Intro to the foot and gait mechanics
The foot is a complex and important landmark in the human body. It is our interface with the ground, and influences the rest of the body's ability (or lack thereof) to move efficiently and effectively.
This relatively small area of the body has 26 bones, 33 joints, over 7,000 nerve endings (in each foot), and some of the thickest, toughest skin on the body, so the foot is meant to possess a high amount of movement, sensation and durability in order to react and adapt to a highly varied surface terrain. Unfortunately since many of the surfaces we walk on in the modern world are flat (paved, tiled, and so on), unless we actively seek out and provide a highly varied environment (grass, dirt, rocks, sand, etc.), our feet do not receive the stimulus they were designed to perceive. This may lead to negative adaptations that can impact alignment and movement up the entire kinetic chain.
Below is a video that introduces some basic concepts of the foot as it relates to the typical human gait cycle.