Isolate vertical acceleration with quaternion and acceleration data

Hi everyone,

I got from an iPhone app : User acceleration (acceleration along the 3 axis of the iPhone without the gravity) and Quaternion.

Considering the iPhone upright:

X : right/left acceleration

Y : vertical acceleration

Z : forward/backward acceleration

I am trying to isolate the vertical acceleration assuming the device may be continuously changing (oscillation).

I am not sure how to proceed, I did that:

    %create a vector with my 3 accelerations vectors 
    AccVector = sqrt(UserX.^2 + UserY.^2 + UserZ.^2);

    %rotate the vector using the quaternion data 
    Acc2Reference_Frame = qvrot(Quat,User);

    %rename with axis
    Acc2Reference_FrameY = Acc2Reference_Frame(:,2);
    Acc2Reference_FrameZ = Acc2Reference_Frame(:,3);

    %compute vertical acceleration 
    VerticalAcceleration = sqrt(Acc2Reference_FrameY.^2 + Acc2Reference_FrameZ.^2);

When I plot the result, it does not look like the good result. I am not sure about how I proceed to achieve my goal.

I am not sure about the qvrot function which I took from the Quaternion toolbox in Mathworks.

Anyone face this problem or have an idea ?

Thank you very much for your help.