Disclosed herein is a device which detects repetitive movement of a user's body part. The device has a sensor which detects G forces along at least two axes when the user repeatedly moves the body part; a memory, which stores reference data corresponding to ideal reference data; a processor/computing unit, which communicates with the sensor and the memory, and receives data associated with the G forces. The processing/computing unit compares the ideal reference data with the data associated with the detected G forces. A feedback component is connected to the processor/computing unit to provide the user with a signal when a target has been achieved. Also disclosed is a method of computing data received by the device and an exerciser device that simulates the movement of a hula hoop.