Creep deformation of viscoelastic lumbar tissue and its implications in biomechanical modeling of the lumbar spine
Date
2024-12
Authors
Kang, Sang Hyeon
Major Professor
Advisor
Mirka, Gary
Dorneich, Michael
Hallbeck, Susan
Gillette, Jason
Norasi, Hamid
Committee Member
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Abstract
This dissertation expands our knowledge of the “creep” deformation of the viscoelastic lumbar tissues through in vivo human experiments and biomechanical modeling. The specific goals of this dissertation were 1) to examine the effects of trunk flexion angle and stress-recovery (work-rest) schedule on the creep deformation response of viscoelastic lumbar tissues and 2) to explore the impact of creep of viscoelastic lumbar tissues on the biomechanical loading of the lumbar spine. Three preliminary studies were conducted to lay the foundation of this dissertation and two main studies (an empirical study and a modeling study) were conducted to achieve these goals.
Preliminary Study I explored the effect of a 12-minute submaximal trunk flexion on lumbar spinal creep. The subject-specific angle was set to be ten degrees less than the trunk flexion angle inducing flexion-relaxation of lumbar extensor muscles. The trunk flexion-extension motions were performed every three minutes to capture the peak lumbar flexion angle and lumbar flexion angle (EMG-off angle) at which flexion-relaxation of lumbar extensors occurred. Results revealed that the 12 minutes of these postures led to significant increases in peak lumbar flexion angle (1.3°) and EMG-off angle (2.9°), denoting the lumbar spinal creep. The results suggest that passive tissue deformation should be considered in submaximal trunk flexion, where the extensor muscles are significantly involved in resisting the external moment.
Preliminary Study II examined cumulative creep deformation of viscoelastic lumbar tissues as a function of work-rest schedule: a) three minutes of maximal trunk flexion followed by twelve minutes of upright standing (3:12)—sequence repeated three times, and b) one minute of maximal trunk flexion followed by four minutes of upright standing (1:4)—sequence repeated nine times. The lumbar flexion angle was continuously captured to quantify lumbar spinal creep during maximal trunk flexion. Results revealed that the change in lumbar flexion angle was significantly greater in the Long (3:12) condition (Δ3.5°) than in the Short (1:3) condition (Δ1.6°). The results demonstrated that the cumulative creep deformation of the lumbar spine can vary with work-rest schedules even when total work and rest times are constant, supporting multidimensional physiological responses of creep in the viscoelastic tissues of the lumbar spine.
Preliminary Study III quantified the time-dependent changes in individual tissue forces/moments at the L4/L5 level during sustained submaximal trunk flexion. A muscle fatigue-modified EMG-assisted biomechanical model with passive tissue components was employed. The kinematic and EMG data collected in Preliminary Study I were used as input variables. Results revealed that sustained trunk postures resulted in a time-dependent increase in the proportion of passive lumbar tissues in resisting the external moment (54.9% to 65.7%)—ultimately leading to increases in the compression force (1480.8 N to 1720.8 N) and anterior-posterior shear force (770.0 N to 889.4 N). These indicate that the passive tissues with shorter moment arms should bear greater amounts of the net internal extensor moment as lumbar flexion gradually increases at the constant external moment. The results demonstrated that the time-dependent approach to EMG-assisted modeling with passive tissue components can improve the accuracy in the estimation of spinal loading by considering the transfers of load from active to passive tissues. This study also suggests that creep should be considered to more accurately estimate spinal loading as a function of time.
Main Study I aimed to explore the interaction between trunk flexion posture and work-rest schedule on lumbar spinal creep. On four different days sixteen participants performed a 30-minute protocol that consisted of 12 minutes of trunk flexion and 18 minutes of upright standing. Two trunk flexion postures (Max [maximum lumbar flexion], SubMax [ten degrees less than flexion-relaxation]) and two work-rest schedules (Long (3:6), Short (1:2)) were considered. Trunk flexion-extension motions were performed before and after the 30-minute protocol to capture the changes in peak lumbar flexion angles and changes in EMG-off angles for L3 and L4 paraspinals. Results revealed that ΔL3 EMG-off angle was significantly smaller in the SubMax/Long condition (0.1°) compared to the other conditions (1.8° on average), denoting a significant interaction effect. The Δpeak lumbar flexion angle and ΔL4 EMG-off angle were greater in the Short (1:2) (1.8°) than in the Long (3:6) condition (1.0°), which is contrary to the results of the Preliminary Study II. The major difference between the two studies is the work-rest ratio (1:2 for this dissertation and 1:4 for the Preliminary Study II). These results suggest that a potential interaction between work-rest schedule and work-rest ratio should be explored to find the optimal work-rest strategy to minimize the cumulative creep response of viscoelastic lumbar tissue. Further analysis of lumbar flexibility showed significant interactions between flexibility and trunk flexion posture (p < 0.05 for EMG-off angles) and flexibility and work-rest schedule (p < 0.05 for all measures) on the cumulative lumbar spinal creep. The low flexible group had significantly greater ΔL3 EMG-off and ΔL4 EMG-off in the Max posture (2.1° on average) than in the SubMax posture (0.8°), while the high flexible group was not affected by postures. In addition, the low flexible group was not affected by work-rest schedules, while the high flexible group experienced greater creep responses in the Short (1:2) condition (2.5° on average) than in the Long (3:6) condition (0.6°). These results indicate that lumbar flexibility can play a significant role in lumbar spinal creep. Collectively, the results of this study suggest that the cumulative creep response of the viscoelastic tissues of the lumbar spine can be affected by trunk flexion angle, work-rest schedule, work-rest ratio, and lumbar flexibility.
Main Study II explored the impact of creep on the quality of internal moment predictions from a biomechanical model of the lumbar spine during sagittally symmetric exertions. Comparison of two different biomechanical models was of primary interest: a) EMG-assisted model with passive tissue components (No-Creep) and b) EMG-assisted model with stiffness-modified passive tissue components (Creep). The Creep model modified the stiffness of each ligament as a function of creep through in vitro data. The kinematic/EMG data, collected during a single trunk flexion motion immediately after 30-minute work-rest schedules under Max conditions in Main Study I, were used as input variables in the biomechanical models. Results revealed a significant difference in mean absolute error between Creep and No-Creep models beyond trunk flexion angle eliciting flexion-relaxation of erector spinae muscles (21.8 Nm vs. 40.3 Nm). Further analysis of spinal loads of an L4/L5 disc at full trunk flexion showed that the Creep model led to 784.7 N (31.7%) and 280.6 N (21.6%) reductions of compression and shear forces of the L4/L5 disc than the No-Creep model. These results indicate that the modulation of the loss of force-producing capability of passive tissues under creep conditions led to more accurate prediction of net internal moment and spinal loads at near full flexion postures.
In conclusion, the creep deformation of viscoelastic lumbar tissues as a function of trunk flexion angle and stress-recovery schedule and its implications in lumbar biomechanical modeling were explored. The results of the empirical studies suggest that the cumulative creep response of the viscoelastic lumbar tissues can be influenced by trunk flexion angle, work-rest schedule, work-rest ratio, and lumbar flexibility, supporting a multidimensional physiological response to creep. The results of the modeling studies suggest the importance of a time-dependent approach to consider load transfer between lumbar tissues and an in vitro data-driven approach to modify passive stiffness in a lumbar biomechanical modeling technique to provide more precision prediction of the internal extensor moment and spinal loading.
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