Anthro leg structure 2

I was asked a few weeks ago to help with creating a set of satyr legs. The main point I outlined was:

only look at the biggest forms when filling out the wire frame with muscles. Keep the curves flowing; show the largest sweeps of these shapes to keep them dynamic; have them show the force of the muscles not just their outlines. Don’t get too hung up on individual muscles at the start, that detail is for later when you’re shading, make sure you’re drawing the big forms to start with.

Some ideas to consider on an unguiligrade biped leg.

Gesture: Centre top is an idea of the flowing curves for a digitigrade and unguligrade type. I wanted to get an idea of the fundamental shapes the curves make as if the legs were mechanical; they would be made of curves that would give them spring as they moved.

Form: On the left an idea of where the muscular bulk is as opposed to the bone and tendon areas. All limbs follow the principle that they get thinner the more distal (further from the torso) they become. Take the human arm as example, muscular bulk decreases further down the arm and the limb becomes more sinuous until we get down to just tendons controlling the hand and fingers. This decrease in muscular bulk is worth noting when drawing so as to keep the limb in proportion. On the right is this idea now shown as contours in the forms of the leg.

Function: At the bottom of the image is a movement sequence. Those areas shaded represent the major muscles that are contracting. The quadriceps, (Q) extend the knee just before the 'contact'. On the 'down', the limb is slowed by the hamstrings (H) as the limb takes the torso's full weight. The elastic energy in taking this weight is channelled down into to fetlock or toe joint by long flexor tendons. This energy is released in the 'up' with the gluteals (G) powering the knee to full extension and the hamstrings extending the hip. This drives the torso forwards as I've talked about in previous posts. Note that the 'knee' and the 'toe' joints flex and extend by the greatest degree, the 'ankle' joint is kept relativity stiff. If the ankle was allowed to flex too much the limb would collapse under the weight of the torso.



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...