Background/Purpose: To evaluate the measurement properties of functional performance measures (FPMs)—broad jump (BJ) and 10-meter run (10MR)—for assessing muscle function and disease activity in children with juvenile dermatomyositis (JDM), and to assess their added value alongside conventional clinical tools.

Methods: This longitudinal cohort study included 39 children with JDM. BJ and 10MR were assessed alongside standard clinical scores. Construct validity and responsiveness were assessed through cross-sectional and longitudinal analyses (linear mixed-effects models, LMMs).

Results: FPMs correlated significantly with established disease activity and muscle function measures (Figure 1). While 10MR and BJ demonstrated moderate responsiveness (standardized response mean = -0.56 and 0.31, respectively), longitudinal analyses (Figure 2) confirmed that changes in FPMs closely tracked with improvements in Childhood Myositis Assessment Scale (CMAS) and manual muscle testing (MMT8) over time. Within the clinically inactive disease (CID) group, functional heterogeneity was evident (Figure 3A). Cluster analysis based on CMAS and MMT8 identified a subgroup with relatively low strength scores and poor FPM performance (Figures 3B-C), while a more comprehensive four-variable clustering using BJ, 10MR, CMAS, and MMT8 further stratified CID patients into three distinct phenotypes (Figure 3D). Notably, one subgroup exhibited high CMAS/MMT8 scores but reduced FPMs, suggesting residual functional impairment not captured by traditional assessments. Low internal consistency (Cronbach’s α =0.44) further suggested that FPMs capture distinct dimensions of muscle function beyond CMAS/MMT8.

Conclusion: FPMs are quick, non-invasive, and clinically feasible tools that demonstrate good measurement properties. By capturing dynamic aspects of muscle function overlooked by CMAS and MMT8, FPMs may enhance clinical assessment and support individualized management in JDM.

Figure 1. Relationship between FPMs and disease activity scores in JDM.

(A-C) Correlation of baseline FPMs (BJ and 10MR) with PGA Muscle (A-B) and CMAS (C). (D-F) Relationship between broad jump and disease activity scores (PGA Muscle, MMT8, and CMAS) across all follow-up data. (G-J) Relationship between the 10-meter run and disease activity scores (PGA, PGA Muscle, MMT8, and CMAS) across all follow-up data. The red line represents the linear regression model fitted to the data, with the shaded area indicating the 95% confidence interval. The regression coefficient (β) and p-value are annotated on the plots. Statistical analyses were performed using Spearman correlation (A-C) and LMM (D-J).

Figure 2. Change in FPMs and disease activity scores between follow-up visits in JDM. (A-B) Relationship between changes in BJ and changes in disease activity scores (ΔCMAS and ΔMMT8, respectively) across all follow-up data. (C-D) Relationship between changes in 10MR and changes in disease activity scores (ΔCMAS and ΔMMT8, respectively) across all follow-up data. The red line represents the linear regression model fitted to the data, with the shaded area indicating the 95% confidence interval. The regression coefficient (β) and p-value are annotated on the plots. Statistical analyses were performed using LMM.

Figure 3. Evaluation of CMAS, MMT8 and functional performances in patients with JDM in clinically inactive disease (CID).

(A) Heatmap depicting two patient clusters within the CID group, based on CMAS and MMT8 scores. (B) Comparisons of BJ and 10MR performance between Cluster 1 and Cluster 2. (C) Comparisons of 10MR and BJ performance between high and low CMAS groups, as well as high and low MMT8 groups, in CID patients across all follow-up data. (D) Heatmap depicting three patient clusters within the CID group, based on CMAS, MMT8 scores, BJ and 10MR (inverted). Statistical analyses were performed using the LMM and Mann-Whitney U test with * p < 0.05 and ** p < 0.01.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/beyond-muscle-strength-functional-performance-measures-enhance-assessment-of-disease-activity-in-juvenile-dermatomyositis/