A Study to Learn About Salanersen's (BIIB115) Effects on Movement and Its Safety in Participants Aged 15 to 60 Years With Spinal Muscular Atrophy (SMA) Who Are Either New to SMA Treatment or Were Previously Treated With Risdiplam

Key Inclusion Criteria: * Participants aged 15 to 60 years, inclusive, at the time of informed consent * Participants with genetic documentation of 5q Spinal Muscular Atrophy (SMA) (homozygous gene deletion or mutation or compound heterozygous mutation). * Participants with clinical signs and symptoms consistent with SMA. * Survival motor neuron 2 (SMN2) copy number ≥ 1. * Participants with baseline Hammersmith Functional Motor Scale - Expanded (HFMSE) total score of ≥ 10 to ≤ 54. * Participants who are able to sit without using support for at least 10 seconds. * Participants with no prior treatment with myostatin inhibitors and a willingness to remain off concurrent myostatin inhibitor therapy for the duration of the study. * Ambulatory and nonambulatory participants: * Ambulatory participants must be able to walk at least 10 meters independently without assistance and are willing and able to complete the 6 Minute Walk Test (6MWT) at Screening. * For participants in the treatment-naïve cohort: * No prior treatment with an approved SMA Disease Modifying Therapy (DMT) or an investigational drug given for the treatment of SMA. * For participants in the risdiplam-treated cohort: * Currently receiving risdiplam treatment and have been on once-daily 5 milligrams (mg) risdiplam treatment for at least 6 months prior to Screening. * Willing to stop risdiplam therapy for the duration of the study. The last dose of risdiplam must be taken the day before the first dose of salanersen. * No prior treatment with nusinersen, onasemnogene abeparvovec-xioi/onasemnogene abeparvovec-brve (OA), other approved DMTs for SMA or investigational drugs given for the treatment of SMA apart from risdiplam. Key Exclusion Criteria: * Respiratory insufficiency at Screening, defined by the medical necessity for invasive or noninvasive ventilation for \> 6 hours during a 24-hour period (except for nocturnal bilevel positive airway pressure). * Medical necessity for a gastric feeding tube, where the majority of nutrition is provided by this route, as assessed by the site Investigator at Screening. * History of brain or spinal cord disease or other contraindications (e.g., severe scoliosis) that would interfere with the lumbar puncture (LP) procedures, Cerebrospinal fluid (CSF) circulation, efficacy assessments, or safety assessments (including a history of hydrocephalus or implanted shunt for CSF drainage), as assessed by the Investigator. * Hospitalization for surgery, a pulmonary event, or nutritional support within 2 months prior to Screening or plans to undergo elective procedures or surgeries at any time after signing the Informed Consent Form (ICF) through the end of the study. Note: If prior scoliosis surgery has been performed, it must be done at least 1 year prior to Screening. * Presence of an active medical issue (e.g., infection, recent fracture) that would make the participant unsuitable for inclusion, as assessed by the Investigator. * Current enrollment or a plan to enroll in any interventional clinical study in which an investigational treatment or approved therapy for investigational use is administered within 90 days or 5 half-lives of the treatment (if known), whichever is longer, prior to Screening. This includes neuromodulation therapy such as spinal cord stimulation. Note: Other protocol-defined inclusion/exclusion criteria will apply.