This study will focus on the pathophysiological underpinnings of reduced exercise capacity and fatigue in ambulatory patients with spinal muscular atrophy (SMA). There has been laboratory evidence to suggest that the molecular mechanisms underlying mitochondrial biogenesis may be vulnerable to survival motor neuron (SMN) protein deficiency. This is an observational, single visit study including 34 ambulatory SMA patients treated with SMN repletion therapies (risdiplam or nusinersen) for at least 6 months at enrollment.