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, 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, HFMSE score at screening between 40 and 66. * SMN-directed therapy inclusion: * Current Evrysdi prescription (Group 1) * Must have Evrysdi prescription through their treating physician * If initiating combined therapy using Evrysdi with Spinraza or Zolgensma, must have not started Evrysdi treatment OR * Current Spinraza or Zolgensma prescription (Group 2) * 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 (Group 3) * 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 (Group 4) Exclusion Criteria: * Any chronic medical condition, planned surgery, or treatment with a medication which would impact safety or participation of the study at the investigator's discretion * Inability to perform reliably the motor function testing or the exercise testing in the MR scanner. * Fat fraction \> 35% in calf or bicep at screening MRI * 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.