Lip force in young adults: descriptive values, within-session repeatability, and associated factors in a cross-sectional study

Lip force is an indicator of perioral muscle performance. Factors influencing lip force in young adults remain insufficiently understood, and within-session repeatability has not been fully established. This study aimed to provide descriptive data on lip force, evaluate within-session repeatability, and identify associated factors in young adults. This single-center cross-sectional study included 100 young adults (40 men, 60 women; aged 19–35 years). Lip force was measured five times using a standardized button-pull method; the primary outcome was the mean of the middle three values after excluding the highest and lowest values. Associations were assessed using Welch’s t-test, Pearson’s correlation analysis, and multiple linear regression with HC3 robust standard errors. Within-session repeatability was assessed using ICC(2,1), ICC(2,5), and Bland–Altman analysis. The mean lip force was 11.69 ± 3.76 N, which was greater in men than in women (mean difference, 3.40 N; 95% CI, 1.93 to 4.86; p < 0.001). Lip force was positively correlated with height, body weight, and BMI (all p < 0.001). No statistically significant associations were detected with the assessed occlusal characteristics or occlusal force. In multivariable analysis, male sex (b = 2.450; p = 0.003) and BMI (b = 0.610 N per kg/m²; p = 0.007) were associated with greater lip force, whereas age and occlusal force were not. ICC(2,1) was 0.760 and ICC(2,5) was 0.940. Bland–Altman analysis of trials 1 and 2 showed a mean difference of − 0.754 N, limits of agreement from − 6.066 to 4.558 N, and proportional bias ( p = 0.031). In this study sample, male sex and BMI were associated with greater lip force, whereas no statistically significant associations were detected with the assessed occlusal characteristics or occlusal force. These findings indicate that sex and BMI should be considered when interpreting lip force measurements. This study provides descriptive data on lip force and characterizes within-session repeatability using a standardized button-pull method, which may inform the interpretation of lip force measurements in clinical practice and future research.