The ability for individuals with spinal cord injury (SCI) to affect changes in standing posture with functional neuromuscular stimulation (FNS) was explored using an anatomically inspired musculoskeletal model of the trunk, pelvis and lower extremities (LE). The model tracked trajectories for anteriorly and laterally shifting movements away from erect stance. Forces were applied to both shoulders to represent upper extremity (UE) interaction with an assistive device (e.g., a walker). The muscle excitations required to execute shifting maneuvers with UE forces <10% body-weight (BW) were determined via dynamic optimization. Nine muscle sets were examined to maximize control of shifting posture. Inclusion of the Psoas and External Obliques bilaterally resulted in the least relative UE effort (0.119, mean UE effort=45.3 N≡5.4% BW) for anterior shifting. For lateral shifting, the set including the Psoas and Latissimus Dorsi bilaterally yielded the best performance (0.025, mean UE effort=27.8 N≡3.3% BW). However, adding the Psoas alone bilaterally competed favorably in overall best performance across both maneuvers. This study suggests suitable activation to specific muscles of the trunk and LE can enable individuals with SCI to alter their standing postures with minimal upper-body effort and subsequently increase reach and standing work volume.
Quiet standing and simple stepping maneuvers can be restored to individuals paralyzed by spinal cord injury (SCI) through neuroprostheses employing functional neuromuscular stimulation (FNS). Existing FNS systems maintain a single erect standing posture by continuously activating the knee, hip and trunk extensors (Jaeger et al., 1989, Yarcony et al., 1990 and Triolo et al., 1996). Current systems essentially lock users into a single upright posture with no means to alter position except by pulling or pushing against the continuously activated muscles with the upper extremities (Kobetic et al., 1999). Furthermore, existing FNS systems are capable of activating only a small number of carefully selected muscles, making advanced functions and finer movements difficult. The purpose of this study was to examine the feasibility of dynamically shifting standing posture with low upper extremity exertion and a minimal number of optimally selected muscles. The ability to dynamically shift posture by appropriately modulating stimulation would expand work volume to allow users to reach and manipulate objects or prepare for anticipated disturbances, thus affording greater control over the environment and reducing the potential for falls.
Simplifying assumptions in prior modeling studies examining standing balance include: actuating the system by joint moments (Hemami and Wyman, 1979, Kim et al., 2006 and Matjacic et al., 2001), representing the body as a multi-joint single inverted pendulum (Soetanto et al., 2001 and Gollee et al., 2004), or combining pelvis and trunk into a single segment (Mihelj and Munih, 2004). While adequate for theoretical investigations into disturbance response and single limb stance, these models were essentially static oversimplifications. Exploring FNS-generated movement in three dimensions is important because muscle actions are not confined to single planes. For instance, stimulating the tibialis anterior after SCI causes the body to both fall forward (a sagittal plane movement) and lean sideways (a coronal plane movement). Moreover, the closed chain defined by maintaining the feet on the ground effectively reduces the system degrees of freedom but couples their individual effects across all movement planes. Thus, stimulation of any muscle about the closed chain can result in complex motor behavior unaccounted for by planar models. Finally, it is necessary to represent the pelvis and trunk separately since important muscles attach to one without spanning the other. A single pelvis–trunk segment could require complex synergy patterns from many muscles to constrain the anatomy to fit the simplified model.
Recent simulation studies with 3D musculoskeletal models have demonstrated the possibility of holding the body statically at discrete bipedal postures with FNS (Heilman et al., 2006 and Gartman et al., 2008). These studies used static optimization to determine optimal muscle sets to maintain the body in different postures. However, the muscle forces calculated to keep the body in static equilibrium are not guaranteed to be sufficient to move the body dynamically from one posture to another.
In our study, a 3D model was used to explore the feasibility of producing dynamic movements with FNS in a typical individual with paralysis. The results will inform future clinical implementation of neuroprostheses designed to restore standing balance after SCI and assist surgeons and rehabilitationists in the selection of optimal muscles and stimulation patterns.
