Points when drawing plantigrade legs

I want to add a few posts about drawing anthro legs again, as they are a tricky bit of anatomy to understand and draw. First off is a few notes on what to keep in mind when drawing human plantigrade legs. It's not a comprehensive guide, but I've tried to focus on some of the key messages that will hopefully help give you a few ideas on what to think about, whether you're drawing from reference or memory.

Tails

Something that's puzzled me for quite a while now is how does a tail work on a biped. Lets take for arguments sake that we are retaining this feature even in some vestigial form. The bones that form the caudal vertebrae of any tail are of course that tiny group of coccyx bones in a human. Far from being just a human vestigial tail, the coccyx is the site for attachment for many important pelvic floor muscles including the muscles of the genitals. How then, could we keep both the functionality of the pelvic floor needed in bipeds to help keep everything 'in' as well as functioning as a birth canal, and the aesthetics of an animal tail. Would caudal bones diverge from the sacrum, or could the sacrum be formed to function like that of the coccyx?

A few sketched ideas on how the pelvic floor could exist in a biped with a tail.

Anthro run cycle

... and now the wire frame for a possible run cycle. Here I've tried to oscillate both hips to reflect their full range of motion during this cycle. The timing isn't finely tuned yet, different moments of the cycle will create faster and slower points in each limbs movement which I haven't really maximised yet. Useful as a reminder that anthro movement is very energetic, likely the arms will have to be swung in arcs to maintain balance.

Ignore the black dots in the middle, they were placeholders for the start and finish of the cycle

Anthro walk cycle

A quick test of a wire frame I made of an anthro walk cycle. I think it's rather species neutral but I had a larger herbivore type in mind; not only is there rise and fall in the hips but due to the mass and stability issues of the legs, each step the leg is going to be thrown forward, creating a far amount of 'hip swagger'. This isn't efficient, as I've mentioned in previous posts, but a digitigrade or unguligrade on 2 legs isn't ever going to be.

Way is knowing the pelvic structure important?

It’s a good idea to note the configuration of the pelvis when creating any character, human or anthro. I'm a firm believer in sketching out the basic bone structure in any character sketch, it helps to create your anchor points and maintain your proportions. There are a number of landmarks of the hip that will dictate the overall look of your characters waist, hips and legs that are worth taking some time to study.

Landmarks from the front:
1. The iliac crest, the most obvious part of the pelvis you can feel under the skin. In an athletic character the crest forms the ridge to the bulging mass of the obliques, the muscles of your waist.
2. The inguinal ligament is formed by the obliques and demarcates the line where the torso ends and legs begin. It creates the distinctive 'V' or 'U' shape seen in the photo below.
3. Pubic tubercle this forward projection of the pubic bone can be felt under the public line and is the attachment of the inguinal ligament. The abdominal muscles also attach to the pubic bone. Below this landmark is the point at which the genitals sit, in the male for example via the suspensory ligament. (Also worth noting is that if your characters are in 'heroic' proportion, that's 8-heads high, this landmark marks the mid point of your character height).
4. The greater trochanter of the femur is the major attachment point of the gluteals. It is clearly visible in a frontal view as a bulge at the top of the leg and more predominant in the female. It adds a distinctive curve to the top of the leg when drawing characters from the front or back.

The distinctive 'V' or 'U' shape of the obliques.

Standing upright - Part III

In Part I I looked at changes in pelvic configuration from early hominids. It’s useful to note these changes as it helps explain why we are good at being bipedal and conversely, what wouldn't work or work very well. I’ve laid out what I’ve found here in terms of adaptations to the pelvis from quadruped to biped, these can act as our ‘ground rules’ for consideration when designing anthro characters or mechanical walkers.

Points to consider:

• Centre of gravity – bipeds must make sure their centre of gravity doesn't shift drastically from side to side with each step, this is destablising and inefficient.
• A tall pelvis has the effect of lengthening the torso, meaning that the centre of gravity moves higher, further away from the hips. This makes the trunk harder to stabilise.
• The ‘S’ curve of the lumbar vertebra lowers the centre of gravity towards the hips, helping stabilise the torso.
• This lumbar curve also gives the vertebral column the flex needed to withstand the pressure of the torso acting through it, the lower lumbar have widened giving a larger surface area for weight transmission.
• A wider sacrum has evolved to accommodate the wider lower lumbar vertebra. The sacroiliac joint (connecting the sacrum and the pelvis) also has a large surface area for weight transmission from the torso through both sides of the pelvis down to the femoral heads.
• The widened sacrum increases the width of the 'true pelvis', (the space through the centre of the pelvis), facilitating the ability to birth offspring with much larger craniums.
• However, a wider pelvis is a problem. During the support phase of locomotion one leg is off the ground. The weight of the torso is now acting on the femoral head of the standing leg. This is an example of a first class lever (levers that balance weight like a child's see-saw). The femoral head is acting as the pivot and the distance from the pivot to the body weight is called the 'load arm'. The ‘force arm’ on the other side of the pivot is our gluteus medius (an abductor muscle). It contracts to counterbalance the load arm. The wider we make our pelvis the longer we make the load arm, putting more pressure on the femoral head. We need a larger force or longer force arm to increase the mechanical advantage of this lever, otherwise we risk damaging the femoral head or having the hips slump with each step, just like the chimpanzee. In the Lucy skeleton, Australopithecus afarensis, it reveals her long load arm was countered by an increased length in the neck of the femur and a flaring of the iliac crest of the pelvis to place the abductors further from the pivot.
• The bicondylar angle is unique in humans. The femurs converge at the knees, bringing the legs close to the midline. This means the feet pass close to the midline and the centre of gravity is maintained directly underneath the torso. This is energy efficient as it doesn’t create a side to side motion of the hips when we walk.

Standing upright - Part II

It isn’t possible to directly compare a set of quadruped ‘buttocks’ to that of a human because for quadrupeds, like the horse in this example, their behinds are not really gluteals, they are hamstrings. I'll bang on about these muscles groups just once more:

Hamstrings: in quadrupeds serve as powerful hip extensors, driving the animal forwards against the ground reaction and pulling the leg up and backwards to take the next stride, whereas in humans their action is similar but less powerful due to them being almost vertical when stood upright. Importantly, in bipeds they counteract the truck from falling forward.

Gluteals: in a quadruped, are powerful locomotors also extending the hip, in humans these would relate to gluteus minimus and medius and are now adapted to stabilise the hip laterally, most notably when we stand with one leg off the ground, rather than being used for locomotion. In bipeds the gluteus maximus takes more of the role of hip extensor via the ilio-tibial band. Gluteus maximus also counteracts the truck from falling forward.

Take a look at the action of the race horses legs, you can see all that ground force coming from the contraction of the hamstrings at the back and gluteals at the top of the hind limb pulling them backwards and driving the horse forwards. 

The gluteus maximus dominates in humans, its function still makes it a powerful hip extensor but it’s role in stabilising, holding the femur and pelvis in alignment, keeps us stood upright. This makes it a very important muscle for bipeds. (Also worth noting that a large gluteal makes it easier for us to sit down).

Dependent on your furry character’s needs, be them straight legged or bent kneed, their gluteals and hamstrings are going to function slightly differently, be sized accordingly and maybe even positioned differently. All that is going to be aided by the configuration of their pelvis.

Standing upright - Part I

Bipedalism in Humans is by no means a well evolved task. We still have issues of lower back pain, knee joint stresses, ankle injuries and hip fractures. Many of these aliments are conditions of aging but it shows during our life time where we are taking the stresses and risks of standing upright.

When we discovered the skeletal remains of Australopithecus afarensis commonly known as ‘Lucy’, we had proof that bipedal hominids were around 3.5million years ago. When we look at her pelvis we can see more in common with our species than that of our distant relations like chimpanzees who are better adapted to climbing and quadruped walking. Lucy’s anatomy showed us that moving from quadruped to biped relied on a reconfiguration of the bony plains of the pelvis and the function of some locomotor muscles to provide lateral support while walking.

The ilia of most quadrupeds are thin and flat to the back of the torso, a gradual bending of these has formed the bony rounded ridge of the iliac crests, giving an anchor point for muscles bearing lateral support, very important for stabilising the hip of a biped.

This video is taken from the BBC’s ‘Prehistoric Autopsy’ Series. You can clearly see how similar Lucy walks compared to modern humans. In contrast the Chimpanzee on the right does not have the required skeletal and muscular configuration of its hip to allow it to walk effectively on two legs.

Pelvic configuration in humans has slowly adapted to provide quite a host of requirements: the most advantageous configuration of musculature for locomotion, transfer of weight during locomotion and to hold the torso upright during locomotion, along with providing the space to adequately hold the internal viscera, and importantly, the birthing passage for offspring. Because of the above, I feel that the pelvis is one of those areas that is quite pivotal in considering an anthro characters design. Without having to delve too deep into bio-mechanics  it’s good to take a look at some of the requirements needed to make a pelvis fit our characters given needs to see what ‘rules’ we may need to follow when making them more anthropomorphic. 

Walking on 2 legs not 4 - Stride and energy

So what would make X-Men's Beast run faster than a human? Well there are a couple simple things that we can observe from those animals that can easily outrun us. Firstly, as a biped plantigrade our maximum stride length is really rather short, a longer stride covers more ground and generally makes a faster runner. (What could break that rule would be an elephant, a plantigrade, that moves it's legs very fast when it charges). 

Digitigrades like a cheetah and unguligrades such as horses have a stride advantage by having longer limbs distal from what would be the knee joint. Simply a lengthening of the metatarsals.

The length of the femurs represented above are equal in length across the different classes for comparison.

That's an advantageous change in bone configuration but driving the power is a muscular change. Secondly, animals like horses have short fibred muscles on their lower limbs that attach to long tendons for elastic energy storage. This increased spring creates a mechanical advantage in the limb, meaning the muscles become more economical as they do not need to generate as much force per stride.

Check out just how thin the lower leg is on a horse, those long tendons and the canon bone are really the only thing they've got; there's no muscle. Find more plates like this Here.

This video is taken from "Inside Nature's Giants - The Race Horse" - (Channel 4). It's a dramatic example of just how much force is stored in the tendons once they are under stress. Energy that would otherwise be lost is recovered via this elastic strain energy. This would make Beast's flat hand very energy inefficient whilst running, and even the bony arch of the human foot is rather inflexible and still a long way from holding the capacity of elastic strain of even a digitigrade.

So if you were designing a character that's a serious fast runner - biological or mech, you might want to give them a shorter thigh in relation to the lower leg to extend that stride and go easy on the musculature of the lower leg, giant muscles don't always create giant forces, that depends on their position on the limb in terms of leverage! Of course, these are not the only things to consider for a set of biped digitigrade legs...

Comparative Skull Proportions

Each skull is divided simply into 3 parts; red - the maxilla and the plains of the 'face'; green - the mandible; blue - the cranial vault; yellow - the Axis (1st cervical vertebra). Each represents to a rough scale the respective size of each skull element in a) human, b) canine and c) equine. Note the cranial vault does not directly represent respective brain size.

I've started with a thought on the sentient issues of anthro' characters. We would expect our characters to the be the singing, dancing types of the average human. So if we take the casual assumption that to have the same level of intelligence means having the same brain size of a human we soon need to adjust the proportions of the skull to accommodate this. Fig 2 indicates  a size of the cranial vault in blue of a human (a) that we'd need to apply to our animal skulls. How then do we balance this with the large jaw and jaw muscles of a herbivore or carnivore? What changes does a larger cranium make to that of the facial plains, particularly the Zygomatric (or cheek bones), the position of the eyes and visual field? We have a number of considerations to make and plenty of permutations dependent on species...

S-spine, C-spine

Comparison of axial skeletons of a human and canine, arrows note the number and directions of each spinal curve. (Not to scale).

We encounter an extreme number of issues when converting quadruped anatomy to that of a biped. Essentially it boils down to the evolution of each systems handling of its centre of mass. Human bipeds balance their weight over 2 legs, with the centre of mass moving between each stance leg, however a quadruped dog shifts its weight between fore and hind limbs during movement.

The spines of each system are therefore adapted to cope with each of these extremes. The S-shape spine of a biped has 4 notable curves, creating it's distinctive S shape. This maintains the centre of gravity when standing from the top of the spine down through the feet. These curves also facilitate weight transference with minimal effort. 

The C-shape spine of quadrupeds have only 3 notable curves that are near opposite to biped curves. The C-shape comes from the thoracic and lumbar regions curving upwards. I've drawn the canine above stood upright, this would bring it's centre of gravity in front of its feet, causing it to be unbalanced and fall forward. The curvature of its spine, particularly at the lumbar region would not support the weight of an upright torso, the curve is in the wrong direction.

This shows that we simply can't start our anthropomorphic characters by just standing a quadruped up on 2 feet. This and other anatomical issues make this a poor starting point, however, should we wish to maintain a degree of our chosen animals features in our anthropomorphic bipeds we can start adapting this anatomy into likely possibles. Lets explore these possibilities...