Across all participants, the average Rift-based full RoM (from one
side to the other side) was close to that measured by the reference
motion tracking system (Table 1). The absolute error of the Rift-based
full RoM was 5.41, 3.71, and 2.31 for lateral bending, axial rotation,
and flexion/extension, respectively. The average error of the unilateral maximum cervical spine movement (from one side to the
neutral posture) across all the participants during the performed task
was within 751, however, with a large inter-participant variance
(Table 1). The RMSE errors for each rotation axis were all within 101.
The average traces of TðtrÞRGoðtrÞ and GoðtÞRHðtÞ across all the
participants were 2.97 (0.03) and 2.98 (0.02), respectively. The
corresponding angles were 9.31 (4.61) and 7.61 (3.41), correspondingly. The average trace of TðtÞRTðtrÞ over time showed that the trunk
slightly moved over time, and the average trace of ISðtÞRGoðtÞ over time
indicated that there was an error associated with drifting from the
inertial sensor (Fig. 2a and b).