An active-marker infrared motion tracking system (Optotrak Certus System,
Northern Digital, Canada), with an accuracy level of 0.1 mm, was used as the 3-D
kinematics ground truth measure of the trunk, head, and the Rift. Clusters of three
markers were attached on the trunk and the Rift. Anatomical landmarks including the
sternal notch, xiphoid process, C7, T8, left and right tragion, and vertex, as well as the
four corners of the Rift, were digitized with participants wearing it in an uprightseated reference posture before the start of the task. The local coordinate system of the
trunk was constructed according to the ISB recommendations (Wu et al., 2005). The
head Z-axis was from the left tragion to the right tragion, and the X-axis was
perpendicular to the plane including the Z-axis and the vertex pointing forward. The
Rift X-axis was perpendicular to the front cover pointing forward, and the Z-axis was
from the lower left corner to the lower right corner. During the neutral posture, for
trunk, head and the Rift, the X-axis pointed forward, the Y-axis pointed upward, and
the Z-axis pointed laterally to the right side (Fig. 1a). The Rift and the motion tracking
system were synchronized at a 60 Hz sample rate.