An Exploratory Analysis of Postural Stability in Acrobatic Shoulder-Stand Pyramids Using Inertial Sensors: The Interplay of Top-Athlete’s Variations and Base-Athlete’s Stance Configurations

Abstract

In static acrobatic gymnastics pyramids, maintaining stability in a multi-person system is critical, yet unconstrained floor baselines and base-to-top pairing mechanics remain poorly quantified. This study evaluated postural stability in shoulder-stand pyramids, examining the interplay of two specific top-athlete variations (lighter Pyramid T1 vs. heavier Pyramid T2) and base-athlete stances (parallel vs. tandem). Five elite base-athletes were monitored while supporting two top-athlete variations, a lighter (Pyramid T1) and a heavier (Pyramid T2) during a standard static shoulder-stand pyramid across parallel and tandem foot placement configurations (three trials in each Pyramid variation). Unconstrained free floor-standing baselines were also recorded for base-athletes and for tops prior to and following pyramid trials. The root mean square (RMS) of the resultant 3D free acceleration (Xsens MTw Awinda inertial sensors sampling at 100 Hz, Xsens MT Manager version 4.6.5 software) positioned at the bases’ and the tops’ shanks (right and left) was used to assess postural stability. A five-point median filter followed by a zero-phase, 2nd-order forward and reverse Butterworth low-pass filter (yielding an effective 4th-order response at 5 Hz and 10 Hz cutoffs) was applied to all signals (MATLAB R2025b). Stance-envelope dimensions were calculated from rectangular boundaries fitted to the base’s foot outlines. Non-parametric Spearman rank correlations (ρ) and parametric correlations (r,R2) were used to test the interbase-consistency and base-to-top coupling. Two-way repeated measures ANOVAs (pyramid x stance configurations) were applied with primary analytical emphasis placed on descriptive effect sizes alongside exact p-values (SPSS v30, p < 0.05). The acrobatic tandem stance expanded the parallel stance-envelope area by 148.3% and its width by 24.7%. When base-athletes transitioned from free standing to pyramids there was a substantial acceleration RMS increase (Pyramid T1: +24.3% to 83.6%, Pyramid T2: 47.2% to 166.2%). Supporting the heavier top-athlete (Pyramid T2) significantly increased the bases’ resultant acceleration RMS by +37% to +48% across both filter cutoff thresholds (p<0.05). Furthermore, top athletes exhibited differential behaviors: while Top 2 displayed lower acceleration RMS than Top 1, she experienced a greater acceleration surge when in Pyramid (+238.4% to +266.2%). The bases’ and the tops’ acceleration profiles did not exhibit parallel responses, indicating decoupled rather than mirrored stability adjustments. Furthermore, when the acceleration RMS was normalized to stance-envelope dimensions, significant pyramid x stance interaction (p<0.05) was observed in the anteroposterior but not the mediolateral acceleration. Base-athlete stability varies significantly across top-athlete variations and stance geometries. Evaluating unconstrained floor baselines alongside spatial stance boundaries is essential for capturing structural loading dynamics in multi-person athletic tasks.