Muscle lines-of-action affect predicted forces in optimization-based spine muscle modeling

Modeling Human Suboptimal Control: A Review

This review paper provides an overview of the approaches to model neuromuscular control, focusing on methods to identify nonoptimal control strategies typical of populations with neuromuscular disorders or children. Where possible, the authors tightened the description of the methods to the mechanisms behind the underlying biomechanical and physiological rationale. They start by describing the first and most simplified approach, the reductionist approach, which splits the role of the nervous and musculoskeletal systems. Static optimization and dynamic optimization methods and electromyography-based approaches are summarized to highlight their limitations and understand (the need for) their developments over time. Then, the authors look at the more recent stochastic approach, introduced to explore the space of plausible neural solutions, thus implementing the uncontrolled manifold theory, according to which the central nervous system only controls specific motions and tasks to limit energy consumption while allowing for some degree of adaptability to perturbations. Finally, they explore the literature covering the explicit modeling of the coupling between the nervous system (acting as controller) and the musculoskeletal system (the actuator), which may be employed to overcome the split characterizing the reductionist approach.

EMG optimization in OpenSim: A model for estimating lower back kinetics in gait

Participant-specific musculoskeletal models are needed to accurately estimate lower back internal kinetic demands and injury risk. In this study we developed the framework for incorporating an electromyography optimization (EMGopt) approach within OpenSim (https://simtk.org/projects/emg_opt_tool) and evaluated lower back demands estimated from the model during gait. Kinematic, external kinetic, and EMG data were recorded from six participants as they performed walking and carrying tasks on a treadmill. For evaluation, predicted lumbar vertebral joint forces were compared to those from a generic static optimization approach (SOpt) and to previous studies. Further, model-estimated muscle activations were compared to recorded EMG, and model sensitivity to day-to-day EMG variability was evaluated. Results showed the vertebral joint forces from the model were qualitatively similar in pattern and magnitude to literature reports. Compared to SOpt, the EMGopt approach predicted larger joint loads (p<.01) with muscle activations better matching individual participant EMG patterns. L5/S1 vertebral joint forces from EMGopt were sensitive to the expected variability of recorded EMG, but the magnitude of these differences (±4%) did not impact between-task comparisons. Despite limitations inherent to such models, the proposed musculoskeletal model and EMGopt approach appears well-suited for evaluating internal lower back demands during gait tasks.

The impact of anatomical uncertainties on the predictions of a musculoskeletal hand model - a sensitivity study

Outputs of musculoskeletal models should be considered probabilistic rather than deterministic as they are affected by inaccuracies and estimations associated with the development of the model. One of these uncertainties being critical for modeling arises from the determination of the muscles' line of action and the physiological cross-sectional area. Therefore, the aim of this study was to evaluate the outcome sensitivity of model predictions from a musculoskeletal hand model in comparison to the uncertainty of these input parameters. For this purpose, the kinematics and muscle activities of different hand movements (abduction of the fingers, abduction of the thumb, and flexion of the thumb) were recorded. One thousand simulations were calculated for each movement using the Latin hypercube sampling method with a corresponding variation of the muscle origin/insertion points and the cross-sectional area. Comparing the standard hand to simulations incorporating uncertainties of input parameters shows no major deviations in on- and off-set time point of muscle activities. About 60% of simulations are located within a ± 30% interval around the standard model concerning joint reaction forces. The comparison with the variation of the input data leads to the conclusion that the standard hand model is able to provide not over-scattered outcomes and, therefore, can be considered relatively stable. These results are of practical importance to the personalization of a musculoskeletal model with subject-specific bone geometries and hence changed muscle line of action.

A new musculoskeletal AnyBody™ detailed hand model

Musculoskeletal research questions regarding the prevention or rehabilitation of the hand can be addressed using inverse dynamics simulations when experiments are not possible. To date, no complete human hand model implemented in a holistic human body model has been fully developed. The aim of this work was to develop, implement, and validate a fully detailed hand model using the AnyBody Modelling System (AMS) (AnyBody, Aalborg, Denmark). To achieve this, a consistent multiple cadaver dataset, including all extrinsic and intrinsic muscles, served as a basis. Various obstacle methods were implemented to obtain with the correct alignment of the muscle paths together with the full range of motion of the fingers. These included tori, cylinders, and spherical ellipsoids. The origin points of the lumbrical muscles within the tendon of the flexor digitorum profundus added a unique feature to the model. Furthermore, the possibility of an entire patient-specific scaling based on the hand length and width were implemented in the model. For model validation, experimental datasets from the literature were used, which included the comparison of numerically calculated moment arms of the wrist, thumb, and index finger muscles. In general, the results displayed good comparability of the model and experimental data. However, the extrinsic muscles showed higher accordance than the intrinsic ones. Nevertheless, the results showed, that the proposed developed inverse dynamics hand model offers opportunities in a broad field of applications, where the muscles and joint forces of the forearm play a crucial role.

Application of MR-derived cross-sectional guideline of cervical spine muscles to validate neck surface electromyography placement

The importance of surface-EMG placement for development and interpretation of EMG-assisted biomechanical models is well established. Since MR has become a reliable noninvasive cervical spine musculoskeletal diagnostic tool, this investigation attempted to illustrate the anatomical relationships of individual cervical spine muscles with their paired surface-EMG electrodes. The secondary purpose of this investigation was to provide an MR cross-sectional pictorial and descriptive guideline of the cervical spine musculature. MR scans were performed on a healthy adult male subject from skull to manubrium of the sternum. Prior to scanning, MR safe markers were placed over neck muscles following surface EMG placement recommendations. Twenty-three neck muscles were traced manually in each of 267 scan slices. 3-D models of the neck musculoskeletal structure were constructed to aid with understanding the complex anatomy of the region as well as to identify correct EMG electrode locations and to identify muscles' curved lines-of-action. 3D models of the MR-safe markers were constructed relative to the target muscles. Based on the findings of this study, muscle palpation and bony landmarks can be used to effectively identify appropriate surface EMG electrode locations to record upper trapezius, middle trapezius, semispinalis capitis, splenius capitis, levator scapulae, scalenus, sternocleidomastoid and hyoid muscles activities.

Implementation and validation of the extended Hill-type muscle model with robust routing capabilities in LS-DYNA for active human body models

BACKGROUND

In the state of the art finite element AHBMs for car crash analysis in the LS-DYNA software material named *MAT_MUSCLE (*MAT_156) is used for active muscles modeling. It has three elements in parallel configuration, which has several major drawbacks: restraint approximation of the physical reality, complicated parameterization and absence of the integrated activation dynamics. This study presents implementation of the extended four element Hill-type muscle model with serial damping and eccentric force-velocity relation including [Formula: see text] dependent activation dynamics and internal method for physiological muscle routing.

RESULTS

Proposed model was implemented into the general-purpose finite element (FE) simulation software LSDYNA as a user material for truss elements. This material model is verified and validated with three different sets of mammalian experimental data, taken from the literature. It is compared to the *MAT_MUSCLE (*MAT_156) Hill-type muscle model already existing in LS-DYNA, which is currently used in finite element human body models (HBMs). An application example with an arm model extracted from the FE ViVA OpenHBM is given, taking into account physiological muscle paths.

CONCLUSION

The simulation results show better material model accuracy, calculation robustness and improved muscle routing capability compared to *MAT_156. The FORTRAN source code for the user material subroutine dyn21.f and the muscle parameters for all simulations, conducted in the study, are given at https://zenodo.org/record/826209 under an open source license. This enables a quick application of the proposed material model in LS-DYNA, especially in active human body models (AHBMs) for applications in automotive safety.

MRI cross sectional atlas of normal canine cervical musculoskeletal structure

Although magnetic resonance imaging (MRI) has been increasingly used as a diagnostic tool for cervical spine injuries in canines, a comprehensive normal MRI anatomy of the canine cervical spine muscles is lacking. Therefore, the purpose of this study was to build a magnetic resonance imaging atlas of the normal cross sectional anatomy of the muscles of the canine cervical spine. MRI scans were performed on a canine cadaver using a combination of T1 and T2-weighted images in the transverse, sagittal and dorsal planes acquired at a slice thickness of 1mm. Muscle contours were traced manually in each slice, using local osseous structures as reference points for muscle identification. Twenty-two muscles were traced in 401 slices in the cervical region. A three dimensional surface model of all the contoured muscles was created to illustrate the complex geometrical arrangement of canine neck muscles. The cross-sectional area of the muscles was measured at the mid-level of each vertebra. The accuracy of the location of the mapped muscles was verified by comparing the sagittal view of the 3D model of muscles with still photographs obtained from anatomic canine cadaver dissection. We believe that this information will provide a unique and valuable resource for veterinary researchers, clinicians and surgeons who wish to evaluate MRI images of the cervical spine. It will also serve as the foundation for ongoing work to develop a computational model of the canine cervical spine in which anatomical information is combined with electromyographic, kinematic and kinetic data.

A physically motivated constitutive model for 3D numerical simulation of skeletal muscles

A detailed numerical implementation within the FEM is presented for a physically motivated three-dimensional constitutive model describing the passive and active mechanical behaviors of the skeletal muscle. The derivations for the Cauchy stress tensor and the consistent material tangent are provided. For nearly incompressible skeletal muscle tissue, the strain energy function may be represented either by a coupling or a decoupling of the distortional and volumetric material response. In the present paper, both functionally different formulations are introduced allowing for a direct comparison between the coupled and decoupled isochoric-volumetric approach. The numerical validation of both implementations revealed significant limitations for the decoupled approach. For an extensive characterization of the model response to different muscle contraction modes, a benchmark model is introduced. Finally, the proposed implementation is shown to provide a reliable tool for the analysis of complex and highly nonlinear problems through the example of the human mastication system by studying bite force and three-dimensional muscle shape changes during mastication.

The relationship between sagittal curvature and extensor muscle volume in the lumbar spine

A previous modelling study predicted that the forces applied by the extensor muscles to stabilise the lumbar spine would be greater in spines that have a larger sagittal curvature (lordosis). Because the force-generating capacity of a muscle is related to its size, it was hypothesised that the size of the extensor muscles in a subject would be related to the size of their lumbar lordosis. Magnetic resonance imaging (MRI) data were obtained, together with age, height, body mass and back pain status, from 42 female subjects. The volume of the extensor muscles (multifidus and erector spinae) caudal to the mid-lumbar level was estimated from cross-sectional area measurements in axial T1-weighted MRIs spanning the lumbar spine. Lower lumbar curvature was determined from sagittal T1-weighted images. A stepwise linear regression model was used to determine the best predictors of muscle volume. The mean lower lumbar extensor muscle volume was 281 cm(3) (SD = 49 cm(3)). The mean lower lumbar curvature was 30 ° (SD = 7 °). Five subjects reported current back pain and were excluded from the regression analysis. Nearly half the variation in muscle volume was accounted for by the variables age (standardised coefficient, B = -3.2, P = 0.03) and lower lumbar curvature (B = 0.47, P = 0.002). The results support the hypothesis that extensor muscle volume in the lower lumbar spine is related to the magnitude of the sagittal curvature; this has implications for assessing muscle size as an indicator of muscle strength.

On validation of multibody musculoskeletal models

We review the opportunities to validate multibody musculoskeletal models in view of the current transition of musculoskeletal modelling from a research topic to a practical simulation tool in product design, healthcare and other important applications. This transition creates a new need for justification that the models are adequate representations of the systems they simulate. The need for a consistent terminology and established standards is identified and knowledge from fields with a more progressed state-of-the-art in verification and validation is introduced. A number of practical steps for improvement of the validation of multibody musculoskeletal models are pointed out and directions for future research in the field are proposed. It is hoped that a more structured approach to model validation can help to improve the credibility of musculoskeletal models.

The development of lower limb musculoskeletal models with clinical relevance is dependent upon the fidelity of the mathematical description of the lower limb. Part 2: patient-specific geometry

Musculoskeletal models have the potential to evolve into sensitive clinical tools that provide relevant therapeutic guidance. A key impediment to this is the lack of understanding as to the function of such models. In order to improve this it is useful to recognise that musculoskeletal modelling is the mathematical description of musculoskeletal movement – a process that involves the construction and solution of equations of motion. These equations are derived from standard mechanical considerations and the mathematical representation of anatomy. The fidelity of musculoskeletal models is highly dependent on the assumption that such representations also describe the function of the musculoskeletal geometry. In addition, it is important to understand the sensitivity of such representations to patient-specific variations in anatomy. The exploration of these twin considerations will be fundamental to the creation of musculoskeletal modelling tools with clinical relevance and a systematic enquiry of these key parameters is recommended.

Computational model of the lumbar spine musculature: implications of spinal surgery

BACKGROUND

The development of a comprehensive and detailed model of the musculature of the lumbar region is required if biomechanical models are to accurately predict the forces and moments experienced by the lumbar spine.

METHODS

A new anatomical model representing the nine major muscles of the lumbar spine and the thoracolumbar fascia is presented. These nine muscles are modeled as numerous fascicles, each with its own force producing potential based on size and line of action. The simulated spine is fully deformable, allowing rotation in any direction, while respecting the physical constraints imposed by the skeletal structure. Maximal moments were predicted by implementing the model using a pseudo force distribution algorithm. Three types of surgery that affect the spinal musculature were simulated: posterior spinal surgery, anterior surgery, and total hip replacement.

FINDINGS

Predicted moments matched published data from maximum isometric exertions in male volunteers. The biomechanical changes for the three different types of surgery demonstrated several common features: decreased spinal compression and production of asymmetric moments during symmetric tasks.

INTERPRETATION

This type of analysis provides new opportunities to explore the effect of different patterns of muscle activity including muscle injury on the biomechanics of the spine.

Analytical and numerical analysis of inverse optimization problems: conditions of uniqueness and computational methods

One of the key problems of motor control is the redundancy problem, in particular how the central nervous system (CNS) chooses an action out of infinitely many possible. A promising way to address this question is to assume that the choice is made based on optimization of a certain cost function. A number of cost functions have been proposed in the literature to explain performance in different motor tasks: from force sharing in grasping to path planning in walking. However, the problem of uniqueness of the cost function(s) was not addressed until recently. In this article, we analyze two methods of finding additive cost functions in inverse optimization problems with linear constraints, so-called linear-additive inverse optimization problems. These methods are based on the Uniqueness Theorem for inverse optimization problems that we proved recently (Terekhov et al., J Math Biol 61(3):423-453, 2010). Using synthetic data, we show that both methods allow for determining the cost function. We analyze the influence of noise on the both methods. Finally, we show how a violation of the conditions of the Uniqueness Theorem may lead to incorrect solutions of the inverse optimization problem.

Volumic patient-specific reconstruction of muscular system based on a reduced dataset of medical images

Three-dimensional mechanical modelling of muscles is essential for various biomechanical applications and clinical evaluation, but it requires a tedious manual processing of numerous images. A muscle reconstruction method is presented based on a reduced set of images to generate an approximate parametric object from basic dimensions of muscle contours. A regular volumic mesh is constructed based on this parametric object. The approximate object and the corresponding mesh are deformed to fit the exact muscles contours yielding patient-specific geometry. Evaluation was performed by comparison of geometry to that obtained by contouring all computed tomography (CT) slices, and by quantification of the mesh quality criteria. Muscle fatty infiltration was estimated using a threshold between fat and muscle. Volumic fat index (VFI) of a muscle was computed using first all the complete CT scan slices containing the muscle (VFI(ref)) and a second time only the slices used for reconstruction (VFI(recons)). Mean volume error estimation was 2.6% and hexahedron meshes fulfilled quality criteria. VFI(recons) respect the individual variation of fat content.

Experimental evaluation of a computational shoulder musculoskeletal model

BACKGROUND

Many evaluations of shoulder biomechanical models have focused on static exertions in constrained postures, but few have considered tasks that are more complex. This study examines model performance in load delivery tasks for a range of target locations.

METHODS

The study evaluated an optimization-based muscle force prediction model used to assess dynamic load transfer tasks. Model predictions were compared with experimental electromyographic data for two task phases: (1) static hold and (2) dynamic reach.

FINDINGS

Predictions correlated positively over all subjects with electromyographic data for prime movers (deltoid [r=0.53]; infraspinatus [r=0.63]; biceps [r=0.61]), though variations in the correlation existed across subjects and tasks. Conversely, the model predicted electromyographic activity of secondary muscles somewhat less accurately. The model also predicted inactivity for electromyographic inactive muscles.

INTERPRETATION

The model provides important insights into activity levels muscles that most actively respond to external moments during manual load transfer tasks.

Vertebral body integrity: a review of various anatomical factors involved in the lumbar region

The body of the vertebra can be affected in the majority of the conditions involving the lumbar spine. Multiple references, both books and periodicals, have been reviewed, and the anatomical factors responsible for the vertebral body integrity in the lumbar spine have been included under the following important areas, namely, morphology, development, genetics, microscopic examination using histology, structural architecture, blood supply, neuromuscular control, and biomechanics.

INTRODUCTION

The anatomy provides a three-dimensional frame work to support the interaction between the physiological and pathological alterations. The body of the vertebra can be affected in a majority of acute or chronic conditions involving the lumbar spine. The etiology of these conditions is multifactorial, which has been dealt with in previous studies sporadically. This study aims to review and incorporate the important anatomical factors which can influence the integrity of vertebral bodies in the lumbar region and manifest as low back pain.

METHODS

Multiple references, both books and periodicals, have been reviewed for the literature. Electronic databases, including Medline and PubMed, were used to collect the latest information. They were finally arranged in an anatomical framework for the article. An attempt has been made to cover these relevant issues in an integrated way in the article and have been structured into introduction, morphology, development, genetics, microscopic examination using histology, structural architecture, blood supply, neuromuscular control, biomechanics, and conclusion. The aforementioned anatomical aspects, some of which have received less attention in the literature, may be helpful to clinicians for restoring the mobility, stability, and load bearing capacity of the lumbar spine as well as planning better management strategies, especially for the chronic low back pain.

RESULTS

In our article all the anatomical factors affecting the integrity of vertebral body, including the morphology, development, genetics, growth and ossification, blood supply, specifically in the lumbar region, have been described, which were not covered earlier. The limitations of this review is its wide dimensions; hence, there are fair scopes of missing many relevant facts, as all of them cannot be compiled in a single article. We have attempted to confine our views to different anatomical domains only, this is our second limitation. Additional studies are required to incorporate and discuss the uncovered relevant scientific details.

CONCLUSIONS

The integrity of the body of the lumbar vertebra is multifactorial (Fig. 8). The vast spectrum of the anatomical domain influencing it has been summarized. The evolution of erect posture is a landmark in the morphology of human beings and the lumbar lordosis, which has also contributed to the gross design of the vertebral body, is one of the most important adaptations for axial loading and bipedal movements. The role of metamerism in the evolution of vertebrate morphology is repeated in the development of spine. The body of the vertebra is intersegmental in origin, which results in dual vascular and nerve supply, both from superior and inferior aspects of the body of the lumbar vertebrae. The vertebral body ossifies from three primary centers, one for centrum, which will form the major portion of body, and the other two for neural arches. The cartilaginous growth plate is mainly responsible for the longitudinal vertebral growth. Regional differentiation of the vertebral column, and the definite pattern of the structure of the different vertebra, is regulated by a large number of genetic factors, including the Hox genes. The vertebral body design therefore provides the requirements for optimal load transfer by maximal strength with minimal weight. Bone mineral density (BMD), bone quality, microarchitecture, and material properties are the important factors that contribute to bone strength. BMD is highly heritable; bone mineral distribution and architecture are also shown to be under strong genetic influence. All the aforementioned factors finally integrate to ensure mainly the mobility, stability, and load bearing capacity of the lumbar spine.

Thoracic kyphosis affects spinal loads and trunk muscle force

BACKGROUND AND PURPOSE

Patients with increased thoracic curvature often come to physical therapists for management of spinal pain and disorders. Although treatment approaches are aimed at normalizing or minimizing progression of kyphosis, the biomechanical rationales remain unsubstantiated.

SUBJECTS

Forty-four subjects (mean age [+/-SD]=62.3+/-7.1 years) were dichotomized into high kyphosis and low kyphosis groups.

METHODS

Lateral standing radiographs and photographs were captured and then digitized. These data were input into biomechanical models to estimate net segmental loading from T2-L5 as well as trunk muscle forces.

RESULTS

The high kyphosis group demonstrated significantly greater normalized flexion moments and net compression and shear forces. Trunk muscle forces also were significantly greater in the high kyphosis group. A strong relationship existed between thoracic curvature and net segmental loads (r =.85-.93) and between thoracic curvature and muscle forces (r =.70-.82).

DISCUSSION AND CONCLUSION

This study provides biomechanical evidence that increases in thoracic kyphosis are associated with significantly higher multisegmental spinal loads and trunk muscle forces in upright stance. These factors are likely to accelerate degenerative processes in spinal motion segments and contribute to the development of dysfunction and pain.

Model-based estimation of muscle forces exerted during movements

Estimation of individual muscle forces during human movement can provide insight into neural control and tissue loading and can thus contribute to improved diagnosis and management of both neurological and orthopaedic conditions. Direct measurement of muscle forces is generally not feasible in a clinical setting, and non-invasive methods based on musculoskeletal modeling should therefore be considered. The current state of the art in clinical movement analysis is that resultant joint torques can be reliably estimated from motion data and external forces (inverse dynamic analysis). Static optimization methods to transform joint torques into estimates of individual muscle forces using musculoskeletal models, have been known for several decades. To date however, none of these methods have been successfully translated into clinical practice. The main obstacles are the lack of studies reporting successful validation of muscle force estimates, and the lack of user-friendly and efficient computer software. Recent advances in forward dynamics methods have opened up new opportunities. Forward dynamic optimization can be performed such that solutions are less dependent on measured kinematics and ground reaction forces, and are consistent with additional knowledge, such as the force-length-velocity-activation relationships of the muscles, and with observed electromyography signals during movement. We conclude that clinical applications of current research should be encouraged, supported by further development of computational tools and research into new algorithms for muscle force estimation and their validation.

Wrapping of trunk thoracic extensor muscles influences muscle forces and spinal loads in lifting tasks

BACKGROUND

An improved assessment of risk of spinal injury during lifting activities depends on an accurate estimation of trunk muscle forces, spinal loads and stability margin which in turn requires, amongst others, an accurate description of trunk muscle geometries. The lines of action of erector spinae muscles are often assumed to be linear despite the curved paths of these muscles in forward flexion postures.

METHODS

A novel approach was introduced that allowed for the proper simulation of curved paths for global extensor muscles in our Kinematics-driven finite element model. The lever arms of global muscles at different levels were restrained either to remain the same or decrease only by 10% relative to their respective values in upright posture. Based on our earlier measurements, static lifting tasks at two trunk flexions (40 degrees and 65 degrees ) and three lumbar postures (free style, lordotic and kyphotic) with 180 N in hands were analyzed.

FINDINGS

Muscle forces and spinal compression at all levels substantially decreased as the global extensor muscles took curved paths. In contrast, the shear force at lower levels increased. Allowing for a 10% reduction in these lever arms during flexion increased muscle forces and compression forces at all levels. Despite smaller muscle forces, wrapping of global muscles slightly improved the spinal stability.

INTERPRETATION

Consideration of global extensor muscles with curved paths and realistic lever arms is important in biomechanical analysis of lifting tasks. Reduction in the erector spinae lever arms during flexion tasks could vary depending on the lumbar posture. Results advocate small flattening of the lumbar curvature in isometric lifts yielding smaller compression and shear forces at the critical L5-S1 level.

Sensitivity of kinematics-based model predictions to optimization criteria in static lifting tasks

The effect of eight different cost functions on trunk muscle forces, spinal loads and stability was investigated. Kinematics-based approach combined with nonlinear finite element modeling and optimization were used to model in vivo measurements on isometric forward flexions at approximately 40 degrees and approximately 65 degrees in sagittal plane with or without a load of 180N in hands. Four nonlinear (summation stress(3), summation stress(2), summation force(2) and muscle fatigue) and four linear (summation stress, summation force, axial compression and double-linear) criteria were considered. Predicted muscle activities were compared with measured EMG data. All predictions, irrespective of the cost function used, satisfied required kinetic, kinematics and stability conditions all along the spine. Four criteria (summation stress(3), summation stress(2), fatigue and double-linear) predicted muscle activities that qualitatively matched measured EMG data. The fatigue and double-linear criteria were inadequate in predicting greater forces in larger muscles with no consideration for their moment arms. Nearly the same stability margin was computed under these four cost functions. At the lower lumbar levels, the compression forces differed by <20% and the shear forces by <14% as various cost functions were considered. Smaller axial compression and anterior shear forces (by less than or approximately equal 6%) were computed when only the active components rather than the total muscle forces were taken as unknown in the summation stress(3) cost function. Overall, one single cost function of summation stress(2) or summation stress(3) rather than a multi-criteria one was found sufficient and adequate in yielding plausible results comparable with measured EMG activities and disc pressure.

Determination of trunk muscle forces for flexion and extension by using a validated finite element model of the lumbar spine and measured in vivo data

Muscle forces stabilize the spine and have a great influence on spinal loads. But little is known about their magnitude. In a former in vitro experiment, a good agreement with intradiscal pressure and fixator loads measured in vivo could be achieved for standing and extension of the lumbar spine. However, for flexion the agreement between in vitro and in vivo measurements was insufficient. In order to improve the determination of trunk muscle forces, a three-dimensional nonlinear finite element model of the lumbar spine with an internal fixation device was created and the same loads were applied as in a previous in vitro experiment. An extensive adaptation process of the model was performed for flexion and extension angles up to 20 degrees and -15 degrees, respectively. With this validated computer model intra-abdominal pressure, preload in the fixators, and a combination of hip- and lumbar flexion angle were varied until a good agreement between analytical and in vivo results was reached for both, intradiscal pressure and bending moments in the fixators. Finally, the fixators were removed and the muscle forces for the intact lumbar spine calculated. A good agreement with the in vivo results could only be achieved at a combination of hip- and lumbar flexion. For the intact spine, forces of 170, 100 and 600 N are predicted in the m. erector spinae for standing, 5 degrees extension and 30 degrees flexion, respectively. The force in the m. rectus abdominus for these body positions is less than 25 N. For more than 10 degrees extension the m. erector spinae is unloaded. The finite element method together with in vivo data allows the estimation of trunk muscle forces for different upper body positions in the sagittal plane. In our patients, flexion of the upper body was most likely a combination of hip- and lumbar spine bending.

Muscle forces predicted using optimization methods are coordinate system dependent

Optimization methods are widely used to predict in vivo muscle forces in musculoskeletal joints. Moment equilibrium at the joint center (usually chosen as the origin of the joint coordinate system) has been used as a constraint condition for optimization procedures and the joint reaction moments were assumed zero. This study, through the use of a three-dimensional elbow model, investigated the effect of coordinate system origin (joint center) location on muscle forces predicted using a nonlinear static optimization method. The results demonstrated that moving the origin of the coordinate system medially and laterally along the flexion-extension axis caused dramatic variations in the predicted muscle forces. For example, moving the origin of the coordinate system from a position 5mm medial to 5mm lateral of the geometric elbow center caused the predicted biceps force to vary from 12% to 46% and the brachialis force to vary from 80% to 34% of the total muscle loading. The joint reaction force reduced by 24% with this medial to lateral variation of the coordinate system origin location. This data revealed that the muscle forces predicted using the optimization method are sensitive to the coordinate system origin location due to the zero joint reaction moment assumption in the moment constraint condition. For accurate prediction of muscle load distributions using optimization methods, it is necessary to determine the accurate coordinate system origin location where the condition of a zero joint reaction moment is satisfied.

A review of anatomical and mechanical factors affecting vertebral body integrity

Background: The aetiology of osteoporotic vertebral fracture is multifactorial and may be conceptualised using a systems framework. Previous studies have established several correlates of vertebral fracture including reduced vertebral cross-sectional area, weakness in back extensor muscles, reduced bone mineral density, increasing age, worsening kyphosis and recent vertebral fracture. Alterations in these physical characteristics may influence biomechanical loads and neuromuscular control of the trunk and contribute to changes in subregional bone mineral density of the vertebral bodies. Methods: This review discusses factors that have received less attention in the literature, which may contribute to the development of vertebral fracture. A literature review was conducted using electronic databases including Medline, Cinahl and ISI Web of Science to examine the potential contribution of trabecular architecture, subregional bone mineral density, vertebral geometry, muscle force, muscle strength, neuromuscular control and intervertebral disc integrity to the aetiology of osteoporotic vertebral fracture. Interpretation: A better understanding of factors such as biomechanical loading and neuromuscular control of the trunk may help to explain the high incidence of subsequent vertebral fracture after sustaining an initial vertebral fracture. Consideration of these issues may be important in the development of prevention and management strategies.

ISSLS prize winner: A novel approach to determine trunk muscle forces during flexion and extension: a comparison of data from an in vitro experiment and in vivo measurements

STUDY DESIGN

Disc pressure and fixator load were measured in an in vitro setup and compared to in vivo measurements with the identical transducers from the two groups participating in this study.

OBJECTIVES

The goal of this in vitro study was to determine the magnitude of trunk muscle forces during flexion and extension. The loading conditions in this study accounted for body weight, local and global muscles, and forces resulting from the support of the abdominal soft tissue in different postures. Resulting intersegmental motions and intradiscal pressure in each segment and the six load components in both rods of an internal fixator were determined.

SUMMARY OF BACKGROUND DATA

The spine is primarily stabilized by muscle forces, which greatly influence spinal loads. However, little information exists on the magnitudes of trunk muscle forces during postures like flexion and extension of the upper body.

METHODS

Seven human cadaveric lumbar spines were mounted in a spine tester and adjusted to different degrees of flexion and extension of the upper body with different hip flexions. For each specimen, a total of 124 load cases were studied. They included combinations of a vertical compressive load, a follower load and forces pulling with cables at a plate fixed at the cranial end of the specimen to simulate rectus abdominis, erector spinae, and a supporting force of the abdomen. The muscle forces were varied until the external moment, necessary to keep the lumbar spine specimen in the examined posture, was zero. This was achieved with different muscle force combinations. Loads on internal fixators as well as intradiscal pressure and intersegmental rotation at all levels were measured. The muscle force combination that caused intradiscal pressures and loads in the internal fixator closest to those measured in vivo were assumed to be the muscle forces which can be expected in vivo.

RESULTS

Generally, intradiscal pressure was closer to in vivo measurements than the fixator loads. The force in the m. erector spinae increased with the flexion angle but was only slightly influenced by extension. The estimated forces in the erector spinae were 100 N for standing, 130 N for 15 degrees extension, and 520 N for 30 degrees flexion of the upper body. Little influence was found on the intersegmental motion.

CONCLUSION

In vitro loading conditions can be approximated closely to in vivo conditions with the simulation of an axial preload, local, and global muscles. This novel approach can help to estimate muscle forces, which can usually not be measured. The results from this study provide important input for FEM models, which may then allow the investigation of different load cases.

Effect of torso flexion on the lumbar torso extensor muscle sagittal plane moment arms

BACKGROUND CONTEXT

Accurate anatomical inputs for biomechanical models are necessary for valid estimates of internal loading. The magnitude of the moment arm of the lumbar erector muscle group is known to vary as a function of such variables as gender. Anatomical evidence indicates that the moment arms decrease during torso flexion. However, moment arm estimates in biomechanical models that account for individual variability have been derived from imaging studies from supine postures.

PURPOSE

Quantify the sagittal plane moment arms of the lumbar erector muscle group as a function of torso flexion, and identify individual characteristics that are associated with the magnitude of the moment arms as a function of torso flexion.

STUDY DESIGN/SETTING

Utilization of a 0.3 Tesla Open magnetic resonance image (MRI) to image and quantify the moment arm of the right erector muscle group as a function of gender and torso flexion.

METHODS

Axial MRI images through and parallel to each of the lumbar intervertebral discs at four torso flexion angles were obtained from 12 male and 12 female subjects in a lateral recumbent posture. Multivariate analysis of variance was used to investigate the differences in the moment arms at different torso flexion angles, whereas hierarchical linear regression was used to investigate associations with individual anthropometric characteristics and spinal posture.

RESULTS

The largest decrease in the lumbar erector muscle group moment arm from neutral to 45-degree flexion occurred at the L5-S1 level (9.7% and 8.9% for men and women, respectively). Measures of spinal curvature (L1-S1 lordosis), body mass and trunk characteristics (depth or circumference) were associated with the varying moment arm at most lumbar levels.

CONCLUSIONS

The sagittal plane moment arms of the lumbar erector muscle mass decrease as the torso flexes forward. The change in moment arms as a function of torso flexion may have an impact on prediction of spinal loading in biomechanical models.

The mechanics of back-extensor torque production about the lumbar spine

The purpose of this study was to develop and evaluate a biomechanical model of lumbar back extension over a wide range of positions for the lumbar spine, incorporating the latest information on muscle geometry and intra-abdominal pressure (IAP). Analysis of the Visible Human data was utilised in order to obtain anatomical information unavailable from the literature and magnetic resonance imaging was used to generate subject-specific anatomical descriptions. The model was evaluated by comparisons with measured maximal voluntary static back-extension torques. Predicted maximal specific muscle tensions agreed well with in vitro measurements from the literature. When modelling the maximal static back-extension torque production, it was possible to come fairly close to simultaneous equilibrium about all the lumbar discs simply by a uniform muscle activation of all back-extensor muscles (the caudal part showed, however, less agreement). This indicates that equilibrium in the lumbar spine is mainly regulated by passive mechanical properties, e.g. muscle length changes due to postural changes, rather than due to complex muscle coordination, as earlier proposed. The model showed that IAP (measured during torque exertions) contributes about 10% of the total maximal voluntary back-extensor torque and that it can unload the spine from compression. The spinal unloading effect from the IAP was greatest with the spine held in a flexed position. This is in opposition to the effects of changed muscle lever arm lengths, which for a given load would give the largest spinal unloading in the extended position. These findings have implications for the evaluation of optimal lifting techniques.

Biomechanical analysis of the effect of changing patient-handling technique

The objective of the study was to assess the changes in the mechanical load on the low-back when shifting from a self-chosen to a recommended patient-handling technique. Nine female health care workers without formal education in patient-handling carried out 8 different tasks involving moving, turning and lifting situations. By means of a dynamic 3D biomechanical model of the lower part of the body, peak torque, compression and shear forces at the L4/L5 joint were compared using the two different patient-handling techniques. In 5 of the 8 tasks, a significant reduction was observed in spinal loading. Application of the recommended technique decreased the compression value significantly for all tasks with a mean value above 3000 N. For the two tasks with the highest compression values when using the self-chosen technique (4223, 4446 N), the loading was reduced with 36% and 25%, respectively. If the principles behind the recommended technique are implemented and maintained, a decrease in the risk of low-back disorders during patient-handling should thus be expected.

Spine loading characteristics of patients with low back pain compared with asymptomatic individuals

STUDY DESIGN

Patients with low back pain and asymptomatic individuals were evaluated while performing controlled and free-dynamic lifting tasks in a laboratory setting.

OBJECTIVE

To evaluate how low back pain influences spine loading during lifting tasks.

SUMMARY OF BACKGROUND DATA

An important, yet unresolved, issue associated with low back pain is whether patients with low back pain experience spine loading that differs from that of individuals who are asymptomatic for low back pain. This is important to understand because excessive spine loading is suspected of accelerating disc degeneration in those whose spines are damaged already.

METHODS

In this study, 22 patients with low back pain and 22 asymptomatic individuals performed controlled and free-dynamic exertions. Trunk muscle activity, trunk kinematics, and trunk kinetics were used to evaluate three- dimensional spine loading using an electromyography- assisted model in conjunction with a new electromyographic calibration procedure.

RESULTS

Patients with low back pain experienced 26% greater spine compression and 75% greater lateral shear (normalized to moment) than the asymptomatic group during the controlled exertions. The increased spine loading resulted from muscle coactivation. When permitted to move freely, the patients with low back pain compensated kinematically in an attempt to minimize external moment exposure. Increased muscle coactivation and greater body mass resulted in significantly increased absolute spine loading for the patients with low back pain, especially when lifting from low vertical heights.

CONCLUSIONS

The findings suggest a significant mechanical spine loading cost is associated with low back pain resulting from trunk muscle coactivation. This loading is further exacerbated by the increases in body weight that often accompany low back pain. Patient weight control and proper workplace design can minimize the additional spine loading associated with low back pain.

Sensitivity of predicted muscle forces to parameters of the optimization-based human leg model revealed by analytical and numerical analyses

There are different opinions in the literature on whether the cost functions: the sum of muscle stresses squared and the sum of muscle stresses cubed, can reasonably predict muscle forces in humans. One potential reason for the discrepancy in the results could be that different authors use different sets of model parameters which could substantially affect forces predicted by optimization-based models. In this study, the sensitivity of the optimal solution obtained by minimizing the above cost functions for a planar three degrees-of-freedom (DOF) model of the leg with nine muscles was investigated analytically for the quadratic function and numerically for the cubic function. Analytical results revealed that, generally, the non-zero optimal force of each muscle depends in a very complex non-linear way on moments at all three joints and moment arms and physiological cross-sectional areas (PCSAs) of all muscles. Deviations of the model parameters (moment arms and PCSAs) from their nominal values within a physiologically feasible range affected not only the magnitude of the forces predicted by both criteria, but also the number of non-zero forces in the optimal solution and the combination of muscles with non-zero predicted forces. Muscle force magnitudes calculated by both criteria were similar. They could change several times as model parameters changed, whereas patterns of muscle forces were typically not as sensitive. It is concluded that different opinions in the literature about the behavior of optimization-based models can be potentially explained by differences in employed model parameters.

Modelling the line of action for the oblique abdominal muscles using an elliptical torso model

When modelling the line of action of a muscle, anatomical considerations must be included if the model is to realistically mimic the muscle behaviour. The internal and external oblique muscles are examples of muscles that do not follow a straight line between origin and insertion, instead having to wrap around the torso. A model is presented which describes the shape of the torso using a right elliptical cylinder of varying dimensions. The muscle lines of action are then calculated based on this underlying torso shape. The model has been successfully fitted to the data reported by Stokes and Gardner-Morse (Journal of Biomechanics 32(3) (1999) 311). When compared to a linear model, the use of the torso model results in a 15% increase in the axial twist moment, and decreases in the lateral bend and extension moments (5% and 2%, respectively), able to be generated by the internal and external oblique muscles combined in upright stance. These differences become larger (up to 37%) when the torso is flexed, extended or twisted. The structure of the torso model allows it to be used to model any posture without significant increases in the overall model complexity.

Comparative ability of EMG, optimization, and hybrid modelling approaches to predict trunk muscle forces and lumbar spine loading during dynamic sagittal plane lifting

OBJECTIVE

To compare the ability of three modelling approaches to resolve the muscle and joint forces in a lumbar spine model during dynamic sagittal plane lifting.

DESIGN

Trunk muscle forces, spine compression, and coactivity predicted through double linear optimization, EMG-assisted, and EMG assisted by optimization approaches were compared.Background. The advantages of EMG-based approaches are known from static task analyses. Limited assessment has been made for dynamic lifting.

METHODS

Eleven male subjects performed sagittal plane lifting-lowering at fixed cadence from 0 degrees to 45 degrees of trunk flexion with and without an external load of 12 kg. Three-dimensional kinematics and dynamics as well as surface EMG provided inputs to a 12 muscle lumbar spine model.

RESULTS

Trunk muscle coactivity was different between the modelling approaches but spine compression was not. Both EMG-based approaches were sensitive to trunk muscle coactivity and imbalance in left-right muscle forces during sagittal plane lifting. Overall, the best correlations between predicted forces and EMG as well as between forces predicted by different modelling approaches were obtained with the EMG-based models. Only the EMG assisted by optimization approach simultaneously satisfied mechanical and physiological validity.

CONCLUSIONS

Both EMG-based approaches demonstrated their potential to detect individual trunk muscle strategies. A more detailed trunk anatomy representation would improve the EMG-assisted approach and reduce the adjustment to muscle force gain through EMG assisted by optimization.

RELEVANCE

Injury to the lumbar spine could command alternative strategies of motion to attenuate pain and damage. To understand these strategies, the ideal lumbar spine model should predict individual muscle force patterns and satisfy mechanical equilibrium.

Progression of idiopathic thoracolumbar scoliosis after breast reconstruction with a latissimus dorsi flap: a case report

STUDY DESIGN

A report of a patient in whom progressive symptomatic thoracolumbar scoliosis developed after breast reconstruction with a latissimus dorsi myocutaneous flap.

OBJECTIVES

To present the first reported case of progressive symptomatic scoliosis after breast reconstruction with a latissimus dorsi myocutaneous flap and to suggest that latissimus flap harvest may be contraindicated in patients with preexisting scoliosis.

SUMMARY OF BACKGROUND DATA

Latissimus dorsi myocutaneous flap harvest incorporated into several surgical operations including breast reconstruction has been presented as a relatively benign procedure without significant biomechanical consequence. Nevertheless, various anatomic and animal studies have suggested an important role for balanced latissimus function in terms of proper spinal alignment. Long-term follow-up evaluation of patients after latissimus flap harvest is insufficient and fails to address the specific issue of spinal deformity.

METHODS

Postoperative radiographs demonstrated significant progression of the patient's thoracolumbar scoliosis as compared with radiographs taken before her latissimus harvest. Curve progression accompanied by development of severe and disabling back pain were considered indications for surgical curve correction and stabilization.

RESULTS

At the time of 1-year follow-up assessment after posterolateral spinal fusion and instrumentation, the patient had experienced complete relief from her back pain and satisfactory spinal fusion.

CONCLUSIONS

Although a cause and effect relation cannot be established, this case study suggests that latissimus harvest may have a destabilizing effect on the thoracolumbar spine in the long term, especially in patients with preexisting scoliosis. Alternative procedures should be considered in these patients.

Biomechanical analysis of materials handling manipulators in short distance transfers of moderate mass objects: joint strength, spine forces and muscular antagonism

Although often suggested as a control measure to alleviate musculoskeletal stresses, the use of mechanical assistance devices (i.e. manipulators) in load transfers has not been extensively studied. Without data describing the biomechanical effects of such devices, justification for decisions regarding implementation of such tools is difficult. An experimental study of two types of mechanical manipulators (articulated arm and overhead hoist) was conducted to determine whether biomechanical stresses, and hence injury risk, would be alleviated. Short distance transfers of loads with moderate mass were performed both manually and with manipulator assistance under a variety of task conditions. Using analysis and output from new dynamic torso models, strength demands at the shoulders and low back, lumbar spine forces, and lumbar muscle antagonism were determined. Strength requirements decreased significantly at both the shoulders and low back when using either manipulator in comparison with similar transfers performed manually. Peak spine compression and anterior-posterior (a-p) shear forces were reduced by about 40% on average, and these reductions were shown to be primarily caused by decreases in hand forces and resultant spinal moments. Two metrics of muscular antagonism were defined, and analysis showed that torso muscle antagonism was largest overall when using the hoist. The results overall suggest that hoist-assisted transfers, although better in reducing spine compression forces, may impose relatively higher demands on coordination and/or stability at extreme heights or with torso twisting motions. The relatively higher strength requirements and spine compression associated with the articulated arm may be a result of the high inertia of the system. Potential benefits of practice and training are discussed, and conclusions regarding implementation of mechanical manipulators are given.

Stoop or squat: a review of biomechanical studies on lifting technique

OBJECTIVE

To assess the biomechanical evidence in support of advocating the squat lifting technique as an administrative control to prevent low back pain.

BACKGROUND

Instruction with respect to lifting technique is commonly employed to prevent low back pain. The squat technique is the most widely advised lifting technique. Intervention studies failed to show health effects of this approach and consequently the rationale behind the advised lifting techniques has been questioned.

METHODS

Biomechanical studies comparing the stoop and squat technique were systematically reviewed. The dependent variables used in these studies and the methods by which these were measured or estimated were ranked for validity as indicators of low back load.

RESULTS

Spinal compression as indicated by intra-discal pressure and spinal shrinkage appeared not significantly different between both lifting techniques. Net moments and compression forces based on model estimates were found to be equal or somewhat higher in squat than in stoop lifting. Only when the load could be lifted from a position in between the feet did squat lifting cause lower net moments, although the studies reporting this finding had a marginal validity. Shear force and bending moments acting on the spine appeared lower in squat lifting. Net moments and compression forces during lifting reach magnitudes, that can probably cause injury, whereas shear forces and bending moments remained below injury threshold in both techniques.

CONCLUSION

The biomechanical literature does not provide support for advocating the squat technique as a means of preventing low back pain.

RELEVANCE

Training in lifting technique is widely used in primary and secondary prevention of low back pain, though health effects have not been proven. The present review assesses the biomechanical evidence supporting the most widely advocated lifting technique.

Total trunk muscle force and spinal compression are lower in asymmetric moments as compared to pure extension moments

The aim of the present study was to test the assumption that asymmetric trunk loading requires a higher total muscle force and consequently entails a higher compression forces on the spine as compared to symmetric loading. When the trunk musculature is modelled in sufficient detail, optimisation shows that there is no mechanical necessity for an increase in total muscle force (or compression force) with task asymmetry. A physiologically based optimisation does also not predict an increase in total muscle force or spinal loading with asymmetry. EMG data on 14 trunk muscles collected in eight subjects showed antagonistic coactivity to be present in both conditions. However, estimates of total muscle force based on the EMG were lower when producing an asymmetric moment. In conclusion, producing an asymmetric moment appears to cause slightly lower forces on the lumbosacral joint as compared to a symmetric moment. Only lateral shear forces increase with asymmetry but these remain well below failure levels.

Quantitative anatomy of the lumbar musculature

This paper describes the anatomy of the musculature crossing the lumbar spine in a standardized form to provide data generally suitable for static biomechanical analyses of muscle and spinal forces. The muscular anatomy from several sources was quantified and transformed to the mean bony anatomy of four young healthy adults measured from standing stereo-radiographs. The origins, insertions and physiological cross-sectional area (PCSA) of 180 muscle slips which act on the lumbar spine are given relative to the bony anatomy defined by the locations of 12 thoracic and five lumbar vertebrae, and the sacrum, and the shape and positions of the 24 ribs. The broad oblique abdominal muscles are each represented by six vectors and an appropriate proportion of the total PCSA was assigned to each to represent the muscle biomechanics.

Sensitivity of single-equivalent trunk extensor muscle models to anatomical and functional assumptions

Single-equivalent muscle models are often used to estimate loads on the lumbosacral joint after net extension moments have been calculated by means of inverse dynamics. These models usually ignore the effects trunk flexion has on the extensor lever arm. In addition, no systematic analysis of the sensitivity to the anatomical and functional assumptions made in these models is available. In the present study a series of single-equivalent models incorporating trunk flexion dependence was derived from a detailed description of the trunk musculature. Each model was based on different anatomical and functional assumptions. The differences of estimates of compression and shear forces on the lumbosacral disc during a lifting movement resulting from these models were analysed. The results show that these load estimates heavily depend on assumptions regarding anthropometry, lumbar curvature and coactivity of abdominal muscles and only moderately on assumptions regarding force sharing between extensor muscles. Fairly simple single-equivalent models with the net moment and thorax orientation as input can be used to predict lumbosacral compression and shear.

Low-back stresses when learning to use a materials handling device

This study examines the potential effect of short-term practice on low-back stresses during manual lifting and lowering of a 15 kg load, and while using two different types of materials handling devices (MHDs) to lift and lower a 40 kg load. The two MHDs used were an articulated balance arm and a pneumatic hoist. The expectation was that low-back dynamic moments, EMG measured torso muscle antagonism, and EMG predicted L4/L5 disc compression forces would rapidly decrease with practice, and that the manual lift-lower activities would be learned faster than the MHD-assisted exertions. Four naïve male college age subjects performed 40 lift and lower exertions, both manually and with the two MHDs for a total of 24 experiments. Non-linear regressions of the peak and average low-back moments, EMGs and disc compression values revealed only small decreases in the values (from 2 to 14%) over the 40 trials, and it was only statistically significant for five of the 48 regressions. This would seem to indicate that if learning is present in these tasks it is going to be very slow learning, and thus future studies will need to include a much larger number of trials. The effects of MHDs on the learning rates when compared to manual lifting learning rates was not statistically significant. It was shown, however, that MHDs had a particularly beneficial effect on reducing L4/L5 compression forces during load lowering activities despite the MHD load being much heavier than the manual load. It also was found that the level of torso muscle co-contraction increased significantly (2-4 times) when MHD handling was involved compared to manual lifting and lowering.

Estimation of trunk muscle forces and spinal loads during fatiguing repetitive trunk exertions

STUDY DESIGN

The effects of human trunk extensor muscle fatigue on the estimated trunk muscle forces and spinal loading were investigated during the performance of repetitive dynamic trunk extension.

OBJECTIVE

To evaluate if alterations in the trunk muscle recruitment patterns resulted in a greater estimated active loading of the spine and, in turn, an increased risk of injury.

SUMMARY OF BACKGROUND DATA

Epidemiologic studies highlight the increased risk of low back injury during repetitive lifting, implicating fatigue of muscles and/or passive tissues as causes of such injury. Increased trunk muscle activity or altered recruitment patterns resulting from fatigue in the primary trunk extensor muscles may indicate an increase in the active loading of the spine, which could contribute to an increased risk of injury.

METHODS

Sixteen healthy study participants performed repetitive isokinetic trunk extension endurance tests at two load levels and two repetition rates, while their net muscular torque output and trunk muscular activity were measured. During each exertion, trunk torque, position, and velocity were controlled, so that any change in muscle activity could be attributed to fatigue. An electromyography-assisted model, adapted to accommodate the decline in maximum muscular tension generation resulting from fatigue, was used to estimate the 10 trunk muscle forces and spinal loading. Linear regression was used to quantify the rate of change in muscle force and spinal loading resulting from fatigue, while analysis of variance was used to determine if the rate of change was dependent on the task conditions (load and repetition rate).

RESULTS

Significant elevations were estimated for the latissimus dorsi and external oblique muscle forces in more than 70% of the endurance tests, whereas significant reductions in the erector spinae muscle force were predicted in 75% of the trials. The magnitude of the range of change of the erector spinae and latissimus dorsi muscle forces was dependent on the load level and repetition rate. The reduction in erector spinae forces offset the augmented force in the other muscles, because the net changes in compression and lateral shear forces on the spine were not significant, and the anteroposterior shear was reduced.

CONCLUSION

The results of the study do not suggest that an increase in the muscular loading of the spine occurs as a result of changing trunk muscular recruitment patterns. Therefore, future studies should focus on injury mechanisms that may occur as a result of a change in the viscoelastic passive tissue responses, muscular insufficiency, or a decline in neuromuscular control and coordination.

Effect of electromyogram-force relationships and method of gain estimation on the predictions of an electromyogram-driven model of spinal loading

STUDY DESIGN

An experimental study of fatiguing isometric trunk extension was conducted to investigate the spinal loading estimated from an electromyogram-assisted biomechanical model.

OBJECTIVE

To evaluate the sensitivity of the model outputs to two crucial assumptions: electromyogram-force relationship and method of gain estimation.

SUMMARY OF BACKGROUND DATA

In the proposed electromyogram-assisted models of the trunk, the nature of the electromyogram-force relationship and the wide variation in reported muscle gains can result in a wide variation in estimates of spinal loading. Given the absence of any gold standard for validation of muscle forces, the delineation of confidence intervals for the estimated loads has become critical.

METHODS

Ten subjects performed a fatiguing isometric trunk extension while the net muscular torque output and trunk muscular activity were measured. An electromyogram-assisted model was used to estimate the torque output and spinal loading. Linear and nonlinear erector spinae electromyogram-force relationships and three methods for gain estimation were investigated: constant gain determined from an initial maximum extension exertion, constant gain based on the fatiguing exertion, and a time-varying gain from the fatigue test.

RESULTS

The predicted torque was not sensitive to the electromyogram-force relationship; the nonlinear model produced 10% lower estimates of peak spinal compression force and 14% higher estimates of peak anterior shear force. The gain determined from an initial calibration exertion underestimated the external torque and underpredicted the peak compression force by 20%, compared with gains calculated in the fatigue test.

CONCLUSION

The nature of the electromyogram-force relationship and of the method for estimating the gain significantly affect the outcomes of an electromyogram-assisted model of spinal loading.

Development and evaluation of a scalable and deformable geometric model of the human torso

A model is developed to provide a geometric representation of the human spine including thoracic and lumbar motion segments, lumbar muscles, ribs, sternum, sacrum, and pelvis. An existing model was modified in order to allow for scaling using standard anthropometric measures, deformation to specific 3-dimensional postures using surface markers, and incorporation of muscle length-tension and motion segment passive bending properties. Experiments were performed to evaluate the accuracy of model postural predictions. Analysis of surface marker displacements demonstrated that the thoracic spine deforms only minimally over a range of flexion, extension, and lateral bending torso postures, suggesting that it can be treated as essentially rigid during low-weight lifting over the range of passive flexibility. Locations of bony landmarks were accurately reproduced (mean errors 2.9-6.8 mm) as were several body dimensions (mean differences 2.6-15.4 mm). It is concluded that linear scaling to subject-specific anthropometry and the use of specific surface markers provides an accurate and direct technique for describing spinal geometry. Predicted passive spinal moments were found to be comparable to those required to support body weight in different extreme postures. It is recommended that data obtained from this type of model be incorporated in future investigations of spinal loading.