In evaluating human motion, its useful to think of a force as a push or pull by one object (Thing 1) on another object (Thing 2). Whenever two “things” make contact, they are exerting forces on each other.
Whether any of your body segments is in motion or at rest depends on the various forces exerted on it.
External forces are pushes or pulls on the body that come from sources outside the body. The best example of this is gravity.
Internal forces are forces that act on the body but come from sources within the human body. The best example of this are muscles.
All forces are vectors. This means they are characterized by (1) a point of application on the object being acted on, (2) an action line and direction indicating a pull toward or a push away from the source, and (3) a magnitude (the quantity of force being exerted).