Patients with dermatomyositis (DM) and polymyositis (PM) have reduced muscle endurance. The aim of this study was to streamline the Functional Index-2 (FI-2) by developing the Functional Index-3 (FI-3) and to evaluate its measurement properties, content and construct validity, intra- and inter-rater reliability.A data set of the previously performed and validated FI-2 (n=63) was analyzed for internal redundancy, floor and ceiling effects. The content of the FI-2 was revised into the FI-3. Construct validity and intra-rater reliability of FI-3 were tested on 43 DM and PM patients at two rheumatology centers. Inter-rater reliability was tested in 25 patients. The construct validity was compared with the Myositis Activities Profile (MAP), Health Assessment Questionnaire (HAQ), and Borg CR-10 using Spearman's correlation coefficient.Spearman's correlation coefficients of 63 patients performing FI-3 revealed moderate to high correlations between shoulder flexion and hip flexion tasks and similar correlations with MAP and HAQ scores; there were lower correlations for neck flexion task. All FI-3 tasks had very low to moderate correlations with the Borg scale. Intra-class correlation coefficients (ICC) of FI-3 tasks for intra-rater reliability (n = 25) were moderate to good (range: 0.88-0.96). ICC of FI-3 tasks for inter-rater reliability (n = 17) were fair to good (range: 0.83-0.96).The FI-3 is an efficient and valid method for clinically assessing muscle endurance in DM and PM patients. FI-3's construct validity is supported by the significant correlations between functional tasks and the MAP, HAQ, and Borg CR-10 scores.