Modeling of the human trunk: the impact of stiffness gain and lumbar lordosis on stability

Low back pain and injuries are prevalent and costly musculoskeletal conditions in our society, afflicting most Americans at some point throughout their lifetime. In an effort to develop effective treatment and rehabilitation methods, researchers have continued in their investigation of the potential risk factors and causes of low back pain through use of spine models and experimental data collection. The focus of this work is the development and utilization of an 18 degree-of-freedom stability-based trunk model with 90 muscle fascicles, reflexes, and a lumped parameter intervertebral disc model to explore the potential impacts stiffness gain magnitude and lumbar lordosis angle have on the model’s predictions for stability. Throughout the development of this model, a thorough verification procedure was utilized to minimize errors. It was determined that as stiffness gain in the short-range muscle stiffness model increased from 0.9 to 40, the magnitude of the required metabolic power and muscle force to stabilize the system decreased. Above stiffness gains with a magnitude of 20, increasing the stiffness gains had little impact on the required metabolic power and muscle force. Additionally, for a stiffness gain of 0.5, the model predicted instability whereas all stiffness gains greater than or equal to 0.9 resulted in a stabilized system. It is evident that stiffness gain has the ability to influence model predictions, including the required metabolic power and muscle force. In our investigation with lumbar lordosis, it was determined that the hyperlordosis case required more metabolic power and additional recruitment of the flexor muscles in order to stabilize in comparison to the other lordosis cases. The hypolordosis cases required less metabolic power and additional recruitment of extensor muscles to stabilize in comparison to the other lordosis cases. The additional recruitment of the flexor muscles needed to stabilize the hyperlordotic spine and the additional recruitment of the extensor muscles needed to stabilize the hypolordotic spine can be explained by the line of gravity being positioned posteriorly and anteriorly for the lordosis cases respectively. Future investigations should further explore the impact of lumbar lordosis angle on spine stability, especially experimentally and with EMG-based models, and should consider investigating the impacts of varying stiffness gains on spinal stability. Lastly, additional loading tasks should be simulated with the model, such as asymmetric tasks, differing load magnitudes and differing load application points