The way our bodies move is extremely dependent on how our skeleton is set up, and in fact, the skeleton is unique to bipedal characters, allowing us to move in a specific way.
The Synovial Joints
There are three different types of joints: the fibrous (immoveable), the cartilaginous (partially moveable), and the synovial (freely moveable). The synovial joints are the most relevant for artists, and these are the ones we'll be talking about today. Knowing where each joint is located will help you to understand how the body moves.
There are six types of synovial joints, and they fall into two different categories. Some are revolute, which means they can rotate (ball and socket, pivot and hinge joints). The others are prismatic, which means they extend and contract.
Hinge - rotates in only one axis (flexion and extension only), ex: elbow joint
Pivot - rotates around the y axis only (the two extremes of rotation are called pronation and supanation), ex: radius joint (the radius joint in the forearm rotates in and out from the top, while the ulna stays in place; this is what allows the forearm to twist)
Ball and Socket - can move/rotate in all axes, the size of the socket limits the range of motion, ex: hip and shoulder (the shoulder has a larger socket and therefore larger range of motion, which is why it can get dislocated more easily)
Ellipsoid - similar to ball and socket, but is oval shaped and prevents clockwise/
counterclockwise rotation, ex: carpal bones in the wrist
Saddle - rotates a little in most directions and shifts (translates) to stay in the socket, ex: base of the thumb
Plane (Gliding) - two flat surfaces that can glide or rotate (clockwise/counterclockwise) on top of each other, ex: carpals of the hands and tarsals of the feet
When animating, keep these types of joints in mind, and make sure that you're rotating the joint in the right direction and with a reasonable amount. However, keep in mind that especially with cartoony animations, you can push things farther than you would find in real life. You may also find that you can get more accurate rotations using forward kinematics (FK) on the arms, rather than inverse kinematics (IK).
Bipedalism
There are a number of skeletal features that distinguish a biped from a quadruped, which are important to keep in mind, as an animator. In this post, I will be focusing on biped anatomy only, but if you'd like to learn more about quadrupeds, see THIS POST.
Angle of the Femur - In bipeds, the femur has uneven condyles (the bumps at the base of the bone). Because of this, when the femur is held perpendicular to a flat surface, it tilts so that the knee is closer in toward the center, rather than pointing straight up, as it does with quadrupeds. This allows humans to keep their weight over their center of gravity and is especially beneficial to walking. Unlike quadrupeds, we do not have to shift side to side too much when standing on two legs, since our weight is more centered.
The Foramen Magnum - The foramen magnum is the hole at the base of the skull that indicates the location of the spine. The location of this hole is important for bipeds, since our skull/brain is heavy. Since this hole is right at the base of our skull, it allows us to keep this weight over our center of gravity. The neck muscles also don't have to be as strong to hold up the head because of this.
Lumbar Lordosis - The lumbar lordosis is the anterior (toward the front of the body) curve of the spine, which helps to create our s shaped spine. This is a feature not found in quadrupeds. The purpose is to keep the upper body over the lower limbs by positioning the torso right above the hips. This reduces the amount of muscular effort required to balance your weight over your hips.
Anterior Inferior Iliac Spine - The is a little bump on the blade of the pelvis bone, where the iliofemoral ligament attaches. This ligament tenses when we're upright and releases tension when we sit down. Again, this feature is only found in bipeds, and it allows us to maintain an upright position, without needing a lot of muscle contraction.
The Skeleton in Animation
Keep in mind these anatomy traits when animating bipedal characters. This will allow you to create realistic and believable motion and to use weight correctly in your animation. I also encourage you to pay attention to how your own body works and apply this to your animation as well!
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