This study is being conducted to test whether exercise can be effectively used as an intervention to treat Spinal Muscular Atrophy (SMA). In order to answer this question, the investigators will enroll 14 subjects with SMA between ages 8 and 50 and ask them to complete an 18 month training schedule. At some points subjects will be asked to closely follow a specific training regimen and at other points they may be asked to exercise in the same manner they do normally. The exercises they will be asked to perform include biking on a stationary cycle and lifting hand weights. Subjects will be asked to come in to the clinic seven times over the course of the study to perform tests. These tests include motor function measures, a physical exam, questionnaires, a exercise capacity test which involves riding a stationary bicycle, and test where the subject is asked to walk as far as they can in six minutes. The main goal of the study is to see if the subjects who participate in the exercise protocol have larger increases in the distance they can walk in six minutes than those who do not.

Inclusion Criteria:

1. Weakness and hypotonia consistent with the clinical diagnosis of SMA type 3, i.e. having achieved the ability to walk independently
2. Laboratory documentation of homozygous absence of SMN1 exon 7
3. ability to walk at least 25 meters without assistance
4. Aged 8 to 50 years at the time of enrollment
5. Ability to tread the stationary cycle ergometer
6. Written informed consent of patient (if ≥ 18 years of age) or parents/guardian (if < 18 years of age), and assent for participants who are of minor age.

Exclusion Criteria:

1. Inability to walk independently at least 25 meters
2. Any acute co-morbid condition interfering with study participation in the judgment of the investigators within 7 days of enrollment including bacterial infection, viral infectious process, food poisoning, temperature > 99.0ºF, need for acute treatment or observation due to any other reason, as judged by the investigator; patient can be included after resolution of the acute event
3. Use of investigational medications intended for the treatment of SMA including riluzole, valproic acid, hydroxyurea, oral use of albuterol, sodium phenylbutyrate, butyrate derivatives, creatine, carnitine, growth hormone, anabolic steroids, or agents with known or presumed histone deacetylase (HDAC) inhibition, within 30 days prior to study entry. After signing informed consent, participants may discontinue prohibited medications and will then qualify for a screening visit after the 30 day wash-out period
4. Inability to meet study visit requirements, or cooperate reliably with functional testing and exercise protocol
5. Coexisting medical conditions that contraindicate travel, testing or moderate intensity exercise
6. Pregnant or breastfeeding women, or those intending to become pregnant during the course of the study.