As a continuation of my post on the bipedal skeleton, I'd like to talk about muscles, since they allow us to move our joints and affect the way our body moves. A knowledge of muscles will help you ground your animations in realism and prevent you from overextending and breaking your character's body anatomy.
Muscle Anatomy
Types of Muscles
There are three different types of muscles:
smooth muscle (digestive system)
cardiac muscle (heart muscle)
skeletal muscle (everything else in the body; acts similarly to rubber bands)
Smooth muscle and cardiac muscle are considered involuntary muscles, since they are not controlled consciously. (They contract due to impulses sent by the nervous system.) As animators, we will only be working with skeletal muscle, so that's what we'll be talking about today.
Muscle Actions
Muscles act on synovial joints to move the body. These joints are freely movable, and we discussed them in detail in my post about the Bipedal Skeleton. To move these joints, the muscles attach to the bone at two or more places. These places are either referred to as the origin (if the attachment is on a bone that is immobile during an action) or the insertion (if the attachment is on a bone that moves during an action).
There are many different types of muscle actions, listed below, and these actions are usually paired. I've grouped the paired motions for you.
Flexion + Extension (forward and backward movement)
flexion - bending (decreasing angle between bones)
extension - straightening (increasing angle between bones); hyper extension results from extension of a joint beyond it's normal range of motion Abduction + Adduction (side-to-side movement)
abduction - moving away from body's midline
adduction - moving toward the body's midline
Elevation + Depression (up and down movement)
elevation - moving a body part up
depression - moving a body part down
Pronation + Supination (forearm movement)
pronation - rotating the forearm so the palm is facing down
supination - rotating the forearm so the palm is facing up
Protraction + Retraction (jaw or shoulder movement)
protraction - moving a bone forward without changing the angle
retraction - moving a bone backward without changing the angle
Excursion (jaw movement)
excursion - side to side movement of the jaw
Inversion + Eversion (ankle movement)
inversion - turning the the foot inward
eversion - turning the foot outward
Dorsiflexion + Plantar Flexion (ankle movement)
dorsiflexion - rotating the foot up from the heel
plantar flexion - rotating the foot up from the toe or pointing the toes down
Rotation (spine, hip, and limb movement)
rotation - twisting movement
Muscle Contraction
Now that we know how the muscles are attached, we can talk about how they rotate our joints. This is achieved by the contraction of the muscles.
Muscle contraction begins with a signal from the nervous system (called an action potential). When this signal ends, the muscle then relaxes. There are many steps between when the signal is sent out and when it ends, taking place within the cells of the muscle, but as artists, all we need to know is that muscles turn energy into motion.
Types of Contraction
Muscles are often paired to counteract each other. Muscles called prime movers provide the primary force of an action. These muscles usually pair with an antagonist on the other side of the joint that provides resistance. This way, you can release a contraction in a controlled way. There are often one or more synergists (smaller muscles) that assist the prime mover as well. Last, we have stabilizer muscles, which keep the bones immobile when needed.
From this, we get three main types of contraction:
concentric contraction - the muscle shortens to move the joint (ex: contraction of hamstring to raise your leg)
eccentric contraction - the muscle lengthens in a controlled way; in other words, it controls the movement (ex: keeps the forearm from dropping down after a bicep curl)
isometric contraction - static contractions; the muscle maintains it's position (ex: gymnasts use this to stay in a pose for an extended period of time)
Limbs as Levers
You can also think of limbs as levers, where the fulcrum of the lever is at the joint location, allowing your muscles to lift the weight of the limb.
The human arm is a third class lever, where a large effort force acts over a small distance to move a small load over a large distance. As you can see, in this type of lever the effort arm is shorter than the load arm, and as a result, it is inefficient but fast to move our arms. In general with our muscles, speed is more important than efficiency.
This video from Alejandro Garcia explains this idea in more detail:
Putting it into Practice
What does all of this mean and how is it useful to animators? First, knowing the ways in which a muscle can move a joint tells us what's possible in an animation. Although animation is an exaggeration of reality, we do want to ground our work in reality, since it makes our characters more believable. It's also important to understand how muscles move, since this will inform the body mechanics of characters. Knowing that muscles contract and release in a controlled way helps us figure out what our spacing should look like. Altogether, this information helps us to better understand why we make certain choices in an animation.
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