Sway measures in Mobility Lab

Why does Mobility Lab 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.

These acceleration based measures are difficult to understand for many users, as they have complicated units of measurement. For example, the accelerometer based "Sway Area" measure in used the units m^2/s^5.

For Mobility Lab, 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. Mobility Lab provides the option to use either type of result (acceleration or lean angle).