Background: Non-ambulatory children with a neuromuscular disability such as spinal muscular atrophy (SMA) are at significant risk for poor bone health as defined by low bone mineral density (BMD) and increased propensity to fracture. Poor bone health is thought to be related, at least in part, to abnormally low levels of load experienced by the skeleton. A common physical approach for increasing bone density is to stimulate the musculoskeletal system by increasing the amount and duration of weight-bearing in the lower extremities. For non-ambulatory individuals, this takes the form of using an assisted standing device to enable the child to spend time in a standing position with some degree of weight placed on the lower limbs. Some of these physical interventions demonstrate variable improvement in BMD in children with neuromuscular conditions, and some do not. A serious limitation in the previous work in this area is a failure to objectively measure the magnitude and duration of the loading experienced by the lower extremities. Thus, a lack of change in BMD may be due to the extremities experiencing only a fraction of the body weight (due to load-sharing with the assistive device) for an inadequate duration of standing time.

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.

Implications: Successful completion of this proposed project will provide the necessary groundwork to move forward with a multicenter clinical trial on a large number of subjects with a variety of neuromuscular conditions. The measurement device will allow us to correlate change in BMD with the specific load history for each participant, thereby allowing us to determine whether physical interventions can lead to increased BMD in individuals with neuromuscular disabilities. Analysis of the data will help define a dose-dependent relationship between load, duration and changes in BMD, thereby guiding providers and therapists in prescribing standing interventions.