As described in the following article:
The spatio-temporal gait analysis algorithm used in Mobility Lab and Moveo Explorer do not depend on the magnetometer, and these applications can be used on subjects on a treadmill.
The joint angle estimation algorithms, however, rely on the local magnetic field for the heading component of each Opal's estimated orientation. This, in turn, has a potentially large effect on the resulting joint angles, which are largely derived from the relative differences in Opal orientations from different body segments. It is therefore not recommended to estimate joint angles when using a treadmill.
The following article provides more information on how the orientation of an Opal is estimated: