This study evaluated the progression of muscle changes observed in patients with myotonic dystrophy type 1 (DM1) at baseline (T1) and their 5-year follow-up (T5) via whole body muscle magnetic resonance imaging (MRI). A total of 14 patients with DM1 were recruited in this longitudinal study. Implemented motor function scales included a 6-minute walk test (6MWT), Medical Research Council Sum-Score (MRCSS), and Muscular Impairment Rating Scale (MIRS). Thirty-five upper and lower limb muscles were semi-quantitatively evaluated using Mercuri scores.At T1, the calf muscles including gastrocnemius, soleus, and tibialis anterior muscles were the most severely affected, compared to those in other muscle groups. The changes in these muscles were significantly correlated with the 6MWT results, MIRS, and CTG repeat number; however, at T5, these parameters only showed a correlative tendency. Among upper extremity muscles, the latissimus dorsi, serratus, and rhomboid muscles showed prominent involvement at T1 and significant progression at T5, while the teres muscles showed minimal changes after 5 years.This study illustrates the slow progression of DM1 via whole body muscle MRI, showing significant progression in calf muscle changes. More studies are warranted to understand the clinical and imaging correlations in patients with DM1.