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 50 Hz
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