We present an electromagnetic energy harvester using rotational eccentric mass structure that generates power from extremely low-frequency motion of a driven mechanical swing-arm (i.e. pendulum) that is intended to mimic wrist motion during regular walking. It consists of two aligned eccentric rotors, which rotate together, containing multiple N52 permanent magnet pole-pairs with back iron shields. The shields allow the magnetic flux densities to increase in the middle of the rotor structure, where the coils are located, by 63%. The stator is formed by multiple series connected self-supported copper coils in a PCB interconnect. Voltage is induced in the coils due to magnetic flux change while the eccentric rotor moves because of swing-arm motion. The output power of a fabricated prototype was measured for various swing-arm motions and compared with the simulation. The prototype generates maximum 55 μW average power at ±25 degree rotational amplitude and 0.8 sec period (1.25 Hz).