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A rotational electromagnetic energy harvester is designed to collect the mechanical energy of human motion at a low frequency. Linear motion can be converted to high speed rotation with an inertial system, which is mainly composed of a twist driving structure and a ratchet-clutch structure. When the twist rod is compressed by a footstep, the ratchet can keep rotating for about 20 s inertially, and an overall energy of 85.2 mJ can be harvested. The peak power output can reach 32.2 mW and a root mean square power of 7.7 mW is achieved. The maximum speed of the ratchet would be as high as 3700 revolutions per minute. When driven by the human footstep at a frequency of 1 Hz, an electronic hygrothermograph and 70 light-emitting diodes (LEDs) could be easily powered, which demonstrates the promising application of self-powered microelectronic devices.