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Following a spinal cord injury (SCI), the paralyzed extremities undergo muscle atrophy and decrease in bone mineral density (BMD) due in part to the loss of physiological loading. It is crucial to prevent musculoskeletal deterioration so the population is less susceptible to fractures, and could take advantage of stem cell treatment if it becomes available. Functional electrical stimulation (FES) has been shown to advantageously train the paralyzed extremities. However, there is a risk of fracture during FES due to low BMD of individuals with SCI. Therefore, the forces generated during FES need to be modeled so researchers and clinicians safely administer this intervention.
The purpose of this project was to develop a biomechanical or mathematical model to estimate the internal compressive and shear forces at the distal femur, a common fracture site for individuals with SCI during FES. Therefore, a two-dimensional static model was created of the lower extremity in the supine and seated positions. The compressive and shear forces at the distal femur were estimated for both positions during FES. These internal compressive and shear forces estimated at the distal femur by the supine model were compared to those estimated by the standing model. Also, for the seated model, the compressive and shear forces at the distal femur estimated by a tetanic muscle contraction were compared to those estimated by a doublet muscle contraction. Finally, the supine model was validated using experimental testing.
The primary findings are 1) the standing model estimated more compressive force and less shear force at the distal femur compared to the supine model when position and quadriceps muscle force remain constant and 2) for the seated model, a tetanic quadriceps muscle contraction predicts greater compressive and shear at the distal femur compared to a doublet muscle contraction. Also the validation testing revealed a 3.4% error between the supine model and the experimental testing. These models provide valuable insights into the internal forces at the distal femur during FES for those with SCI.