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4-Legged Creatures page.
A Dog's Elbow
Index:
|> Introduction
|> Elbow During Step Recovery Phase
|> More on Elbow Bending
|> Horse Advantage
|> References

[click for column]
What does a dog think about its elbows? In fact, they appear to be the most "expressive" part of its locomotor apparatus. They're used to stand up on, to rest upon, to point with, to beg with, to doggie-paddle, and for play. This page shows a few ways the dog bends and uses its legs in general, and elbows in particular.

In general, the dog seems to perform the major amount of front-leg bending at the elbow joint, compared to the other joints, during many movements. Note the "pointing" or reaching movement at the left - and others below.
[candy, man]

Humerus and elbow angles. We're especially interested in the bending angles of the various leg bones, for adaptation to walking robots. The picture at the right shows that in many large dogs, the elbow and humerus (upper leg bone) are angled slightly backwards when the animal is standing up straight. Compare to the dog skeleton on the anatomy page. Note that, in this position, the foreleg bones (radius and ulna) are vertical, while the shoulder blade (scapula) angles backwards above the shoulder joint about the same as the humerus below the shoulder joint, such that a vertical line can be drawn from the top of the scapula straight down to the feet. Despite a couple of bends, the front leg overall is built much like a vertical column.

<| Elbow During Step Recovery Phase
The leg-elbow arrangement is of interest, because we've observed that larger dogs (especially) rarely move the humerus much more forward than "vertical" (cf, leg mechanics page) when the leg is moved forwards in normal walking, but will rotate it quite far backwards and upwards at the end of the stroke phase of the step, when beginning the next forward movement. In other words, the humerus movement is not symmetrical around the vertical, and this is illustrated in the diagrams of Jessie the Shepherd doing the trotting gait [eg, positions 1 + 3]. Leg movements during the walk are similar, but the overall leg extensions are not as great as in the trot.
['pointing' at game]

[elbow movement back+up]
We haven't been able to find really good pictures which illustrate the full range of humerus movement during slower walks, but "field observations" revealed there is typically a distinct backwards-and-upwards movement of the humerus and elbow during slow walking, when the dog begins to take a new step. This is easier to observe on larger dogs, especially setters and retrievers. The 2 pictures here illustrate how far back and up a dog can pull its elbow. In both cases, the humerus is almost horizontal. Neither is a slow walk, but they do illustrate the range of movement. Likewise, the dog below left, walking through a narrow opening with a raised bottom panel (ie, a dog door), pulls its leg in close to clear the panel while positioning its other front leg centrally for balance.

[clever girl]
So why is this important? The reason is that it relates to the ground clearance of the dog's front legs during walking. The dog above right clearly has a large amount of clearance when its leg is fully pulled up and in. In addition, during a fast walk, the dog above left gains ground clearance by pulling its elbow up and bending the paw under with its front "knee" [pastern joint]. This is the critical point in the forward movement of the leg, where it will either impact or clear ground obstacles. Slightly later, the elbow will bend and ground clearance will quickly increase. The dog immediate left is going through a similar sequence of movements.

Deer Creep. On a similar note, in Colorado the Mule Deer are everywhere. They have rather long lower leg segments, and spend a lot of time grazing in tall grass, etc. Because of their long legs, they are actually a bit cumbersome at walking. When grazing, they move slowly with their heads down, and use a walking gait similar to the creep, where they move only 1 leg at a time, while maintaining a tripod stance with the other 3 legs. However, even with heads down they keep their bodies fully erect, and lift each leg high to clear the deep grass. Rather interesting to watch. The actually "lift" the lower leg segment up, forward, and over the grass, mainly by sharp bending at the "elbow" joint. They perform a similar movement with the rear leg, using deep bending at the "knee" - which is comparable height above the ground as the front elbow. (cf, points 5 and 13 on the horse).

Robots. This interest in elbow movement was inspired by the fact that our first upright-leg robot walkers had problems with ground clearance of the legs - especially during the recovery [forward movement] phases. We originally tried symmetrical movements of the humerus (and femur in back), and had consistent drag problems. And the problems were even worse with the octopod Gimlee-U8 than with Nico the quadruped - and worse yet when longer tibias were used. The dog has an "additional" bending joint in the pastern ["knee"] in front, to help increase ground clearance, as well as the hock joint in back. Our robots have only 2 joints per leg, and adding another one represents a significant increase in mechanical complexity.

We needed a way to increase ground clearance, and started looking at dogs doing slow walking movements, where there is minimal up-and-down bouncing of the body. The solution seems to be to add an additional draw-back movement of the elbow after the power stroke is finished, in order to pull the foot far enough above the ground that it doesn't scrape when the lower leg section is rotated forwards. In other words, we originally tried to emulate something akin to Jake's casual walk, but with only 2 joints in the robot's legs, we couldn't get a sufficient amount of ground clearance. In short, symmetrical femur movements don't work well with simpler legs.

<| More on Elbow Bending
[nice day in the neighborhood]
During leisure time, a dog rests on its lower leg segments, with its elbows heavily bent. During play time in the grass, a young dog naturally bends its elbows sharply to pull its legs in close to its body (below right).
[deep elbow-bends]

Deep Elbow Bends. A dog, investigating items at ground level, performs what amounts to deep knee-bends (right), except they are actually "deep elbow-bends". Here, the two front feet are placed at staggered positions, so the midline of the shoulders is balanced between the feet. From the position of the right foot, it can be ascertained that the right elbow is heavily bent and the right humerus is close to horizontal. Sometimes a dog will simultaneously bend both front legs in the manner the left leg is bent here.

[lift first, jump second]
Dogs Rising. Because of the multiple bends in its legs (cf, dog skeleton and leg bending), at top of shoulder, ball-and-socket shoulder joints, elbow joints, and pastern joints, as well as the relatively short lower leg segments below the pastern joint, getting up from the sitting position for the dog is easy, and involves mainly a net "vertical" push of all leg segments - similar to an accordion opening. Bending as regards a vertical column keeps the tops of the shoulders directly above the feet, and balance is easily maintained. In comparison, the horse, with longer and less flexible lower leg segments, has much more trouble rising - as anyone knows who has watched a horse trying to do this. Similarly, a robot built with "accordion" style leg bending around a symmetrical "vertical column" should rise up easily.

Wobbly Deer Rising. Observations on the local Mule deer show that they have a more difficult time than dogs when rising from a prone position. First off, when sitting, they are not nearly as comfortable looking as the dog, and tend to sit with one front and one rear leg tucked under, and with the other rear extended out sideways and the other front extended out forwards. Then, upon rising, they go through a somewhat wobbly sequence of raising their rear ends first using their front knees as pivot point, and then raising their front ends one leg at a time. The front leg is bent under at the knee, since a straight knee would apparently present too much stress considering the long length of the lower leg segments - (this may have some bearing on robot leg design). This situation is clearly related to the metacarpal and metatarsal (cannon) bones of the deer (cf, horse points 8, 17) being so much longer than those of the dog [points 20, 28]. Note the very short segment length between the shepherd's front "knee" (actually, pastern joint point 18, comparable to the human wrist) and its paw (picture, above left).

[stretch]
[float like a butterfly]

Dogs Jumping. During jumping, the major amount of bending is in the elbow joint to get the front legs to clear the hurdle. At the right, the upper leg (ie, humerus) is barely forward of vertical, while the lower leg segments are horizontal. The dog at left has its front legs pulled up even farther, due to the height of the jump. Most pictures of dogs jumping are similar to these.

<| Horse Advantage
[0.052 sec later in time] [earlier in time]
<<<---- movement right to left
In the ground clearance department, the horse has it somewhat better than the dog, and much better than our robots. The horse has a relatively short humerus, compared to the overall length of the lower leg segments - and the very lowest segments [points 9-10, 15-16] including the hoof, are relatively elongated compared to the homologous paws [points 21, 29] of dogs and cats.

The pictures to the right capture a trotting horse in 2 successive frames (movement right to left), and show how ground clearance is increased by sharp bending of the lowest segment. Clearance is about double what it would be were the segment not bent back. Because of the extra segment bending, the horse has very high ground clearance - even with the humerus (above the elbow) at approx 45 degree angle to vertical, as shown.

[long humerus + forearm, relatively short cannon]
Leg Tucks in Jumping Horses. Many pictures of horses running and jumping show similar extreme bending of the lower leg. Whereas most dogs, like those above, hold their paws forward when jumping over an obstacle, in contrast, most horse pictures show the hooves bent far backwards - the more so, the higher the jump.

In the horse, a long armbone / humerus (meaning at least 50-60% of the length of the scapulus) is considered to be advantageous regards both increased movement of the elbow forwards during striding, and also tucking of the limbs under during jumping [Loving97]. A short armbone leads to short choppy strides, less ability for lateral movements, less ability to tuck the legs under during jumping, and also more impact stress on the front limbs. In addition, Loving describes that a long and sloping shoulder (scapula), with about 45 degree angle, is advantageous over a more vertical shoulder, regards range of front-back movement, stride length, ground impact, and tucking of the legs. Notice the scapula angle on the horse skeleton. The sloping shoulder and long humerus work together, allowing the legs to tuck in further during flexion and extend farther forward during running.

In passing, we note that Loving indicates that both short and long cannon bones are common to horses, but that long forearms (radius+ulna) coupled with short cannon bones are most preferable in giving good mechanical advantage. A good ratio is seen on the horse skeleton diagram. Longer cannons reduce mechanical advantage, increase stress on the front legs, and result in more injuries. Interestingly, speeder Pronghorns, and also deer, have long and thin cannon bones. Last note - ideal pasterns are 50-75% the length of the cannon bones, and slope backwards at 30-40 degrees or so from vertical for optimal shock impact.

In summary, the strong rear-end muscles ultimately provide the power and range for jumping (and for galloping), but the conformation and range of movement of the front legs are critical to getting the body over the hurdles.

<| References

  • Eadweard Muybridge (1830-1904) - Animal Locomotion: Gallery, posters [1] [2]
  • [Loving97] Conformation and Performance by Nancy S. Loving, Breakthrough Publications (1997).
  • Go the Distance, the Complete Resource for Endurance Horses, by Nancy S. Loving, Trafalgar Square pub (1997).

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    © Oricom Technologies, April 2004