The presence of repetitive symmetry in continua and structures allows for the modeling of a single symmetric cell. Typically, the symmetry conditions are enforced with homogeneous kinematic constraints, but these may preclude some otherwise admissible response. Also, in large deformation situations, the original symmetric boundaries can distort significantly. The objective of this work is to provide a means for explicit finite element methods to ensure periodic symmetry while resultant loads may be prescribed on the symmetric boundaries. A method is devised to determine external forces that concurrently maintain dynamic equilibrium of the system with compatible deformations at the boundaries. The method requires only internal forces and nodal positions, which are accessible in an explicit finite element formulation.