Biological legs vs Cheetah prosthetic legs - wich is better for running?

  • #1

Main Question or Discussion Point

Biological legs vs Cheetah prosthetic legs - which is better for running?

EDIT: I misspelled "which" in the subject line, how embarrassing.

You may have heard of Oscar Pistorius, a double below-knee amputee from South Africa who recently won the silver medal in the 400m at the South African senior athletics championships (did I mention everyone else in the race was running on his natural legs?). However, he may be excluded from further competing in "able-bodied" events because some feel that his artificial legs give him an unfair advantage by virtue of being longer then natural legs. Others say that his legs are a disadvantage, since unlike natural legs, they are basically just springs and can not generate energy. Here is the statement about this that seems to be widely circulating:

In running the human ankle complex returns over 250% of the energy absorbed at heel strike. The prosthetic foot complex is a passive system, and cannot return more enegry than what is put into it. You can think of it as a spring. No matter how good a spring you design, it cannot return more than 100% of the energy it absorbs.
You are correct in your statement about energy return of the Cheetah. When comparing apples to apples (Cheetah to foot/anke/calf complex), the Cheetah will return around 95% of the force applied (there will always be some loss to heat), while the foot/ankle/calf has the ability to create energy in response to load (250%)... The key here though, is that the foot/ankle/calf complex has a about a 2.5 to 1 energy return ratio when compared to the cheetah.
To me, these statements seem badly incomplete at best, and purely nonsensical at worst. Supposedly, the natrual ankle returns 250% of the stored energy. BUT HOW CAN YOU EVEN TALK ABOUT THE ENERGY RETURN OF A SYSTEM WHICH NOT ONLY STORES ENERGY, BUT ALSO GENERATES IT? I would think the generated energy wouldn't depend too much on the stored energy, so if there's little stored energy, then the generated energy can be ten times greater than the stored energy. On the other hand, if there's a lot of stored energy, then the energy generated would be much less than the energy stored. Anyway, is the ankle even good at storing energy? Try jumping of a height of only two feet or so and landing on your feet WITHOUT bending any other joints. You probably can't do this. I think the reason for this is that your body knows that your feet alone simply cannot handle this impact and it brings your knees and hips into action to absorb the shock. So it seems that the natural ankle isn't very good at all at absorbing and storing energy. On the other hand, if you were jumping on very springy artificial feet, you could conceivably jump from the second floor, land without bending any joints, and be thrown back up to the second floor (slightly lower due to energy absorption). Of course, I was discussing jumping, and running is not the same thing as jumping, but thankfully, basic mechanics applies to running as well. So what's going on here? What in the world do people mean when they say that the biological ankle returns 250% of the stored energy?
 
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Answers and Replies

  • #2
Danger
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Welcome to PF. I'd like to see the whole story, with pictures if possible, so a link or address would be nice.
Perhaps the confusion comes from people thinking of the ankle as just a little joint at the end of the leg. Fact is, it's operated by that honking big calf muscle above it. While there's a bit of 'springiness' in the connective tissues, it certainly isn't a 'closed system' as far as the 2nd law is concerned.
 
  • #3
Welcome to PF. I'd like to see the whole story, with pictures if possible, so a link or address would be nice.
Perhaps the confusion comes from people thinking of the ankle as just a little joint at the end of the leg. Fact is, it's operated by that honking big calf muscle above it. While there's a bit of 'springiness' in the connective tissues, it certainly isn't a 'closed system' as far as the 2nd law is concerned.
I'm sure (or hope, anyway) people know that the ankle is operated by the calf muscle.


Here are some links, you can find more by googling "Oscar Pistorius":

http://sport.independent.co.uk/olympics/article2377707.ece [Broken]

http://www.wired.com/wired/archive/15.03/blade.html

http://www.spectrum.ieee.org/print/2189

http://www.yourrunning.com/story-amputee_runner_may_qualify_for_olympics_but_be_denied_chance_to_compete [Broken]


Another question to ask here is: if Oscar Pistorius qualifies for the Olympics, should he be allowed to compete?
 
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  • #4
Danger
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I'm sure (or hope, anyway) people know that the ankle is operated by the calf muscle.
It disgusts me to mention how frequently this doesn't apply. PF is a special community, as are probably yourself and your friends. As a general rule, people are stupid.
Thanks for the links; I look forward to reading the whole score. I cannot, however, offer an opinion upon his eligibility. Sports rules are way out of my scene.
 
  • #5
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well its same story in a different guise. People balked at the idea of engineered running tracks which provide rebound and in a semi resonant way for the right stride freq/amplituse. People object at doping.

And no muscles and tendons have no super elastic tendencies and are no better than springs at storing energy, in fact there is a lot of viscosity in the way. And so theres merit to the claimthat without dashpots he does have an advantage, in addition to the less mass he has to accelerate by way of carbon fiber bones.

Two competions: anything goes in one category vs natural man in the other.
 
  • #6
Danger
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Two competions: anything goes in one category vs natural man in the other.
I like that idea, but it brings up the disturbing concept of rocket suppositories...
 
  • #7
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I'm certain that prosthetics provide an advantage for any runner. That's why they don't let bicycle-enhanced runners compete against non-bicycle-enhanced runners. It is the same principle with this springy device, it stores and returns energy much more efficiently than a natural leg. I dismiss the 250% nonsense comparison of apples and oranges. The initial accelerating force comes from energy generated by the athlete during the first few seconds of the race, then whoever can maintain this top speed is likely to win the race. Springs and wheels are more efficient at this than human legs.
 
  • #8
I'm certain that prosthetics provide an advantage for any runner. That's why they don't let bicycle-enhanced runners compete against non-bicycle-enhanced runners. It is the same principle with this springy device, it stores and returns energy much more efficiently than a natural leg. I dismiss the 250% nonsense comparison of apples and oranges. The initial accelerating force comes from energy generated by the athlete during the first few seconds of the race, then whoever can maintain this top speed is likely to win the race. Springs and wheels are more efficient at this than human legs.
I don't disagree with you about the possibility that prosthetics do provide an advantage for maintaining speed (although certainly not for accelerating, to accelerate quickly with prosthetics, they would need a built in power source). However, you do have to consider that all other double below-knee amputees are nowhere near as fast as Oscar Pistorius. In fact, as far as I know, many double-below knee amputees have trouble walking, not to mention running. Why the heck are these people even issued disabled parking tags if their artificial legs provide a locomotive advantage?
 
  • #9
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However, you do have to consider that all other double below-knee amputees are nowhere near as fast as Oscar Pistorius. In fact, as far as I know, many double-below knee amputees have trouble walking, not to mention running.
No doubt, all amputees have various degrees of abilities just like all non-amputees. Maybe not everyone can balance on a cougar.


Why the heck are these people even issued disabled parking tags if their artificial legs provide a locomotive advantage?
Just how good is that gadget for going down a flight of stairs? Or walking on a slippery surface?
 
  • #10
Just how good is that gadget for going down a flight of stairs? Or walking on a slippery surface?
Um, and how many parking lots have STAIRS? Parking spots for the "disabled" are merely closer to the entrance than the other ones, but it's not like if you park somewhere else you have to go down an extra flight of stairs. And I think if you add spikes to that thing it would be better for walking on slippery surfaces than a natural leg in a regular shoe.

I think it's pretty funny how some people who might actually have relatively significant physical limitations compared to the general population try to prove that they're not "disabled", never use "disabled" parking spaces, etc. While other people who might be missing a part of one leg insist on using the "disabled" parking spaces and think they have a right to park there despite not really being "more disabled" than anyone else. But to each his own, I guess. Oscar Pistorius did have the following to say about this, "There’s nothing that cheeses me off worse than seeing somebody pull into a disabled spot, then get out of their car and walk off."
 
  • #11
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Hey, I know this is a VERY old thread, but I thought maybe an update is due. We were discussing this in the lab the other day, and today I just watched Daily Planet, where a physiologist/biomechanist (sorry, I'm completely blanking on the name-- maybe someone can help me out on that?) came to a "definitive conclusion" on these prosthetic devices. He concluded that they were advantageous when compared to biological limbs, mainly due to their reduced weight. He suggested that because of the lightweight material, they could be repositioned for the next stride in less time. Also, he states that the flexible prosthetic remains in contact with the ground for a significantly longer period of time than the stiff biological limbs, allowing the user more time to push off.

I can't say I agree 100% with this, but it does provide a decent argument. Thoughts?
 
  • #12
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Thats what I was taught, greater angular accceleration about the joint and better energy storage=faster than your average bear.
 

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