Secondary Consequences of Juvenile Idiopathic Arthritis in Youth with Knee Joint Involvement Compared to Healthy Controls

Background: Juvenile Idiopathic Arthritis (JIA) is the most common childhood rheumatic disease. Given the heterogeneity of JIA and advances in treatment approaches, there is a persistent lack of consensus regarding the secondary consequences of JIA. This study assessed body structure and function and physical activity outcomes in youth with JIA compared to their healthy peers. Methods: Youth with JIA (n=25; 10-20 years old) and age and sex matched healthy peers (n=25) participated in two data collections (one week apart) (Ethics ID# REB15-3125). Day 1 testing included a triple single leg hop test (TSLH) and an incremental bike test. Participants then recorded their physical activity for one week using the waist worn ActiGraph. Day 2 testing consisted of a dual-energy x-ray absorptiometry scan. Outcomes included: moderate to vigorous physical activity (MVPA), peak oxygen uptake (VO2peak), maximal TSLH distance (% leg length), and fat mass index (FMI). Data were assessed for normality and outliers. Pair differences were analyzed in R (R Core Team, Austria) using t-confidence intervals (CI) or the Hodges-Lehmann method with Bonferroni correction. A significant difference was accepted if CIs excluded zero. The effect of sex on outcomes was examined using pair difference medians and first and third quartiles. Results: CIs for all study outcomes contained zero and did not meet criteria for significant differences between pairs. However, based on the large lower limit and small upper limit of the CIs for MVPA (median -18.6min 97.5%CI [-44.3,2.0]), a larger sample might have concluded that youth with JIA perform substantially less MVPA than their healthy peers. Further, based on pair difference point estimates and interquartile ranges, sex may be a confounder with regards to physical activity participation and may be an effect modifier of the TSLH. Conclusions: The results of this study provide further evidence on the consequences of JIA on physical activity participation in youth. These findings highlight the importance of considering the effects of sex, particularly with regards to physical activity and TSLH outcomes. Further research across JIA subtypes is needed to inform the targeted exercise therapy programs to restore healthy patterns of physical activity participation.