Abstract Reduced quality of life in patients with myositis is partly due to impairments in muscle strength, muscle endurance and functional capacity. In a recent randomised controlled trial (RCT) (NCT04486261) we showed that high-intensity resistance training improved quality of life, muscle strength, and endurance. This follow-up study aimed to investigate if these improvements remained persistent one year after completing 16 weeks of high-intensity resistance training. A total of 32 participants (intervention group (IG): 15; control group (CG): 17) were enrolled in the RCT, with 27 (IG: 13; CG: 14) completing the 1-year follow-up. Outcomes were assessed at three time points: baseline (weeks − 4 − 0), post-intervention (weeks 17–18), and 1-year follow-up (weeks 52–54). Outcomes included quality of life (QoL, SF36), functional capacity, muscle endurance (functional index 3 (FI3)), body composition (DEXA), and disease activity/damage, including manual muscle test 8 (MMT8). Training-induced improvements in muscle endurance (FI3) remained at 1-year follow-up, with a mean change of 10.7 (CI95: 2.2;19.1) in favour of IG (p = 0.01). Within-group improvements in IG were sustained for QoL (SF36, physical component summary) (4.8 (CI95: 0.9;8.7), p = 0.02), muscle strength (MMT8) (1.8 (CI95: 0.8; 2.9), p < 0.01), and functional capacity measures (p ≤ 0.04), although not significantly different from CG at 1-year follow-up. Disease activity and disease damage were similar between IG and CG at 1-year follow-up. Patients with myositis completing 16 weeks of high-intensity resistance training showed sustained improvements in muscle endurance at 1-year follow-up compared to controls. Indications of lasting enhancements in quality of life and strength were also present, with no increase in disease activity or damage. NCT04486261.