Development of a multiscale model of the human lumbar spine for investigation of tissue loads in people with and without a transtibial amputation during sit-to-stand.

Quantification of lumbar spine load transfer is important for understanding low back pain, especially among persons with a lower limb amputation. Computational modeling provides a helpful solution for obtaining estimates of in vivo loads. A multiscale model was constructed by combining musculoskeletal and finite element (FE) models of the lumbar spine to determine tissue loading during daily activities. Three-dimensional kinematic and ground reaction force data were collected from participants with ([Formula: see text]) and without ([Formula: see text]) a unilateral transtibial amputation (TTA) during 5 sit-to-stand trials. We estimated tissue-level load transfer from the multiscale model by controlling the FE model with intervertebral kinematics and muscle forces predicted by the musculoskeletal model. Annulus fibrosis stress, intradiscal pressure (IDP), and facet contact forces were calculated using the FE model. Differences in whole-body kinematics, muscle forces, and tissue-level loads were found between participant groups. Notably, participants with TTA had greater axial rotation toward their intact limb ([Formula: see text]), greater abdominal muscle activity ([Formula: see text]), and greater overall tissue loading throughout sit-to-stand ([Formula: see text]) compared to able-bodied participants. Both normalized (to upright standing) and absolute estimates of L4-L5 IDP were close to in vivo values reported in the literature. The multiscale model can be used to estimate the distribution of loads within different lumbar spine tissue structures and can be adapted for use with different activities, populations, and spinal geometries.

Validation of lumbar spine loading from a musculoskeletal model including the lower limbs and lumbar spine.

Low back mechanics are important to quantify to study injury, pain and disability. As in vivo forces are difficult to measure directly, modeling approaches are commonly used to estimate these forces. Validation of model estimates is critical to gain confidence in modeling results across populations of interest, such as people with lower-limb amputation. Motion capture, ground reaction force and electromyographic data were collected from ten participants without an amputation (five male/five female) and five participants with a unilateral transtibial amputation (four male/one female) during trunk-pelvis range of motion trials in flexion/extension, lateral bending and axial rotation. A musculoskeletal model with a detailed lumbar spine and the legs including 294 muscles was used to predict L4-L5 loading and muscle activations using static optimization. Model estimates of L4-L5 intervertebral joint loading were compared to measured intradiscal pressures from the literature and muscle activations were compared to electromyographic signals. Model loading estimates were only significantly different from experimental measurements during trunk extension for males without an amputation and for people with an amputation, which may suggest a greater portion of L4-L5 axial load transfer through the facet joints, as facet loads are not captured by intradiscal pressure transducers. Pressure estimates between the model and previous work were not significantly different for flexion, lateral bending or axial rotation. Timing of model-estimated muscle activations compared well with electromyographic activity of the lumbar paraspinals and upper erector spinae. Validated estimates of low back loading can increase the applicability of musculoskeletal models to clinical diagnosis and treatment.

Lumbar loads and trunk kinematics in people with a transtibial amputation during sit-to-stand.

People with a transtibial amputation have numerous secondary health conditions, including an increased prevalence of low back pain. This increased prevalence may be partially explained by altered low back biomechanics during movement. The purpose of this study was to compare trunk kinematics and L4-L5 lumbar loads in people with and without a transtibial amputation during sit-to-stand. Motion capture, ground reaction force and electromyographic data were collected from eight people with a unilateral transtibial amputation and eight people without an amputation during five self-paced sit-to-stand motions. A musculoskeletal model of the torso, lumbar spine, pelvis, lower limbs, and 294 muscles was used in a static optimization framework to quantify L4-L5 loads, low back muscle forces, and trunk kinematics. Participants with an amputation had greater peak and average L4-L5 loading in compression compared to control participants, with peak loading occurring shortly after liftoff from the chair. At the instant of peak loading, participants with an amputation had significantly greater segmental trunk lateral bending and trunk-pelvis axial rotation toward the intact side, and significantly greater segmental trunk axial rotation toward the prosthetic side compared to control participants. Participants with an amputation also had greater peak frontal plane and transverse plane segmental trunk angular velocity. The postural differences observed in people with a transtibial amputation were consistent with their ground reaction force asymmetry. The cumulative effects of the altered movement strategy used by people with an amputation may result in an increased risk for low back pain development over time.