**Background:** Physical activity has well-established health benefits, yet activity levels decline during childhood and adolescence. The relationship between physical activity and sedentary behavior is complex—they are not mutually exclusive, and multiple behavioral clusters exist. Psychological stress is bidirectionally associated with activity-related behaviors, but evidence on dominant stressors in preadolescence and their link to activity patterns is scarce. This study aimed to identify clusters of physical activity/sedentariness among preadolescents and investigate their association with self-reported sources of stress.
**Methods:** This school-based cross-sectional study was conducted during the 2014–2016 school period. Forty-seven primary schools were randomly selected from the Peloponnese peninsula (Sparta, Kalamata, Pyrgos), Heraklion (Crete), and greater Athens, covering approximately 75% of the total Greek population. All children aged 10–12 years in fifth and sixth grades were invited; 1,728 children (54% girls) were enrolled with a participation rate of 95–100% across schools. Physical activity was assessed using the validated Physical Activity and Lifestyle Questionnaire (PALQ), capturing time spent in physical activity, screen-based sedentary activities (TV, computer, electronic games), and non-screen-based sedentary activities (studying, reading, board games). Three composite variables were created: frequency of physical activity (minutes/week), screen-based sedentary time, and non-screen-based sedentary time. Sources of stress were evaluated using questions adapted from the Adolescent Stress Questionnaire (ASQ) covering: parents' requirements/expectations, teachers' requirements/expectations, school performance, pressed schedule of activities, and peer pressure. K-means cluster analysis with K-nearest-means classifier was applied to identify activity patterns, run 100 times with two to five clusters tested. A three-cluster solution was deemed optimal based on dendrogram and Bayesian Information Criterion. Associations were evaluated using Pearson Chi-square tests, one-way ANOVA, and logistic regression.
**Key Results:** Three clusters were identified: Cluster 1 (Inactive-Non-Sedentary, 43% of children) had both physical activity and sedentary time below average; Cluster 2 (Active-Non-screen Sedentary, 45%) had physical activity above average and screen-based sedentary time below average; Cluster 3 (Inactive-Sedentary, 12%) had sedentary time above average and physical activity below average. Children in Cluster 3 reported the highest screen-based sedentary time (3,292.4 min/week, SD 1,274.1) and non-screen-based sedentary time (2,327.3 min/week, SD 1,365.0), while Cluster 2 had the highest physical activity time (268.8 min/week, SD 128.7). Significant differences were found across clusters for sleep duration (p=0.002; Cluster 3: 9.3 hours vs. 9.7 hours in Clusters 1 and 2), fast food consumption (p<0.001; 53.8% of Cluster 3 consumed fast food 1–2 times/week vs. 33.1% in Cluster 1 and 27.9% in Cluster 2), and KIDMED Mediterranean diet score (p=0.003; Cluster 2: 5.0, Cluster 3: 4.7, Cluster 1: 4.5). Cluster 3 was dominated by boys (62.7% vs. 40.6% in Cluster 1 and 47.7% in Cluster 2; p<0.001). Overall, 87.9% of children reported having stress, with no significant difference across clusters (p=0.135). However, parental requirements/expectations were significantly associated with cluster membership (p=0.009): 47.6% of children in Cluster 3 reported stress from parental expectations, compared to 36.7% in Cluster 1 and 30.5% in Cluster 2. Logistic regression showed that compared to Cluster 3, children in Cluster 1 had 36% lower odds of being stressed by parental expectations (OR=0.64, 95% CI 0.41–0.99), and children in Cluster 2 had 51% lower odds (OR=0.49, 95% CI 0.32–0.76). No other sources of stress showed significant associations with cluster membership.
**Clinical Implications:** This study demonstrates that parental expectations are the key stressor differentiating activity-related behavioral clusters in Greek preadolescents, with the highest burden in the Inactive-Sedentary group. The clustering of unhealthy behaviors—sedentary patterns, poor diet, and shorter sleep—in the same children underscores the need for integrated rather than single-behavior interventions. Tailor-made counseling for parents about how their expectations may shape children's activity behaviors is warranted. Limitations include the cross-sectional design (causality cannot be inferred), potential self-report bias, lack of accelerometer data, and limited generalizability beyond the Greek preadolescent population. Despite these limitations, identifying parental expectations as a modifiable psychosocial factor offers a concrete target for public health interventions aimed at promoting physical activity and reducing sedentary behavior in this age group.