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  #1  
Old 01-10-2001, 01:25 PM
purplepuma purplepuma is offline
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I am taller than the average person at 6' 5" and when I am walking next to a friend who is much shorter, they always say that they have trouble keeping up because my legs are longer. Or they other one is, that I can run faster
than most people. I do not think this matters though, because while each step I take covers a greater distance, it also takes more energy. So a smaller person's legs cover less distance but take less energy for each step,
therefore the overall walk or run is not easier for me. I try to make the analogy to a multi-geared bike to my short friends, but they tell that its not the same thing. On a bike a smaller gear setting takes less energy to
turn with the peddles but turns the wheel a shorter distance, and visa versa for the larger gear. Is this right or do tall people really have an advantage walking and running?

Thanks
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  #2  
Old 01-10-2001, 02:05 PM
Giraffe Giraffe is online now
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I think you've answered your own question. Can someone ride a bike faster on the highest gear than on the lowest gear? The answer is clearly yes. It's called a mechanical advantage.

I don't know much about physiology, but I wouldn't be surprised to learn that our bodies have an optimal speed that depends on our size and build. It may take more power for a tall person to run than a short person, but they may also be able to run faster. And it may not take proportionally more power, i.e. if I can run twice as fast as someone, it may not cost me twice as much energy per second. I don't actually know, though.

I'm 6'8", and I find that on average, I can outrun almost everyone I've played with on a soccer field or basketball court. There have been a few notable exceptions, though, one of whom was a 5'4" guy.
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  #3  
Old 01-10-2001, 02:36 PM
purplepuma purplepuma is offline
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I agree that on a bike a higher gear makes you go faster, but it is at the expense of more energy. Therefore if there where two people with the same amount of avaliable energy, one using a higher gear and the other using a lower gear, riding on a flat surface, they would go the same distance. I should have been more specific when I asked about tall people running faster. I should have asked: over a distance using the same amount of energy, does a taller person have an advantage in the distance they can cover? This this is true than wouldn't the best distance runners and walkers all be extreamly tall?
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  #4  
Old 01-10-2001, 02:55 PM
BobT BobT is offline
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Speaking as another 6'5" person, people say I walk fast because my legs are longer, which is a fallacy. I actually have relatively short legs for my body (inseam length 33").

I think everyone walks at almost identical paces, but the difference is in your own physical conditioning. Unless you are very short, e.g. a midget, you will probably be able to walk at the same pace as a person of any height. I doubt that my stride length makes that much of a difference.

On a related note, it is much harder for taller people to develop good golf swings because the club travels through a much longer arc than a smaller person and there are more places for the swing to go awry.
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Old 01-10-2001, 03:17 PM
Irishman Irishman is offline
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Giraffe, 6'8"? That have something to do with your nickname?

Applying general physics knowledge to biometrics problem. Running speed is controlled by two factors - stride length and rate of pumping legs.

The longer your stride, the more ground you cover per step, and thus the faster you get from point A to point B. This generally would indicate the advantage to the taller person, who typically has the longer legs, and thus longer stride. Michael Johnson, the olympic athelete, contributes his speed to his unique running style that is almost reclining. He says it allows his legs to stretch further, and gives him a longer stride. Given his success, some younger runners are now copying his style - I don't know about their success or not with the technique.

The other factor given was rate of pumping legs. That's how fast you swing your legs. The faster you swing, the more steps you take in a given amount of time.

Combining the two is a more complicated process. This is complex because what controls the natural pumping speed ability is not fully understood. One major contributer is muscle strength - size and tone. Thus runners work out and get really large thigh and calf muscles that make give them more power. However, natural ability may play a role in the ability to condition your leg strength. Not sure how to quantify that. Some people naturally get bigger, denser muscles than others. I know a few guys with "chicken legs" - they are bony and slim even though they exercise.

purplepuma, regarding your standard walking they are probably right because your natural stride is probably longer than theirs, so though your rate of swing might be the same or even a little slower, the stride length would counter that.

About the energy required... I'm more or less guessing on this part. If we're talking total energy, you are probably correct, but I don't think the effective energy is much different. What I mean is, being bigger, your metabolism is geared to handle your energy requirements, and you have to eat more (larger caloric intake) to provide the energy needs. Assuming similar levels of physical activity for each person. So you won't feel the energy output as more significant. If you did, you would probably reduce your stride rate to something far slower than the shorter people, and I don't think this is the case. Perhaps you could test that with some of your friends. Have someone comparison measure your stride rate and stride length to various of your friends.

Another factor that comes to mind just now is how hard a person pushes off with their foot. Walking (and running) is the act of pushing with the back foot and landing on the front foot, then shifting weight from back to front as you lift and swing back foot to front, and repeat with the other side. Your forward motion is caused by the push you give the ground. The harder you push, the faster your body moves forward, and thus you either have to cycle your legs faster or lengthen your stride. Running is, in fact, both.

Try this: start walking, then speed up so that you are walking faster and faster, but keeping your stride long. You will reach a point that you cannot cycle your legs any faster and keep the same stride length. You can shorten your step and cycle faster. Or you can start running. In order to run, you have to push off with enough force to get you slightly airborn. With your weight going up into the air, you have more time before you have to place your foot, and more time to complete the leg swing, and the distance between each step is farther apart. The harder you run, the more you are pushing forward and upward - there's a balance. If you push too much upward, then you are burning energy fighting gravity and having gravity pull you back down rather than pushing forward. But if you don't get enough lift, your stride doesn't have enough room.

I think I'm going on and on and going way past the point of your question. I believe the answer you are looking for is buried above, but specifically, you might be burning more energy, but you probably don't feel any more exertion, and your stride rate is probably similar but your stride length is longer, causing your companions to have to hurry to keep up. Taller people may burn more energy, but they also eat more and their metabolism can handle the extra load.
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Old 01-10-2001, 03:32 PM
Irishman Irishman is offline
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While composing my response, you posted again.

I think the question you are asking is making a fallacious assumption - that two people would consume the same amount of energy, and that energy would be the limiting factor. I mentioned that a bigger person burns more energy, but also eats more, so has more energy to burn. Also, conditioning is a factor. Muscle burns more energy than fat, so a person with firmer or bigger muscles will have a higher metabolism. Similarly, a taller person has more body to burn energy, so their needs are higher. Thus two runners won't burn the same amount of energy, and maybe not even the same percentage of their available energy, for a given run.

Thus long-legged people have the advantage of available stride. All other things being equal (natural inclination, leg muscle strength and conditioning, plenty of fluids and food, good rest, mental state, whatever, plus stride rate ability), the longer legged person will be able to run faster.

BobT, you may be correct and still be wrong. You claim your legs are actually short, with an inseam of only 33" despite a height of 6'5". However, stride length is not solely dependent on leg length. It also matters on the individual comfort in taking steps. I am only 5'10", with an inseam of 32", however in the past I was often accused of being too speedy, and one explanation often given was my legs were longer. Admittedly the complaints were by women 5'2" to 5'4", but I've done some unofficial comparisons and given the differences in anatomy, I usually find my legs aren't significantly longer. However, I take long steps, so my stride length is longer. This is especially easy when said female is wearing heels, or skirts, as these impede her ability to take long steps. And I suspect some women are then conditioned to take smaller steps regularly, because of habit.
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  #7  
Old 01-10-2001, 03:54 PM
keeper0 keeper0 is offline
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I know from talking to several Biomechanical Engineers that a given human tends to make the same number of strides every minute when walking. Speed is regulated by increasing the length of the stride.

If you recall Galileo looking at the swinging lamp, he realized that the natural frequency of a swinging object (like a pendulum)was independent of mass and length of travel, and that it depended only on the length.
Galileo and the pendulum

By moving our legs at their natural frequency, we can accomplish walking at a given speed for the least expenditure of energy.

If, as a first approximation, we ignore the knees and ankles and act as if the leg was a rigid object (ie. a pendulum), we can start to get a handle on the issue.

The frequency of a simple pendulum under the small angle assumption is sqrt(g/l) (or sqrt(g/l)/2*pi to get it in Hz. instead of radians/sec). g is the force of gravity, which (for today at least) is not a variable.
Discussion of Pendulums

So, the frequency decreases as l increases.

Men's inseams run from about 30 to 40. Add four inches to go from crotch to hip and hem to heel. Convert to metric, and we have a range of legs from 86 cm to 112 cm.

Let's assume the leg is uniform density, so the center of gravity is halfway down the leg.

The natural frequency of the shorter leg is then
sqrt(9.81/.43)/2*pi or .76 strides per second.

The natural frequency of the longer leg is then
sqrt(9.81/.56)/2*pi or .66 strides per second.

For a given angle of stride (say 10 degrees), the shorter leg will cover 86*sin(10 deg) or 14.9 cm per stride. The longer leg will cover 112*sin(10 deg) or 19.4 cm per stride.

In ten seconds the shorter leg will take 10*.76 or 7.6 strides, and therefore move 7.6*14.9 or 113 cm.

The longer leg will take 10*.66 = 6.6 strides for 6.6*19.4 = 128 cm.

So, for a given stride angle, the longer legged person will move faster at their natural frequency.


"AHA", you say, "Will they use the same angle?"

"I don't know," I say.

So, let assume each person puts the same energy into lifting their leg. The potential energy of the shorter leg at 15 degrees is m*g*.43*(1 - cos(15 deg)).

Assuming the mass of the legs are proportional to their length, the same potential energy of the longer leg is
achieved at 112*.56*(1 - cos(x)) = 86*.43*(1 - cos(15 deg))

Solving for x, I get 11.5 degrees.

6.6 strides of a long leg at 11.5 degrees gives a distance
of 147.3 cm

7.6 strides of a short leg at 15 degrees gives a distance of 169.1 cm.


So, for a given leg angle the longer legged person will move faster. But for a given energy input, the shorter legged person will move faster.


(now you see where I get the first part of my .sig)
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  #8  
Old 01-10-2001, 04:19 PM
ENugent ENugent is offline
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I and others have given this a moderate amount of thought in the context of ballroom dancing. There, stride frequency is the same for everyone (you're all dancing to the same music), so movement is a function of stride length. One interesting conclusion is that you can get a rather long, smooth stride without a whole lot of leg division if you use your feet and ankles strongly. If you roll through the whole foot to push out of a pointed toe with each step, you get quite a bit more distance than if you just leave your feet like lumps at the end of your legs. For example, I can walk with about a 3-foot stride length while keeping both knees locked.

Because of this, I tend to walk faster than non-dancers with similar or longer leg lengths, even if I use the same stride frequency. I also can follow a novice leader who has not yet learned to fully use the foot action, even though his legs are a foot longer than mine and I am wearing heels and dancing backwards (I've done it).

This type of walking action also seems to use less energy for the same amount of propulsion - I'm not sure why.
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  #9  
Old 01-10-2001, 06:44 PM
kanicbird kanicbird is online now
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Going back to the bike example
IIRC the total energy input to an ideal gearset should be the same no matter what the gear. By ideal I mean ignoring frictional losses. The problem comes in with the 'power plant' which can efficently transmit the energy only at a certain range of rpm's.
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