The development and implementation of the smartphone application was described previously in the context of an evaluation of sit-to-stand sequences [20]. The application was developed on the basis of the mobile operating system Android (Google Inc., Mountain View, CA, USA) and made use of the smartphone’s integrated movement sensors. Acceleration sensors were used to measure the device’s acceleration, while rotation sensors were used to measure the device’s rotation and to identify orientation fluctuations of the smartphone. In contrast to accelerometers, which are hardware sensors, rotation sensors are software-based (virtual) sensors. These rotation sensors use acceleration and magnetic field measurements in order to compute device orientation. Orientation fluctuations of the smartphone can be corrected through transformation of raw acceleration data from device coordinates to world coordinates.
The smartphone’s sampling frequency was inconsistent since the sampling mechanism was event- rather than time-based. However, the data was automatically re-sampled at 50Hz immediately after recording.
Raw acceleration data (further referred to as uncorrected data) as well as position-corrected acceleration data (further referred to as corrected data) were saved on the smartphone as comma-separated value (csv) files. For verification purposes, a graphical view of acceleration data was displayed immediately after recording.