question: If almost every large animal with mobility has forward bending knees, why do so many advanced Boston dynamics type robots have rear facing knees?
comment: The answer is: [because it's more efficient]( URL_0 )! In the simplest sense: figures 21 and 22 in the linked study show that if you eliminate hip movement, the backward bending leg can still make progression towards the following step. The forward bending leg can't. So the forward bending leg will always require more hip movement than the backward bending leg. The data in the experiments indeed show that the hip movement is much less important in backward bending legs than forward bending legs. Also, there is a slight advantage in shock damping. EDIT: Sorry, forgot I was on the university network at the time of writing, so you probably won't be able to see the full article (the main idea is explained in the abstract). Will try to provide some more information tomorrow. EDIT2: Fixed link (thanks u/quote_engine) : Interpretation of the results starting p10 is where it's most interesting.
passages: Human knee joints are enlarged for the same reason as the hip – to better support an increased amount of body weight. The degree of knee extension (the angle between the thigh and shank in a walking cycle) has decreased. The changing pattern of the knee joint angle of humans shows a small extension peak, called the “double knee action,” in the midstance phase. Double knee action decreases energy lost by vertical movement of the center of gravity. Humans walk with their knees kept straight and the thighs bent inward so that the knees are almost directly under the body, rather than out to the side, as is the case in ancestral hominids. This type of gait also aids balance.
   Gorillas use the form of walking on all fours with the fingers on the hands of the front two limbs folded inward. A gorilla’s forearm and wrist bones lock together to be able to sustain the weight of the animal and create a strong supporting structure. Gorillas use this form of walking because their hips are attached differently than humans so standing on two legs for a long period of time would eventually become painful. Humans would have to walk on all fours as well if they had the same type of hip placement. Gorillas sometimes do walk upright in instances where dangers are present. Walking upright is a way to improve the ability to watch for danger, those who could do so for longer without any pains have a better survival advantage and evolved into bipeds.
   Having long hindlimbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches. Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired. This is because their femurs are not adapted for bipedalism. Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee, thus making human knees closer together and under the body’s center of gravity. This adaptation lets humans lock their knees and stand up straight for long periods of time without much effort from muscles. The gluteus maximum became a major role in walking and is one of the largest muscles in humans. This muscle is much smaller in chimps, which shows that it has an important role in bipedalism. When humans run, our upright posture tends to flex forward as each foot strikes the ground creating momentum forward. The gluteus muscle helps to prevent the upper trunk of the body from "pitching forward" or falling over.
   The ancestors of gorillas and chimpanzees are suggested to have become more specialised in climbing vertical tree trunks, using a bent hip and bent knee posture that matches the knuckle-walking posture they use for ground travel. This was due to climate changes around 11 to 12 million years ago that affected forests in East and Central Africa, so periods occurred when openings prevented travel through the tree canopy, and at these times, ancestral hominids could have adapted the erect walking behaviour for ground travel. Humans are closely related to these apes, and share features including wrist bones apparently strengthened for knuckle-walking.
   Heinrich Mallison (in 2009) was the first to study the physical potential for various sauropods to rear into a tripodal stance. Mallison found that some characters previously linked to rearing adaptations were actually unrelated (such as the wide-set hip bones of titanosaurs) or would have hindered rearing. For example, titanosaurs had an unusually flexible backbone, which would have decreased stability in a tripodal posture and would have put more strain on the muscles. Likewise, it is unlikely that brachiosaurids could rear up onto the hind legs, as their center of gravity was much farther forward than other sauropods, which would cause such a stance to be unstable.
   Non-avian theropods were first recognized as bipedal during the 19th century, before their relationship to birds was widely accepted. During this period, theropods such as carnosaurs and tyrannosaurids were thought to have walked with vertical femurs and spines in an upright, nearly erect posture, using their long, muscular tails as additional support in a kangaroo-like tripodal stance. Beginning in the 1970s, biomechanical studies of extinct giant theropods cast doubt on this interpretation. Studies of limb bone articulation and the relative absence of trackway evidence for tail dragging suggested that, when walking, the giant, long-tailed theropods would have adopted a more horizontal posture with the tail held parallel to the ground. However, the orientation of the legs in these species while walking remains controversial. Some studies support a traditional vertically oriented femur, at least in the largest long-tailed theropods, while others suggest that the knee was normally strongly flexed in all theropods while walking, even giants like the tyrannosaurids. It is likely that a wide range of body postures, stances, and gaits existed in the many extinct theropod groups.
   Section::::"Procoptodon goliah".:Physiology.:Mobility.
  
  "Procoptodon goliah" were unable to hop as a mode of transportation, and would have been unable to accelerate sufficiently due to their mass. Broad hips and ankle joints adapted to resist torsion or twisting, point to an upright posture where weight is supported by one leg at a time. Their broad hips also allowed for another important modification: large buttocks - a feature shared with other walking species.
   The vertebral column of humans takes a forward bend in the lumbar (lower) region and a backward bend in the thoracic (upper) region. Without the lumbar curve, the vertebral column would always lean forward, a position that requires much more muscular effort for bipedal animals. With a forward bend, humans use less muscular effort to stand and walk upright. Together the lumbar and thoracic curves bring the body's center of gravity directly over the feet. Also, the degree of body erection (the angle of body incline to a vertical line in a walking cycle) is significantly smaller to conserve energy.
   Nevertheless, in the recent study, researchers Martin Hora and Vladimir Sladek of Charles University in Prague show that Neanderthal lower limb configuration, particularly the combination of robust knees, long heels and short lower limbs, increased the effective mechanical advantage of Neanderthal knee and ankle extensors, thus reducing the force needed and the energy spent for locomotion significantly. The walking cost of the Neanderthal male is now estimated to be 8–12% higher than that of anatomically modern males, whereas the walking cost of the Neanderthal female is considered to be virtually equal to that of anatomically modern females.
   Section::::Significance.
  
  Even with much modification, some features of the human skeleton remain poorly adapted to bipedalism, leading to negative implications prevalent in humans today. The lower back and knee joints are plagued by osteological malfunction, lower back pain being a leading cause of lost working days, because the joints support more weight. Arthritis has been a problem since hominids became bipedal: scientists have discovered its traces in the vertebrae of prehistoric hunter-gatherers. Physical constraints have made it difficult to modify the joints for further stability while maintaining efficiency of locomotion.
  
  Section::::See also.
  
  BULLET::::- Happisburgh footprints
  
  BULLET::::- Ileret footprints
   Upright bipedal walking is commonly thought to have evolved from knuckle-walking with bent legs, in the manner used by chimpanzees and gorillas to move around on the ground, but fossils such as "Orrorin tugenensis "indicate bipedalism around 5 to 8 million years ago, in the same general period when genetic studies suggest the lineage of chimpanzees and humans diverged. Modern apes and their fossil ancestors show skeletal adaptations to an upright posture used in tree-climbing, and upright, straight-legged walking has been proposed to have originally evolved as an adaptation to tree-dwelling.
   Like the early tetrapods, early archosaurs had a sprawling gait because their hip sockets faced sideways, and the knobs at the tops of their femurs were in line with the femur.
  
  In the early to middle Triassic, some archosaur groups developed hip joints that allowed (or required) a more erect gait. This gave them greater stamina, because it avoided Carrier's constraint, i.e. they could run and breathe easily at the same time. There were two main types of joint which allowed erect legs:
   Climbing is an especially difficult task because mistakes made by the climber may cause the climber to lose its grip and fall. Most robots have been built around a single functionality observed in their biological counterparts. Geckobots typically use van der waals forces that work only on smooth surfaces. Stickybots, and use directional dry adhesives that works best on smooth surfaces. Spinybot and the RiSE robot are among the insect-like robots that use spines instead. Legged climbing robots have several limitations. They cannot handle large obstacles since they are not flexible and they require a wide space for moving. They usually cannot climb both smooth and rough surfaces or handle vertical to horizontal transitions as well.
   The first definitive evidence of habitual orthograde posture in human evolutionary lineage begins with Ardipithecus ramidus kadabba, dating between 5.2 to 5.8 million years ago. The skeletal remains of this hominid exhibit a mosaic of morphological characteristics that would have been both adapted to an arboreal environment and walking upright terrestrially. The earliest evidence of a hominid exhibiting skeletal morphology capable of achieving orthograde posture dates to 9.5 million years ago, with the discovery of a Miocene ape, Dryopithecus in Can Llobateres, Spain.
  
  Section::::Morphological characteristics.
   The feet of "H. floresiensis" were unusually flat and unusually long in relation with the rest of the body. As a result, when walking, it would have had to bend its knees further back than modern people do. This forced a high-stepping gait and low walking speed. The toes had an unusual shape and the big toe was very short.
  
  Section::::Scandal over specimen damage.
   BULLET::::- A study on the variation in the structure of trabecular bone and joint loading in the humeral head of extant hominoids, spider monkeys and "Australopithecus africanus" is published by Kivell "et al." (2018), who interpret their findings as indicating that "A. africanus" may have still used its forelimbs for arboreal locomotion.
  
  BULLET::::- Description of a nearly complete, 3.32-million-year-old foot of a juvenile "Australopithecus afarensis" from Dikika (Ethiopia) is published by DeSilva "et al." (2018).
   "The Front Legs- The Knee"
  
  Medial Carpal Deviation/ Carpus Valgus/ Knock-Kneed  
  
  BULLET::::- One or both knees deviate inward toward each other, with the lower leg angles out, resulting in a toed-out stance. Occurs because of an unequal development of the growth plate of distal radius, with the outside growth plate growing faster than inside. The bottom of the forearm seems to incline inward.
  
  BULLET::::- Any horse can inherit this, but it may also be acquired from imbalanced nutrition leading to developmental orthopedic disease (DOD) or a traumatic injury to growth plate.
   It is not yet agreed to what extent the early human ancestor "Australopithecus afarensis", (3.75 million years ago onwards) had acquired a functionally human-like foot, but the medial twist of the forefoot evident in fossil footbones of this species, and in the Laetoli footprint trail in Tanzania generally attributed to this species, certainly appears less marked than is evident in fossil footbones of "Homo erectus" (sometimes called "Homo georgicus") from Dmanisi, Georgia (c. 1. 8 million years ago) and the roughly contemporaneous fossil footprint trail at Ileret, Kenya attributed to "Homo erectus ergaster".
  
  Section::::See also.
  
  BULLET::::- Pes cavus
   Section::::Curved.
  
  The curved knee (also sometimes called a side knee or roundhouse knee) is similar to the front knee except that it does not use a forward thrusting motion, but is instead rotated from the outside. Whereas the front knee needs some space in between the combatants to be performed, the curved knee can be executed from a minimal distance, such as in a close clinch. This technique is often employed when an opponent is attempting to protect from a straight knee. Typical targets include floating ribs, hips, and the side of the abdomen.
  
  Section::::Flying.
   No machine yet exists that can maneuver in the “scansorial” regime – that is, perform nimbly in general vertical terrain environments without loss of competence in level ground operation. Two major research challenges face the development scansorial robotics: First, we seek to understand, characterize and implement the dynamics of climbing (wall reaction forces, limb trajectories, surface interactions, etc.); and second, we must design, fabricate and deploy adhesive patch technologies that yield appropriate adhesion and friction properties to facilitate necessary surface interactions.
   In humans, the term "knee" refers to the joints between the femur, tibia, and patella, in the leg.
  
  In quadrupeds such as dogs, horses, and mice, the homologous joints between the femur, tibia, and patella, in the hind leg, are known as the stifle joint. Also in quadrupeds, particularly horses, ungulates, and elephants, the layman's term "knee" also commonly refers to the forward-facing joint in the foreleg, the carpus, which is homologous to the human wrist.
   As progress continues in legged robotics, research has begun to focus on developing robust climbers. Various robots have been developed that climb flat vertical surfaces using suction, magnets, and arrays of small spines, to attach their feet to the surface.
  
  Section::::Applications.:Robotics.:RiSE platform.
   BULLET::::- A study on the intervertebral joints in the necks and tails of sauropod dinosaurs, characterized by having the convex articular face directed away from the body and the concave articular face directed toward the body, is published by Fronimos, Wilson & Baumiller (2016), who argue that these joints evolved to prevent possible joint failure caused by rotation, providing stability with greater mobility and facilitating the evolution of elongated necks and tails in sauropods.
   To avoid this issue knees requiring sharper curves are made using methods which ensure that the wood grain and direction of load are closely aligned. This can be achieved by steam bending, laminating, or selecting a natural crook with matching grain - a "grown knee". Grown knees are generally considered as the "best" method among boat builders and have a strong traditions associated with their use, but they may not achieve the same strength as a good laminated knee.
  
  Section::::Types.
   Most raised surfaces at the appropriate height can be used as seats for humans, whether they are made for the purpose, such as chairs, stools and benches, or not. While the buttocks are nearly always rested on the raised surface, there are many differences in how one can hold one's legs and back.
  
  There are two major styles of sitting on a raised surface. The first has one or two of the legs in front of the sitting person; in the second, sitting astride something, the legs incline outwards on either side of the body.
  
raw_presuppositions: Almost every large animal with mobility is built for maximal efficiency already with forward bending knees.
raw_corrections: The mechanics of large animals with mobility can be made even more efficient with rear facing knees.