Compositional patterns of device-measured movement behaviour in juvenile idiopathic arthritis: results from the multicentre ActiMON study

Children and adolescents with juvenile idiopathic arthritis (JIA) are at increased risk for long-term physical and psychosocial complications, making physical activity (PA) and sedentary behaviour (SB) key modifiable lifestyle factors. Although increasingly acknowledged as relevant, data on the daily distribution of these behaviours in JIA remain scarce. This study aimed to (1) describe the time-use composition of SB and PA intensities in young people with JIA, (2) identify correlates of greater relative time spent in SB, and (3) compare movement behaviour patterns to matched population controls using a compositional data analysis (CoDA) approach. Patients aged 10–20 years with JIA and individually matched population controls wore hip-worn accelerometers (ActiGraph wGT3X-BT) for eight consecutive days. Movement behaviours were categorized into SB, light-intensity PA, and moderate-to-vigorous PA (MVPA) using validated, age-specific cut-points. CoDA with log-ratio transformations was used to model associations and compare groups. Diifferences in movement composition were assessed using adjusted multivariate analysis of variance (MANOVA). Data from 126 matched pairs (mean age: 15.0 ± 2.1 years; 67% female) were analysed. Patients spent, on average, 86% of waking time in SB, 8% in light-intensity PA, and 6% in MVPA. Overall, 76% did not meet the WHO recommendation of an average of ≥ 60 min of MVPA per day. Among those who did, 87% still spent ≥ 75% of their wear time sedentary. A greater proportion of SB relative to PA was associated with female gender (B = 0.13; p = 0.042), higher age (B = 0.06; p < 0.001), and higher BMI (B = 0.01; p = 0.049). Compared to controls, patients spent more time in SB, less in light-intensity PA, and slightly more in vigorous PA (all p < 0.001). Group differences remained significant after adjustment and were consistent across weekdays and weekends. Young people with well-controlled JIA show a distinctly unbalanced movement behaviour composition compared to controls, marked by a predominance of sedentary behaviour—even among those meeting PA guidelines. This highlights the limitations of threshold-based definitions and underscores the importance of assessing full daily movement patterns. Promoting light-intensity PA may offer a feasible strategy to reduce sedentary time, particularly in girls, older adolescents, and individuals with higher BMI. The study was registered in the German Clinical Trials Register (DRKS00022258).