World reference values for the accelerometer sensor when walking

Question

I have been trying to develop a Pedestrian Dead Reckoning application for Android, and after taking care of the step detection and step length components, I have decided to tackle the orientation determination problem.

After stumbling on a couple of posts regarding coordinate transformation (and even chatting with a frequent answerer), I have been getting gradually better results, but the are still some things that bother me.

The experiment:

I walked forward Northward, turned back, and walked back Southward. Repeated the procedure towards West , then East.

Issues:

1. I expected, while walking straight in several directions, to have values of the X and Y values oscillate with the footsteps, and have a relatively stable Z value throughout. Instead, the Y values behave this way, with the Z value having its expected behavior. How come? Does it have anything to do with me not using remapCoordinates()? (see Graph 1)

2. I expected the angle plots to jump around 180º and -180º, but why do they also do it around 35º? (see Graph 2)

Notes:

• I am using gravity and magnetometer values to compute the rotation matrix, and multiplying it using OpenGL's multiplyMV();
• I am not using remapCoordinates(), because I thought I didn't need to: the phone is upright in my pocket (Y points up/down, Z usually forward) and should displace itself 45º forwards backwards and forwards, at worst;
• Azimuth values seem ok, and do not have the oscillation described in issue 2. (see Graph 3)

Graphs:

World Reference Gravity Acceleration (blue is X, red is Y, green is Z)

World Reference Gravity Angles (blue is atan2(Y/X), red is atan2(Z/Y) and green is atan2(Z/X) )

Orientation Values (blue is azimuth, red is pitch and green is roll)