Effect of low back pain on the kinetics and kinematics of the lumbar spine - a combined in vivo and in silico investigation.

Lifting is a significant risk factor for low back pain (LBP). Different biomechanical factors including spinal loads, kinematics, and muscle electromyography (EMG) activities have previously been investigated during lifting activities in LBP patients and asymptomatic individuals to identify their association with LBP. However, the findings were contradictory and inconclusive. Accurate and subject-specific prediction of spinal loads is crucial for understanding, diagnosing, planning tailored treatments, and preventing recurrent pain in LBP patients. Therefore, the present study aimed to estimate the L5-S1 compressive and resultant shear loads in 19 healthy and 17 non-specific chronic LBP individuals during various static load-holding tasks (holding a 10 kg box at hip, chest, and head height) using full-body and personalized musculoskeletal models driven by subject-specific in vivo kinematic/kinetic, EMG, and physiological cross-sectional areas (PCSAs) data. These biomechanical characteristics were concurrently analyzed to identify potential differences between the two groups. Statistical analyses showed that LBP had almost no significant effect on the range of motion (trunk, lumbar, pelvis), PCSA, and EMG. There were no significant differences (p > 0.05) in the predicted L5-S1 loads. However, as the task became more demanding, by elevating the hand-load from hip to head, LBP patients experienced significant increases in both compressive (33 %, p = 0.00) and shear (25 %, p = 0.02) loads, while asymptomatic individuals showed significant increases only in compressive loads (30 %, p = 0.01). This suggests that engaging in more challenging activities could potentially magnify the effect of LBP on the biomechanical factors and increase their discrimination capacity between LBP and asymptomatic individuals.

Comparing spinal loads in individuals with unilateral transtibial amputation with and without chronic low back pain: An EMG-informed approach.

Chronic low back pain (cLBP) is highly prevalent after lower limb amputation (LLA), likely due in part to biomechanical factors. Here, three-dimensional full-body kinematics and kinetics during level-ground walking, at a self-selected and three controlled speeds (1.0, 1.3, and 1.6 m/s), were collected from twenty-one persons with unilateral transtibial LLA, with (n = 9) and without cLBP (n = 12). Peak compressive, mediolateral, and anteroposterior L5-S1 spinal loads were estimated from a full-body, transtibial amputation-specific OpenSim model and compared between groups. Predicted lumbar joint torques from muscle activations were compared to inverse dynamics and predicted and measured electromyographic muscle activations were compared for model evaluation and verification. There were no group differences in compressive or anterior shear forces (p > 0.466). During intact stance, peak ipsilateral loads increased with speed to a greater extent in the cLBP group vs. no cLBP group (p=0.023), while during prosthetic stance, peak contralateral loads were larger in the no cLBP group (p=0.047) and increased to a greater extent with walking speed compared to the cLBP group (p=0.008). During intact stance, intact side external obliques had higher activations in the no cLBP group (p=0.039), and internal obliques had higher activations in the cLBP group at faster walking speeds compared to the no cLBP group. Predicted muscle activations demonstrated similar activation patterns to electromyographic-measured activations (r = 0.56-0.96), and error between inverse dynamics and simulated spinal moments was low (0.08 Nm RMS error). Persons with transtibial LLA and cLBP may adopt movement strategies during walking to reduce mediolateral shear forces at the L5-S1 joint, particularly as walking speed increases. However, future work is needed to understand the time course from pain onset to chronification and the cumulative influence of increased spinal loads over time.

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.

Sensitivity of subject-specific upper body musculoskeletal model predictions to mass scaling methods.

Accurate predictions of spinal loads in subject-specific musculoskeletal models require precise body segment parameters, including segment mass and center of mass (CoM) locations. Existing upper body models often assume a constant percentage of total body mass to calculate segmental masses, disregarding inter-individual variability and limiting their predictive capacity. This study evaluated the sensitivity of subject-specific upper body musculoskeletal model predictions to body mass scaling methods. The upper body segmental masses and corresponding CoM of six male subjects with varying body mass indices were computed using two mass scaling methods: the constant-percentage-based (CPB) scaling method, commonly used in AnyBody software; and our recently developed body-shape-based (BSB) method. Subsequently, these values were used by a validated musculoskeletal model to predict the muscle and disc forces in upright and flexed postures. The discrepancies between the results of the two scaling methods were compared across subjects and postures. Maximum deviations in thorax masses reached up to 7.5% of total body weight (TBW) in overweight subjects, while maximum CoM location differences of up to 35 mm were observed in normal weight subjects. The root mean squared errors (RMSE) of the CPB results, calculated with the BSB results as baseline, showed that the muscle and shear forces of the two scaling methods were quite similar (<4.5% of TBW). Though, there were small to moderate differences in compressive forces (6.5-16.0% of TBW). Thus, the compressive forces predicted with CPB method should be used with caution, particularly for overweight and obese subjects.

Trunk muscle forces and spinal loads during heavy deadlift: Effects of personalization, muscle wrapping, muscle lever arm, and lumbopelvic rhythm.

Heavy deadlift is used as a physical fitness screening tool in the U.S. Army. Despite the relevance of such a screening tool to military tasks performed by Service Members, the biomechanical impact of heavy deadlift and its risk of low-back injury remain unknown. A kinematics-driven musculoskeletal model of spine was implemented to investigate biomechanics of the lower back in a volunteer (23 years old, height of 1.82 m, and body mass of 98.8 kg) during a 68 kg deadlift. In search of protective mechanisms, effects of model personalization and variations in trunk musculature and lumbopelvic rhythm were also investigated. The net moment, compression and shear forces at the L5-S1 reached peaks of 684 Nm, 17.2 and 4.2 kN, respectively. Geometrical personalization and changes in lumbopelvic rhythm had the least effects on predictions while increases in muscle moment arms (40%) had the largest effects that caused, respectively, 32% and 36% decrease in the maximum compressive and shearing forces. Initiating wrapping of back muscles at farther distances from the spine had opposing effects on spinal loads; peak compression at the L5-S1 decreased by 12% whereas shear increased by 19%. Despite mechanisms considered, spinal loads during heavy deadlift exceed the existing evidence concerning the threshold of injury for spinal segments, suggesting the vulnerability to injury. Chronic exposure to such high-spinal loads may lead to (micro) fractures, degeneration, pathoanatomical changes and finally low-back pain.

Association between sagittal alignment and loads at the adjacent segment in the fused spine: a combined clinical and musculoskeletal modeling study of 205 patients with adult spinal deformity.

PURPOSE: Sagittal malalignment is a risk factor for mechanical complications after surgery for adult spinal deformity (ASD). Spinal loads, modulated by sagittal alignment, may explain this relationship. The aims of this study were to investigate the relationships between: (1) postoperative changes in loads at the proximal segment and realignment, and (2) absolute postoperative loads and postoperative alignment measures. METHODS: A previously validated musculoskeletal model of the whole spine was applied to study a clinical sample of 205 patients with ASD. Based on clinical and radiographic data, pre-and postoperative patient-specific alignments were simulated to predict loads at the proximal segment adjacent to the spinal fusion. RESULTS: Weak-to-moderate associations were found between pre-to-postop changes in lumbar lordosis, LL (r = - 0.23, r = - 0.43; p < 0.001), global tilt, GT (r = 0.26, r = 0.38; p < 0.001) and the Global Alignment and Proportion score, GAP (r = 0.26, r = 0.37; p < 0.001), and changes in compressive and shear forces at the proximal segment. GAP score parameters, thoracic kyphosis measurements and the slope of upper instrumented vertebra were associated with changes in shear. In patients with T10-pelvis fusion, moderate-to-strong associations were found between postoperative sagittal alignment measures and compressive and shear loads, with GT showing the strongest correlations (r = 0.75, r = 0.73, p < 0.001). CONCLUSIONS: Spinal loads were estimated for patient-specific full spinal alignment profiles in a large cohort of patients with ASD pre-and postoperatively. Loads on the proximal segments were greater in association with sagittal malalignment and malorientation of proximal vertebra. Future work should explore whether they provide a causative mechanism explaining the associated risk of proximal junction complications.

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.

The Validity and Inter-Rater Reliability of a Video-Based Posture-Matching Tool to Estimate Cumulative Loads on the Lower Back.

Background: Low back pain (LBP) is known as one of the most common work-related musculoskeletal disorders. Spinal cumulative loads (CLs) during manual material handling (MMH) tasks are the main risk factors for LBP. However, there is no valid and reliable quantitative lifting analysis tool available for quantifying CLs among Iranian workers performing MMH tasks. Objective: This study aimed to investigate the validity and inter-rater reliability of a posture-matching load assessment tool (PLAT) for estimating the L5-S1 static cumulative compression (CC) and shear (CS) loads based on predictive regression equations. Material and Methods: This experimental study was conducted among six participants performing four lifting tasks, each comprised of five trials during which their posture was recorded via a motion capture (Vicon) and simultaneously a three-camera system located at three different angles (0°, 45°, and 90°) to the sagittal plane. Results: There were no significant differences between the two CLs estimated by PLAT from the three-camera system and the gold-standard Vicon. In addition, ten raters estimated CLs of the tasks using PLAT in three sessions. The calculated intra-class correlation coefficients for the estimated CLs within each task revealed excellent inter-rater reliability (> 0.75), except for CS in the first and third tasks, which were good (0.6 to 0.75). Conclusion: The proposed posture-matching approach provides a valid and reliable ergonomic assessment tool suitable for assessing spinal CLs during various lifting activities.

Sex-Dependent Estimation of Spinal Loads During Static Manual Material Handling Activities-Combined in vivo and in silico Analyses.

Manual material handling (MMH) is considered as one of the main contributors to low back pain. While males traditionally perform MMH tasks, recently the number of females who undertake these physically-demanding activities is also increasing. To evaluate the risk of mechanical injuries, the majority of previous studies have estimated spinal forces using different modeling approaches that mostly focus on male individuals. Notable sex-dependent differences have, however, been reported in torso muscle strength and anatomy, segmental mass distribution, as well as lifting strategy during MMH. Therefore, this study aimed to use sex-specific models to estimate lumbar spinal and muscle forces during static MHH tasks in 10 healthy males and 10 females. Motion-capture, surface electromyographic from select trunk muscles, and ground reaction force data were simultaneously collected while subjects performed twelve symmetric and asymmetric static lifting (10 kg) tasks. AnyBody Modeling System was used to develop base-models (subject-specific segmental length, muscle architecture, and kinematics data) for both sexes. For females, female-specific models were also developed by taking into account for the female's muscle physiological cross-sectional areas, segmental mass distributions, and body fat percentage. Males showed higher absolute L5-S1 compressive and shear loads as compared to both female base-models (25.3% compressive and 14% shear) and female-specific models (41% compressive and 23.6% shear). When the predicted spine loads were normalized to subjects' body weight, however, female base-models showed larger loads (9% compressive and 16.2% shear on average), and female-specific models showed 2.4% smaller and 9.4% larger loads than males. Females showed larger forces in oblique abdominal muscles during both symmetric and asymmetric lifting tasks, while males had larger back extensor muscle forces during symmetric lifting tasks. A stronger correlation between measured and predicted muscle activities was found in females than males. Results indicate that female-specific characteristics affect the predicted spinal loads and must be considered in musculoskeletal models. Neglecting sex-specific parameters in these models could lead to the overestimation of spinal loads in females.

Comparison of different lifting analysis tools in estimating lower spinal loads - Evaluation of NIOSH criterion.

Excessive loads on the human spine is recognized as a risk factor for back injuries/pain. Various lifting analysis tools such as musculoskeletal models, regression equations and NIOSH (National Institute for Occupational Safety and Health) lifting equation (NLE) have been proposed to evaluate and mitigate associated risks during manual material handling activities. Present study aims to compare predicted spinal loads from 5 different lifting analysis tools as well as to critically evaluate the NIOSH recommended weight limit (RWL). Spinal loads were estimated under different symmetric/asymmetric lifting tasks in which hand-load mass at each task was set based on RWL from NLE. Estimated intradiscal pressures (IDPs) of various tools were also compared with in vivo measurements. We compared RWL by NLE versus our estimations of RWL calculated from our regression equations using biomechanical criteria (compression <3400 N with/without shear <1000, 1250 or 1500 N). Our regression equations followed by OpenSim, AnyBody, simple polynomial and 3DSSPP satisfactorily predicted L4-L5 IDPs. Lifting analysis tools estimated comparable spinal compression forces (mean Pearson's r = 0.80; standard deviation of relative difference = 26%) while in shear, differences were greater (mean Pearson's r = 0.68; standard deviation of relative difference = 56%). NLE estimations of RWL were conservative in comparison with our estimations for lean individuals (BMI < 25 kg/m2) when compression <3400 N and shear <1250 N were considered as the biomechanical criteria. For heavier individuals, however, NLE estimations of RWL generated spinal compression >3400 N (NIOSH biomechanical safety threshold) as well as shear >1000 N. Although RWLs estimated by NLE was body weight independent, body weight substantially altered RWLs estimated from our regression equations. For improved estimation of the risk of injury, more accurate failure criteria for spinal segments are essential.

A numerical study to determine the effect of strengthening and weakening of the transversus abdominis muscle on lumbar spine loads.

This work aimed to determine the influence of strengthening or weakening of the transversus abdominis (TrA) muscle on loads in the lumbar spine using musculoskeletal modelling methods. The input kinematic data of two positions (a standing position and a position during a sitting-down task) were angles in the elbow joint (0°;4°), shoulder joint (0°;3°), hip joint (0°;75°) and knee joint (0°;69°) as well as the torso tilt angle (0°;32°). It was shown that a change in the TrA physiological cross section area (PCSA) has a crucial impact on lumbar spine loads (2xTrA PCSA causes a reduction in the force in joint L5-S1 by 11% for a standing position and by 25% for a sitting-down position) and value of intra-abdominal pressure.

Evolution of Fatigue Damage in the L5-S1 Intervertebral Disc Resulting from Walking Exposures Among Persons with Lower Limb Loss.

The risk of fatigue damage within the L5-S1 spinal disc was calculated for a sample of 52 individuals with unilateral limb loss (26 transtibial; 26 transfemoral) and 26 uninjured controls using a non-linear multi-axial fatigue model of the spine motion segments. Time to complete damage was calculated for each participant and walking pace; the influences of walking activity were determined by varying daily step counts. Assuming similar activity across groups (10,000 steps per day), times to failure were not different between persons with and without limb loss (50 [23] vs. 46 [24] years, respectively); walking faster was associated with shorter times to failure. Greater daily step counts similarly decreased time to failure in all groups. While such a similarity in fatigue damage evolution does not necessarily downplay the role of biomechanical factors, it may highlight the important influences of psychosocial factors in the development of low back pain in persons with limb loss. Nevertheless, with additional work to include activities beyond walking, computational damage models can provide a predictive platform for evaluating specific clinical interventions (e.g., behavior modification, movement retraining, prosthetic devices) that are ultimately intended to mitigate physical risk factors for low back pain following limb loss.

Modified electromyography-assisted optimization approach for predicting lumbar spine loading while walking with backpack loads.

This study modified an electromyography-assisted optimization approach for predicting lumbar spine loading while walking with backpack loads. The modified-electromyography-assisted optimization approach eliminated the electromyography measurement at maximal voluntary contraction and adopted a linear electromyography-force relationship. Moreover, an optimal lower boundary condition for muscle gain was introduced to constrain the trunk muscle co-activation. Anthropometric information of 10 healthy young men as well as their kinematic, kinetic, and electromyography data obtained while walking with backpack loads were used as inputs in this study. A computational algorithm was used to find and analyse the sensitivity of the optimal lower boundary condition for achieving minimum deviation of the modified-electromyography-assisted optimization approach from the electromyography-assisted optimization approach for predicting lumbosacral joint compression force. Results validated that the modified-electromyography-assisted optimization approach (at optimal lower boundary condition of 0.92) predicted on average, a non-significant deviation in peak lumbosacral joint compression force of -18 N, a standard error of 9 N, and a root mean square difference in force profile of 73.8 N. The modified-electromyography-assisted optimization approach simplified the experimental process by eliminating the electromyography measurement at maximal voluntary contraction and provided comparable estimations for lumbosacral joint compression force that is also applicable to patients or individuals having difficulty in performing the maximal voluntary contraction activity.

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.

In vivo hip and lumbar spine implant loads during activities in forward bent postures.

Long-term measurements on the lumbar spinal alignment during daily life revealed that humans spent 90% of the day in a forward bent posture. Compared to standing, this posture leads to a substantial increase in spinal loading. The lumbar spine and pelvis, however, contribute differently to the total amount of flexion, which could possibly indicate a different timing of maximum loads in both structures during flexion. This study aimed to evaluate the in vivo implant forces in the hip and lumbar spine during activities in forward bent postures. This work utilized data collected in earlier in vivo measurements on patients either with telemeterized hip endoprostheses (HE) or vertebral body replacements (VBR). The following activities were investigated: standing, upper body flexion with and without weights in the hands using different lifting techniques (straight and bent knees). The maximum resultant forces in VBR were considerably lower than in HE. Increases in flexion inclinations caused direct increases of the resultant forces within VBR, followed by a plateau or even a decrease of the force until maximum inclination. The resultant force in HE displayed an almost continuous increase until the maximum inclination. This general curve behavior resulted in different HE-VBR load ratios, which were affected by lifting additional weights or different lifting techniques. The results emphasize that maximum loads in the spine, in contrast to the hip, do not necessarily occur at maximum upper body flexion as normally expected, rather already at intermediate flexion angles in VBR patients. The results form the basis for more detailed insilico analyzes.

EMG-based lumbosacral joint compression force prediction using a support vector machine.

Electromyography-assisted optimization (EMGAO) approach is widely used to predict lumbar joint loads under various dynamic and static conditions. However, such approach uses numerous anthropometric, kinematic, kinetic, and electromyographic data in the computation process, and thus makes data collection and processing complicated. This study developed an electromyography-based support vector machine (EMGB_SVM) approach for predicting lumbar spine load during walking with backpack loads. The EMGB_SVM is simple and uses merely the electromyographic data. Anthropometric information of 10 healthy male adults as well as their kinematic, kinetic, and electromyographic data acquired during walking exercises with no-load and with various backpack loads (5%, 10%, 15%, and 20% of their body weight) were used as the inputs of a biomechanical model, which was then used for predicting the lumbosacral joint compression force. The efficacy of the EMGB_SVM was investigated by comparing the force profiles obtained using this model with those obtained using the current EMGAO approach. On average, the EMGB_SVM obtained deviations in the peak and minimum forces of -3.3% and 5.1%, respectively, and a root mean square difference in the force profile of 7.5%. The EMGB_SVM is a comparable estimator in terms of its slight bias, favourable consistency, and efficiency at predicting the lumbosacral joint compression force.

Twente Spine Model: A thorough investigation of the spinal loads in a complete and coherent musculoskeletal model of the human spine.

Although in vivospinal loads have been previously measured, existing data are limited to certain lumbar and thoracic levels. A detailed investigation of spinal loads would assist with injury prevention and implant design but is unavailable. In this study, we developed a complete and coherent musculoskeletal model of the entire human spine and studied the intervertebral disc compression forces for physiological movements on three anatomical planes. This model incorporates the individual vertebrae at the cervical, thoracic, and lumbar regions, a flexible ribcage, and complete muscle anatomy. Intradiscal pressures were estimated from predicted compressive forces, and these were generally in close agreement with previously measured data. We found that compressive forces at the trunk discs increased during trunk lateral bending and axial rotation of the trunk. During flexion, compressive forces increased in the thoracolumbar and lumbar regions and slightly decreased at the middle thoracic discs. In extension, the forces generally decreased at the thoracolumbar and lumbar discs whereas they slightly increased at the upper and middle thoracic discs. Furthermore, similar to a previous biomechanical model of the cervical spine, our model predicted increased compression forces in neck flexion, lateral bending, and axial rotation, and decreased forces in neck extension.

Trunk muscle forces and spinal loads in persons with unilateral transfemoral amputation during sit-to-stand and stand-to-sit activities.

BACKGROUND: Alterations and asymmetries in trunk motions during activities of daily living, involving lower extremities, are suggested to cause higher spinal loads in persons with unilateral lower limb amputation. Given the repetitive nature of most activities of daily living, knowledge of the amount of increase in spinal loads is important for designing interventions aimed at prevention of secondary low back pain due to potential fatigue failure of spinal tissues. The objective of this study was to determine differences in trunk muscle forces and spinal loads between persons with and without lower limb amputation when performing sit-to-stand and stand-to-sit tasks. METHODS: Kinematics of the pelvis and thorax, obtained from ten males with unilateral transfemoral lower limb amputation and 10 male uninjured controls when performing sit-to-stand and stand-to-sit activities, were used within a non-linear finite element model of the spine to estimate trunk muscle forces and resultant spinal loads. FINDINGS: The peak compression force, medio-lateral (only during stand-to-sit), and antero-posterior shear forces were respectively 348 N, 269 N, and 217 N larger in person with vs. without amputation. Persons with amputation also experienced on average 171 N and 53 N larger mean compression force and medio-lateral shear force, respectively. INTERPRETATION: While spinal loads were larger in persons with amputation, these loads were generally smaller than the reported threshold for spinal tissue injury. However, a rather small increase in spinal loads during common activities of daily living like walking, sit-to-stand, and stand-to-sit may nevertheless impose a significant risk of fatigue failure for spinal tissues due to the repetitive nature of these activities.

Effect of whole-body vibration and sitting configurations on lumbar spinal loads of vehicle occupants.

Whole-body vibration (WBV) has been identified as one of the serious risk factors leading to spinal disorders, particularly in professional drivers. Although the influential factors in this area have been investigated epidemiologically, finite element (FE) modeling can efficiently help us better understand the problem. In this study, a modified HYBRID III dummy FE model which was enhanced by detailed viscoelastic discs in the lumbar region was utilized to simulate the effect of WBV on lumbar spine loads. Spinal responses to the vertical sinusoidal vibrations of a generic seat were obtained and spinal injury risk factors were calculated. Effects of variation of excitation frequencies, three different seatback inclinations and four pre-defined occupant postures on the spinal loads were investigated as influential variables. Results showed that under sinusoidal loading with a frequency of 5 Hz and in a typical sitting configuration, disc forces remained in a safe range (<1700 N) for short term. Collagen Fibers strain (<0.3%) and intradiscal pressure (<1.15 MPa) also indicated that the spinal loads were in a safe range. Additionally, calculating the risk factor according to ISO 2631-5 (about R = 0.8) confirmed the low probability of an adverse health effect due to WBV in long term. Frequency-domain analysis showed the resonance frequency to be at f = 6.27 Hz. Although according to ISO/CD 2631-5 standard, the occupant experienced the highest risk of injury at f = 7 Hz, it was found that spinal compression load at f = 6 Hz was 7.7% higher than the compression load at f = 7 Hz. Seatback oriented at 75° exhibited the highest risk of injury, nevertheless, maximum von-Mises stress in disc annulus was observed at 70°. In the evaluation of occupant posture, lordotic and slouching postures were compared and the latter exhibited higher stress ranges resulting in higher injury risk factor. Results of the model demonstrated its aptness to predict the spinal disc injuries in response to various vibrational loading and boundary conditions.

The effect of muscle ageing and sarcopenia on spinal segmental loads.

PURPOSE: The interrelations between age-related muscle deterioration (sarcopenia) and vertebral fractures have been suggested based on clinical observations, but the biomechanical relationships have not been explored. The study aim was to investigate the effects of muscle ageing and sarcopenia on muscle recruitment patterns and spinal loads, using musculoskeletal multi-body modelling. METHODS: A generic AnyBody model of the thoracolumbar spine, including > 600 fascicles representing trunk musculature, was used. Several stages of normal ageing and sarcopenia were modelled by reduced strength of erector spinae and multifidus muscles (ageing from 3rd to 6th life decade: ≥ 60% of normal strength; sarcopenia: mild 60%, moderate 48%, severe 36%, very severe 24%), reflecting the reported decrease in cross-sectional area and increased fat infiltration. All other model parameters were kept unchanged. Full-range flexion was simulated using inverse dynamics with muscle optimization to predict spinal loads and muscle recruitment patterns. RESULTS: The muscle changes due to normal ageing (≥ 60% strength) had a minor effect on predicted loads and provoked only slightly elevated muscle activities. Severe (36%) and very severe (24%) stages of sarcopenia, however, were associated with substantial increases in compression (by up to 36% or 318N) at the levels of the upper thoracic spine (T1T2-T5T6) and shear loading (by up to 75% or 176N) along the whole spine (T1T2-L4L5). The muscle activities increased for almost all muscles, up to 100% of their available strength. CONCLUSIONS: The study highlights the distinct and detrimental consequences of sarcopenia, in contrast to normal ageing, on spinal loading and required muscular effort. These slides can be retrieved under Electronic Supplementary Material.