Background: Manual muscle testing has been widely used for the evaluation of muscle strength in myositis, yet less attention has been devoted to the evaluation of muscle function and endurance. Objective: Our objective was therefore to compare the responsiveness to change of muscle strength, endurance and functional testing following induction therapy for severe myositis flare (requiring high-dose corticosteroids and combined immunotherapy) in patients with a diagnosis of dermatomyositis, immune-mediated necrotizing myopathy, or overlap myositis. Methods: Muscle status was assessed at baseline and after mean 6.4 ± 1.9 months, using the MRC-5 scale, along with endurance (Barre and Mingazzini maneuvers) and functional evaluation (e.g. chair rise) and responsiveness to change was evaluated using the Standardized Response Mean (SRM) and Standardized Mean Difference. Results: Among the 49 patients included, the strongest responsiveness to change was observed in the muscle testing of the psoas and deltoids (SRM: 1.23 and 1.16, respectively). Noticeably, endurance testing also demonstrated strong responsiveness (SRM: 1.05 and 0.96, respectively), compensating for the poor discriminatory ability of muscle testing and permitting to overcome its ceiling effect. Conclusion: Functional and endurance testing provide simple and reliable measures complementing the testing of select proximal muscle groups to evaluate responsiveness to intervention in myositis patients in daily clinical practice. Interest of functional and endurance testing should be evaluated prospectively as outcome measures in myositis clinical trials.