Spinal stability and role of passive stiffness in dynamic squat and stoop lifts

The spinal stability and passive-active load partitioning under dynamic squat and stoop lifts were investigated as the ligamentous stiffness in flexion was altered. Measured in vivo kinematics of subjects lifting 180 N at either squat or stoop technique was prescribed in a nonlinear transient finite element model of the spine. The Kinematics-driven approach was utilized for temporal estimation of muscle forces, internal spinal loads and system stability. The finite element model accounted for nonlinear properties of the ligamentous spine, wrapping of thoracic extensor muscles and trunk dynamic characteristics while subject to measured kinematics and gravity/external loads. Alterations in passive properties of spine substantially influenced muscle forces, spinal loads and system stability in both lifting techniques, though more so in stoop than in squat. The squat technique is advocated for resulting in smaller spinal loads. Stability of spine in the sagittal plane substantially improved with greater passive properties, trunk flexion and load. Simulation of global extensor muscles with curved rather than straight courses considerably diminished loads on spine and increased stability throughout the task.     

Model and in vivo studies on human trunk load partitioning and stability in isometric forward flexions

To resolve the trunk redundancy to determine muscle forces, spinal loads, and stability margin in isometric forward flexion tasks, combined in vivo-numerical model studies was undertaken. It was hypothesized that the passive resistance of both the ligamentous spine and the trunk musculature plays a crucial role in equilibrium and stability of the system. Fifteen healthy males performed free isometric trunk flexions of approximately 40 degrees and approximately 65 degrees +/- loads in hands while kinematics by skin markers and EMG activity of trunk muscles by surface electrodes were measured. A novel kinematics-based approach along with a nonlinear finite element model were iteratively used to calculate muscle forces and internal loads under prescribed measured postures and loads considered in vivo. Stability margin was investigated using nonlinear, linear buckling, and perturbation analyses under various postures, loads and alterations in ligamentous stiffness. Flexion postures significantly increased activity in extensor muscles when compared with standing postures while no significant change was detected in between flexed postures. Compression at the L5-S1 substantially increased from 570 and 771 N in upright posture, respectively, for +/-180 N, to 1912 and 3308 N at approximately 40 degrees flexion, and furthermore to 2332 and 3850 N at approximately 65 degrees flexion. Passive ligamentous/muscle components resisted up to 77% of the net moment. In flexion postures, the spinal stability substantially improved due both to greater passive stiffness and extensor muscle activities so that, under 180 N, no muscle stiffness was required to maintain stability. The co-activity of abdominal muscles and the muscle stiffness were of lesser concern to maintain stability in forward flexion tasks as compared with upright tasks. An injury to the passive system, on one hand, required a substantial compensatory increase in active muscle forces which further increased passive loads and, hence, the risk of injury and fatigue. On the other hand, it deteriorated the system stability which in turn could require greater additional muscle activation. This chain of events would place the entire trunk active-passive system at higher risks of injury, fatigue and instability.     

Relative efficiency of abdominal muscles in spine stability

Using an iterative kinematics-driven nonlinear finite element model, relative efficiency of individual abdominal muscles in spinal stability in upright standing posture was investigated. Effect of load height on stability and muscle activities was also computed under different coactivity levels in abdominal muscles. The internal oblique was the most efficient muscle (compared with the external oblique and rectus abdominus) in providing stability while generating smaller spinal loads with lower fatigue rate of muscles. As the weight was held higher, stability deteriorated requiring additional flexor-extensor activities. The stabilising efficacy of abdominal muscles diminished at higher activities. The difference in critical loads in frontal and sagittal planes computed in the absence of abdominal coactivity disappeared under prescribed coactivities suggesting an optimal system in stability. The central nervous system may settle for a less stable spine in favour of lowering the risk of injury. Findings could help introduce stability criterion in optimisation models.     

Comparison between the effectiveness of expiration and abdominal bracing maneuvers in maintaining spinal stability following sudden trunk loading

The purpose of this study was to clarify the effectiveness of expiration and abdominal bracing maneuvers in response to sudden trunk loading in healthy subjects. Fifteen healthy male subjects were anteriorly loaded under different experimental conditions. Tests were conducted at rest and while performing each of the stabilization maneuvers (expiration and abdominal bracing) at 15% of the maximal voluntary isometric contraction of the internal oblique muscle. Subjects had no knowledge of the perturbation timing. An electromyographic biofeedback system was used to control the intensity of internal oblique muscle activation. Muscular pre-activation of three trunk muscles (internal oblique, external oblique, and L3 erector spinae muscles) and lumbar acceleration in response to loading were measured. The expiration and abdominal bracing maneuvers promoted torso co-contraction, reduced the magnitude of lumbar acceleration, and increased spinal stability compared to the resting condition. There were no differences between the expiration and abdominal bracing maneuvers in the pre-activation of the three trunk muscles or in lumbar acceleration in response to loading. It appears that both expiration and abdominal bracing maneuvers are effective in increasing spinal stability in response to sudden anterior loading.     

Trunk response to sudden forward perturbations – Effects of preload and sudden load magnitudes,posture and abdominal antagonistic activation

Unexpected loading of the spine is a risk factor for low back pain. The trunk neuromuscular and kinematics responses are likely influenced by the perturbation itself as well as initial trunk conditions. The effect of four parameters (preload, sudden load, initial trunk flexed posture, initial abdominal antagonistic activity) on trunk kinematics and back muscles reflex response were evaluated. Twelve asymptomatic subjects participated in sudden forward perturbation tests under six distinct conditions. Preload did not change the reflexive response of back muscles and the trunk displacement; while peak trunk velocity and acceleration as well as the relative load peak decreased. Sudden load increased reflex response of muscles, trunk kinematics and loading variables. When the trunk was initially flexed, back muscles latency was delayed, trunk velocity and acceleration increased; however, reflex amplitude and relative trunk displacement remained unchanged. Abdominal antagonistic preactivation increased reflexive response of muscles but kinematics variables were not affected. Preload, initial flexed posture and abdominal muscles preactivation increased back muscles preactivity. Both velocity and acceleration peaks of the trunk movement decreased with preload despite greater total load. In contrast, they increased in the initial flexed posture and to some extent when abdominal muscles were preactivated demonstrating the distinct effects of pre-perturbation variables on trunk kinematics and risk of injury.     

Role of trunk muscles in generating follower load in the lumbar spine of neutral standing posture

Recently, experimental results have demonstrated that the load carrying capacity of the human spine substantially increases under the follower load condition. Thus, it is essential to prove that a follower load can be generated in vivo by activating the appropriate muscles in order to demonstrate the possibility that the stability of the spinal column could be maintained through a follower load mechanism. The aim of this study was to analyze the coordination of the trunk muscles in order to understand the role of the muscles in generating the follower load. A three-dimensional finite element model of the lumbar spine was developed from T12 to S1 and 117 pairs of trunk muscles (58 pairs of superficial muscles and 59 pairs of deep muscles) were considered. The follower load concept was mathematically represented as an optimization problem. The muscle forces required to generate the follower load were predicted by solving the optimization problem. The corresponding displacements and rotations at all nodes were estimated along with the follower forces, shear forces, and joint moments acting on those nodes. In addition, the muscle forces and the corresponding responses were investigated when the activations of the deep muscles or the superficial muscles were restricted to 75% of the maximum activation, respectively. Significantly larger numbers of deep muscles were involved in the generation of the follower load than the number of superficial muscles, regardless of the restriction on muscle activation. The shear force and the resultant joint moment are more influenced by the change in muscle activation in the superficial muscles. A larger number of deep trunk muscles were activated in order to maintain the spinal posture in the lumbar spine. In addition, the deep muscles have a larger capability to reduce the shear force and the resultant joint moment with respect to the perturbation of the external load or muscle fatigue compared to the superficial muscles.     

Coupled objective function to study the role of abdominal muscle forces in lifting using the kinematics-driven model

To circumvent the existing shortcoming of optimisation algorithms in trunk biomechanical models, both agonist and antagonist trunk muscle stresses to different powers are introduced in a novel objective function to evaluate the role of abdominal muscles in trunk stability and spine compression. This coupled objective function is introduced in our kinematics-driven finite element model to estimate muscle forces and to identify the role of abdominal muscles in upright standing while lifting symmetrically a weight at different heights. Predictive equations for the compression and buckling forces are developed. Results are also compared with the conventional objective function that neglects abdominal muscle forces. An overall optimal solution involving smaller spinal compression but higher trunk stability is automatically attained when choosing muscle stress powers at and around 3. Results highlight the internal oblique muscle as the most efficient abdominal muscle during the tasks investigated. The estimated relative forces in abdominal muscles are found to be in good agreement with the respective ratios of recorded electromyography activities.     

Trunk response analysis under sudden forward perturbations using a kinematics-driven model

Accurate quantification of the trunk transient response to sudden loading is crucial in prevention, evaluation, rehabilitation and training programs. An iterative dynamic kinematics-driven approach was used to evaluate the temporal variation of trunk muscle forces, internal loads and stability under sudden application of an anterior horizontal load. The input kinematics is hypothesized to embed basic dynamic characteristics of the system that can be decoded by our kinematics-driven approach. The model employs temporal variation of applied load, trunk forward displacement and surface EMG of select muscles measured on two healthy and one chronic low-back pain subjects to a sudden load. A finite element model accounting for measured kinematics, nonlinear passive properties of spine, detailed trunk musculature with wrapping of global extensor muscles, gravity load and trunk biodynamic characteristics is used to estimate the response under measured sudden load. Results demonstrate a delay of ～200 ms in extensor muscle activation in response to sudden loading. Net moment and spinal loads substantially increase as muscles are recruited to control the trunk under sudden load. As a result and due also to the trunk flexion, system stability significantly improves. The reliability of the kinematics-driven approach in estimating the trunk response while decoding measured kinematics is demonstrated. Estimated large spinal loads highlight the risk of injury that likely further increases under larger perturbations, muscle fatigue and longer delays in activation.     

Estimation of trunk muscle forces using the finite element method and in vivo loads measured by telemeterized internal spinal fixation devices.

Direct quantitative measurement of muscle forces is not possible. Forces in the trunk muscles were estimated for standing and flexion of the upper body using three-dimensional, nonlinear finite element models of the lumbar spine with and without an internal spinal fixation device. Muscle forces assumed were two pairs dorsally and one ventrally, each representing several muscles. Muscle forces in the model with internal fixators were varied in discrete steps until the implant loads calculated closely corresponded to those measured in a patient with an instrumented implant. The calculated angles between adjacent lumbar vertebrae were compared with corresponding values measured on X-ray films of a patient as well as with literature values and served as a second criterion for predicting muscle forces. For the model without an implant, the muscle forces of the first model were slightly varied until the lumbar spine shape and the intradiscal pressure were physiological. The abdomen was shown to have a considerable supporting function for flexion.     

A frontal plane model of the lumbar spine subjected to a follower load: implications for the role of muscles

Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles under a vertical load of 80-100 N. Conversely, the whole lumbar spine can support physiologic compressive loads without large displacements when the load is applied along a follower path that approximates the tangent to the curve of the lumbar spine. This study utilized a two-dimensional beam-column model of the lumbar spine in the frontal plane under gravitational and active muscle loads to address the following question: Can trunk muscle activation cause the path of the internal force resultant to approximate the tangent to the spinal curve and allow the lumbar spine to support compressive loads of physiologic magnitudes? The study identified muscle activation patterns that maintained the lumbar spine model under compressive follower load, resulting in the minimization of internal shear forces and bending moments simultaneously at all lumbar levels. The internal force resultant was compressive, and the lumbar spine model, loaded in compression along the follower load path, supported compressive loads of physiologic magnitudes with minimal change in curvature in the frontal plane. Trunk muscles may coactivate to generate a follower load path and allow the ligamentous lumbar spine to support physiologic compressive loads.     

Computation of trunk equilibrium and stability in free flexion-extension movements at different velocities

Velocity of movement has been suggested as a risk factor for low-back disorders. The effect of changes in velocity during unconstrained flexion-extension movements on muscle activations, spinal loads, base reaction forces and system stability was computed. In vivo measurements of kinematics and ground reaction forces were initially carried out on young asymptomatic subjects. The collected kinematics of three subjects representing maximum, mean and minimum lumbar rotations were subsequently used in the kinematics-driven model to compute results during the entire movements at three different velocities. Estimated spinal loads and muscle forces were significantly larger in fastest pace as compared to slower ones indicating the effect of inertial forces. Spinal stability was improved in larger trunk flexion angles and fastest movement. Partial or full flexion relaxation of global extensor muscles occurred only in slower movements. Some local lumbar muscles, especially in subjects with larger lumbar flexion and at slower paces, also demonstrated flexion relaxation. Results confirmed the crucial role of movement velocity on spinal biomechanics. Predictions also demonstrated the important role on response of the magnitude of peak lumbar rotation and its temporal variation.     

Elevation and orientation of external loads influence trunk neuromuscular response and spinal forces despite identical moments at the L5–S1 level

A wide range of loading conditions involving external forces with varying magnitudes, orientations and locations are encountered in daily activities. Here we computed the effect on trunk biomechanics of changes in force location (two levels) and orientation (5 values) in 4 subjects in upright standing while maintaining identical external moment of 15 Nm, 30 N m or 45 Nm at the L5–S1. Driven by measured kinematics and gravity/external loads, the finite element models yielded substantially different trunk neuromuscular response with moderate alterations (up to 24% under 45 Nm moment) in spinal loads as the load orientation varied. Under identical moments, compression and shear forces at the L5–S1 as well as forces in extensor thoracic muscles progressively decreased as orientation of external forces varied from downward gravity (90°) all the way to upward (−25°) orientation. In contrast, forces in local lumbar muscles followed reverse trends. Under larger horizontal forces at a lower elevation, lumbar muscles were much more active whereas extensor thoracic muscle forces were greater under smaller forces at a higher elevation. Despite such differences in activity pattern, the spinal forces remained nearly identical (<6% under 45 Nm moment). The published recorded surface EMG data of extensor muscles trend-wise agreed with computed local muscle forces as horizontal load elevation varied but were overall different from results in both local and global muscles when load orientation altered. Predictions demonstrate the marked effect of external force orientation and elevation on the trunk neuromuscular response and spinal forces and questions attempts to estimate spinal loads based only on consideration of moments at a spinal level.     

Load-sharing in the lumbosacral spine in neutral standing & flexed postures – A combined finite element and inverse static study

Understanding load-sharing in the spine during in-vivo conditions is critical for better spinal implant design and testing. Previous studies of load-sharing that considered actual spinal geometry applied compressive follower load, with or without moment, to simulate muscle forces. Other studies used musculoskeletal models, which include muscle forces, but model the discs by simple beams or spherical joints and ignore the articular facet joints.This study investigated load-sharing in neutral standing and flexed postures using a detailed Finite Element (FE) model of the ligamentous lumbosacral spine, where muscle forces, gravity loads and intra-abdominal pressure, as predicted by a musculoskeletal model of the upper body, are input into the FE model. Flexion was simulated by applying vertebral rotations following spine rhythm measured in a previous in-vivo study, to the musculoskeletal model. The FE model predicted intradiscal pressure (IDP), strains in the annular fibers, contact forces in the facet joints, and forces in the ligaments. The disc forces and moments were determined using equilibrium equations, which considered the applied loads, including muscle forces and IDP, as well as forces in the ligaments and facet joints predicted by the FE model. Load-sharing was calculated as the portion of the total spinal load carried along the spine by each individual spinal structure. The results revealed that spinal loads which increased substantially from the upright to the flexed posture were mainly supported by the discs in the upright posture, whereas the ligaments’ contribution in resisting shear, compression, and moment was more significant in the flexed posture.     

An improved multi-joint EMG-assisted optimization approach to estimate joint and muscle forces in a musculoskeletal model of the lumbar spine

Muscle force partitioning methods and musculoskeletal system simplifications are key modeling issues that can alter outcomes, and thus change conclusions and recommendations addressed to health and safety professionals. A critical modeling concern is the use of single-joint equilibrium to estimate muscle forces and joint loads in a multi-joint system, an unjustified simplification made by most lumbar spine biomechanical models. In the context of common occupational tasks, an EMG-assisted optimization method (EMGAO) is modified in this study to simultaneously account for the equilibrium at all lumbar joints (M-EMGAO). The results of this improved approach were compared to those of its conventional single-joint equivalent (S-EMGAO) counterpart, the latter method being applied to the same lumbar joints but one at a time. Despite identical geometrical configurations and passive contributions used in both models, computed outcomes clearly differed between single- and multi-joint methods, especially at larger trunk flexed postures and during asymmetric lifting. Moreover, muscle forces predicted by L5-S1 single-joint analyses do not maintain mechanical equilibrium at other spine joints crossed by the same muscles. Assuming that the central nervous system does not attempt to balance the external moments one joint at a time and that a given muscle cannot exert different forces at different joints, the proposed multi-joint method represents a substantial improvement over its single-joint counterpart. This improved approach, hence, resolves trunk muscle forces with biological integrity but without compromising mechanical equilibrium at the lumbar joints.     

Spinal loads and trunk muscles forces during level walking – A combined in vivo and in silico study on six subjects

During level walking, lumbar spine is subjected to cyclic movements and intricate loading of the spinal discs and trunk musculature. This study aimed to estimate the spinal loads (T12–S1) and trunk muscles forces during a complete gait cycle.Six men, 24–33 years walk barefoot at self-selected speed (4–5 km/h). 3D kinematics and ground reaction forces were recorded using a motion capturing system and two force plates, implemented in an inverse dynamic musculoskeletal model to predict the spinal loads and trunk muscles forces. Additionally, the sensitivity of the intra-abdominal pressure and lumbar segment rotational stiffness was investigated.Peak spinal loads and trunk muscle forces were between the gait instances of heel strike and toe off. In L4–L5 segment, sensitivity analysis showed that average peak compressive, antero-posterior and medio-lateral shear forces were 130–179%, 2–15% and 1–6%, with max standard deviation (±STD) of 40%, 6% and 3% of the body weight. Average peak global muscles forces were 24–55% (longissimus thoracis), 11–23% (iliocostalis thoracis), 12–16% (external oblique), 17–25% (internal oblique) and 0–8% (rectus abdominus) of body weight whereas, the average peak local muscles forces were 11–19% (longissimus lumborum), 14–31% (iliocostalis lumborum) and 12–17% (multifidus). Maximum ± STD of the global and local muscles forces were 13% and 8% of the body weight.Large inter-individual differences were found in peak compressive and trunk muscles forces whereas the sensitivity analysis also showed a substantial variation.     

A novel approach to evaluate abdominal coactivities for optimal spinal stability and compression force in lifting

A novel optimisation algorithm is developed to predict coactivity of abdominal muscles while accounting for both trunk stability via the lowest buckling load (P _cr) and tissue loading via the axial compression (F _c). A nonlinear multi-joint kinematics-driven model of the spine along with the response surface methodology are used to establish empirical expressions for P _cr and F _c as functions of abdominal muscle coactivities and external load magnitude during lifting in upright standing posture. A two-component objective function involving F _c and P _cr is defined. Due to opposite demands, abdominal coactivities that simultaneously maximise P _cr and minimise F _c cannot exist. Optimal solutions are thus identified while striking a compromise between requirements on trunk stability and risk of injury. The oblique muscles are found most efficient as compared with the rectus abdominus. Results indicate that higher abdominal coactivities should be avoided during heavier lifting tasks as they reduce stability margin while increasing spinal loads.     

Effects of abdominal stabilization maneuvers on the control of spine motion and stability against sudden trunk perturbations.

Much discussion exists about which is the most effective technique to improve spine stability. The purpose of this study was to evaluate the effectiveness of abdominal bracing and abdominal hollowing maneuvers to control spine motion and stability against rapid perturbations. Eleven healthy males were posteriorly loaded in different experimental conditions: resting with no knowledge of the perturbation timing; performing each of the stabilization maneuvers at 10%, 15% and 20% of internal oblique maximum voluntary contraction with no knowledge of the perturbation timing; and naturally coactivating the trunk muscles when perturbation timing was known. An EMG biofeedback system was used to control the pattern and intensity of abdominal coactivation. The muscular preactivation of seven trunk muscles (bilaterally registered), the applied force, and the torso muscular and kinematic responses to loading were measured; and the spine stability and compression were modeled. The hollowing maneuver was not effective for reducing the kinematic response to sudden perturbation. On the contrary, the bracing maneuver fostered torso cocontraction, reduced lumbar displacement, and increased trunk stability, but at the cost of increasing spinal compression. When the timing of the perturbation was known, the participants were able to stabilize the trunk while imposing smaller spine compressive loads.     

Effect of load position on muscle forces, internal loads and stability of the human spine in upright postures

A novel kinematics-based approach coupled with a non-linear finite element model was used to investigate the effect of changes in the load position and posture on muscle activity, internal loads and stability margin of the human spine in upright standing postures. In addition to 397 N gravity, external loads of 195 and 380 N were considered at different lever arms and heights. Muscle forces, internal loads and stability margin substantially increased as loads displaced anteriorly away from the body. Under same load magnitude and location, adopting a kyphotic posture as compared with a lordotic one increased muscle forces, internal loads and stability margin. An increase in the height of a load held at a fixed lever arm substantially diminished system stability thus requiring additional muscle activations to maintain the same margin of stability. Results suggest the importance of the load position and lumbar posture in spinal biomechanics during various manual material handling operations.     

Trunk strength,muscle activity and spinal loads in maximum isometric flexion and extension exertions: A combined in vivo-computational study

Determination of the trunk maximum voluntary exertion moment capacity and associated internal spinal forces could serve in proper selection of workers for specific occupational task requirements, injury prevention and treatment outcome evaluations. Maximum isometric trunk exertion moments in flexion and extension along with surface EMG of select trunk muscles are measured in 12 asymptomatic subjects. Subsequently and under individualized measured harness-subject forces, kinematics and upper trunk gravity, an iterative kinematics-driven finite element model is used to compute muscle forces and spinal loads in 4 of these subjects. Different co-activity and intra-abdominal pressure levels are simulated. Results indicate significantly larger maximal resistant moments and spinal compression/shear forces in extension exertions than flexion exertions. The agonist trunk muscles reach their maximum force generation (saturation) to greater extent in extension exertions compared to flexion exertions. Local lumbar extensor muscles are highly active in extension exertions and generate most of the internal spinal forces. The maximum exertion attempts produce large spinal compression and shear loads that increase with the antagonist co-activity level but decrease with the intra-abdominal pressure. Intra-abdominal pressure decreases agonist muscle forces in extension exertions but generally increase them in flexion exertions.