In several industrial applications, drivetrains impose highly dynamic oscillating motions to inertial loads. By introducing springs, the system's natural frequencies can be matched to the required operating frequencies, lowering energy consumption of the drivetrain. However, fixed-stiffness springs only have a positive effect in a limited range of frequencies. To solve this problem, variable stiffness springs are proposed. A discussion of the effect of the series and parallel topology, as well as the possibility of adapting the spatio-temporal characteristics of the motion to the spring, is presented. Furthermore, a practical implementation of a variable stiffness spring is proposed. Its effectiveness is validated in experiments.