Internal forces are essential to humans because we can’t rely on external forces for movement. The muscles are the most important source of internal forces.
Muscles are the “motors” of the movement system and pull on bone segments.
Muscles produce forces by contracting.
Simply, we can place the point of application for the muscle vector on the bone being moved at the muscle’s attachment site. The action line can be approximated by being drawn up the middle of the muscle’s fibers.
A useful way to think of a muscle is to think of a stretched rubber band. The force produced is being directed along the rubber band. Additionally, the rubber band can only “pull” and “wants” to un-stretch.
A contracting muscle exerts a force on and pulls all the bone segments it is attached to. It’s pulling the two attachment points closer together whenever it contracts.¹