BACKGROUND: In individuals with neuromuscular diseases (NMD), symptoms of muscle weakness, fatigue and pain may limit physical activity. Inactivity leads to reduced physical fitness, which further complicates daily life functioning. Due to inconclusive evidence regarding exercise in NMD, the optimal training approach and strategies to preserve an active lifestyle remain to be determined. The physical activity programme I'M FINE, consisting of individualized aerobic exercise to improve physical fitness and coaching to preserve an active lifestyle, was therefore developed. The primary objective of this study will be to evaluate the efficacy of the I'M FINE programme in terms of improved physical fitness in individuals with slowly progressive NMD, compared to usual care. METHODS: A multicentre, assessor-blinded, two armed, randomized controlled trial will be conducted in a sample of 90 individuals with slowly progressive NMD. Participants motivated to improve their reduced physical fitness will be randomized (ratio 1:1) to the I'M FINE intervention or usual care. The I'M FINE intervention consists of a six-month physical activity programme, including individualized home-based aerobic exercise to improve physical fitness (i.e. peak oxygen uptake), and motivational interviewing coaching (e.g. goal setting, self-management) to adopt and preserve an active lifestyle. Measurements will be performed at baseline, post-intervention, and at 12- and 18-months follow-up. The primary outcome is peak oxygen uptake (VO2 peak) directly post intervention. Main secondary outcomes are physical capacity, muscle strength, self-efficacy, daily activity, quality of life and markers of metabolic syndrome. The primary analysis compares change in VO2 peak post-intervention between the intervention and usual care group, with analysis of covariance. DISCUSSION: The I'M FINE study will provide evidence regarding the efficacy of a physical activity intervention on the physical fitness and active lifestyle over the short- and long-term in individuals with slowly progressive NMD. These outcomes could potentially improve the (inter)national guidelines for efficacy of aerobic exercise programmes and provide insight in achieving a more active lifestyle in NMD. TRIAL REGISTRATION: (5/11/2018): Netherlands Trial Register NTR7609 (retrospectively registered), https://www.trialregister.nl/trial/7344. However, the Ethics Review Committee of the Amsterdam Medical Center (AMC) approved the study protocol on 7/11/2017. No adjustments were made to the approved study protocol before the first participant enrolment and registration. Registration was done after the second participant enrolment and the information in the register corresponds one on one with the approved study protocol.