Pedometer, tri-axial accelerometer, and measuring system of $CO_2$ generated during metabolism have mainly been used to measure the quantity of motion and inhaling capacity while walking or running. However, these sensors are heavy, expensive, and uncomfortable to wear and difficult to check the work-out condition. In an attempt to overcome these limitations, a soft, flexible, and lightweight piezoelectric polymer-based sensor has been prepared. The PVDF film sensor with thin silver-paste layers in both sides was coated with silicon rubber to amplify stress by the external power such as tension and pressure. Thus, movement of thigh muscles was monitored and physical activity was predicted during walking and running, with an elastic belt embedded with the prepared PVDF film sensor, which was named as a physiological sensing belt (PSB), strapped on the thigh.