The human skeleton is a system of levers. A lever can transmit and modify force and motion.
Bones are levers that rotate around an axis.
Forces applied to a bone will produce either no motion or movement. This depends on the combination of forces (both internal forces and external forces) acting at different points on a lever.
A first class lever exists when forces are applied on either side of an axis at some distance from that axis, creating rotation in opposite directions.¹
A second class lever exists when two forces are applied so that the resistance lies between the effort force and the axis of rotation.¹
A third class lever exists when the forces on a lever are applied so that the effort force lies closer to the axis of the lever than does the resistance.¹
The effort force is the force that is causing or attempting to cause the motion.
The resistance force is the force that is opposing the movement.
The lever arm is the distance from the axis to the point at which a force (effort or resistance) is applied to the lever.
The effort arm is the lever arm of the effort force.
The resistance arm is the lever arm of the resistance force.
Most muscles/bones in the human in human body are third class lever systems. Thus, when a muscle contracts and pulls on a bone, it can be viewed as the effort force working against some resistance force (like gravity or another external force).¹