Local Dynamic Stability During the 1-Minute Sit-to-Stand Test: Directional Differences and Associations with Body Composition in Healthy Adult Women

<b>Background:</b> The 1 min sit-to-stand test (1-MSTST) is a widely used functional assessment involving repetitive sit-to-stand transitions. This study examined local dynamic stability during the 1-MSTST across three acceleration directions, compared young and middle-aged women, and explored associations between body composition and stability. <b>Methods:</b> Twenty-four young adult women (24.1 ± 5.2 years) and twenty-four middle-aged women (51.4 ± 5.9 years) performed the 1-MSTST. Trunk accelerations were recorded using a tri-axial accelerometer at L5. Local dynamic stability was quantified using the largest Lyapunov exponent (LyE), and movement magnitude using root mean square (RMS). Directional, group, and correlational analyses were performed with correction for multiple testing. <b>Results:</b> Significant directional differences were observed for both LyE and RMS, with all pairwise contrasts between mediolateral (ML), anteroposterior (AP), and vertical (VT) directions remaining significant after correction (<i>p</i> < 0.001). Apparent age effects in LyE were no longer significant after adjusting for cadence, BMI, and multiple testing, indicating no robust age-related difference in local dynamic stability. Body fat percentage showed moderate positive correlations with LyE in the VT (<i>p</i> = 0.003) and AP (<i>p</i> = 0.003) directions. Muscle mass percentage showed a moderate positive correlation with VT LyE (<i>p</i> = 0.002) and moderate negative correlations with ML (<i>p</i> = 0.002) and AP LyE (<i>p</i> = 0.002). <b>Conclusions:</b> Stability during the 1-MSTST differs by direction, with the greatest variability in the mediolateral axis. No independent age effect was found. Higher body fat relates to poorer stability, while greater muscle mass supports better movement control.