The purpose of this study was to describe functional ability, muscle strength, forced vital capacity, and clinical events in participants with Duchenne muscular dystrophy (DMD) or spinal muscular atrophy (SMA) in the non-ambulatory stages of the diseases. Nineteen non-ambulatory participants with DMD (all males; 13 to 24 years) and 13 with SMA (six males, seven females; 11 to 57 years) were assessed once a year over 5 years. The assessments comprised functional ability measured with the EK scale and upper extremity grade, muscle strength measured with the manual muscle test, and forced vital capacity defined as a percentage of normal values (FVC%). In the DMD group all variables measured deteriorated and there was a direct correlation between them. In the SMA group only muscle strength and FVC% deteriorated and there was no close relation between the variables measured. In the DMD group, 16 participants had cardiorespiratory clinical events leading to death in five cases. In the SMA group only four participants had respiratory clinical events, none leading to death. Although the participants with SMA had been extremely weak and non-ambulatory since early childhood they were older and less exposed to life-threatening events than the participants with DMD.