  Background: Skin marker-based analysis of functional spinal movement is a
promising method for quantifying longitudinal effects of treatment
interventions in patients with spinal pathologies. However, observed day-to-day
changes might not necessarily be due to a treatment intervention, but can
result from errors related to soft tissue artifacts, marker placement
inaccuracies or biological day-to-day variability. Research question: How
reliable are skin marker-derived three-dimensional spinal kinematics during
functional activities between two separate measurement sessions? Methods:
Twenty healthy adults (11f/9m) were invited to a movement analysis laboratory
for two visits separated by 7-10 days. At each visit, they performed various
functional activities (i.e. sitting, standing, walking, running, chair rising,
box lifting and vertical jumping), while marker trajectories were recorded
using a skin marker-based 10-camera optical motion capture system and used to
calculate sagittal and frontal plane spinal curvature angles as well as
transverse plane segmental rotational angles in the lumbar and thoracic
regions. Between-session reliability for continuous data and discrete
parameters was determined by analyzing systematic errors using one sample
T-tests as well as by calculating intraclass correlation coefficients (ICCs)
and minimal detectable changes (MDCs). Results and Significance: The analysis
indicated high relative consistency for sagittal plane curvature angles during
all activities, but not for frontal and transverse plane angles during walking
and running. MDCs were mostly below 15{\deg}, with relative values ranging
between 10% and 750%. This study provides important information that can serve
as a basis for researchers and clinicians aiming at investigating longitudinal
effects of treatment interventions on spinal motion behavior in patients with
spinal pathologies.
