Respiratory muscle activity and fatigue response during respiratory endurance testing in patients with spinal muscular atrophy

Objective: To explore respiratory muscle activity and fatigue response during a respiratory endurance test (RET) in patients with spinal muscular atrophy (SMA). Methods: Fifty-five participants with SMA types 2 and 3 performed a respiratory endurance test (RET) at 20 %-70 % of their maximum inspiratory mouth pressure (PImax). We recorded surface electromyography (sEMG) of the diaphragm, intercostal, and scalene muscles and evaluated muscle activity at onset and during the test. Respiratory muscle fatigability (RMF) was defined as the inability to complete 60 consecutive breaths during the RET. Results: The diaphragm showed a significantly lower variance in activity at onset of the test compared to the intercostal and scalene muscles (p = 0.002). Participants with RMF showed fatigue of the diaphragm, indicated by a significant decrease in root mean square amplitude (p = 0.012) and median frequency (p < 0.001), compared to those without RMF. Conclusion: The relative activity of the diaphragm at onset of a RET remained stable at higher intensity levels, while the activity of the intercostal and scalene muscles became more variable. The diaphragm showed significant electrophysiological signs of fatigue, while accessory inspiratory muscles showed no clear electrophysiological fatigue signs. Significance: The results of this study provide more insight into respiratory muscle function and fatigability in patients with SMA.