This paper is to extend the authors' previous work in 2010 to understand the perspective of aerodynamic force decompositions of the dynamic stall phenomenon induced by an oscillating rigid airfoil and a deformable airfoil at low speed. The dynamic stall of an oscillating airfoil was computed and validated based on a grid independence study. Mainly here, the force-element method is applied to evaluate the evolution of the aerodynamic force during the process of unsteady stall phenomenon when the airfoils are oscillating. The results of unsteady air-load hysteresis curves at the cases of dynamic stall are analyzed. The analysis shows that the lift and drag can be, respectively, categorized into the four elements associated with the effects of volume vorticity, rotational velocity, angular acceleration, and surface friction of the airfoil. In the simulation of the oscillating rigid airfoils, the distributions of force elements show that only the volume vorticity produces a great positive source of the lift and drag. The other three effects have little contribution in the formation of the aerodynamic forces. However, the positive contributions on propulsive forces clearly come from the influences of rotational acceleration and volume vorticity produced by the oscillating flexible airfoil. The effects associated with the surface friction and rotational velocity are weak in the formation of the propulsive and later forces. [ABSTRACT FROM AUTHOR]