This paper presents a motion-tracking sensor for the continuous monitoring of human physical activities. The mechanical modulation of REWOD without any external bias provides the supply voltage for the motion-sensing read-out circuit with the help of a CMOS energy harvester including a rectifier, a charge-pump circuit, and an LDO. Eventually, the motion-sensing electrodes along with the on-chip circuitry will be implemented on a flexible substrate for wearable applications.