Simulation of functional neuromuscular stimulation assisted sit-to-stand movements.

Functional neuromuscular stimulation (FNS) has the potential to enhance the capability for individuals with a spinal cord injury (SCI) to perform activities of daily living. Individuals with SCI have successfully used FNS to stand, but the transition from sitting to standing is often awkward and may require excessive amounts of arm support. The long-term goal of this research project is to provide stimulation patterns that reduce the demands on an FNS user's arms and provide a smooth, stable sit-to-stand transition. A multi-segment, dynamic model has been developed to simulate different combinations of stimulated muscles and stimulation patterns. The model includes individualized anthropometric parameters, active and passive muscle and joint mechanics, and arm support forces (experimentally measured or an assumed pattern). Stimulated muscle properties were derived using SIMM software (Musculographics, Chicago, IL), and the simulations were run in Matlab (Mathworks, Natick, MA). Initial simulation results have estimated that ramped stimulation of the vastus lateralis and semimembranosus muscles reduces the peak vertical arm support forces from 91% to 42% of body weight as compared to unassisted sit-to-stand. Kinematic results also suggested that the hand support forces remained excessive due to a lack of joint coordination and insufficient ankle joint stiffness. In further simulations, additional muscles will be stimulated to increase joint stiffness and stimulation patterns will be designed to mimic able-bodied sit-to-stand motions. This model will be validated and improved using results of sit-to-stand experiments with FNS users once an optimized set of muscles and stimulation patterns has been determined.