Shape-model scaled gait models can neglect segment markers without consequential change to inverse kinematics results

Inverse Kinematics (IK) is an optimisation to estimate joint angles from motion capture data, where marker trajectories and weighting strategies determine the outcome. Skin-mounted markers are subject to Soft Tissue Artefact (STA), particularly thigh markers. Our first aim was to test the effect of neglecting thigh markers on IK results across different markersets. Our second aim was to investigate inter-markerset differences using varying weighting strategies. Twenty participants participated in a treadmill walking motion capture session. Inverse kinematic analysis was performed using three markersets, termed Contemporary (segment clusters), NoThigh (Contemporary without thigh markers), and Traditional (modified Helen Hayes). Seven weighting schemes were used with varying magnitudes with each markerset. Joint angles (comprising tri-planar hip angles, and sagittal knee and ankle angles) were compared across all three conditions. NoThigh and Traditional generated joint angle results that differed from the Contemporary markerset by a median of 1.2° and 1.5°, respectively. Non-sagittal hip angles differed the most. Most average joint angle differences were smaller than previously-estimated STA error. NoThigh generated less difference from Contemporary than Traditional for the hip and knee joints. Intuitively, weighting strategies that heavily favour shared markers resulted in IK results with closer inter-markerset agreement. Thigh markers can be neglected without introducing more error than STA, and different markersets (with validation) can be compared against each other when using strategic weighting. This creates new research pathways for collaboration and data re-use, as well as freeing researchers (and participants) of their attachment to thigh markers.