Recently, self-powered flexible sensors have shown important application value in sports training. Thus, we design a triboelectric nanogenerator based on PET/Graphite composite film (PG-TENG) to harvest human motion energy and monitor football player leg muscle health. The polytetrafluoroethylene (PTFE) film and PET/graphite composite film serve as the triboelectric pairs; meanwhile, the PET/Graphite composite film also plays the role of conductive electrode. Moreover, the PET/graphite composite film can be prepared by a simple reverse molding process. According to the results, the instantaneous power density of PG-TENG can arrive at 5.94 mW/m2 at 130 MΩ. The PG-TENG can serve as the motion player to monitor the health of football players’ leg muscles and various football sports postures, including the posture of bouncing and dribbling. This research will promote the application of self-driving sensors in sports monitoring.