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

Effect of muscular fatigue on the cumulative lumbar damage during repetitive lifting task: a comparative study of damage calculation methods

Several methods have been put forward to quantify cumulative loads; however, limited evidence exists as to the subsequent damages and the role of muscular fatigue. The present study assessed whether muscular fatigue could affect cumulative damage imposed on the L5-S1 joint. Trunk muscle electromyographic (EMG) activities and kinematics/kinetics of 18 healthy male individuals were evaluated during a simulated repetitive lifting task. A traditional EMG-assisted model of the lumbar spine was modified to account for the effect of erector spinae fatigue. L5-S1 compressive loads for each lifting cycle were estimated based on varying (i.e. actual), fatigue-modified, and constant Gain factors. The corresponding damages were integrated to calculate the cumulative damage. Moreover, the damage calculated for one lifting cycle was multiplied by the lifting frequency, as the traditional approach. Compressive loads and the damages obtained through the fatigue-modified model were predicted in close agreement with the actual values. Similarly, the difference between actual damages and those driven by the traditional approach was not statistically significant (p = 0.219). However, damages based on a constant Gain factor were significantly greater than those based on the actual (p = 0.012), fatigue-modified (p = 0.017), and traditional (p = 0.007) approaches.Practitioner summary: In this study, we managed to include the effect of muscular fatigue on cumulative lumbar damage calculations. Including the effect of muscular fatigue leads to an accurate estimation of cumulative damages while eliminating computational complexity. However, using the traditional approach also appears to provide acceptable estimates for ergonomic assessments.

Kinetic and kinematic variables affecting trunk flexion during level walking in patients with lumbar spinal stenosis

Lumbar spinal stenosis causes cauda equina and nerve root compression, resulting in neurological symptoms. Although trunk flexion during level walking may alleviate these symptoms by enabling spinal canal decompression, some patients do not use this strategy. We aimed to identify the kinetic and kinematic variables that affect trunk flexion in patients during level walking. Gait was recorded in 111 patients using a three-dimensional motion capture system and six force plates. From the data recorded, walking velocity, bilateral step length, cycle time, maximum trunk flexion angle, forward pelvic tilt angle, pelvic rotation angle, maximum and minimum joint angles, and moment and power of the lower limb were calculated. Then a step-wise multiple regression analysis was conducted to identify kinetic and kinematic variables affecting trunk flexion. The maximum hip extension angle (β = 0.416), maximum hip flexion moment (β = -0.348), and step length (β = 0.257) were identified as variables significantly affecting the trunk flexion angle. The coefficient of determination adjusted for the degree of freedom was 0.294 (p < 0.05). Our results suggest that patients with lumbar spinal stenosis choose one of two strategies to alleviate symptoms during walking. One strategy is gait with trunk flexion posture to increase step length and hip extension angle. The other strategy is gait with trunk upright posture to decrease step length and hip extension angle.

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

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

Variations in trunk muscle activities and spinal loads following posterior lumbar surgery: A combined in vivo and modeling investigation

BACKGROUND

Iatrogenic injuries to paraspinal muscles during posterior lumbar surgery cause a reduction in their contractile cross-sectional area and thus presumably their postoperative activation. This study investigates the effect of such intraoperative injuries on postoperative patterns of muscle activations and spinal loads during various activities using a combined modeling and in vivo MR imaging approach.

METHODS

A three-dimensional, multi-joint, musculoskeletal model was used to estimate pre- and postoperative muscle forces and spinal loads under various activities in upright and flexed postures. According to our in vivo pre- and postoperative (~6 months) measurements in six patients using a 3-Tesla-MR scanner, physiological cross-sectional areas of multifidus and erector spinae were reduced in the postoperative model by 26 and 11%, respectively.

FINDINGS

Postoperative trunk extension strength was predicted to decrease by ~23% from 215 Nm in the intact model to 165 Nm in the postoperative model. Postoperative force in multifidus fascicles decreased by ~21-40% in flexion tasks and by ~14-35% in upright tasks. In contrast, the sum of the forces in all other intact and less injured extensor muscles slightly increased (by <6%) in the postoperative model. Postoperative L5-S1 compressive and shear loads varied slightly (by ~3%).

INTERPRETATION

Intraoperative injuries induced a shift in load-sharing from the most injured muscle (multifidus) toward other less injured and intact muscles during all simulated activities. Postoperative rehabilitation programs should therefore strengthen and facilitate (while avoiding muscle imbalance) not only the injured multifidus but also other intact and less injured trunk muscles that play a compensatory role after the operation.

Predictive equations for lumbar spine loads in load-dependent asymmetric one- and two-handed lifting activities

BACKGROUND

Asymmetric lifting activities are associated with low back pain.

METHODS

A finite element biomechanical model is used to estimate spinal loads during one- and two-handed asymmetric static lifting activities. Model input variables are thorax flexion angle, load magnitude as well as load sagittal and lateral positions while response variables are L4-L5 and L5-S1 disc compression and shear forces. A number of levels are considered for each input variable and all their possible combinations are introduced into the model. Robust yet user-friendly predictive equations that relate model responses to its inputs are established.

FINDINGS

Predictive equations with adequate goodness-of-fit (R(2) ranged from ~94% to 99%, P≤0.001) that relate spinal loads to task (input) variables are established. Contour plots are used to identify combinations of task variable levels that yield spine loads beyond the recommended limits. The effect of uncertainties in the measurements of asymmetry-related inputs on spinal loads is studied.

INTERPRETATION

A number of issues regarding the NIOSH asymmetry multiplier are discussed and it is concluded that this multiplier should depend on the trunk posture and be defined in terms of the load vertical and horizontal positions. Due to an imprecise adjustment of the handled load magnitude this multiplier inadequately controls the biomechanical loading of the spine. Ergonomists and bioengineers, faced with the dilemma of using either complex but more accurate models on one hand or less accurate but simple models on the other hand, have hereby easy-to-use predictive equations that quantify spinal loads under various occupational tasks.