Muscle function in children with juvenile idiopathic arthritis: the Linking Exercise, Activity and Pathophysiology (LEAP) study

To assess muscle function during physiologic movements in children with juvenile idiopathic arthritis (JIA), and associated factors. Children with JIA aged 8–16 years participating in the multicentre Linking Exercise, Activity and Pathophysiology (LEAP) study underwent jumping mechanography measuring peak power (Watts per kilogram of body mass in 2-legged jump) and peak force (Newtons per kilogram of body mass in 1-legged jump) at baseline. Measurements were converted to age- and sex-specific Z-scores using a reference population of healthy children. One sample Wilcoxon tests assessed the hypothesis that measured values were different than those of the reference population. Multivariable linear regression assessed the association of disease activity, pain, fatigue, and usual physical activity with peak muscle power and force. We included 199 children with JIA recruited at 3 LEAP centres at a median age of 12.3 years (IQR 9.7, 14.8) and a median of 3.6 years after JIA diagnosis (IQR 0.4, 6.7); 63.4% were female and 40.2% had oligoarthritis. Both power (median Z-score − 0.6, IQR: -1.4, 0.1; p < 0.001) and force (median Z-score − 0.8, IQR: -2.0, 0.3; p < 0.001) were lower in children with JIA than in the reference population. Lower levels of physical activity were significantly associated with both lower power and lower force. Higher levels of pain were associated with lower power. In this cross-sectional study, a detailed analysis of muscle function showed significant deficits in JIA, emphasizing the need for optimal control of arthritis and monitoring of physical activity.