The invention is directed to a system and method using the system for guiding a person in correct activation of her or his core muscles for a sport or other exercise session The system is based on an elastic belt to be closely adhered to the lower abdomen of the human being and adapted to follow an inward movement of the lower abdominal wall without movements of the spine and pelvis effectively activating the core muscles responsible for low back control, sensor means provided at the belt for capturing extensions and contractions of the belt following said inward movement of the lower abdominal wall, means for evaluating the extensions and contractions of the belt following said lower abdominal wall region and for outputting evaluation data, means for comparing the evaluation data attained thereby to data representing correct activation of the core muscles, and feedback means provided with the belt for providing feedback to said result of said comparison to said person. In accordance with the invention the sensor means comprise fused together by a data fusion algorithm a 3D accelerometer sensor for capturing user's pelvis movements, a 3D gyroscope sensor for capturing user's pelvis movements, and/or a 3D Magnetometer sensor for capturing user's pelvis movements. The method evaluates and monitors based on the system said person's exercises by calculating the optimal position of the person in correct activation of her or his core muscles out of two positions taken up by the person in a calibration process.
