Why does Mobility Lab v2 offer both acceleration and angular measures of sway?
The sway measures in Mobility Lab are computed directly from the acceleration of the lumbar sensor in the transverse plane. The attached publication by Martina Mancini provides a technical description of the implementation that is used.
In Mobility Lab v1 (MLv1), these acceleration based measures were difficult to understand for many users, as they had complicated units of measurement. For example, the "Sway Area" measure in MLv1 used the units m^2/s^5.
For Mobility Lab v2 (MLv2), we chose to re-represent a subset of these measures to make them easier to interpret. Since the accelerometer can detect the direction and magnitude of Earth's gravity, you can calculate a lean angle. These results highly correlate with the accelerometer based results when the angles are relatively small and the translational acceleration is minimal, as is the case for Sway tests. Below is a figure showing normalized versions of both the acceleration based and sway angle based stabilograms on top of each other. One is red and the other is blue. They are virtually indistinguishable. MLv2 provides the option to use either type of result (acceleration or lean angle).
Can I compare MLv1 to MLv2 acceleration based Sway measures?
The acceleration based sway algorithm in MLv2 is a faithful implementation of the algorithm described in the attached paper. The MLv1 implementation had a different, and undocumented, scaling factor which makes direct comparison to the MLv2 version impossible. For the purposes of reproducibility and transparency, we chose to use the more faithful implementation in MLv2.