Definition of the neutral body posture through three-dimensional local Cardan joint angles

The neutral body posture (NBP) is the result of an equilibrium of internal forces, based on the primarily passive stress and strength of muscles and ligaments, without the influence of external forces, such as gravity. It can be observed in humans with a relaxed muscle tone floating freely in space or in water. Although the NBP has been researched since 1975 for applications in space-related ergonomics, its current definitions are subject to significant limitations, including a small number of observed subjects participating in studies, manual pose detection methods and ambiguous joint angles for pose description. This contribution aims to overcome these limitations and provide a novel NBP definition based on 17 three-dimensional local Cardan joint angles with statistical characterisation. To derive the averaged angle values, a deep learning framework for markerless pose estimation is applied, utilising image data from the literature for individual postures in space and during a parabolic flight for 17 subjects. The resulting angle values and the associated body shape appearances are compared to those in previous publications, showing a reasonable level of agreement. However, accurately representing the small and distal joints of the extremities requires additional specific experimental datasets of these body regions to enable further NBP refinement if needed. The formulated description of the NBP was homogenised and specified unambiguously to facilitate its application in space, ergonomics, and human factors, as well as other possible space-borne and Earth-based scenarios involving computational human body models or digital twins.