Methods: In order to investigate the efficacy of standing treatment for increasing BMD, we will develop a simple, portable and inexpensive transducer that will measure the magnitude and time course of the load experienced by the lower extremities of individuals with SMA who use a stationary assisted standing device. This transducer will consist of an array of sensors embedded within a mat that will be placed under an individual's feet while they are in assisted standing. A prototype will be validated by comparing the output of this load-sensing mat against known loads that will be applied to the surface of the mat statically. The specific characteristics of the loading in the lower extremities of subjects can then be correlated with changes in their BMD to obtain a true outcome assessment. As a demonstration of this approach, we will recruit 3 children with SMA from Gillette Children's Specialty Healthcare who are currently participating in a standing program. Once enrolled and informed consent is obtained, the participants will use the mat provided to them to collect load and duration data during a baseline period where they will continue their standing program, and during a subsequent intervention period where they will increase the duration of their standing. The specific goal of this proposed project will be to develop, validate and establish the initial feasibility of such a measurement device.