Introduction: Three-dimensional (3D) gait analysis using optoelectronic stereophotogrammetry and force platforms is widely employed to assess joint loading and movement patterns [1]. However, marker-based motion capture systems are susceptible to measurement errors, particularly soft tissue artefacts (STA) arising from skin movement relative to the underlying bone [2]. These errors are even more pronounced in osteoarthritis (OA) research, where nearly 90% of participants have a body mass index (BMI) of 30 or higher, classifying them as obese [3, 4]. Accurate tracking of anterior pelvic anatomical landmarks is therefore crucial, especially in individuals with higher BMI. This study evaluated the impact of two methods for anterior pelvic landmark tracking on gait kinematics across different BMI
groups.
Methods: Twenty participants were evenly divided into two groups based on their BMI, classified as ”low BMI” (19–27.5 kg/m2) or ”high BMI” (>27.5 kg/m2). Each participant completed a 3D gait analysis on an instrumented treadmill using a modified CGM2.4 marker set, which included additional sacrum markers and pointer-based ASIS tracking. Two methods for anterior pelvic landmark tracking were compared: skin-mounted ASIS markers (ASIS𝑠) and pointer-based virtual ASIS tracking (ASIS𝑣). Their effects on the estimation of ASIS location, pelvic orientation, and hip joint centre (HJC) were
evaluated. Furthermore, intra-rater reliability of the pointer technique was assessed using intraclass correlation coefficients (ICC) and standard error of measurement (SEM), and the relationship between
BMI and pointing errors was analysed. The effect of using multiple pointer measurements on reliability was also examined.
Results: Data from twenty participants showed no statistically significant differences between ASIS𝑠 and ASIS𝑣 within the low BMI group. In contrast, the high BMI group exhibited statistically significant
and clinically relevant differences across all directions in ASIS trajectories, pelvic obliquity, and HJC estimation. A positive correlation was found between BMI and the mean absolute error in ASIS
dentification, with the largest deviations occurring in the anterior-posterior direction. Intra-rater reliability was generally high (ICC > 0.90); however, the medial-lateral direction within the high BMI group showed notably lower reliability, with ICC improving from 0.29 to 0.77 and SEM decreasing from 11.92 mm to 6.79 mm after averaging four pointer measurements.
Conclusion: Within the low BMI group, both ASIS𝑠 and ASIS𝑣 could be used for gait kinematics assessment. However, within the high BMI group, the two methods could not be used interchangeably due to statistically significant differences in ASIS tracking, pelvic orientation, and HJC estimation. ASIS𝑣 appeared to better estimate pelvic kinematics, as it was less affected by STA associated with skinmounted markers. Nevertheless, the accuracy and reliability of ASIS𝑣 require further validation using imaging techniques such as fluoroscopy, enabling direct measurement and potential correction of STA.

Due to neuromuscular capacity decline, sit-to-stand, an essential daily life activity, is increasingly difficult to perform for elderly. In practice, elderly frequently use compensatory arm-strategies in sit-to-stand. However, the specific advantages of these compensatory strategies are unclear. The focus of this research is to study the influence of thigh push-off, a common compensatory strategy, on the stability of sit-to-stand for both the young and elderly. Motion data was retrieved from 50 young and elderly participants who performed sit-to-stand with and without thigh push-off. To mimic realistic daily-life strategies, participants were not restricted in sit-to-stand style in any way but arm-use. The experimental sit-to-stand data was fitted to a pendulum model with feedforward and feedback control to simulate the stability limits of the observed sit-to-stand strategies. The model allows us to explore stability limits without the need of perturbation. For each arm-strategy and participant a stability basin, i.e. all potential trajectories of motion, was formed. We compared the stability basins between arm-strategies and between age-groups. The size of the computed stability basin for thigh push-off was larger than for no arm push-off implicating thigh push-off is a more stable sit-to-stand strategy. The difference between arm strategies was larger for the elderly than for the young participants. Overall, the stability basins of the elderly were larger compared to those of the young participants, for both arm strategies. The resulting stability basins suggest that thigh push-off increases the stability of sit-to-stand and thus could indeed be an effective compensatory strategy. Stability basin shape differences indicate that without thigh push-off elderly compensate for possible early sit-down and a step halfway sit-to-stand, and with the thigh push-off strategy only for early sit-down. The age-groups differences confirm stability basins quantify the stability of the observed movement strategy rather than the stability of the participants. We observed that elderly use more precautious sit-tot-stand movements.