Backpack energy harvesting systems are very promising solutions for powering portable devices without primary batteries, especially in excursionist and military applications. During human walking, they allow the conversion of the backpack mechanical vibration energy into electricity. In this paper the attention is focused on a backpack regenerative system based on a mechanical motion rectifier, which is able to convert the up-and-down oscillation of the backpack into a unidirectional rotation of a DC generator. Such a system needs a power electronic interface for optimizing the operating point that maximizes the extracted power. In particular, two types of ideal power electronic interfaces are analyzed and compared. The first one is based on the adaptation of the voltage at the DC generator output, while the second one is based on the matching of the resistance at the DC generator output. Through a numerical analysis, the average power extracted by the two interfaces are compared under different input displacement characteristics. The proposed analysis can be useful for the identification of the optimal voltage profile to set at the DC generator output for the maximization of the extracted electrical energy.