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.

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...

Some ideas on larger herbivore types. That cervical curve is really going to dip if we make a character with those distinctive rounded shoulders with a neck in a 'stoop'. The nuchal ligaments are going to have to act like a crane arm to help stop that neck collapsing - it's rather unstable. So I've also highlighted some context as to large stabilising muscle shapes of the neck. I think this would really limit head rotation. Also the cervical vertebra in this configuration would have to be bulkier to take the stress of the lateral rotation.

Great video on cervical spine rotation.

Skull to Neck

As heads get bigger, so does the weight of keeping them upright. Here i'm showing the nuchal ligament as being an important part of keeping a larger head upright.

We need to strike a careful balance with how we expect larger anthro skulls to sit on a biped spine. The cervical vertebrae is the most flexible part of the spine but this comes at a trade-off with strength. Strong yet flexible interspinal ligaments hold the vertebrae from palling apart and in many quadrupeds the nuchal ligament is very important in holding the head and neck from collapsing under it's weight. In humans this tough ligament is less important, our skulls are balanced on the 1st cervical vertebrae, the Atlas, with stability aided by numerous muscles. An anthro skull by contrast would likely be heavier, larger facial features, big jaw. This upsets the balance and we can either reposition the skull on the atlas like a pivot or we recruit ligaments and muscles to help stability. Nuchal ligaments would act like a suspension bridge, pulling from the processes on the thoracic vertebrae to a prominent crest on the part of the Occipital bone of the skull.

What does this mean for character designs: 

consider the weight of your characters heads, this includes big horns and teeth! 

How would the cervical vertebrae curve, is it realistic

Where is the load of the head being distributed

Spinal processes are going to be visible along their backs

How will bigger back muscles such as the trapezius look and how is that head able to flex and move?!