We previously reported that patients with spinal muscular atrophy do not lose muscle strength over time as measured quantitatively. However, we noted that many patients with spinal muscular atrophy suffer from what they call fatigue. We wondered if we could measure fatigue during a single maximal voluntary contraction, whether fatigue might increase with time, independent of muscle strength, and whether increasing fatigue might correlate with loss of function in some patients. We measured fatigue during a single maximal voluntary contraction in a cohort of patients having spinal muscular atrophy using quantitative strength testing. We included only patients with spinal muscular atrophy aged 5 years or older, so they could follow instructions regarding muscle contraction, and who were followed for at least 2 years. Seventy-six children with spinal muscular atrophy and 24 untrained individuals, aged 5 to 57 years (mean, 16.8 years), were studied. There was no discernible abnormal fatigue in patients with spinal muscular atrophy compared to untrained controls using our methodology. Thus, spinal muscular atrophy may not be associated with fatiguability. Moreover, spinal muscular atrophy does not appear to cause progressive muscle fatigue with age or loss of function. It is possible that fatigue was undetectable by our methods. An alternative explanation is that what patients describe as fatigue may be caused by factors outside the neuromuscular system. Such factors may include chronic respiratory insufficiency with hypoventilation and carbon dioxide retention as well as chronic malnutrition and negative nitrogen balance.