Author(s): VUILLOD, N., PERTHUY, B., DALIBOT, A., DEGRANGE, M., VINZANT, H., Institution: EPFL, Country: SWITZERLAND, Abstract-ID: 917

INTRODUCTION:
Muscular exhaustion impairs postural control, but quantifying fatigue in daily routine is often impractical. Static postural sway is a promising fatigue-sensitive outcome, traditionally measured with force plates to obtain center of pressure (CoP). With force plates rarely available for field testing, recent work suggests center of mass (CoM) may be a practical alternative measure of balance when CoP is unavailable (Richmond et al., 2021). This study aims to assess pre- to post- training changes in CoM displacement features across sessions during a static postural control task, with CoM computed from markerless motion capture.
METHODS:
9 elite athletes performed 1-min quiet stance trials pre- and post-training. Post-training assessments were conducted 10–20 min post-session to avoid hyperventilation (Sakellari et al., 1997) and limiting muscle stiffness (Kawama et al., 2024). Session intensity was assessed on a 1-10 scale by athletes and coach. Primary outcomes were sway metrics derived from CoM time series, evaluated in the medio-lateral (ML), antero-posterior (AP), and radial directions using spatio-temporal features and non-linear measures: Stabilogram Diffusion Analysis (SDA) (Quijoux et al., 2021) and Sample Entropy (Ramdani et al., 2009). CoM was computed using Body-Segment Inertial Parameters (BSIP) (Dumas and Wojtusch, 2018) from segment positions reconstructed with 3D keypoints. Baseline test–retest reliability across subjects and sessions was assessed using ICC (3,k) and training-induced changes were analyzed as post–pre differences, estimated with a linear mixed-effects model and reported as estimated mean change with 95% confidence intervals.
RESULTS:
We recorded 18 training sessions (mean intensity = 6.94, std = 1.47). Each athlete averaged 11 sessions (std = 2.31) and 7 athletes completed at least 8 high-intensity (≥ 7) sessions.
- At baseline, variance components analyses indicated that session and residual sources accounted for most variability, while the proportion attributable to stable between-subject differences remained small (ICC subject ≈ 0.02–0.26 across metrics).
- At high intensity, a significant pre-to-post change was observed for the long-term SDA slope in the AP axis, with a negative mean ∆ (post < pre). Other metrics showed no clear change (95% CI crossed 0).
CONCLUSION:
CoM-based sway metrics extracted from markerless motion capture showed low subject-specific stability across sessions, indicating that inter-session variability exceeded inter-individual variability, with variability largely driven by session and residual effects. A significant decrease was observed for the long-term SDA slope in the AP axis, indicating reduced long-term drift and improved postural regulation. As quiet standing may be insufficiently challenging for young elite athletes, more demanding dual-task balance conditions (eyes closed, joint feet, etc.) are the next step to enhance sensitivity to individual signatures and fatigue-related changes.