Muscle endurance deficits in myositis patients despite normal manual muscle testing scores

Introduction: It is unclear whether quantitating muscle endurance adds nonredundant information useful for the care of patients with muscular disease. Methods: Records were retrospectively reviewed for all Johns Hopkins Myositis Center patients with a muscle endurance assessment (n = 128, 226 patient-visits). Muscle endurance and strength were quantitated with the Myositis Functional Index-2 (FI2) and manual muscle testing (MMT), respectively. Results: Composite FI2 muscle endurance scores were comparable in inclusion body myositis (n = 58), dermatomyositis (n = 31), and polymyositis (n = 39). Overall, muscle endurance correlated with and evolved similarly to strength, inversely to serum creatine kinase. However, in patients with normal or near-normal strength (mean MMT > 9.75/10), muscle endurance was typically abnormal and highly variable (mean FI2, 5.6/10; interquartile range, 3.3-7.8/10). Discussion: Muscle endurance testing may identify muscle impairment inadequately described by MMT, particularly in patients with high MMT scores. Muscle Nerve 59:70–75, 2019.