Lip Force in Healthy Young Adults: Reliability, Reference Values, and Associated Factors in a Cross-Sectional Study

<title>Abstract</title> <p>Background Lip force is an important component of oral function and has been used as an indicator of perioral muscle performance. Although lip force has been associated with oral functional conditions, factors influencing lip force in healthy young adults remain insufficiently understood. Furthermore, the reliability of lip force measurements is not fully established. Therefore, this study aimed to establish reference values for lip force, evaluate measurement reliability, and identify factors associated with lip force in healthy young adults. Methods This cross-sectional study included 100 healthy adults (40 men, 60 women; aged 20–35 years). Lip force was measured using a standardized button-pull method with a digital force gauge. Associations between lip force and sex, body size, occlusal characteristics, and occlusal force were assessed using Welch’s t-test, Pearson’s correlation analysis, and multiple linear regression analysis. Measurement reliability was evaluated using the intraclass correlation coefficient (ICC[2,1]) and Bland–Altman analysis. Results Mean lip force was 11.66 ± 3.74 N. Men exhibited significantly greater lip force than women (13.69 ± 3.98 N vs. 10.30 ± 2.89 N, p &lt; 0.001; Cohen’s d = 1.01). Lip force was positively correlated with height, body weight, and body mass index (BMI; all p &lt; 0.001). No significant associations were observed between lip force and occlusal characteristics or occlusal force (r = 0.01, p = 0.921). Multiple linear regression analysis adjusted for age demonstrated that male sex (β = 2.461, p = 0.002) and BMI (β = 0.596, p = 0.007) were independently associated with greater lip force, whereas age and occlusal force were not. The button-pull method demonstrated good reliability, with an ICC(2,1) of 0.815. Bland–Altman analysis revealed a small fixed bias (− 0.53 N) and no proportional bias. Conclusions Lip force in healthy young adults was associated with systemic physical characteristics, particularly sex and BMI, but not with occlusal characteristics or occlusal force. These findings suggest that lip force reflects perioral muscular function and overall physique rather than masticatory muscle performance. The standardized button-pull method demonstrated good reliability and may be useful for both clinical assessment and future research investigating oral muscular function.</p>