The Rift-based neck lateral bending, axial rotation, and flexion/extension were
extracted using Y–X–Z Euler angle sequence from ISðtrÞRISðtÞ , which is the orientation
of the inertial sensor at time t relative to its orientation at the time of resetting the
inertial sensor at the beginning of the task (tr). The reference neck kinematics were
extracted using the same Euler angle sequence from TðtÞRHðtÞ , which is the
orientation of the head relative to the trunk. In order to examine the measurement
accuracy of the Rift, RoM derived from both the Rift and the infrared motion
tracking system was calculated, as well as the error during the maximum rotation
and the root-mean-square error (RMSE) of the Rift.