Segment-embedded frame definition affects the hip joint centre precision during walking.

Due to marker-specific soft tissue artefacts, the choice of the markers defining the segment-embedded frame affects the functional joint centre location, with subsequent error propagation to joint kinematics and kinetics in gait analysis. Our aim was to assess the effect of the number and placement of markers on the precision of the hip joint centre (HJC) location during walking. Twelve markers (2x6) were attached to the pelvis and left thigh of 15 young male subjects. Set-up movements were collected to locate an optimised functional HJC. For all permutations of three from six markers, a HJC was located and subsequently reconstructed in a static trial and during walking. Precision measures with two different definitions of the origin, namely a single maker or their mean-point, and using three, four, five and six were calculated. Finally, marker triads that reduced the variability of the HJC location were determined. Both the number of markers and method for defining the origin significantly affected the HJC precision during static and walking trials. For walking, precision of 39 mm using three markers improved to 5mm using redundant markers and the mean marker position as the segment origin. Markers placed close to the joint gave more consistent results.