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  #1  
Old 01-04-2009, 05:14 PM
hamishritchie hamishritchie is offline
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Animal knees vs. human knees

Whose knees are backwards: mine or my cat's?
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  #2  
Old 01-04-2009, 05:22 PM
Colibri Colibri is offline
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Both your knees and a cat's knees bend in the same direction. What you think of as the cat's hind knees, which seem to bend backward relative to your knee, is actually the joint between the lower leg bone (tibia) and the ankle. The lower "leg" of the cat is actually the upper part of the foot, composed of elongated metatarsals. Compared to humans, cats walk on tip-toe.

Last edited by Colibri; 01-04-2009 at 05:22 PM..
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Old 01-04-2009, 05:25 PM
Colibri Colibri is offline
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Here's a diagram of a cat's skeleton.
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Old 01-04-2009, 05:26 PM
Alex_Dubinsky Alex_Dubinsky is offline
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I was thinking about this very thing the other day.

Why do almost all animals tiptoe, besides humans?
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Old 01-04-2009, 05:32 PM
Q.E.D. Q.E.D. is offline
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Originally Posted by Alex_Dubinsky View Post
Why do almost all animals tiptoe, besides humans?
Birds don't, generally, either. We have something in common with them.
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  #6  
Old 01-04-2009, 05:36 PM
hamishritchie hamishritchie is offline
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It seems to me that keeping your feet flat on the ground like humans is great for standing around doing nothing, but the tip-toe set-up is more suited for critters who may move around a lot, like my cat.

Last edited by hamishritchie; 01-04-2009 at 05:37 PM..
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Old 01-04-2009, 05:40 PM
Q.E.D. Q.E.D. is offline
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Originally Posted by hamishritchie View Post
It seems to me that keeping your feet flat on the ground like humans is great for standing around doing nothing....
I can assure you our early human and proto-human ancestors spent precious little time "standing around doing nothing," as that was likely to have gotten them on a predator's menu.
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Old 01-04-2009, 05:44 PM
Alex_Dubinsky Alex_Dubinsky is offline
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Plus, lots of herbivores stand around doing nothing, and they also walk on their toes (even when those toes are hooves).

It just seems having that extra joint there is useful for a lot of animals to be quicker and nimbler. And we just go around stomping on things.

Last edited by Alex_Dubinsky; 01-04-2009 at 05:48 PM..
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  #9  
Old 01-04-2009, 05:44 PM
Colibri Colibri is offline
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Originally Posted by Alex_Dubinsky View Post
I was thinking about this very thing the other day.

Why do almost all animals tiptoe, besides humans?
It's not true of all animals, but rather mainly those adapted for running. Long legs are more efficienct for running, and elongating the foot bones is a good way to achieve longer legs. The animals we are most familiar with, ungulates and carnivores, are generally adapted for running. Dogs and cats walk on the "balls" of their feet; cattle and horses actually walk on the tips of their toes. Bears are an exception; they walk flat-footed like us.

Humans are adapted to some extent for running, but in our case we had to make do with a basic primate foot, and achieved a longer leg by elongating the leg bones alone.

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Originally Posted by Q.E.D. View Post
Birds don't, generally, either. We have something in common with them.
Birds also walk on "tip-toe" as it were; they walk on their metatarsals (actually fused with part of the ankle bones). That's why a bird's "knee" also appears to be backwards relative to ours; the true knee is close to the body and concealed by feathers.

Last edited by Colibri; 01-04-2009 at 05:45 PM..
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  #10  
Old 01-04-2009, 05:53 PM
Q.E.D. Q.E.D. is offline
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Originally Posted by Colibri View Post
Birds also walk on "tip-toe" as it were; they walk on their metatarsals (actually fused with part of the ankle bones). That's why a bird's "knee" also appears to be backwards relative to ours; the true knee is close to the body and concealed by feathers.
Neat. I always though bird feet were true feet, if there is such a thing.
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  #11  
Old 01-04-2009, 06:07 PM
Colibri Colibri is offline
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Originally Posted by Q.E.D. View Post
Neat. I always though bird feet were true feet, if there is such a thing.
Technically, in vertebrate anatomy the manus (front foot) and pes (hind foot) include the metacarpals (front foot) and metatarsals (hind foot) plus the phalanges (finger/toe bones). In humans the metacarpals make up the palm of the hand, and the metatarsals the sole of the foot. The manus and pes do not include the carpals (wrist) and tarsals (ankle); however in humans and many other animals they are effectively part of the hand and foot. Our heel, for example, is composed of one of the ankle bones.

Bird feet are composed of just the phalanges, attached to the bottom of a bone called the tarsometatarsus, which represents a fusion of the metatarsals with some of the tarsal elements.
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  #12  
Old 01-04-2009, 06:21 PM
Spoke Spoke is offline
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Originally Posted by hamishritchie View Post
Whose knees are backwards: mine or my cat's?
Ask your elephant.
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Old 01-04-2009, 06:37 PM
Satellite^Guy Satellite^Guy is offline
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Quote:
Originally Posted by Alex_Dubinsky View Post
I was thinking about this very thing the other day.

Why do almost all animals tiptoe, besides humans?
My WAG at this is that since most animals walk on all fours, this position is conducive to walking on one's toes (try walking on hands and knees, with your feet flat on the ground. now, try again on your tiptoes. the latter, of course, being much easier)
Bears and elephants, owing to their mass, adapted differently, to better support their weight.

Again, just a WAG.

S^G
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  #14  
Old 01-04-2009, 06:43 PM
Sage Rat Sage Rat is online now
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The joints are actually roughly the same. Dogs and cat are essentially running on their fingers/toes. The first joint above their paw is equivalent to our wrist/heel.
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Old 01-04-2009, 09:18 PM
LawMonkey LawMonkey is offline
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All of the primates are plantigrade though, correct? One of our own little family peculiarities, so to speak.
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  #16  
Old 01-04-2009, 09:23 PM
tim314 tim314 is offline
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This thread actually makes me wonder the opposite question as the OP: Why do all animals legs bend the same way? That is, why does everything have the vertex of the knee on the front of the leg, and the vertex of the ankle on the back. It seems like this could just as well be reversed. Is it just too hard for an animal with reversed legs to run? Or is it just that everything evolved from a common ancestor with legs going one way (cartoon-style legged fish?), and no one ever had the right mutation to flip them around?
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  #17  
Old 01-04-2009, 09:36 PM
GuanoLad GuanoLad is offline
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Originally Posted by tim314 View Post
Is it just too hard for an animal with reversed legs to run?
The direction of the knees determines the direction of the walking. If the knees were swapped to point the other way, then you'd walk the other way, and therefore the head would need to face the other way, in which case it ends up all the same as the current arrangement.
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  #18  
Old 01-04-2009, 09:43 PM
tim314 tim314 is offline
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Originally Posted by GuanoLad View Post
The direction of the knees determines the direction of the walking. If the knees were swapped to point the other way, then you'd walk the other way, and therefore the head would need to face the other way, in which case it ends up all the same as the current arrangement.
But I can walk backwards with my current legs, so presumably I could also walk forwards with reversed legs. Admittedly, walking backwards is more difficult, but I think this is mostly because I can't see where I'm going.

Perhaps walking backwards (or forwards with backwards legs) isn't as energy efficient?
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  #19  
Old 01-04-2009, 10:46 PM
Tom Tildrum Tom Tildrum is offline
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Originally Posted by Q.E.D. View Post
I can assure you our early human and proto-human ancestors spent precious little time "standing around doing nothing," as that was likely to have gotten them on a predator's menu.
I am descended in part from the sloth.
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  #20  
Old 01-04-2009, 11:13 PM
Colibri Colibri is offline
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Quote:
Originally Posted by tim314 View Post
Or is it just that everything evolved from a common ancestor with legs going one way (cartoon-style legged fish?), and no one ever had the right mutation to flip them around?
This is essentially the answer. The first vertebrates on land had joints that flexed a certain way, and their descendants have essentially followed suit. This goes for flying and swimming forms too.

One exception is bats, which have the hind legs rotated at the hip joint so that they can hang by their feet. Note how in this photo of a vampire bat the hind legs and feet are facing backward from the normal mammalian position.
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  #21  
Old 01-04-2009, 11:17 PM
Alex_Dubinsky Alex_Dubinsky is offline
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Quote:
Originally Posted by GuanoLad View Post
The direction of the knees determines the direction of the walking. If the knees were swapped to point the other way, then you'd walk the other way, and therefore the head would need to face the other way, in which case it ends up all the same as the current arrangement.
Birds' "knees" point the other way. Take a look at deer, their hind legs also have "knees" going the other way. I guess whenever an animal needed to have its knees point the other way, it'd use its angle. There's definitely a benefit even if you walk forward.

But it seems there is something fundamentally hard, genetically, to re-engineer the walking fish's innovations. Vertebrates share a lot of features among their bones.

Last edited by Alex_Dubinsky; 01-04-2009 at 11:22 PM..
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  #22  
Old 01-04-2009, 11:20 PM
Alex_Dubinsky Alex_Dubinsky is offline
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Then again, if you're a human and you try to walk on all-fours, your true knees (meaning elbows) will turn the other way! Are humans the only vertebrates whose knees ever [came close to] rotating since the aforementioned stinking fish?

Last edited by Alex_Dubinsky; 01-04-2009 at 11:21 PM..
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  #23  
Old 01-04-2009, 11:46 PM
Colibri Colibri is offline
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Originally Posted by Colibri View Post
One exception is bats, which have the hind legs rotated at the hip joint so that they can hang by their feet. Note how in this photo of a vampire bat the hind legs and feet are facing backward from the normal mammalian position.
I should clarify this a bit. A bat's knees and ankles flex the same way relative to the leg as a normal mammal's. It's the hip joint that's changed, so that the leg as a whole is reversed.
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  #24  
Old 01-04-2009, 11:48 PM
Colibri Colibri is offline
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Originally Posted by Alex_Dubinsky View Post
Birds' "knees" point the other way. Take a look at deer, their hind legs also have "knees" going the other way. I guess whenever an animal needed to have its knees point the other way, it'd use its angle. There's definitely a benefit even if you walk forward.
The direction a joint flexes is rather irrelevant to the direction of walking. After all, the elbow and the knee flex in opposite directions, yet the animal moves forward. And as has been mentioned, the joints you mention are not knees at all, but ankles (more or less).
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Old 01-04-2009, 11:53 PM
waterj2 waterj2 is offline
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Originally Posted by Alex_Dubinsky View Post
Then again, if you're a human and you try to walk on all-fours, your true knees (meaning elbows) will turn the other way! Are humans the only vertebrates whose knees ever [came close to] rotating since the aforementioned stinking fish?
Look at the diagram of the cat skeleton Colibri gave in his second post. The joint between the humerus and the forearm bones (radius and ulna) points the same way my elbows do if I try to walk on all fours.
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  #26  
Old 01-04-2009, 11:53 PM
Colibri Colibri is offline
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Originally Posted by LawMonkey View Post
All of the primates are plantigrade though, correct? One of our own little family peculiarities, so to speak.
Not all. The tarsier has an extremely elongated hind foot, and climbs using mainly the toes alone.
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Old 01-05-2009, 02:46 AM
GuanoLad GuanoLad is offline
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Quote:
Originally Posted by Colibri View Post
The direction a joint flexes is rather irrelevant to the direction of walking. After all, the elbow and the knee flex in opposite directions, yet the animal moves forward.
But the joints are consistent to both forelegs and back legs, it's only the proportions that are different. Aren't they? The elbow is technically opposite to the knee, but its equivalent in a quadruped is the ankle, which bends the same way.

Er... Maybe I'm proving your point by arguing the opposite.
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Old 01-05-2009, 03:05 AM
tim314 tim314 is offline
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Originally Posted by GuanoLad View Post
But the joints are consistent to both forelegs and back legs, it's only the proportions that are different. Aren't they? The elbow is technically opposite to the knee, but its equivalent in a quadruped is the ankle, which bends the same way.
Surely the equivalent of the ankle is the wrist, no?
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Old 01-05-2009, 04:39 AM
GuanoLad GuanoLad is offline
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Colibri said the joints of the legs can be any direction, and his example was that the foreleg joint, the elbow, is in reverse to the back leg knee.

But effectively the actual joint directional equivalent on each leg, is the foreleg's elbow to the back leg's ankle. The forward motion is achieved by having the two sets of limbs effectively jointed in the same '<' formation.

Yes, they aren't the equivalent in parallel joint comparison, but they are equivalent in relative placement from the ground up.
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Old 01-05-2009, 06:10 AM
hamishritchie hamishritchie is offline
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Originally Posted by tim314 View Post
This thread actually makes me wonder the opposite question as the OP: Why do all animals legs bend the same way?
Because of intelligent design, duh.

The Aristocrats!
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  #31  
Old 01-05-2009, 08:13 AM
Colibri Colibri is offline
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Originally Posted by GuanoLad View Post
But effectively the actual joint directional equivalent on each leg, is the foreleg's elbow to the back leg's ankle. The forward motion is achieved by having the two sets of limbs effectively jointed in the same '<' formation.
Not at all. This depends entirely on the leg proportions of the animal. Some have elbows and knees at the same level, others don't. Differences in the direction of flexion of a limb at a particular point above the ground are achieved by lengthening or shortening the limb bones, not by changing the direction of flexion of the joint itself.
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Old 01-05-2009, 08:24 AM
Zsofia Zsofia is offline
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Originally Posted by tim314 View Post
But I can walk backwards with my current legs, so presumably I could also walk forwards with reversed legs. Admittedly, walking backwards is more difficult, but I think this is mostly because I can't see where I'm going.

Perhaps walking backwards (or forwards with backwards legs) isn't as energy efficient?
People run backwards marathons. Did you know that? Because I think those people are screwed up.
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  #33  
Old 01-05-2009, 08:39 AM
GuanoLad GuanoLad is offline
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Originally Posted by Colibri View Post
Some have elbows and knees at the same level, others don't.
It's not so much the same level, though that does play into it, as that they do the same effective work; which is, by bending in the same direction they achieve the most efficient stride. For that reason I consider those joints as being the equivalent of each other.

But I accept that you know more about animal physiology than me, so I will bow to your superior knowledge.
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  #34  
Old 01-05-2009, 09:06 AM
Colibri Colibri is offline
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Originally Posted by GuanoLad View Post
It's not so much the same level, though that does play into it, as that they do the same effective work; which is, by bending in the same direction they achieve the most efficient stride. For that reason I consider those joints as being the equivalent of each other.

But I accept that you know more about animal physiology than me, so I will bow to your superior knowledge.
I think to understand the mechanics you need to actually look at a photo of the animals in question. In this pronghorn, the elbows and the knees are at the same level (the joints closest to the body), as are the wrists and ankles. Note that the joint in the middle of the fore leg is the wrist, and the joint in the middle of the hind leg is the ankle. In both cases the joints at the same level bend in opposite directions. The most efficient stride is achieved by having the corresponding leg segments approximately the same length; it has little to do with which direction the joints bend.

The pronghorn's legs display adaptation for fast running. Most of the heavy muscle mass for moving the legs is up near the body, in the thighs and shoulders. The lower leg segments are thin and elongated in order to lighten them so they can be moved rapidly with a minimum of effort.

Last edited by Colibri; 01-05-2009 at 09:07 AM..
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  #35  
Old 01-05-2009, 12:22 PM
Quercus Quercus is offline
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Anyway, on a 'selective advantage' level of 'why don't humans tiptoe when cats do?', I presume the reason is that cats don't need to spend any time balancing on only one foot, therefore can get away with a small point of contact (since there's always two or three other feet on the ground).
Humans, by contrast, need both toes and heels on the ground in order to stay balanced, since often there's no other limb on the ground.
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Old 01-05-2009, 01:22 PM
Elendil's Heir Elendil's Heir is offline
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Originally Posted by GuanoLad View Post
The direction of the knees determines the direction of the walking. If the knees were swapped to point the other way, then you'd walk the other way, and therefore the head would need to face the other way, in which case it ends up all the same as the current arrangement.
Somewhere, at sometime, there must have been a mad scientist who tried this.
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Old 01-05-2009, 01:36 PM
Polycarp Polycarp is offline
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Just for the record, the appropriate technical terms are:

Plantigrade: Walks on "full foot" (in human sense) with both heel and toe touching ground. Forefoot is equivalent to full hand. (May adopt a different gait when running.) Examples include humans and many other primates, ursids, proboscideans.

Digitigrade: Animal's "foot" is ball of foot and toes, fingers and fleshy pad wher they join palm. Ankle and wrist constitute distalmost joints of legs.

Unguligrade: Animal runs on (a) a hoof at the end of the third finger and toe, all others being lost or reduced to splints (equids), or (b) a double (cloven) hoof at the end of two conjoined fingers or toes (most artiodactyls), same caveat.
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