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.