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.

Anthro-grade stance

Been a few weeks from my last post, had a few things on my mind but it's given me time to sketch out more ideas, particularly on the subject of tails and anthro legs.

To keep the anthro 'stoop' we need to change some of the anatomy. Quadriceps shown in green, gluteals in shades of red, hamstrings in purple and the ilio-tibial tract in cream.

Anthro leg anatomy issues really come down to antho characters standing in a constant 'stoop'. I think that this is some sort of visual misnomer in that it's drawn to emulate the shape of an animals leg but with limited consideration on the extent of its impact to the anatomy. 

Holding a stooped position as a human is hard work, it requires a great deal of force from the quadriceps to keep the knee in partial flexion and the gluteals and hamstrings are under stress to hold the femur at an angle. This is because the weight of the torso is placing the centre of gravity too far behind the feet, the pelvis needs to tilt the torso forward to counterbalance this, bringing the centre of gravity back under the torso. That's why if you try squats in the gym, releasing the stress on your glutes can be done by leaning forward. This stoop is very energy inefficient, and clearly will lead to characters walking not only on bent knees but in a bent forward stance.

There could be a couple of ways to fix this if you wanted to keep the 'dog leg'. Either increase the muscular force the leg can muster (but this still does not change the energy required to hold the leg up, it just means the muscle will be slower to fatigue). Or you shorten the length of the femur. Shortening the distance of the load arm (shown above) makes the stance more energy efficient. This is because the load (the weight of the torso) is now much closer to the pivot (the knee joint), it means that less force is require to lift the load. Much like when using a wheel barrow, this is an example of a second class lever.

Shortening the thigh doesn't solve the problem but it certainly would help.

'A' represents a possible digitigrade stance with a vertical femur. 'B' represents a possible unguligrade stance where the femur is in a stoop. I was not trying to compare digitigrade and unguligrade here, only a vertical femur and one in a stoop.

Taking the stance as a whole it's easier to see that 'B', stood in a stoop with the torso fully upright, would result in the characters centre of gravity being behind it's foot, it's still below the torso but the torso is now more inclined to want to fall backwards because of the stoop, the quadriceps are taking the strain to hold this stance upright.

'A' is digitigrade and the femur is vertical but the knee is still slightly in flexion. If the quadriceps contract too much it will cause the trunk to fall forward. From a balance perspective the centre of gravity is better maintained under the feet of 'A' since the leg is much more vertical but it will of course require a greater level of tone to hold this stance than that of a human plantigrade biped. 

Combining the ideas above:

• shorter thigh
• thigh more vertical than stooped
• larger musculature to lower the fatigue 
• maintains a stance directly under the torso

Is this a more likely anatomically correct look for an antho? One of many configurations for sure!

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...