Regarded by many as the most important principle “Bounce, Squash and Stretch”, the purpose of which is to give a sense of weight and flexibility to drawn objects. This can be applied to simply objects like the bouncing of a ball, also more complex constructions like the muscles on a face when someone is talking. This can also be taken to the extreme point to create a comical effect.
In realistic animation the most important aspect of this principle is that the volume of the object does not change when squashed or stretched. If the length of a ball is stretched vertically, its width (in three dimensions, also its depth) needs to contract correspondingly horizontally.
Below is a image showing the different stages of the principle.
Below is a example that I have created
Below is a animation of a horse running so that you can see how the muscles are moving and how the principles apply to this to