This clinical trial aims to test a new physio-therapeutic approach tailored to type 2 and 3 Spinal Muscular Atrophy patients, based on physical training in swimming-pool. This specific exercise should promote motor skills of trained patients, as we have observed in different mouse models. Patient's motor skills will be assessed using different scales including MFM and Hammersmith.

This clinical trial attempts to develop a new non-invasive motor scale with sophisticated instruments.

This scale will be useful in future clinical trials on SMA, given the lack of sensitivity of currently available scales.

In addition, the study attempts to validate a questionnaire on post-exercise physical well-being.

This multicenter study is based on a longitudinal assessment of the natural disease progression and on the effects of exercise protocol in a swimming pool. This study will compare patients to themselves after determining motor skills and the disease natural course for 12 months.

The study will compare different scales (MFM, Hammersmith) to our new innovative scale based on non-invasive analysis of motor skills.

30 patients will be enrolled and tested with MFM, Hammersmith and our non-invasive scale every 6 months for a total of 18 to 36 months due to the slow natural progression of the disease.

Among these 30 patients, 10 patients will be selected to perform a 6 months training in a swimming pool, from 12 to 18 months or 18 to 24 months or 24 to 30 months after enrollment, in defined and reproducible conditions.

These trained patients will finally be reassessed at 18 and 24 months or 24 and 30 months or 30 and 36 months to test the therapeutic effect of the training compared to the slope of the natural disease progression, and other parameters.

Only patients who have undergone training in the swimming pool will be reevaluated at 24 or 30 or 36 months (6 months post-training) to assess long term exercise effects.

Inclusion Criteria:

1. Patient with type II SMA who can sustain a sitting position without orthopedic corset or type III SMA, with genetic confirmation of the disease.
2. Age from 5 to 10 years old.
3. Patient able to understand and execute training instructions, and perform predetermined movements.
4. Patient able to achieve physical exercises in a swimming pool.
5. MFM score > 35.
6. Patient having any contraindication to physical rehabilitation in a swimming pool.
7. Consent signed by the holders of parental responsibility for children participating in the study.
8. Patient with social security insurance.

Exclusion Criteria:

1. Pneumonia: the inclusion is to be delayed for at least 3 months after recovery of infection.
2. Heart, renal or hepatic failure.
3. Patient undergoing a major surgery during the study: spinal fusion.
4. Patient under the care of a guardian.
5. Inability to cooperate.
6. Medication potentially interfering with the SMA (i.e., riluzole, salbutamol, phenylbutyrate, sodium valproate) started less than 6 months before inclusion.