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4-Legged Creatures page.
Basic Anatomy of Mammals
|> Basics
|> The Horse
|> Cats and Dogs
|> Summary

<| Basics
Basic Anatomy. Shown here is the skeletal outline of a generalized mammalian vertebrate. This architecture is probably not too dissimilar from that of the very first mammals, like the 10-cm long Megazostrodon [1] [2] [3], that appeared 200 million years ago. Clearly, this animal is designed for crawling, digging, pulling vegetation, etc, and not for speed and power, as are the horses, dogs, and cats discussed below.

It is easy to relate the leg joints of the shrew to those of humans - shoulder, elbow, wrist, and hand of the front leg, and hip, knee, ankle, and foot of the back leg. The feet have the same distinquishing characteristics - wrist, palm, knuckles, fingers, and ankle, sole, toes.

Leg Design. Of interest to the roboticist is the general configuration of the legs. The front and back legs, not counting the feet, are essentially mirror images of each other. The shape of the legs - with both middle joints angled inwards - clearly centers the weight of the body over them, and provides front-back stability to the frame. The legs are oriented under the body, rotated so that elbow and knee on the same side "point" at each other. The animal's weight is overtop the legs.

The mirror-image arrangement of the legs would appear to have several other advantages to the general welfare of the mammal. First, the animal can easily fold and tuck them underneath the body, to conserve heat in the cold and to prevent other animals from grabbing it by the legs while sleeping. Secondly, raising and lowering the body amounts to scissoring the legs up and down, and this anatomy keeps the COG (center of gravity) of the body centered over the legs during this movement.

<| The Horse
Bones: ilium [2-4], femur [4-5], tibia+fibula [5-6], metatarsal [8], pastern [9-10, 15-16], scapula [2-12], humerus [12-13], forearm (radius+ulna) [13-14], metacarpus (cannon) [17], pedal [3rd phalanx, 16]. Points + joints: knee [5], hock [7], "point-of-the-hock" [bone extension above 7], fetlock [9, 15], hoof [10, 16], shoulder joint [12], elbow [13], knee (carpus) [14].
In the horse, the leg-foot configuration is somewhat different from the shrew. First, the joints corresponding to the shrew (and human) knee [5] and elbow [13] are "relatively" shorter and closer to the main trunk. Secondly, the lower legs [6-10 and 14-16] are much elongated, and look similar to that as if the shrew were walking on its tippy-toes all of the time.

An Extra Joint? Overall, this at first makes it appear as if the horse has an extra joint in its legs [at 6 and 14], but in fact, if a count is begun, starting at the shoulder, it is seen that both shrew and horse have the same number. The geometries are very different, however. Point [7] corresponds to the rear "heel" of the shrew, and point [14] to the "wrist", ie, heel of the front leg.

Compare back legs to a ballerina standing on tippy-toe - by counting joints starting at the hip. The lower legs of the horse are characterized by long and powerful tendons [dark areas in the above figure], which correspond to the tendons in the hands and feet of humans. The knee of the horse's front leg [14] actually corresponds to the wrist of the human. To the human concept of aesthetics, the ballerina would look very strange indeed if her hands and feet were as long as her lower leg and arm sections - ie, having "horsy" proportions.

Different Foot Types. From a comparative viewpoint, the design of a vertebrate's foot is commensurate with its lifestyle. The hind limb bones of a deer [left], dog [middle], and badger [right] are shown in the drawing on the right. These designs are called unguligrade ["hoof-walking"], digitigrade ["toe-walking"], and plantigrade ["sole-walking"], respectively. The badger is mainly a walker and seldom runs, while the deer is a highly-adapted runner. The dog is in-between. Humans are also plantigrade walkers, wherein the heel hits the ground first.

The diagram clearly shows that the 2 upper leg segments are least modified, while the foot and toe area is most modified. Note the relative position of the "heel" in each case. The highly-elongated "foot" of the deer greatly increases its stride and speed. Although the proportions vary, on an overall basis regarding number of joints and segments, however, the leg designs are all similar.

Stability. Back to the horse, what is of prime interest here is that, like the shrew, the upper leg segments [4-5, 12-13] of the horse both angle inwards, centering the weight of the torso and giving the frame front-back stability. One might surmise that a similar arrangement would prove useful in the design of a robot quadraped, and in fact, this biological lesson was not lost on the designers of the Sony Aibo robotic dog.

Power and Speed. The figure at the right shows how a horse's legs bend at the "knees" [points 5, 14] while galloping. Notice how closely built the knee of the back leg is to the torso, compared to the ballerina's.

Clearly, the long lower leg sections, the leverage afforded by the powerful tendons, the low degree of weight in the legs, and the huge muscles located in the upper legs and torso all contribute to the speed and power of the horse. Because the upper leg sections are relatively short, the major muscle groups (and weight) are all located close in to the body, improving the angular acceleration of the legs for speed, while the elongation of the lower leg segments greatly increases the stride.

What is clear is that the mirror image geometry of the legs keeps them symmetrically-positioned with respect to the COG of the torso at all points in the stride - and this helps keep the horse on an even kilter as it runs. Compare running dog with running horse and panther - front-back legs movement are similarly symmetrical in all cases.

Standing Up. The mirror image geometry of the legs certainly helps keep the COG of the horse's torso centered during the stride, but the long lower leg segments represent a difficulty when the horse (and similar animals) try to rise from a kneeling or sitting position. That the animal's major muscle groups reside near the body, and the animal essentially has to "lever" itself up, certainly contributes to the problem. Animals with shortened forelimbs, like the dog and cat, have a much easier time of it. This is just a passing observation that may have some bearing on the design of robot legs.

<| Cats and Dogs

The anatomy of cats and dogs is somewhat inbetween the shrew and the horse. The lower section of their rear legs are elongated and quite vertical, like the horse, but their front legs are somewhat foreshortened, like the shrew. This is especially clear in the lion skeleton shown at the right, and the German Shepherd shown on the other pages. The top of the rear legs of dogs and cats attach up near the tail, while its front legs attach lower down, near the bottom of the neck.

Shrews are probably closer to humans than dogs and cats when it comes to fine motor use of the front claws, and dogs and cats are closer to the horse when it comes to running fast and strong.

Even the Great Dane, shown at right, has a foreshortened front leg compared to the horse. However, its overall anatomy is more similar to horses than are most other dogs and cats, likely because it is more adapted for high-speed running than the others. Notice on the skeleton of the dire wolf how the well-developed scapula (shoulder blade) dominates the front-end of the animal frame. Also notice the rear-ward extensions of bone at the front elbow and rear hock, which serve as torque arms for muscle and tendon attachments.

Leg Attachments. Note that on horse, cat, and dog skeletons, the leg attachments are located high with respect to the main mass of the torso. This prevents the animals from being top heavy.

Joints. All of these animals have similar leg-joint anatomies. Looking at the figure at the right, the rear legs consist of 5 major segments - pelvis, femur [thigh bone], tibia-fibula [lower leg], tarsals-metatarsals and phalanges [toes]. The area between points [26] and [28] actually corresponds to the bottom of the foot in humans. The front leg has a similar number of segments, of note being the heavy scapula [shoulder blade], which improves the range of motion of the front leg.

The legs, front to back, have a mirror-image conformation as in the horse above, and the joints bend in a similar manner. High up are the front elbow [at 16-17], angled backwards, and rear knee [24], angled forwards. Of note is that the rear hock joint [27] bends backwards only, while the front pastern [19] bends mainly forwards like a knee, but also backwards to a small extent for compliance during foot impact in the stride - see galloping dog, lower front leg at positions 6 and 11.

The front pastern and lower segment correspond to the wrist and hand of humans; the similarity in bending is obvious. Similarly, the bending of the rear hock joint is similar to ankle bending in humans. The dog standing here is similar to a human standing with its weight bearing on the inside pads of the knuckles of its hands and the pads at the front of the soles of its feet.

Foot-Paw Dexterity. It is not for no reason the lion is named king of the jungle - possibly the ultimate (non-mechanized) predator still in existence since certain dinosaurs. Animals adapted for speed, like horses and some dogs, have relatively longer lower limb segments, and major leg muscles located close to the torso. This allows longer strides and higher speeds. However, their paws and feet also tend to be poorly designed for tasks requiring manual dexterity. The dog's paws are not as highly adapted as the ungulate's hooves, but still they are useful for little besides digging and locomotion. The dog cannot really grasp things with its feet.

On the other hand, the big cats have speed, power, and incredible acceleration. Compared to the dog and horse, the cat's legs are relatively shorter in the lower segments, and more heavily muscled towards the periphery. They are typically slower (at 30 MPH), much lighter (at 250 lbs), and have less stamina than the prey they hunt, like zebras (at over 40 MPH and 500 lbs), but their powerful leg muscles give them such rapid acceleration they can catch the prey before it can hit top speed. [survival of the fittest]

A major advantage cats have is that their paws are articulated enough that they can grasp and hold things. They also have more latitude in moving their front legs in and out sideways, and in rotating the "wrist" joints, compared to dogs and horses. This means they can groom themselves, grasp and swat their prey, wrap their paws around larger objects, and use their claws more effectively. The common house cat has an agility and climbing ability no home dog can match. Just watch a cat climb a tree by holding its claws out sideways, and pulling inwards - this is similar to how a lion grabs a wildebeest. All in all, cats have a body design which makes them an almost ideal land predator. If they also had wings, it would be all over for the rest of us.

<| Summary

  • Mammals have legs with 5 major segments and 5 major joints.
  • Certain animals, like horses and some dogs and cats, have an extended foot that gives them a lower leg segment similar in length to the other segments, and they appear to walk on their tippy-toes with their heels [ie, hocks] held high.
  • The legs have a mirror-image front-back geometry, with corresponding joints bending opposite to each other.
  • All legs, be they front or back, have a joint which bends like a knee [rear knee, front pastern, human wrist] and another which bends like an elbow [rear hock, rear ankle, front elbow]; the net effect is the legs more or less "scissor" up and down in multiple segments.
  • The challenge to the roboticist is to come up with a leg design that provides the flexibility of the 5-segment animal leg but is simpler in design than what nature created.

    <| References

  • "The Dog in Action", by McDowell Lyon, 1950, Howell Book House.
  • "Horse Gaits, Balance, and Movement", by S.E. Harris, 1993, Howell Book House.
  • "The Vertebrate Body", by A.S. Romer & T.S. Parsons, 1977, W.B.Saunders pub.
  • "The Raptor and the Lamb", by C. McGowan, 1997, H.Holt pub.

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    © Oricom Technologies, Sept 2001, updated Sept 2002