 70 to 150 N/mm, which is 70 to 150 KN/m

The stiffness of concrete and asphalt are pretty high, above 1000 KN/m. I think that the gel inserts in running shoes are about 100 KN/m.

You can actually measure your own leg stiffness with a VERY simple measurement. It requires a stopwatch and about 30 seconds of running-in-place without leaving the ground. Your leg stiffness would equal 39.4*StepFrequencySquared*Mass in kg. Your mass in kg would be your weight in lbs divided by 2.2. StepFrequency is measured in steps per second. (Be sure to start the count at “zero” and not “one”, if you try this. StepFrequency should be in the area of 2 to 2.5 steps/sec. Remember to square it in the calculation.)

The natural step frequency is directly connected to the leg stiffness. Testing has confirmed that the leg stiffness (along the axis of the compressing leg) is pretty constant and that it doesn’t change much from its initial running-in-place value. It may go up by 10% from the running-in-place value even up to the maximum speed. So, leg stiffness does not seem to explain the existence of the Preferred Step Frequency at each speed.

There is some thought that the preferred step frequency (PSF) may be related to leg swing and not leg compression. Just like any pendulum, the leg has a natural swing frequency. Unfortunately the leg naturally swings very slowly. You can test yours. Just stand on a step and let your leg hang down. If you keep it swinging it will normally do about 20 full swings in 30 seconds. That’s 40 strides per minute, or 80 steps per minute or 1.33 steps per second. Normal “running” step numbers are 80 strides per minute, or 160 steps per minute or 2.66 steps per second which is about twice the natural swing frequency of the leg. It looks like the natural swing frequency of the leg is not the issue that it selecting the PSF.

There is still a glimmer of hope for the swing leg issue. To make the leg swing faster than its natural frequency takes energy. The energy input grows very rapidly as the driving swing frequency moves away from the natural swing frequency. The faster the legs swing, the more energy is required to make it swing. If the extra swing power could be modeled and calculated it might explain the upper limit on the preferred step frequency. Unfortunately, this is a VERY difficult model to construct. (I’ve been working on it for about 2 months.)

If it works this approach does explain why there would be an upper limit on the PSF. Unfortunately, it does not explain why there would be a lower limit on the PSF at every speed.

I do not understand what happens to the gait when a runner tries to run with a step frequency that is slower than the PSF at a given speed. I know that they do take longer steps but I don’t understand how they do it. Do they take higher leaps with longer aerial times? Does the leg stiffness change? I don’t see that. I’ll need to do some tests on that.

Ted

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posted May-27-2007 05:14 AM               

Many thanks for your thorough and patient reply. I understand what you are saying about leg stiffness being constant, but I am not ready to give up on the possibility of variable stiffness yet. It's too temping an explanation for a few different phnomena.

I got the idea from this paper. I know it is not that recent:

Running in the real world: adjusting leg stiffness for different surfaces

I will read some more and maybe will find something else interesting.

BTW - I have no problems with the metric system

-b

[This message has been edited by brianfie (edited May-27-2007).]

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