Exploring the Physiologic, Pharmacodynamic, and Clinical Responses of Skeletal Muscle in Patients With Spinal Muscular Atrophy Treated With SMN-Directed Therapies

Inclusion Criteria: * Genetic confirmation of SMA with homozygous deletion of SMN1 or compound heterozygous deletion/mutation of SMN1 * Two, three, or four copies of SMN2 * Age 5 to 20 years * Non-ambulatory participants: maximum function sitting or standing with support, never walked independently, still able to sit independently for 5 seconds at screening, with active ankle plantar flexion strength of at least 3 N with hand-held myometry and capable of performing repetitive maximal plantar flexion effort for 120 seconds. HFMSE score at screening between 10 and 45 points. * Ambulatory participants: minimum function of independent walking, able to walk unassisted a minimum of 100 meters at screening, ankle plantar flexion strength of at least 10 N with hand-held myometry and capable of performing repetitive maximal plantar flexion for 120 seconds. HFMSE score at screening between 40 and 60. * SMN-directed therapy inclusion: * Current Evrysdi prescription * Must have Evrysdi prescription through their treating physician but have not yet initiated treatment OR * Current Spinraza or Zolgensma prescription * For patients on Spinraza, must have been taking Spinraza for at least 12 months at screening (4 loading and 2 maintenance doses) and following the FDA-recommended dosing schedule * For patients on Zolgensma, must have been dosed at least one year prior to screening * Must have Spinraza or Zolgensma prescription through their treating physician OR * Changing from Spinraza or Zolgensma to Evrysdi * For patients on Spinraza, must have been taking Spinraza for at least 12 months at screening (4 loading and 2 maintenance doses) and following the FDA-recommended dosing schedule * For patients on Zolgensma, must have been dosed at least one year prior to screening * Must have voluntarily decided to switch therapies based on discussion with their treating physician * Must have Evrysdi prescription through their treating physician but have not yet initiated treatment OR * Have never received any SMN-directed therapies Exclusion Criteria: * Labs at screening that are abnormal and identified as clinically significant by the PI: CBC, and CMP, liver function tests (over twice the upper limit of normal), PT/PTT, urine protein of 2+ or greater. * Inability to perform reliably the motor function testing or the exercise testing in the MR scanner. * Treatment with a possible SMA-enhancing or mitochondrial-enhancing medication, unless discontinued within 3 months prior to screening: oral albuterol, hydroxyurea, phenylbutryate, valproic acid, creatine, l-carnitine, or other mitochondrial type supplement (riboflavin, lipoic acid, etc.). A daily multivitamin and Vitamin D supplement and intermittent inhaled albuterol are permitted if the dosage is unchanged during the study. * Need for routine non-invasive ventilation support. * Non-oral nutritional support, e.g., gastrostomy tube feeding. * Any ferrous metal implants (e.g., spinal rods) that preclude testing in a MR scanner.