Elevator movement is due to two effects.
1. Changes in elevator aerodynamic hinge moment due to changes in local airflow as the aircraft pitches.
2. Mass unbalance effects in the control circuit, which can cause the controls to move under varying g loads. For example if the control yoke's neutral position is not vertical, or if there is a bobweight. The elevator itself might not be mass balanced and so move under g loads.
Hinge moment changes cannot effect irreversible powered controls. Mass unbalance effects can if they are "upstream" of the servo valve.
If you are going to account for these forces in the elevator, you might also want to account for similar effects in the rudder during yaw oscillations or sideslip for example.