Associations of Physical Activity, Sedentary Behaviour, and Sleep with Somatotype Components and a Body Shape Index in Young Adults

Background: The Body Shape Index (ABSI) and somatotype components represent different approaches to assessing body morphology. This study aimed to examine the associations between physical activity, sedentary behaviour, sleep duration, ABSI, and somatotype components in young adults. Methods: A cross-sectional study was conducted among 371 university students from Poland and Slovenia (202 men and 169 women; mean age: 21.46 ± 1.07 years). ABSI and somatotype components were determined based on anthropometric measurements. Data on physical activity, sedentary behaviour, and sleep duration were obtained using a seven-day activity record. Associations were assessed using Spearman’s rank correlation coefficients and multiple linear regression models adjusted for sex, age, and country of origin. Results: Weak negative correlations were observed between walking time and ABSI (rs = −0.135; p < 0.01) and between sleep duration and mesomorphy (rs = −0.111; p < 0.05). Additional weak associations were identified in analyses conducted separately for women and men. After adjustment for potential confounders, walking time was positively associated with endomorphy (β = 0.158; p = 0.009) and mesomorphy (β = 0.117; p = 0.041), and negatively associated with ectomorphy (β = −0.192; p = 0.002). All significant associations were characterized by small effect sizes. No significant associations were found for ABSI in the primary adjusted model for the total sample. Conclusions: Despite the small magnitude of the observed associations, different patterns of associations were observed for ABSI and the somatotype profile, reflected by the combined pattern of its endomorphic, mesomorphic, and ectomorphic components. However, the identified associations were few in number and weak in strength; therefore, further research is needed to better explain their significance in young adult populations.