Methods: Patients with spinal muscle atrophy type II and III will be recruited. Electrocardiogram signals will be recorded for analyses of heart rate variability during air stacking in supine and sitting position. Data will collected before, during and after air stacking and will be compared using Anova Repeated Measures or Kruskal-Wallis Anova on Ranks, followed by Tukey test. The relationship between heart rate variability indexes and age will be evaluated by Pearson correlation. Significant level will be set at 5%.