• The feet and legs.
  The feet and legs propel the body forward. To keep your charcter looking natural, you should always keep the joints bent slightly, even at full leg extension.
  
  
• The hips, spine, and shoulders.
  The body’s center of gravity is at the hips; all balance starts there, as does the rest of the body’s motion. During a walk, it’s best to think of the hips’ motion as two separate, overlapping rotations. First, the hips rotate along the axis of the spine, forward and back with the legs. If the right leg is forward, the right hip is rotated forward. Second, at the passing position, the free leg pulls the hip out of center, forcing the hips to rock from side to side. These two motions are then transmitted through the spine to the shoulders, which mirror the hips to maintain balance.
  
  
• The arms.
  Unless the character is holding something or gesturing, its arms hang loose at the sides. When walking, they act like pendulums, dragging a few frames behind the motion of the hips and shoulders. Even at full extension, try keeping the arms slightly bent at the elbows, to make them look natural.
  
• The head and spine from the side.
  If you look at a character’s spine from the side, you can see its general posture, which may be very stiff or slouched over, depending on the character’s attitude. The spine also absorbs some of the shock transmitted to the hips from the legs, making it flex from front to back a bit. In a standard walk, the head tries to stay level, with the eyes pointing in the direction of the walk, but it bobs around slightly to stay balanced. If a character is excited, this bobbing will be more pronounced. The head may also hang low for a sad character, or may look around if the scene requires it.

Principles of Traditional Animation

Squash and Stretch -

Defining the rigidity and mass of an object by distorting its shape during an action

Timing and Motion

Spacing actions to define the weight and size of objects and the personality of characters