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Thread: Skeletal Muscle Movement and Foreshortening

  1. #1
    Super Senior Member CypressDahlia's Avatar
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    Jan 2011

    Skeletal Muscle Movement and Foreshortening

    Okay, so. Today we're gonna discuss skeletal muscle movement since it is a vital part of creating believable poses and drawing believable anatomy. I notice that, even among some pros such as Akira Toriyama and Tite Kubo, manga artists tend to overlook the fact that muscles constantly change shape as the human body moves. Tite Kubo in particular draws very static anatomy that shows no evidence of muscle movement, even in dramatic poses. Muscles need to contract and tense up for our bodies to move. Toriyama, on the other hand, draws in a way where all of the muscles are tense all the time, which is not necessarily correct, either. Though it may work for these artists, chances are it's not easy to overlook if you're going to create a more realistic shonen manga like Baki the Grappler or TOUGH. 90% of the time there are well-muscled men duking it out topless, so muscle movement definitely needs to be conveyed otherwise it just doesn't look natural.

    Now, the first step to understanding muscle movement is understanding how muscles work. Skeletal muscles are chunks of fibrous tissue attached to bone by tendons. You pretty much have at least two muscles everywhere you have a joint. They're attached to both components of a joint (beit a two part hinge or a ball and socket) and, when contracted, cause the joint to flex or extend.

    The most common example is probably the biceps and triceps. The biceps are responsible for the flexion of the elbow joint and the triceps are responsible for its extension. No one muscle is responsible for both the flexion and extension of any joint. This is because muscles do not push, they only pull. But we'll leave that subject for later.

    This here is the main focus of the tutorial: conveying the appearance of muscles. Though we may understand muscles, the anatomy we draw is not accurate until we can show that they are working. The first example on the left shows static anatomy. The muscle movement is not represented and, thus, the anatomy is overall incorrect. The anatomy in the following example, though, is correct because the movement of the bicep is represented. Because it has been contracted, the muscle's mass is displaced, causing it to bulge. It's similar to how a water balloon is affected when you press it against a hard surface. This is because water balloons, like muscles, are elastic in nature. Muscles also become firm when contracted, as does a water balloon when squeezed. We won't go into the science of muscle movement since this tutorial is purely about appearance.


    Onto the hard part: understanding which muscles are contracted and when. Though you could figure this out by building a mental reference library, it's best to understand the rules just in case you don't have a convenient reference for what you're trying to achieve. The biggest rule to remember is this: muscles DO NOT PUSH, they ONLY PULL. Muscles are not capable of exerting force. A common misconception is that we are using our muscles to push things. While this is true in a topical sense, the fact of the matter is that our muscles are not exerting force, our joints are. In fact, the only thing our muscles do is contract. They are simply contracting in such a way that it puts force behind our movements.

    The example above shows a boxer executing a straight jab. The misconception would be that he's pushing his fist forward with his biceps. The truth is, his biceps are not involved in the movement at all. This is why it's important to understand this concept because you might be drawing a fight scene and end up defining all of the wrong muscles. The second rule is that a joint moves in the direction of the muscle being contracted. With this knowledge, you can construct poses from scratch or deconstruct them from references.

    The muscles highlighted in green are the ones mainly involved in the movement. The straight jab is the result of a complex pipeline of muscle contractions, not exerted force.

    A.) The right abdominal obliques contract, causing the waist to pivot counterclockwise.
    B.) The upper trapezius muscle contracts, shrugging up the shoulder of the punching arm.
    C.) The back muscles, namely the infraspinatus and teres major and minor, pull back the shoulder slightly, lining it up with the pivoted torso.
    D&E.) Once the body is poised, the deltoids and triceps flex, causing the arm to spring forward. The momentum built up through the pipeline follows through to the fist and into the punching bag.

    So, just to hit the point home, muscles PULL, not PUSH. It's up to you to learn which muscles are attached to where and what they look like, though.


    The next subject is foreshortening. A lot of people seem to have trouble foreshortening when the contours of an object are totally lost to perspective. This is understandable. Foreshortening is hard to get right, especially with organic shapes.

    This is an icosikaihexagon--a 26-sided polygon. Using this shape, we can create a mathematical approach to foreshortening for those of us who can't eyeball it. I wanted to use a rigid object because it demonstrates the effect of perspective on aspect ratios. And, like the draw-through of the human head done in my other tutorial, we can build upon this object to create more complex ones. We're really only using this to touch on the principle.

    First, we need to define "aspect ratio".
    SPOILER! :
    An aspect ratio is a ratio that expresses the relationship between the width and height of an object (in that order) without giving specific measurements. For example, 4:3 is an aspect ratio wherein the height of an object is three-fourths its width. Easy, right? I chose aspect ratios because the principle behind foreshortening is based on visual relationships as opposed to measurements.

    Moving on. This shape has 8 congruent, square faces that wrap around it, each tilted in increments of 45 degrees. The aspect ratio of each face is technically 1 (or 1:1) since they are square but, because of perspective, that's not what we see. If we were to skewer this object on a horizontal axis and set it at eye-level, we would see the first picture on the right. The red face reads as a square since we're looking directly at it, but the green and blue faces do not.

    This is due to Point of View. The camera represents our eyes and the horizontal lines represent our line of vision. The red face is currently perpendicular to our line of vision and therefore perfectly readable. The green and blue faces are tilted back at a 45 degree angle, therefore we read them as being half their height and half their width respectively. The purple faces are entirely invisible to us because they are perfectly parallel to our line of vision.

    As we continue to rotate the object, the upper green face seems to get shorter and shorter. At 30 degrees, its aspect ratio is 3:1, or one-third of its original height. At 45 degrees, it completely disappears. What conclusions can we draw from this?

    1.) A surface on an object at eye-level that is parallel to your line of vision is not visible. To test this, write something on a card or piece of paper and hold it up at eye-level so that its faces are parallel to your vision. You can't read it, right?

    2.) Surfaces on an object at eye-level that are not perpendicular to your line of vision will have reduced dimensions. A surface tilted on its x-axis will appear shorter. A surface tilted on its y-axis will appear narrower. The less perpendicular (or more parallel) these surfaces are, the smaller they appear.

    But what do we do when an object has no visible surfaces? Well, take a look at the baseball above. A baseball is a sphere, so it has no flat surfaces. Despite that, we can still apply the above rules to it. By laying some flat surfaces over the drawing, I can give myself a vague impression of how the baseball will look foreshortened. Usually you wouldn't have to worry about foreshortening a sphere but a baseball has stitches so it needs to be properly foreshortened for the stitches to look accurate. The purple surface is the most perpendicular to our line of vision so the stitches there are drawn in full. As they wrap around the ball, they begin to change shape and become more dense. Observe the red surface on the right. It is heavily foreshortened so the stitches there are very close together. This is the effect that foreshortening has on detail density.

    3.) The less perpendicular a surface is to your line of vision, the smaller it appears. Thus, the details on that surface seem much more dense as they are being compressed into a smaller area.

    Now you know the 3 basic rules of foreshortening. Here they are in application:

    Using the three rules stated above and the draw-through method, I was able to apply foreshortening to this shoe. I used horizontal guidelines to keep the shoe detail-accurate. Keep in mind, though, that this example is at eye-level. In a larger composition, not everything will be at eye-level. Therefore, we have to make proper use of perspective in conjunction with foreshortening. Large objects that are heavily foreshortened should also be in perspective to maintain a believable scale. The only times perspective should be ignored is if you're making diagrammatic drawings such as concept turnarounds or drawing relatively small objects at eye-level. Here is an old drawing I did of Dizzy from Guilty Gear:

    Notice that the torso is foreshortened while the legs are not, therefore the torso is placed on a separate perspective grid. If perspective hadn't been applied, the drawing wouldn't accurately reflect the slight bird's eye PoV and the foreshortening wouldn't work.

    Anyway, that's that. Hope this helped. Good luck.

    Last edited by CypressDahlia; 10-19-2011 at 08:00 PM.

  2. #2
    Teacup Ninja Tots Cloudy's Avatar
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    Dec 2010
    Thanks cype~

  3. #3
    Junior Member kaspar's Avatar
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    Oct 2011
    Thanks Cypress!!!
    I sure can use this...

  4. #4
    Zeta Members ram's Avatar
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    Apr 2011
    in your heart
    This really beats the drawing books I got, All of the info here is completely new to me.

  5. #5
    Fifty Fifty Member Bacon_Barbarian's Avatar
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    Dec 2010
    Elizabeth's Court
    Can you do a grid for the big muscle guy?
    My AA thread - Updated 06/28/14

    Quote Originally Posted by Celestial-Fox View Post
    You're my favorite.

  6. #6
    Three Trio Tres Member BozeSG's Avatar
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    Dec 2010
    3.0001th Dimension
    whoa!!! this is super helpful, thanks!!!


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