Trying to find distance using a 9dof imu

Question

I am doing a research project to determine, acceleration, velocity, and distance of a weightlifting bar doing a clean. I have looked into using a 9dof imu sensor, however I have found out that calculating the distance the bar has traveled is not accurate, since the sensor is not always parallel during the lift to the ground and issues with the double integral. I have thought about using an ultrasonic range finder to get the distance however, if connected to the bar we run into the same problem as above. I would like some other ideas.