How are we representing the trunk? A narrative review on marker sets for kinematics analysis of neurological conditions.

BACKGROUND: Trunk movement plays a vital role in daily activities, influencing efficiency, balance, and musculoskeletal health-especially in individuals with neurological conditions. However, three-dimensional (3D) motion analysis of the trunk lacks standardisation, limiting both cross-study comparisons and clinical application. This review synthesises current approaches to trunk modelling, including marker setups, segment definitions, and kinematic outcomes, in studies involving participants with neurological conditions. METHODS: A systematic search of six databases, including MEDLINE and Scopus, was conducted (June 2021, updated October 2025i) without date restrictions. Eligible studies assessed 3D trunk kinematics using marker-based systems in participants of any age with neurological or neuromuscular conditions. Extracted data included marker configurations, task types, segment definitions, and reported kinematic outcomes. FINDINGS: Fifty-eight studies met the inclusion criteria. Over half (51.72%) modelled the trunk as a single segment, 43.1% focused solely on the upper trunk, and only one study used a multi-segmental model. The most reported movements were flexion-extension (n = 40), lateral bending (n = 29), and axial rotation (n = 28). Overall, gait analysis studies used more markers on the spine but still reported the trunk as a single segment. Marker configurations and outcome metrics reported varied significantly for similar tasks, highlighting a lack of standardisation. INTERPRETATION: Standardised marker setups and segment definitions are essential for improving comparability and clinical relevance. Although multi-segmental models provide deeper insights into compensatory movement patterns, they remain underused. Future research should focus on refining trunk modelling techniques and assessing their impact on rehabilitation to enhance objective clinical evaluation of interventions and disease progression.