Spinal muscular atrophy (SMA) is a genetic neurodegenerative disease impacting spinal cord motor neurons, leading to motor and respiratory issues and, ultimately, death. With emerging therapies, a need arises to enhance motor function assessment in severely hypotonic infants (SMA type 1) as traditional scales on examination tables lack completeness due to gravity's influence. The study team has developed a bath test to observe infants' motor skills in water, eliminating gravity's effects. This test aims to detect subtle movements using inertial sensors, potentially revealing more active motor neurons in aquatic conditions. It aids in identifying infants with motor improvement potential, even if they show limited mobility outside water, and tracks disease progression and therapy responses. Presently, pediatric neurologists in France use parent-provided bathing videos for evaluations, but these lack standardization and precision. The study aims to establish a standardized evaluation protocol with quantifiable data. The study's key objective is to evaluate severely hypotonic SMA infants using inertial sensors, including accelerometers, gyrometers, and magnetometers. The study will conduct dry and water assessments using a specially designed bathtub. This method's goal is to quantify water-based movements accurately. Simultaneously, the study seeks to establish semi-quantitative evaluation criteria to create a clinical assessment scale for infant motor function in bathtubs. This scale will aid doctors in therapeutic decisions. The study will not influence the treatment or therapeutic decisions made for the children being tested. Collected data from dry and water conditions will be statistically analyzed and compared to reference motor assessment scales (e.g., CHOP INTEND and HINE) and electromyography (CMAP-EMG) results, commonly used in diagnosis and monitoring. Blurred video recordings will assist in protocol monitoring and sensor data analysis.