Trunk instability is a major problem for many people with spinal cord injuries that affects their independence and ability to perform everyday activities. The long-term objective of this project is to produce a new device that will bolster sitting stability by stimulating paralyzed trunk muscles in a manner that is practical and efficient. This will necessarily improve the ability of people with spinal cord injuries to perform such tasks as reaching and wheeling. The first task of this program is an in-depth study of the mechanisms of balance in the trunk, and to analyze the consequences of muscle paralysis on these mechanisms. A biomechanical model of the trunk will be developed in order to identify the muscle control strategies observed in able-bodied subjects. It will also be used to evaluate the balance deficiencies observed in people with spinal cord injury. An instrumented perturbation apparatus will be constructed to provide precise measurements of forces, movements and muscle actions that occur when a sitting subject is maintaining balance. This analysis should provide a valuable basis for the development of an electrical stimulation system that will activate paralyzed trunk muscles in a way that promotes stability and function. The model will be used to test muscle stimulation strategies, the most promising of which will be applied to human subjects will spinal cord injury.