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

It is unclear whether quantitating muscle endurance adds nonredundant information useful for the care of patients with muscular disease.

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.

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).

Muscle endurance testing may identify muscle impairment inadequately described by MMT, particularly in patients with high MMT scores. Muscle Nerve 59:70-75, 2019.