Gravity is the most important external force experienced by the human body.
It is the pull of the earth on a body or its segments.
While gravity acts at all points on an object, its point of application is called the center of gravity of that object or segment.
The action line of gravity always points down. The gravity vector is called the line of gravity.
Each segment in the body is acted on by the force of gravity and has its own center of gravity. However, if two or more segments move together as a single segment, gravity can be represented by a single center of gravity.
For an object to be stable, its line of gravity must fall within its base of support. When the line of gravity falls outside of the base of support, the object will tend to fall.
Other external forces include wind, other humans, etc.