Background/Purpose: Idiopathic inflammatory myopathies (IIM) are characterized by symmetrical proximal muscle weakness. Accurate and reliable assessment of muscle strength is important for diagnosis, monitoring, and clinical trials. Manual Muscle Testing (MMT) is widely used due to its simplicity but has notable limitations, particularly in inter-rater reliability. (1) Isokinetic dynamometry, the gold standard, is accurate but impractical for routine use. Hand-held dynamometry (HHD) offers a portable and objective alternative, and studies have shown that HHD outperforms MMT in intra-rater reliability and clinical responsiveness in IIM. However, prior studies were limited by homogenous evaluator profiles. (2)

Methods: This study was aimed to evaluate the inter-rater reliability of HHD and its correlation with MMT scores. Forty-six patients who were evaluated at the University of Pittsburgh Myositis Center between July 2020 and March 2021 and classified as IIM (DM/PM/NM) as per 2017 EULAR/ACR Classification criteria were included. Muscle strength of bilateral deltoid and iliopsoas groups was assessed using a Micro FET2 dynamometer and MMT of 9 muscle group (MMT-9). HHD was conducted sequentially by two blinded examiners—one experienced male attending and one female rheumatology fellow who underwent a 20 minute training on assessing HHD. Inter-rater reliability was analysed using intraclass correlation coefficients (ICCs) and Pearson correlation.

Results: In the cohort of 46 patients, the mean age was 56 years with 80% (n=37) females and 80% (n=37) Caucasians. Dermatomyositis (n=23) and necrotizing myopathy (n=12) were the most common subtypes of IIM. The mean MMT score in the combined cohort was 151.Physician assessment showed excellent inter-rater reliability: ICCs were 0.92 (upper extremity), 0.93 (lower extremity), and 0.95 (four-limb) among the fellow and attending. Correlation coefficients were strong across patient subgroups stratified by sex and race. The correlation between HHD and MMT scores was also strong [r = 0.85 (fellow), r=0.86 (attending)], with consistent findings for both upper and lower extremity muscle groups. A ceiling effect was noted with MMT, as HHD captured a wide range of values even at maximum MMT score. Variability was greater in iliopsoas testing, likely due to the influence of examiner strength when assessing stronger muscle groups. Subgroup analysis of DM and non-DM patients revealed similar reliability and correlation patterns.

Conclusion: This study confirms the high inter-rater reliability of HHD in assessing muscle strength in IIM, even among raters with different levels of clinical experience and gender. HHD correlated well with MMT and demonstrated additional utility in detecting subtle strength variations, supporting its clinical application in myositis care and research.References-1. Rider LG, et al; International Myositis Assessment and Clinical Studies Group. Defining Clinical Improvement in Adult and Juvenile Myositis. J Rheumatol. 2003 Mar;30(3):603-172. Saygin D, et al. Hand-held dynamometry for assessment of muscle strength in patients with inflammatory myopathies. Rheumatology (Oxford). 2021 May 14;60(5):2146-2156

Figure 1:- Correlation plots for handheld dynamometry measurements of fellows and attending for upper and lower extremities as well as four-limb measurement.

Figure 2:- Combined fellow and attending handheld dynamometry readings across four MMT9 scores grading severity of weakness

Figure 3: Correlation of adjusted total MMT score with four-limb HHD readings performed by fellow (A), attending (B), and the mean of both (C).

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/inter-rater-reliability-of-hand-held-dynamometry-in-patients-with-idiopathic-inflammatory-myopathy/