2D Camera motion detection

Hi,

I'm currently developing a mobile app that must detect simple physical exercices using the device's webcam. I'm using OpenCV For Unity asset for now. I started with the squatting movement and figured a cheap way to measure the movement is to detect face position and set up a margin on how much to go down or up to be counted as a squat. The tablet or phone placed on a table using a regular tablet holder .
I'm having trouble with face detection when it comes to bearded men or tall people from table height.
If needed, I could do with simple any motion detection regardless of what moves if you know any solution.
Do you think I should try an other solution with OpenCV or am I in the wrong way here ?

Thanks for your time !