Motion capture of the trunk using three-dimensional optoelectronic systems and skin markers placed on
anatomical landmarks is prone to error due to marker placement, thus decreasing between-day relia-
bility.The influence of these errors on angular output might be reduced by using an overdetermined
number of markers and optimization algorithms, or by defining the neutralposition using a reference
trial. The purpose of this study was to quantify and compare the between-day reliability of trunk
kinematics, when using these methods.
In each of two sessions, 20 subjects performed four movement tasks.Trunk kinematics were
established through the plug-in-gait protocol, the point cloud optimization algorithm, and by defining
upright standing as neutral position. Between-day reliability was analyzed using generalizability theory
and quantified by indexes of dependability.
Across all movement tasks, none of the methods was superior in terms of between-day reliability.The
point cloud algorithm did not improve between-day reliability,but did result in 24.3% greater axialrotation
angles.The definition of neutral position by means of a reference trial revealed 5.8% higher indexes of
dependability for lateral bending and axial rotation angles, but 13.7% smaller indexes of dependability for
flexion angles. Further, using a reference trial resulted in 8.3° greater trunk flexion angles.
Therefore, the selection of appropriate markerplacement and the corresponding calculation of angular
output are dependent on the movement task and the underlying research question.