SELENON-related myopathy (SELENON-RM) and LAMA2-related muscular dystrophy (LAMA2-MD) are congenital neuromuscular disorders presenting with slowly, progressive axial muscle weakness, spinal rigidity, scoliosis and respiratory insufficiency. Currently, no curative treatment options exist, yet promising preclinical trials are ongoing. Clinical trials are expected to start within 5 years. Natural history data and outcome measures for measuring therapy effectiveness were lacking. Therefore, the LAST STRONG Study (a 1.5-year natural history study) started in 2020. With the extended LAST STRONG Study, we aim to further analyze and expand the 1.5-year natural history data on SELENON-RM or LAMA2-MD to provide a detailed clinical description of the Dutch and Flemish cohort. This will enable a smooth transition towards implementation into clinical care and clinical trials. The extended LAST STRONG Study is a prospective, observational natural history study in Dutch-speaking patients of all ages diagnosed with SELENON-RM and LAMA2-MD. Patients will be invited to visit our hospital two times (3- and 5-years) after the first visit in the LAST STRONG Study. During both visits, patients will undergo a subset of tests (neurological examination, functional measurements, questionnaires, muscle ultrasound, MRI, pulmonary assessment and accelerometry). All measurements are adapted to the patient's age and functional disabilities.