Measuring/Tracking arm movements

Hi everyone,

I want to place a sensor in a wristband for a project.
I need to be able to measure when someone raises their arm and when someone moves their arm actively.

(I am doing a project for which I need to measure the personal highlights of someone during a festival)

I already experimented with a MPU 9150 connected to an Arduino UNO for hours and I did not find a solution (yet).
Honestly, I am kind of a noob when it comes to programming..

I am struggling with the fact that when your arm is hanging next to your body, your inner wrist is pointing towards you. While when you raise your arm, your inner wrist is pointing in the other direction. This means that also the relative position of the sensor changes.

Every suggestion, insight or comment is welcome!

I hope I explained my struggles clear enough.. this my first question on a forum ever.

Thank you very much in advance!

While experimenting with the MPU 9150 I found out that I can combine certain outcomes of for example the accelerometer and the magnetometer.

Does anyone has suggestions on how to implement this in the code?