## 2 Materials and Methods
### Statistical Analysis  
The sample size was calculated using the statistical program G*Power v. 3.1.9.4 ([PERSON], Universitat Kiel, Kiel, Germany, accessed on 30 September 2023) to achieve a statistical power of 80%, with an effect size (f) of 0.25 (calculated based on a partial \(\eta\)2 = 0.06) and a statistical significance of \(\alpha\) = 0.05 [21,58]. It was found that a sample of 28 was sufficient to detect significant differences. For descriptive and inferential statistical analysis, the Statistical Package for the Social Sciences (SPSS(r)) software (Version 28.0) was used, with a significance level of 0.05 and a confidence interval of 95%. To assess the normality of data distribution, the Shapiro-Wilk test was initially applied, followed by a visual inspection of histograms. Depending on the data, violations of statistical assumptions of normality required logarithmic or inverse transformations. The presence of extreme outliers was checked, with the criterion of three standard deviations used to consider an extreme outlier. These cases were considered measurement errors or atypical exceptions and were reported as missing. The homogeneity of variance between groups and the sphericity of covariates were determined based on [PERSON]'s test and [PERSON]'s test, respectively. The Greenhouse-Geisser correction was used when sphericity demands were not assumed. Variables normally distributed were described as the mean \(\pm\) SD, and data with deviations from normality were expressed as the median with the interquartile range. Data that followed the assumptions of normality, homogeneity, and sphericity underwent analysis of variance (ANOVA) repeated measures to analyze the effects of breast implants on COP displacement and mean velocity, angles of spinal curvatures, and the activity levels of SCM, ESC, LS, UT, LT, LM, and Tra/IO muscles. \begin{table}
\begin{tabular}{l l} \hline
**Muscle** & **Muscle Testing** \\ \hline Sternocleidomastoid & Lying supine, hold the neck in anterolateral flexion (90\({}^{\circ}\) contralateral rotation) using isometric neck contraction against manual resistance at mid-range [53]. \\ \hline Eretor spinae cervical & Lying prone position, using isometric neck contraction against manual resistance at mid-range [53]. \\ \hline Levator scapulae & Prone T—thumbs up; In the prone position, the shoulder is horizontally abducted and externally rotated (elbow fully extended) as the examiner applies manual pressure downward (above the elbow) to resist adduction of the scapula and extension of the shoulder [54]. \\ \hline Upper trapezius & Seated T; In a seated position, the shoulder is abducted to 90\({}^{\circ}\) (elbow fully extended) as resistance is applied above the elbow in a downward direction (to resist abduction) [54]. \\ \hline Lower trapezius & Prone V—thumbs up; In the prone position, the arm is raised above the head in line with lower trapezius muscle fibers (elbow fully extended) as resistance is applied above the elbow against further arm raise [54]. \\ \hline Pectoralis major (clavicular portion) & The shoulder horizontally abducts with the shoulder and elbow flexed at 90\({}^{\circ}\). The participant provides maximal force while attempting to horizontally adduct the arm [55]. \\ \hline Lumbar Multifidus & Lifting the trunk from de prone position with manual resistance applied to the upper thoracic area [56]. \\ \hline Transversus abdominis/ Internal Obliques & The participant is in a prone position with knees under the hips and hands under the shoulders. The participant raises the dominant arm with a 180\({}^{\circ}\) shoulder flexion and the contralateral leg with hip extension [57]. \\ \hline \end{tabular}
\end{table} 
Table 2: MVIC muscle testing. In variables where the repeated measures ANOVA showed statistical significance, pairwise comparisons with Bonferroni post hoc were conducted to identify specific differences between the implant conditions. Data that did not follow the assumptions of normality and homogeneity underwent non-parametric analysis.