A behavioural intervention, directed to increase physical activity, had a beneficial effect on lower extremity muscle function in patients with myotonic dystrophy type 1 (DM1). Here, we evaluated potential causes of the effect of this intervention with quantitative MRI in the 20 lower extremity muscles of 27 DM1 patients. We showed that it leads to a ~4% average increase in muscle mass, especially in healthy appearing muscle. Fat infiltration was not decelerated. Therefore, we conclude that some muscles in DM1 patients are trainable, preferably early on in the course of the disease.