Evaluation of ground reaction forces and centers of pressure predicted by AnyBody Modeling System during load reaching/handling activities and effects of the prediction errors on model-estimated spinal loads.

Full-body and lower-extremity human musculoskeletal models require feet ground reaction forces (GRFs) and centers of pressure (CoPs) as inputs to predict muscle forces and joint loads. GRFs/CoPs are traditionally measured via floor-mounted forceplates that are usually restricted to research laboratories thus limiting their applicability in real occupational and clinical setups. Alternatively, GRFs/CoPs can be estimated via inverse dynamic approaches as also implemented in the Anybody Modeling System (AnyBody Technology, Aalborg, Denmark). The accuracy of Anybody in estimating GRFs/CoPs during load-handling/reaching activities and the effect of its prediction errors on model-estimated spinal loads remain to be investigated. Twelve normal- and over-weight individuals performed total of 480 static load-handling/reaching activities while measuring (by forceplates) and predicting (by AnyBody) their GRFs/CoPs. Moreover, the effects of GRF/CoP prediction errors on the estimated spinal loads were evaluated by inputting measured or predicted GRFs/CoPs into subject-specific musculoskeletal models. Regardless of the subject groups (normal-weight or overweight) and tasks (load-reaching or load-handling), results indicated great agreements between the measured and predicted GRFs (normalized root-mean-squared error, nRMSEs < 14% and R2 > 0.90) and between their model-estimated spinal loads (nRMSEs < 14% and R2 > 0.83). These agreements were good but relatively less satisfactory for CoPs (nRMSEs < 17% and 0.57 < R2 < 0.68). The only exception, requiring a more throughout investigation, was the situation when the ground-foot contact was significantly reduced during the activity. It appears that occupational/clinical investigations performed in real workstation/clinical setups with no access to forceplates may benefit from the AnyBody GRF/CoP prediction tools for a wide range of load-reaching/handling activities.

Effect of obesity on spinal loads during load-reaching activities: A subject- and kinematics-specific musculoskeletal modeling approach.

Obesity has been associated to increase the risk of low back disorders. Previous musculoskeletal models simulating the effect of body weight on intervertebral joint loads have assumed identical body postures for obese and normal-weight individuals during a given physical activity. Our recent kinematic-measurement studies, however, indicate that obese individuals adapt different body postures (segmental orientations) than normal-weight ones when performing load-reaching activities. The present study, therefore, used a subject- and kinematics-specific musculoskeletal modeling approach to compare spinal loads of nine normal-weight and nine obese individuals each performing twelve static two-handed load-reaching activities at different hand heights, anterior distances, and asymmetry angles (total of 12 tasks × 18 subjects = 216 model simulations). Each model incorporated personalized muscle architectures, body mass distributions, and full-body kinematics for each subject and task. Results indicated that even when accounting for subject-specific body kinematics obese individuals experienced significantly larger (by âˆ¼38% in average) L5-S1 compression (2305 ± 468 N versus 1674 ± 337 N) and shear (508 ± 111 N versus 705 ± 150 N) loads during all reaching activities (p < 0.05 for all hand positions). This average difference of âˆ¼38% was similar to the results obtained from previous modeling investigations that neglected kinematics differences between the two weight groups. Moreover, there was no significant interaction effect between body weight and hand position on the spinal loads; indicating that the effect of body weight on L5-S1 loads was not dependent on the position of hands. Postural differences alone appear, hence, ineffective in compensating the greater spinal loads that obese people experience during reaching activities.

Effects of auxetic shoe on lumbar spine kinematics and kinetics during gait and drop vertical jump by a combined in vivo and modeling investigation.

The present study examined the effects of auxetic shoes on the biomechanics of the spine, as compared to barefoot and conventional shoe conditions, during gait and drop vertical jump (DVJ) activities using a combined in vivo and musculoskeletal modeling approach. Motion and force-plate data as well as electromyographic (EMG) activities of select trunk muscles of 11 individuals were collected during foregoing activities. In DVJ activity, two main phases of first landing (FL) and second landing (SL) were studied. In the FL phase of DVJ noticeable alternations were observed when auxetic shoes were used. That is, compared to the conventional footwear condition, smaller EMG activities in extensor muscles (by ~ 16-29%, p < 0.001), smaller anterior-posterior (AP) distance between the center of pressure of ground reaction force and heel (by ~ 19%, p = 0.002), generally larger maximal hip, knee, and ankle flexion angles (p < 0.005) and finally smaller maximal L5-S1 compression force and maximal external moment (by ~ 12 and 8%, respectively, p < 0.001) were obtained by wearing auxetic shoes. Our results, therefore, indicate that using auxetic shoes can reduce load on the lumbar spine during high-demanding activities such as vertical jump and thus may decrease the musculoskeletal risk of injuries during these activities.

Comparisons of lumbar spine loads and kinematics in healthy and non-specific low back pain individuals during unstable lifting activities.

Evaluation of spinal loads in patients with low back pain (LBP) is essential to prevent further lumbar disorders. Many studies have investigated the relationship between lifting task variables and lumbar spine loads during manual lifting activities. The nature of the external load (stable versus unstable loads) is an important variable that has received less attention. Therefore, the present study aimed to measure trunk kinematics and estimate compressive-shear loads on the lumbar spine under lifting a 120 N stable load and 120 ± 13.63 N sensual unstable load in 16 healthy and 16 non-specific LBP individuals during lifting activities. The maximal lumbar loads were estimated using a quasi-static electromyography (EMG)-driven musculoskeletal model of the spine with 18 degrees of freedom (3 rotational degrees of freedom at 6 lumbar T12-S1 joints), seven rigid bodies (pelvis, thoracic, and five vertebrae), and 76 muscle fascicles. Moreover, the maximum velocity and acceleration of the thorax, lumbar, and pelvis, as well as their timing during the lifting activities were analyzed to investigate trunk kinematics. Results indicated that unstable, as compared to stable, lifting activities caused significantly larger peak L5-S1 (4677 N versus 4446 N, p = 0.021) and L4-L5 (4567 N versus 4366 N, p = 0.024) compressive loads in LBP individuals. Larger co-contraction of trunk muscles were found responsible for the larger compressive loads in LBP patients during unstable load lifting. The hand-load type (stable versus unstable) and group (LBP versus healthy) had no effects on kinematic variables and only the onset of peak kinematic parameters was significantly later in LBP patients. Slower movements with a change in movement strategy were observed in the LBP group. It was concluded that the nature of the external load adversely affected spinal loads in LBP patients thereby increasing the likelihood of further injury or pain.

Novel force-displacement control passive finite element models of the spine to simulate intact and pathological conditions; comparisons with traditional passive and detailed musculoskeletal models.

Passive finite element (FE) models of the spine are commonly used to simulate intact and various pre- and postoperative pathological conditions. Being devoid of muscles, these traditional models are driven by simplistic loading scenarios, e.g., a constant moment and compressive follower load (FL) that do not properly mimic the complex in vivo loading condition under muscle exertions. We aim to develop novel passive FE models that are driven by more realistic yet simple loading scenarios, i.e., in vivo vertebral rotations and pathological-condition dependent FLs (estimated based on detailed musculoskeletal finite element (MS-FE) models). In these novel force-displacement control FE models, unlike the traditional passive FE models, FLs vary not only at different spine segments (T12-S1) but between intact, pre- and postoperative conditions. Intact, preoperative degenerated, and postoperative fused conditions at the L4-L5 segment for five static in vivo activities in upright and flexed postures were simulated by the traditional passive FE, novel force-displacement control FE, and gold-standard detailed MS-FE spine models. Our findings indicate that, when compared to the MS-FE models, the force-displacement control passive FE models could accurately predict the magnitude of disc compression force, intradiscal pressure, annulus maximal von Mises stress, and vector sum of all ligament forces at adjacent segments (L3-L4 and L5-S1) but failed to predict disc shear and facet joint forces. In this regard, the force-displacement control passive FE models were much more accurate than the traditional passive FE models. Clinical recommendations made based on traditional passive FE models should, therefore, be interpreted with caution.

Spinal segment ranges of motion, movement coordination, and three-dimensional kinematics during occupational activities in normal-weight and obese individuals.

Measurements of spinal segment ranges of motion (RoMs), movement coordination, and three-dimensional kinematics during occupational activities have implications in occupational/clinical biomechanics. Due to the large amount of adipose tissues, obese individuals may have different RoMs, lumbopelvic coordination, and kinematics than normal-weight ones. We aimed to measure/compare trunk, lumbar, and pelvis primary RoMs in all anatomical planes/directions, lumbopelvic ratios (lumbar to pelvis rotations at different trunk angles) in all anatomical planes/directions and three-dimensional spine kinematics during twelve symmetric/asymmetric statics load-handling activities in healthy normal-weight and obese individuals. Kinematics/motion data were collected from nine healthy young male normal-weight and nine age/height/sex matched obese individuals via a ten-camera Vicon motion capture system. Obese individuals had significantly smaller (p < 0.05) lumbar flexion (~9° in average) and larger pelvis right lateral bending (~5°) RoMs as well as smaller lumbopelvic ratios (~37%) in lateral bending and axial rotation movements as compared to normal-weight individuals. Moreover, the two groups had generally non-significant different segmental orientations (<20° and in most cases < 10°) in load-handling tasks that depended on the magnitude of load asymmetry angle (p < 0.05). Differences were larger for tasks performed near the floor, away from body, and at larger load asymmetry angles. Biomechanical models simulating pure lateral bending, axial rotation, or tasks involving large load asymmetry may therefore need subject-specific, rather than population-based, motion analysis due to the effects from body weight. In clinical applications, it should be noted that healthy obese individuals may have different RoMs and lumbopelvic rhythms than healthy normal-weight individuals in some anatomical planes/directions.

A comprehensive approach for the validation of lumbar spine finite element models investigating post-fusion adjacent segment effects.

Spinal fusion surgery is usually followed by accelerated degenerative changes in the unfused segments above and below the treated segment(s), i.e., adjacent segment disease (ASD). While a number of risk factors for ASD have been suggested, its exact pathogenesis remains to be identified. Finite element (FE) models are indispensable tools to investigate mechanical effects of fusion surgeries on post-fusion changes in the adjacent segment kinematics and kinetics. Existing modeling studies validate only their intact FE model against in vitro data and subsequently simulate post-fusion in vivo conditions. The present study provides a novel approach for the comprehensive validation of a lumbar (T12-S1) FE model in post-fusion conditions. Sixteen simulated fusion surgeries, performed on cadaveric specimens using various testing and loading conditions, were modeled by this FE model. Predictions for adjacent segment range of motion (RoM) and intradiscal pressure (IDP) were compared with those obtained from the corresponding in vitro tests. Overall, 70% of the predicted adjacent segment RoMs were within the range of in vitro data for both intact and post-fusion conditions. Correlation (r) values between model and in vitro findings for the adjacent segment RoMs were positive and greater than 0.84. Most of the predicted IDPs were, however, out of the narrow range of in vitro IDPs at the adjacent segments but with great positive correlations (r ≥ 0.89). FE modeling studies investigating the effect of fusion surgery on in vivo adjacent segment biomechanics are encouraged to use post-surgery in vitro data to validate their FE model.

A novel coupled musculoskeletal finite element model of the spine - Critical evaluation of trunk models in some tasks.

Spine musculoskeletal (MS) models make simplifying assumptions on the intervertebral joint degrees-of-freedom (rotational and/or translational), representation (spherical or beam-like joints), and properties (linear or nonlinear). They also generally neglect the realistic structure of the joints with disc nuclei/annuli, facets, and ligaments. We aim to develop a novel MS model where trunk muscles are incorporated into a detailed finite element (FE) model of the ligamentous T12-S1 spine thus constructing a gold standard coupled MS-FE model. Model predictions are compared under some tasks with those of our earlier spherical joints, beam joints, and hybrid (uncoupled) MS-FE models. The coupled model predicted L4-L5 intradiscal pressures (R2 â‰… 0.97, RMSE â‰… 0.27 MPa) and L1-S1 centers of rotation (CoRs) in agreement to in vivo data. Differences in model predictions grew at larger trunk flexion angles; at the peak (80°) flexion the coupled model predicted, compared to the hybrid model, much smaller global/local muscle forces (~38%), segmental (~44%) and disc (~22%) compression forces but larger segmental (~9%) and disc (~17%) shear loads, ligament forces at the lower lumbar levels (by up to 57%) and facet forces at all levels. The spherical/beam joints models predicted much greater muscle forces and segmental loads under larger flexion angles. Unlike the spherical joints model with fixed CoRs, the beam joints model predicted CoRs closer (RMSE = 2.3 mm in flexion tasks) to those of the coupled model. The coupled model offers a great potential for future studies towards improvement of surgical techniques, management of musculoskeletal injuries and subject-specific simulations.

Marker-less versus marker-based driven musculoskeletal models of the spine during static load-handling activities.

Evaluation of workers' body posture in workstations is a prerequisite to estimate spinal loads and assess risk of injury for the subsequent design of preventive interventions. The Microsoft Kinect™ sensor is, in this regard, advantageous over the traditional skin-marker-based optical motion capture systems for being marker-less, portable, cost-effective, and easy-to-use in real workplaces. While several studies have demonstrated the validity/reliability of the Kinect for posture measurements especially during gait trials, its capability to adequately drive a detailed spine musculoskeletal model for injury risk assessments remains to be investigated. Lumbosacral (L5-S1) load predictions of a Kinect-driven and a gold-standard marker-based Vicon-driven musculoskeletal model were compared for various standing static load-handling activities at different heights/asymmetry angles/distances. Full body kinematics of eight individuals each performing eighteen activities were simultaneously recorded by a single-front-placed Kinect and a 10-camera Vicon motion capture system and input to AnyBody Modeling System. The predicted spinal loads by the two models were in average different by 17.8 and 25.9% for the L5-S1 disc compressive and shear forces, respectively, with smaller errors for the activities at higher load heights. Some activities performed near the floor could, however, not be recorded by a single-front-placed Kinect sensor due to the joint occlusion. The capability of the Kinect to adequately drive a spine musculoskeletal model depended on the complexity of the activity. While a single front-placed Kinect camera can be used to evaluate spinal loads in a wide range of static/quasi-static activities, cautious should be exercised when evaluating tasks performed near the floor.

Effect of changes in the lumbar posture in lifting on trunk muscle and spinal loads: A combined in vivo, musculoskeletal, and finite element model study.

Irrespective of the lifting technique (squat or stoop), the lumbar spine posture (more kyphotic versus more lordotic) adopted during lifting activities is an important parameter affecting the active-passive spinal load distribution. The advantages in either posture while lifting remains, however, a matter of debate. To comprehensively investigate the role on the trunk biomechanics of changes in the lumbar posture (lordotic, free or kyphotic) during forward trunk flexion, validated musculoskeletal and finite element models, driven by in vivo kinematics data, were used to estimate detailed internal tissue stresses-forces in and load-sharing among various joint active-passive tissues. Findings indicated that the lordotic posture, as compared to the kyphotic one, resulted in marked increases in back global muscle activities (~14-19%), overall segmental compression (~7.5-46.1%) and shear (~5.4-47.5%) forces, and L5-S1 facet joint forces (by up to 80 N). At the L5-S1 level, the lordotic lumbar posture caused considerable decreases in the moment resisted by passive structures (spine and musculature, ~14-27%), negligible reductions in the maximum disc fiber strains (by ~0.4-4.7%) and small increases in intradiscal pressure (~1.8-3.4%). Collectively and with due consideration of the risk of fatigue and viscoelastic creep especially under repetitive lifts, current results support a free posture (in between the extreme kyphotic and lordotic postures) with moderate contributions from both active and passive structures during lifting activities involving trunk forward flexion.

Subject-specific regression equations to estimate lower spinal loads during symmetric and asymmetric static lifting.

Workplace safety assessment, personalized treatment design and back pain prevention programs require accurate subject-specific estimation of spinal loads. Since no noninvasive method can directly estimate spinal loads, easy-to-use regression equations that are constructed based on the results of complex musculoskeletal models appear as viable alternatives. Thus, we aim to develop subject-specific regression equations of L4-L5 and L5-S1 shear and compression forces during various symmetric/asymmetric tasks using a nonlinear personalized finite element musculoskeletal trunk model. Kinematics and electromyography (EMG) activities of 19 young healthy subjects were collected during 64 different symmetric/asymmetric tasks. To investigate the reliability and accuracy of the musculoskeletal model and regression equations, we compared estimated trunk muscle activities and L4-L5 intradiscal pressures (IDPs) respectively with our own electromyography data (EMGs) and reported in vivo pressure measurements. Although in general, six independent rotation components (three trunk T11 rotations and three pelvic S1 rotations) are required to determine kinematics along the spine, only two surrogate variables (trunk flexion and its asymmetric angles) satisfactorily predicted all six rotation components (R2 > 0.94). Regression equations, developed based on subject-specific inputs, predicted spinal loads in satisfactory agreement with IDP measurements (R2 = 0.85). Predicted muscle activities in the personalized musculoskeletal models were in moderate to weak agreements with our measured EMGs in 19 participants. Based on dominance analysis, trunk flexion and its asymmetry angle, hand-load weight, hand-load lever arm, and body weight were the most important variables while the effects of body height and sex on spinal loads remained small.

Coupled artificial neural networks to estimate 3D whole-body posture, lumbosacral moments, and spinal loads during load-handling activities.

Biomechanical modeling approaches require body posture to evaluate the risk of spine injury during manual material handling. The procedure to measure body posture via motion-analysis techniques as well as the subsequent calculations of lumbosacral moments and spine loads by, respectively, inverse-dynamic and musculoskeletal models are complex and time-consuming. We aim to develop easy-to-use yet accurate artificial neural networks (ANNs) that predict 3D whole-body posture (ANNposture), segmental orientations (ANNangle), and lumbosacral moments (ANNmoment) based on our measurements during load-handling activities. Fifteen individuals each performed 135 load-handling activities by reaching (0 kg) or handling (5 and 10 kg) weights located at nine different horizontal and five vertical (0, 30, 60, 90, and 120 cm from the floor) locations. Whole-body posture was measured via a motion capture system and lumbosacral moments were calculated via a 3D top-down eight link-segment inverse-dynamic model. ANNposture, ANNangle, and ANNmoment were trained (RMSEs = 6.7 cm, 29.8°, and 16.2 Nm, respectively) and their generalization capability was tested (RMSE = 7.0 cm and R2 = 0.97, RMSE = 29.9° and R2 = 0.85, and RMSE = 16.5 Nm and R2 = 0.97, respectively). These ANNs were subsequently coupled to our previously-developed/validated ANNload, which predicts spinal loads during 3D load-handling activities. The results showed outputs of the coupled ANNs for L4-L5 intradiscal pressure (IDPs) during a number of activities were in agreement with measured IDPs (RMSE = 0.37 MPa and R2 = 0.89). Hence, coupled ANNs were found to be robust tools to evaluate posture, lumbosacral moments, spinal loads, and thus risk of injury during load-handling activities.

Hypersensitivity of trunk biomechanical model predictions to errors in image-based kinematics when using fully displacement-control techniques.

Recent advances in medical imaging techniques have allowed pure displacement-control trunk models to estimate spinal loads with no need to calculate muscle forces. Sensitivity of these models to the errors in post-imaging evaluation of displacements (reported to be ∼0.4-0.9° and 0.2-0.3 mm in vertebral displacements) has not yet been investigated. A Monte Carlo analysis was therefore used to assess the sensitivity of results in both musculoskeletal (MS) and passive finite element (FE) spine models to errors in measured displacements. Six static activities in upright standing, flexed, and extended postures were initially simulated using a force-control hybrid MS-FE model. Computed vertebral displacements were subsequently used to drive two distinct fully displacement-control MS and FE models. Effects of alterations in the reference vertebral displacements (at 3 error levels with SD (standard deviation) = 0.1, 0.2, and 0.3 mm in input translations together with, respectively, 0.2, 0.4, and 0.6° in input rotations) were investigated on the model predictions. Results indicated that outputs of both models had substantial task-dependent sensitivities to errors in the measured vertebral translations. For instance, L5-S1 intradiscal pressures (IDPs) were considerably affected (SD values reaching 1.05 MPa) and axial compression and shear forces even reversed directions as translation errors increased to 0.3 mm. Outputs were however generally much less sensitive to errors in measured vertebral rotations. Accounting for the accuracies in image-based kinematics measurements, therefore, it is concluded that the current measured vertebral translation errors at and beyond 0.1 mm are too large to drive biomechanical models of the spine.

Trunk Hybrid Passive-Active Musculoskeletal Modeling to Determine the Detailed T12-S1 Response Under In Vivo Loads.

Biomechanical models of the spine either simplify intervertebral joints (using spherical joints or deformable beams) in musculoskeletal (MS) or overlook musculature in geometrically-detailed passive finite element (FE) models. These distinct active and passive models therefore fail to determine in vivo stresses and strains within and load-sharing among the joint structures (discs, ligaments, and facets). A novel hybrid active-passive spine model is therefore developed in which estimated trunk muscle forces from a MS model for in vivo activities drive a mechanically-equivalent passive FE model to quantify in vivo T12-S1 compression/shear loads, intradiscal pressures (IDP), centers of rotation (CoR), ligament/facet forces, and annulus fiber strains. The predicted and in vivo L4-L5 IDP and L1-S1 CoRs showed satisfactory agreements. The FE model under commonly-used in vitro loading (pure moments and follower loads) predicted different kinetics from those of the hybrid model under in vivo loads (muscle exertions and gravity loads) contributing to suggest the inadequacy of such in vitro loads when simulating in vivo tasks. For an improved assessment of the injury risk, evaluation of the internal loads, and design of implants, such hybrid models should therefore be used.

A novel stability-based EMG-assisted optimization method for the spine.

Traditional electromyography-assisted optimization (TEMG) models are commonly employed to compute trunk muscle forces and spinal loads for the design of clinical/treatment and ergonomics/prevention programs. These models calculate muscle forces solely based on moment equilibrium requirements at spinal joints. Due to simplifications/assumptions in the measurement/processing of surface EMG activities and in the presumed muscle EMG-force relationship, these models fail to satisfy stability requirements. Hence, the present study aimed to develop a novel stability-based EMG-assisted optimization (SEMG) method applied to a musculoskeletal spine model in which trunk muscle forces were estimated by enforcing equilibrium conditions constrained to stability requirements. That is, second-order partial derivatives of the potential energy of the musculoskeletal model with respect to its generalized coordinates were enforced to be positive semi-definite. Fifteen static tasks in upright and flexed postures with and without a hand load at different heights were simulated. The SEMG model predicted different muscle recruitments/forces (generally larger global and local muscle forces) and spinal loads (slightly larger) compared to the TEMG model. Such task-specific differences were dependant on the assumed magnitude of the muscle stiffness coefficient in the SEMG model. The SEMG model-predicted and measured L4-L5 intradiscal pressures were in satisfactory agreement during simulated activities.

Three-dimensional primary and coupled range of motions and movement coordination of the pelvis, lumbar and thoracic spine in standing posture using inertial tracking device.

Evaluation of spinal range of motions (RoMs) and movement coordination between its segments (thorax, lumbar, and pelvis) has clinical and biomechanical implications. Previous studies have not recorded three-dimensional primary/coupled motions of all spinal segments simultaneously. Moreover, magnitude/direction of the coupled motions of the thorax/pelvis in standing posture and lumbopelvic rhythms in the frontal/transverse planes have not been investigated. This study, hence, used an inertial tracking device to measure T1, T5, T12, total (T1-T12) thoracic, lower (T5-T12) and upper (T1-T5) thoracic, lumbar (T12-S1), and pelvis primary and coupled RoMs as well as their movement coordination in all anatomical planes/directions in twenty-two healthy individuals. RoMs were statistically compared between the anatomical planes and spinal segments as well as with available data in the literature. The spine had different primary RoMs in different planes/directions (flexion: lumbar: 55.4 ±â€¯12.4°, pelvis: 42.8 ±â€¯21.6°, and T1-T12 thoracic: 19.9 ±â€¯6.4°, extension: lumbar: 23.4 ±â€¯10.1°, thoracic: 11.7 ±â€¯3.4°, and pelvis: 10.2 ±â€¯6.4°, left/right lateral bending: thoracic: 24.5 ±â€¯7.4°/26.5 ±â€¯6.1°, lumbar: 16.4 ±â€¯7.2°/18.3 ±â€¯5.7°, and pelvis: 11.0 ±â€¯4.4°/9.3 ±â€¯6.2°, and left/right axial rotation: thoracic: 33.5 ±â€¯10.0°/37.1 ±â€¯11.7°, pelvis: 31.6 ±â€¯12.5°/27.2 ±â€¯12.0° and lumbar: 7.5 ±â€¯4.5°/9.2 ±â€¯7.3°). Pelvis, lumbar and thoracic spine had different/varying contributions/rhythms to generate total trunk (T1) movement, both within and between planes. Pattern of the coupled motions was inconsistent between subjects but side bending was generally associated with twisting to the same side at the thoracic spine and to the opposite side at the lumbar spine.

Obesity and spinal loads; a combined MR imaging and subject-specific modeling investigation.

Epidemiological studies have identified obesity asa possible risk factor for low back disorders. Biomechanical models can help test such hypothesis and shed light on the mechanism involved. A novel subject-specific musculoskeletal-modelling approach is introduced to estimate spinal loads during static activities in five healthy obese (BMI>30kg/m2) and five normal-weight (200.05). Heavier subjects did not necessarily have larger muscle moment arms (e.g., they were larger in 64kg (BMI=20.7kg/m2) subject than 78kg (BMI=24.6kg/m2) subject) or greater T1-L5 trunk weight (e.g., the 97kg (BMI=31kg/m2) subject had similar trunk weight as 109kg (BMI=33.3kg/m2) subject). Obese individuals had in average greater spinal loads than normal-weight ones but heavier subjects did not necessarily have greater spinal loads (117kg (BMI=40.0kg/m2) subject had rather similar L5-S1 compression as 105kg (BMI=34.7kg/m2) subject). Predicted L4-L5 intradiscal pressures for the normal-weight subjects ranged close to the measured values (R2=0.85-0.92). Obese individuals did not necessarily have greater IDPs than normal-weight ones.

Trunk musculoskeletal response in maximum voluntary exertions: A combined measurement-modeling investigation.

Maximum voluntary exertion (MVE) tasks quantify trunk strength and maximal muscle electromyography (EMG) activities with both clinical and biomechanical implications. The aims here are to evaluate the performance of an existing trunk musculoskeletal model, estimate maximum muscle stresses and spinal forces, and explore likely differences between males and females in maximum voluntary exertions. We, therefore, measured trunk strength and EMG activities of 19 healthy right-handed subjects (9 females and 10 males) in flexion, extension, lateral and axial directions. MVEs for all subjects were then simulated in a subject-specific trunk musculoskeletal model, and estimated muscle activities were compared with EMGs. Analysis of variance was used to compare measured moments and estimated spinal loads at the L5-S1 level between females and males. MVE moments in both sexes were greatest in extension (means of 236 Nm in males and 190 Nm in females) and least in left axial torque (97 Nm in males and 64 Nm in females). Being much greater in lateral and axial MVEs, coupled moments reached ∼50% of primary moments in average. Females exerted less moments in all directions reaching significance except in flexion. Muscle activity estimations were strongly correlated with measurements in flexion and extension (Pearson's r = 0.69 and 0.76), but the correlations were very weak in lateral and axial MVEs (Pearson's r = 0.27 and 0.13). Maximum muscle stress was in average 0.80 ±â€¯0.42 MPa but varied among muscles from 0.40 ±â€¯0.22  MPa in rectus abdominis to 0.99 ±â€¯0.29 MPa in external oblique. To estimate maximum muscle stresses and evaluate validity of a musculoskeletal model, MVEs in all directions with all coupled moments should be considered.

Effects of eight different ligament property datasets on biomechanics of a lumbar L4-L5 finite element model.

Ligaments assist trunk muscles in balancing external moments and providing spinal stability. In absence of the personalized material properties for ligaments, finite element (FE) models use dispersed data from the literature. This study aims to investigate the relative effects of eight different ligament property datasets on FE model responses. Eight L4-L5 models distinct only in ligament properties were constructed and loaded under moment (15Nm) alone or combined with a compressive follower load (FL). Range of motions (RoM) of the disc-alone model matched well in vitro data. Ligament properties significantly affected only sagittal RoMs (∼3.0-7.1° in flexion and ∼3.8-5.8° in extension at 10Nm). Sequential removal of ligaments shifted sagittal RoMs in and out of the corresponding in vitro ranges. When moment was combined with FL, center of rotation matched in vivo data for all models (3.8±0.9mm and 4.3±1.8mm posterior to the disc center in flexion and extension, respectively). Under 15Nm sagittal moments, ligament strains were often smaller or within the in vitro range in flexion whereas some posterior ligament forces approached their failure forces in some models. Ligament forces varied substantially within the models and affected the moment-sharing and internal forces on the disc and facet joints. Intradiscal pressure (IDP) had the greatest variation between models in extension. None of the datasets yielded results in agreement with all reported measurements. Results emphasized the important role of ligaments especially under larger moments and the need for their accurate representation in search for valid spinal models.

A combined passive and active musculoskeletal model study to estimate L4-L5 load sharing.

A number of geometrically-detailed passive finite element (FE) models of the lumbar spine have been developed and validated under in vitro loading conditions. These models are devoid of muscles and thus cannot be directly used to simulate in vivo loading conditions acting on the lumbar joint structures or spinal implants. Gravity loads and muscle forces estimated by a trunk musculoskeletal (MS) model under twelve static activities were applied to a passive FE model of the L4-L5 segment to estimate load sharing among the joint structures (disc, ligaments, and facets) under simulated in vivo loading conditions. An equivalent follower (FL), that generates IDP equal to that generated by muscle forces, was computed in each task. Results indicated that under in vivo loading conditions, the passive FE model predicted intradiscal pressures (IDPs) that closely matched those measured under the simulated tasks (R2=0.98 and root-mean-squared-error, RMSE=0.18MPa). The calculated equivalent FL compared well with the resultant force of all muscle forces and gravity loads acting on the L4-L5 segment (R2=0.99 and RMSE=58N). Therefore, as an alternative approach to represent in vivo loading conditions in passive FE model studies, this FL can be estimated by available in-house or commercial MS models. In clinical applications and design of implants, commonly considered in vitro loading conditions on the passive FE models do not adequately represent the in vivo loading conditions under muscle exertions. Therefore, more realistic in vivo loading conditions should instead be used.