Flexible piezoelectric energy harvesters (f-PEHs) have drawn attention for their potential use as power sources for wearable electronics. However, the amount of power harvested from conventional f-PEHs is still insufficient for achieving energy autonomy; hence, they are only used for limited applications. One of the effective approaches to enhancing the energy extraction from an f-PEH is to optimize the harvesting circuits. In this article, an energy extraction enhancement circuit (EEEC) using an f-PEH based on piezoelectric (PZT) material is reported to improve energy harvesting from irregular human movement of a joint or limb. The proposed EEEC is optimized for harvesting energy from random energy inputs with varying magnitudes and intervals, just like sporadic and irregular human movement. An f-PEH with a total thickness of 170 µm is utilized, which provides sufficient flexibility for attachment on clothes or human skin. By minimizing the capacitive load experienced by the PZT material during deformation, the EEEC maximizes the output voltage and increases the amount of extracted energy with low static power consumption (1.15 nW). Compared with a conventional full bridge rectifier (FBR)-based harvesting circuit, energy extraction is enhanced up to 495%.