Analysis and quantitative myoelectric measurements of loads on the lumbar spine when holding weights in standing postures

Validation of a patient-specific finite element analysis framework for identification of growing rod-failure regions in early onset scoliosis patients

PURPOSE

Growing rods are the gold-standard for treatment of early onset scoliosis (EOS). However, these implanted rods experience frequent fractures, requiring additional surgery. A recent study by the U.S. Food and Drug Administration (FDA) identified four common rod fracture locations. Leveraging this data, Agarwal et al. were able to correlate these fractures to high-stress regions using a novel finite element analysis (FEA) framework for one patient. The current study aims to further validate this framework through FEA modeling extended to multiple patients.

METHODS

Three patient-specific FEA models were developed to match the pre-operative patient data taken from both registry and biplanar radiographs. The surgical procedure was then simulated to match the post-operative deformity. Body weight and flexion bending (1 Nm) loads were then applied and the output stress data on the rods were analyzed.

RESULTS

Radiographic data showed fracture locations at the mid-construct, adjacent to the distal and tandem connector across the patients. Stress analysis from the FEA showed these failure locations matched local high-stress regions for all fractures observed. These results qualitatively validate the efficacy of the FEA framework by showing a decent correlation between localized high-stress regions and the actual fracture sites in the patients.

CONCLUSIONS

This patient-specific, in-silico framework has huge potential to be used as a surgical tool to predict sites prone to fracture in growing rod implants. This prospective information would therefore be vital for surgical planning, besides helping optimize implant design for reducing rod failures.

Injectable biomaterial induces regeneration of the intervertebral disc in a caprine loaded disc culture model

Back pain is the leading cause of disability with half of cases attributed to intervertebral disc (IVD) degeneration, yet currently no therapies target this cause. We previously reported an ex vivo caprine loaded disc culture system (LDCS) that accurately represents the cellular phenotype and biomechanical environment of human IVD degeneration. Here, the efficacy of an injectable hydrogel system (LAPONITE® crosslinked pNIPAM-co-DMAc, (NPgel)) to halt or reverse the catabolic processes of IVD degeneration was investigated within the LDCS. Following enzymatic induction of degeneration using 1 mg mL-1 collagenase and 2 U mL-1 chondroitinase ABC within the LDCS for 7 days, IVDs were injected with NPgel alone or with encapsulated human bone marrow progenitor cells (BMPCs). Un-injected caprine discs served as degenerate controls. IVDs were cultured for a further 21 days within the LDCS. Tissues were then processed for histology and immunohistochemistry. No extrusion of NPgel was observed during culture. A significant decrease in histological grade of degeneration was seen in both IVDs injected with NPgel alone and NPgel seeded with BMPCs, compared to un-injected controls. Fissures within degenerate tissue were filled by NPgel and there was evidence of native cell migration into injected NPgel. The expression of healthy NP matrix markers (collagen type II and aggrecan) was increased, whereas the expression of catabolic proteins (MMP3, ADAMTS4, IL-1β and IL-8) was decreased in NPgel (±BMPCs) injected discs, compared to degenerate controls. This demonstrates that NPgel promotes new matrix production at the same time as halting the degenerative cascade within a physiologically relevant testing platform. This highlights the potential of NPgel as a future therapy for IVD degeneration.

Sagittal Imbalance May Lead to Higher Risks of Vertebral Compression Fractures and Disc Degeneration-A Finite Element Analysis

BACKGROUND

Sagittal balance is an important clinical parameter of the spine for its normal function. Maintenance of the sagittal balance is crucial in the clinical management of spinal problems.

METHODS

Three different finite element models with spinal alignments based on Schwab's classification were developed: (1) Balanced/Normal model (sagittal vertical axis [SVA] = 0 mm, lumbar lordosis [LL] = 50°, thoracic kyphosis [TK] = 25°, pelvic incidence [PI] = 45°, pelvic tilt [PT] = 10°, sacral slope [SS] = 35°); (2) Balanced with compensatory mechanisms/Flatback model (SVA = 50 mm, LL = 20°, TK = 20°, PI = 45°, PT = 30°, SS = 15°); and (3) Imbalanced/Hyperkyphotic model (SVA = 150 mm, LL = -5°, TK = 25°, PI = 45°, PT = 40°, SS = 5°). All 3 models were subjected to the follower loads simulating bodyweight/muscular contractions along with the moments to simulate flexion, extension, lateral bending, and axial rotation. The maximum cortical vertebral stress, annular stress, and intradiscal pressure (IDP) were calculated and compared.

RESULTS

The results showed that the hyperkyphotic model had higher stresses in the vertebrae (25% higher), the annulus fibrosus (48% higher) and the IDP (8% higher) than the normal models in flexion. The segments near the thoracolumbar junction (T10-L1) showed the highest increase in the vertebral body stress, the annulus fibrosus stress, and the IDP.

CONCLUSIONS

This study showed that the imbalance in sagittal alignment might be responsible for disc degeneration and atraumatic vertebral fractures at the thoracolumbar regions, supporting clinical findings.

Finite Element Comparison of the Spring Distraction System and the Traditional Growing Rod for the Treatment of Early Onset Scoliosis

STUDY DESIGN

Finite element analysis (FEA).

OBJECTIVE

The aim of this study was to determine biomechanical differences between traditional growing rod (TGR) and spring distraction system (SDS) treatment of early-onset scoliosis.

SUMMARY OF BACKGROUND DATA

Many "growth-friendly" implants like the TGR show high rates of implant failure, spinal stiffening, and intervertebral disc (IVD) height loss. We developed the SDS, which employs continuous, dynamic forces to mitigate these limitations. The present FEA compares TGR and SDS implantation, followed by an 18-month growth period.

METHODS

Two representative, ligamentous, scoliotic FEA models were created for this study; one representing TGR and one representing SDS. initial implantation, and up to 18 months of physeal spinal growth were simulated. The SDS model was continuously distracted over this period; the TGR model included two additional distractions following index surgery. Outcomes included differences in rod stress, spinal morphology and iVD stress-shielding.

RESULTS

Maximum postoperative von Mises stress was 249MPa for SDS, and 205MPa for TGR. During the 6-month TGR distraction, TGR rod stress increased over two-fold to a maximum stress of 417MPa, compared to a maximum of 262 MPa in the SDS model at 6-month follow-up. During subsequent follow-up periods, TGR rod stress remained consistently higher than stresses in the SDS model. Additional lengthenings in the TGR model led to a smaller residual curve (16.08) and higher T1-S1 growth (359 mm) at 18-month follow-up compared to the SDS model (26.98, 348 mm). During follow-up, there was less stress-shielding of the IVDs in the SDS model, compared to the TGR model. At 18-month follow-up, upper and lower IVD surfaces of the SDS model were loaded more in compression than their TGR counterparts (mean upper: +112 ± 19N; mean lower: +100 ± 17N).

CONCLUSION

In the present FEA, TGR treatment resulted in slightly larger curve correction compared to SDS, at the expense of increased IVD stress-shielding and a higher risk of rod fractures.

LEVEL OF EVIDENCE

N/A.

The efficacy of a lifting strap as an ergonomic intervention for EMS providers: Does it make it easier to raise a supine patient to an upright sitting posture?

Patient handling related musculoskeletal injuries are prevalent among Emergency Medical Service (EMS) providers. The first step in many patient handling situations is where a supine patient on the floor is brought to a sitting position. This study investigated whether a strap, placed under the patient's torso and long enough that EMS providers can perform the patient raising task in a standing posture, reduced muscular effort. Fifteen participants raised a simulated patient, with the help of an assistant, using the strap method and a traditional method (grasping the shoulders) in an open area, a restricted space, and in a bathtub. Torso postures improved in all location conditions when using the strap. The muscle activation data showed mixed results. While EMG responses from the latissimus dorsi muscles were reduced, EMG activity of the erector spinae muscles increased when the strap was used. Perceived effort assessments supported the use of the strap.

Towards a validated patient-specific computational modeling framework to identify failure regions in traditional growing rods in patients with early onset scoliosis

BACKGROUND

While growing rods are an important contribution to early-onset scoliosis treatment, rod fractures are a common complication that require reoperations. A recent retrieval analysis study performed on failed traditional growing rods revealed that there are commonalities among patient characteristics based on the location of rod fracture. However, it remains unknown if these locations correspond to high stress regions in the implanted construct.

METHODS

A patient-specific finite element scoliotic model was developed to match the pre-operative (pre-op) scoliotic curve of a patient as described in previously published articles, and by using the patient registry information along with biplanar radiographs. A dual stainless-steel traditional growing rod construct was implanted into this scoliotic model and the surgical procedure was simulated to match the post-operative (post-op) scoliotic curve parameters. Muscle stabilization and gravity was simulated through follower load application. Rod distraction magnitudes were chosen based on pre-op to post-op cobb angle correction, and flexion bending load was simulated to identify the high stress regions on the rods.

RESULTS

The patient-specific finite element model identified two high stress regions on the posterior surface of the rods, one at mid construct and the other adjacent to the distal anchors. This correlated well with the data obtained from the retrieval analysis performed by researchers at U.S. Food and Drug Administration (FDA) which showed the posterior surface of the rod as the fracture initiation site, and the three locations of failure as mid-construct, adjacent to distal anchors, and adjacent to tandem connector.

CONCLUSIONS

The result of this study confirms that the high stress regions on the growing rods, as identified by the FEA, match the fracture prone sites identified in the retrieval analysis performed at the FDA. This proof-of-concept patient-specific approach can be used to predict sites prone to fracture in growing rods.

What is the best method to assess the abdominal wall? Restoring strength does not mean functional recovery

BACKGROUND

Restoring the contractile function to the abdominal wall is a major goal in hernia repair. However, the core understanding is required when choosing the method for outcome assessment.

AIM

To assess the role of the anterolateral abdominal muscles on abdominal wall function in patients undergoing hernia repair by analysis of correlation between the surface electromyography activation signal of these muscles and torque produced during validated strength tests.

METHODS

Activation of the rectus abdominis, external oblique, and internal oblique/transverse abdominis muscles was evaluated by surface electromyography during two validated tests: Step: 1-A, isometric contraction in dorsal decubitus; 1-B, isometric contraction in lateral decubitus; 2-A, isokinetic Biodex testing; and 2-B, isometric Biodex testing.

RESULTS

Twenty healthy volunteers were evaluated. The linear correlation coefficient between root mean square/peak data obtained from surface electromyography signal analysis for each muscle and the peak torque variable was always <0.2 and statistically non-significant (p<0.05). The agonist/antagonist ratio showed a positive, significant, weak-to-moderate correlation in the external oblique (Peak, p=0.027; root mean square, 0.564). Surface electromyography results correlated positively among different abdominal contraction protocols, as well as with a daily physical activity questionnaire.

CONCLUSIONS

There was no correlation between surface electromyography examination of the anterolateral abdominal wall muscles and torque measured by a validated instrument, except in a variable that does not directly represent torque generation.

Outcomes of Optimal Distraction Forces and Frequencies in Growth Rod Surgery for Different Types of Scoliotic Curves: An In Silico and In vitro Study

OBJECTIVE

Analyze the effects of the distraction forces and frequencies on multiple representative scoliotic curves and to establish a relationship between high distraction forces and screw loosening.

STUDY DESIGN

Multiple representative finite-element models of a juvenile scoliotic spine were used to study the effects of the magnitude and frequency of distraction on growth rods.

METHODS

Simulation of 6 months of growth under various distraction forces to analyze the effects of distraction forces on the biomechanics of the scoliotic spine and growth rod instrumentation; simulation of 24 months of growth under various intervals of distraction to analyze the effects of the distraction interval on the propensity for rod fracture; in vitro study to assess screw loosening after 6 months.

RESULTS

For all scoliotic spine model instrumented with growth rods, an optimal distraction force existed at which normal T1-S1 growth was sustained, along with minimum stresses on the rods, the lowest load at the screw-bone interface, and the least alteration in the sagittal contour. The results followed similar trends for each model, with the numerical values of optimal distraction forces in proximity for all representative scoliotic spine models. The in vitro study proved that the pullout strength of pedicle screws reduced significantly after 6 months of fatigue at higher distraction forces (in comparison with optimal distraction forces). This corroborated the finite-element findings for lower loads at the screw-bone interface with optimal distraction forces.

CONCLUSIONS

This study concludes that the optimal distraction forces exists for all types of scoliotic curves that have been instrumented with growth rods, which exhibits reduction of stresses on the rods with frequent distractions. This study also links the second most common complication, screw loosening, with high distraction forces. Therefore, optimizing the biomechanical environment of the dual growth rods could drastically reduce the biomechanical complications associated with growth rods.

Distraction magnitude and frequency affects the outcome in juvenile idiopathic patients with growth rods: finite element study using a representative scoliotic spine model

BACKGROUND CONTEXT

Growth rods are used to limit the progression of scoliosis without restraining the opportunity for the spine to grow. However, major complications like rod breakage, screw loosening, and altered sagittal contour have been encountered.

OBJECTIVE

To analyse the effect of the magnitude of distraction forces on the T1-S1 growth, maximum von Mises stresses on the rods, sagittal contours, and load at the pedicle screw-bone interface and quantify the maximum stresses on the rod for a period of 24 months using different frequencies of distraction in a representative scoliotic spine model.

STUDY DESIGN

A representative finite element model of a juvenile scoliotic spine was used to study the effect of magnitude and frequency of distraction on growth rods.

METHODS

A representative scoliotic model was developed and instrumented using proximal foundation, distal foundation, and rods. Part 1: simulation steps comprised 6 months of growth under various distraction forces to analyze effects of distraction force on the biomechanics of the spine and instrument. Part 2: simulation steps comprised 24 months of growth under various intervals of distraction to analyze effects of distraction interval on the propensity of rod fracture.

RESULTS

Part 1: an optimal distraction force exists for which the growth is sustained with minimum stress on the rod, lower loads at screw-bone interface, and unaltered sagittal contours. Part 2: the stresses on the rods were highest for 12-month distraction (2 distractions in 2 years) and lowest for 2-month distraction (12 distractions in 2 years).

CONCLUSIONS

The data and trend suggest that as the distraction forces vary so do the effects on spinal growth. The results of this study also signify the importance of shorter distraction period in reducing the stresses on the rods.

Smaller Interval Distractions May Reduce Chances of Growth Rod Breakage Without Impeding Desired Spinal Growth: A Finite Element Study

BACKGROUND

Growth rods allow regular distraction of the spine to compensate for growth. Traditionally such distractions are performed every 6 months via open surgery. However with the advent of minimally invasive techniques like magnetically controlled growing rods, the distractions can be performed non-surgically. This also implies that the interval of distraction could be changed or customized based on individual patient's need.

HYPOTHESIS

In this study we have hypothesized that the distraction at shorter intervals reduces the stresses on the rods which in turn reduces the chance of rod failure.

OBJECTIVE

A finite element model of a juvenile spine was instrumented with growth rods and distractions were applied at different frequencies (2 months, 3 months, 6 months, and 12 months) for a period of two years to study the effects of frequency of distraction on maximum von Mises stresses on the rods for different loading conditions were studied.

RESULTS

The stresses on the rods were highest for 12-month distraction (2 distractions in 2 years) and lowest for 2-months distraction (12 distractions in 2 years).

CONCLUSION

It was found that the shorter intervals of distraction led to reduction of stresses on the rod for same spinal height gain in two years.

Trunk muscle activity is modified in osteoporotic vertebral fracture and thoracic kyphosis with potential consequences for vertebral health

This study explored inter-relationships between vertebral fracture, thoracic kyphosis and trunk muscle control in elderly people with osteoporosis. Osteoporotic vertebral fractures are associated with increased risk of further vertebral fractures; but underlying mechanisms remain unclear. Several factors may explain this association, including changes in postural alignment (thoracic kyphosis) and altered trunk muscle contraction patterns. Both factors may increase risk of further fracture because of increased vertebral loading and impaired balance, which may increase falls risk. This study compared postural adjustments in 24 individuals with osteoporosis with and without vertebral fracture and with varying degrees of thoracic kyphosis. Trunk muscle electromyographic activity (EMG) associated with voluntary arm movements was recorded and compared between individuals with and without vertebral fracture, and between those with low and high thoracic kyphosis. Overall, elderly participants in the study demonstrated co-contraction of the trunk flexor and extensor muscles during forwards arm movements, but those with vertebral fractures demonstrated a more pronounced co-contraction than those without fracture. Individuals with high thoracic kyphosis demonstrated more pronounced alternating flexor and extensor EMG bursts than those with less kyphosis. Co-contraction of trunk flexor and extensor muscles in older individuals contrasts the alternating bursts of antagonist muscle activity in previous studies of young individuals. This may have several consequences, including altered balance efficacy and the potential for increased compressive loads through the spine. Both of these outcomes may have consequences in a population with fragile vertebrae who are susceptible to fracture.

Effect of Distraction Force on Growth and Biomechanics of the Spine: A Finite Element Study on Normal Juvenile Spine With Dual Growth Rod Instrumentation

BACKGROUND

Growth rods are used to limit the progression of scoliosis without restraining the opportunity for the spine to grow. The growth is sustained by consecutive distraction at intervals of 6 months. The optimal distraction force for a scoliotic patient is not defined adequately and rod breakage, screw loosening, stimulation of growth and altered sagittal contour has been observed.

HYPOTHESIS

The hypothesis of this study is that for every patient with dual growth rods treatment there exists a distraction force that will sustain the growth of that patient's spine equal to normal growth with minimum changes in sagittal contours, results in lower von Mises stresses on the rods and minimum force at the pedicle screw-bone interface at 6 months.

OBJECTIVE

In this finite element study, we undertook an objective to identify the effect of magnitude of distraction forces on the T1-S1 growth, maximum von Mises stresses on the rods, sagittal contours, and the load at the pedicle screw-bone interface.

RESULTS

An optimal distraction force exists for which the growth is sustained with minimum stress on the rod, lower loads at screw-bone interface and unaltered sagittal contours. Another observation was that higher distraction forces (in the given range) didn't produce stresses on rod or load on screw that could result in failure of the implant.

CONCLUSION

Restoration of sagittal contour along with height restoration could guide the clinical practice, for the given range of distraction force.

Biomechanical evaluation of the Eco-Pick lift assist: A device designed to facilitate product selection tasks in distribution centers

Developing lift assist devices to aid workers performing case tasks in distribution centers has been challenging given the movement of workers through a distribution facility. The objective of this work was to biomechanically evaluate a lift assist that can be integrated with pallet jacks and thereby move through the facility with the workers. Twelve participants transferred 16.4 kg boxes from one pallet to another manually and using the Eco-Pick lift assist. Electromyographic (EMG) activities were measured bilaterally in the Bicep, Deltoid, Latissimus Dorsi, and Erector Spinae muscles. The analysis showed that the 90th percentile normalized EMG values were significantly reduced in 4 of the 8 muscles sampled when using the Eco-Pick. Likewise, the 50th percentile normalized EMG data were significantly reduced for the Erector Spinae, Latissimus Dorsi, and the Bicep muscles when using the assist. For some of the muscles the advantage of the Eco-Pick was dependent upon the initial lift height or the placement height. Overall, it appears that this evaluation of the Eco-Pick's efficacy indicates that the device has the potential to be an effective means of reducing worker exposure to risk factors associated with back and shoulder injuries experienced during manual palletizing tasks found in distribution centers.

A biomechanical and subjective assessment and comparison of three ambulance cot design configurations

Effects of ambulance cot design features (handle design and leg folding mechanism) were evaluated. Experienced ambulance workers performed tasks simulating loading and unloading a cot to and from an ambulance, and a cot raising task. Muscle activity, ratings of perceived exertion, and performance style were significantly affected by cot condition (p < 0.05). Erector Spinae activity was significantly less when using Cot-2's stretcher-style handles. Shoulder muscle activity was significantly less when using Cot-2's loop handle. During loading and unloading, operators allowed the cot to support its own weight most often with Cot-2's stretcher-style handles. Preference for Cot-2 (either handles) over Cot-1 (with loop handle) was consistent across tasks. Handle effects were influenced by operator stature; taller participants received more benefit from Cot-2's stretcher-style handles; shoulder muscles' demands were greater for shorter participants due to handle location. Providing handle options and automatic leg folding/unfolding operation can reduce cot operator's effort and physical strain. Practitioner Summary: Paramedics frequently incur musculoskeletal injuries associated with patient-handling tasks. A controlled experiment was conducted to assess effects of ambulance cot design features on physical stress of operators, as seen through muscle activity and operator's perceptions. Differences between cots were found, signalling that intentional design can reduce operator's physical stress.

Biomechanical analysis and modeling of different vertebral growth patterns in adolescent idiopathic scoliosis and healthy subjects

Background

The etiology of AIS remains unclear, thus various hypotheses concerning its pathomechanism have been proposed. To date, biomechanical modeling has not been used to thoroughly study the influence of the abnormal growth profile (i.e., the growth rate of the vertebral body during the growth period) on the pathomechanism of curve progression in AIS. This study investigated the hypothesis that AIS progression is associated with the abnormal growth profiles of the anterior column of the spine.

Methods

A finite element model of the spinal column including growth dynamics was utilized. The initial geometric models were constructed from the bi-planar radiographs of a normal subject. Based on this model, five other geometric models were generated to emulate different coronal and sagittal curves. The detailed modeling integrated vertebral body growth plates and growth modulation spinal biomechanics. Ten years of spinal growth was simulated using AIS and normal growth profiles. Sequential measures of spinal alignments were compared.

Results

(1) Given the initial lateral deformity, the AIS growth profile induced a significant Cobb angle increase, which was roughly between three to five times larger compared to measures utilizing a normal growth profile. (2) Lateral deformities were absent in the models containing no initial coronal curvature. (3) The presence of a smaller kyphosis did not produce an increase lateral deformity on its own. (4) Significant reduction of the kyphosis was found in simulation results of AIS but not when using the growth profile of normal subjects.

Conclusion

Results from this analysis suggest that accelerated growth profiles may encourage supplementary scoliotic progression and, thus, may pose as a progressive risk factor.

Ultrasound assessment of the anatomical validity of T3 and L4 as sEMG recording sites

The accuracy of surface EMG measurement is dependent upon minimizing potential crosstalk from other muscles. Although they are deeply situated, in places the erector spinae are covered with electrically silent aponeuroses rather than active muscle tissue. Theoretically these aponeuroses can serve as windows for sEMG recordings. A recent anatomical study concluded that T3 and L4 are ideal sites for recording the ES because the superficial muscle aponeuroses are wide at these sites. The aim of this prospective study was to investigate these sites in vivo using real time ultrasound. Ultrasound images from 20 subjects (10<30 years and 10>70 years; equal numbers of males and females in each group) were acquired during rest and in prone extension with the arms in three different positions. The most superficial aponeurosis widths were measured. The mean T3 aponeurosis width reduced significantly in extension from 4.4±4.7mm at rest to 1.8±2.6mm in extension (p<0.0001). Males had significantly smaller T3 aponeurosis widths than females (p=0.049). The mean L4 aponeurosis width also significantly decreased in extension from 35.5±7.0mm at rest to 29.9±7.2mm in extension (p<0.0001) due to 'doming' of the aponeurosis. Our results demonstrate that T3 is not a reliable site over which to record the ES because the aponeurosis width is too narrow. L4 is a good site if the electrodes are placed no more than 20mm from the midline.

Development and validation of a predictive bone fracture risk model for astronauts

There are still many unknowns in the physiological response of human beings to space, but compelling evidence indicates that accelerated bone loss will be a consequence of long-duration spaceflight. Lacking phenomenological data on fracture risk in space, we have developed a predictive tool based on biomechanical and bone loading models at any gravitational level of interest. The tool is a statistical model that forecasts fracture risk, bounds the associated uncertainties, and performs sensitivity analysis. In this paper, we focused on events that represent severe consequences for an exploration mission, specifically that of spinal fracture resulting from a routine task (lifting a heavy object up to 60 kg), or a spinal, femoral or wrist fracture due to an accidental fall or an intentional jump from 1 to 2 m. We validated the biomechanical and bone fracture models against terrestrial studies of ground reaction forces, skeletal loading, fracture risk, and fracture incidence. Finally, we predicted fracture risk associated with reference missions to the moon and Mars that represented crew activities on the surface. Fracture was much more likely on Mars due to compromised bone integrity. No statistically significant gender-dependent differences emerged. Wrist fracture was the most likely type of fracture, followed by spinal and hip fracture.

Applications of biomechanics for prevention of work-related musculoskeletal disorders

This paper summarises applications of biomechanical principles and models in industry to control musculoskeletal disorders of the low back and upper extremity. Applications of 2-D and 3-D biomechanical models to estimate compressive force on the low back, the strength requirements of jobs, application of guidelines for overhead work and application of strain index and threshold limit value to address distal upper extremity musculoskeletal disorders are presented. Several case studies applied in the railroad industry, manufacturing, healthcare and warehousing are presented. Finally, future developments needed for improved biomechanical applications in industry are discussed. The information presented will be of value to practising ergonomists to recognise how biomechanics has played a significant role in identifying causes of musculoskeletal disorders and controlling them in the workplace. In particular, the information presented will help practising ergonomists with how physical stresses can be objectively quantified.

Postural taping decreases thoracic kyphosis but does not influence trunk muscle electromyographic activity or balance in women with osteoporosis

BACKGROUND

Greater thoracic kyphosis is associated with increased biomechanical loading of the spine which is potentially problematic in individuals with osteoporotic vertebral fractures. Conservative interventions that reduce thoracic kyphosis warrant further investigation. This study aimed to investigate the effects of therapeutic postural taping on thoracic posture. Secondary aims explored the effects of taping on trunk muscle activity and balance.

METHODS

Fifteen women with osteoporotic vertebral fractures participated in this within-participant design study. Three taping conditions were randomly applied: therapeutic taping, control taping and no taping. Angle of thoracic kyphosis was measured after each condition. Force plate-derived balance parameters and trunk muscle electromyographic activity (EMG) were recorded during three static standing tasks of 40s duration.

RESULTS

There was a significant main effect of postural taping on thoracic kyphosis (p=0.026), with a greater reduction in thoracic kyphosis after taping compared with both control tape and no tape. There were no effects of taping on EMG or balance parameters.

CONCLUSIONS

The results of this study demonstrate that the application of postural therapeutic tape in a population with osteoporotic vertebral fractures induced an immediate reduction in thoracic kyphosis. Further research is needed to investigate the underlying mechanisms associated with this decrease in kyphosis.

Pedicle growth asymmetry as a cause of adolescent idiopathic scoliosis: a biomechanical study

Over the last century the neurocentral junction (NCJ) has been identified as a potential cause of adolescent idiopathic scoliosis (AIS). Disparate growth at this site has been thought to lead to pedicle asymmetry, which then causes vertebral rotation and ultimately, the development of scoliotic curves. The objectives of this study are (1) to incorporate pedicle growth and growth modulation into an existing finite element model of the thoracic and lumbar spine already integrating vertebral body growth and growth modulation; (2) to use the model to investigate whether pedicle asymmetry, either alone or combined with other deformations, could be involved in scoliosis pathomechanisms. The model was personalized to the geometry of a nonpathological subject and used as the reference spinal configuration. Asymmetry of pedicle geometry (i.e. initial length) and asymmetry of the pedicle growth rate alone or in combination with other AIS potential pathogenesis (anterior, lateral, or rotational displacement of apical vertebra) were simulated over a period of 24 months. The Cobb angle and local scoliotic descriptors (wedging angle, axial rotation) were assessed at each monthly growth cycle. Simulations with asymmetrical pedicle geometry did not produce significant scoliosis, vertebral rotation, or wedging. Simulations with asymmetry of pedicle growth rate did not cause scoliosis independently and did not amplify the scoliotic deformity caused by other deformations tested in the previous model. The results of this model do not support the hypothesis that asymmetrical NCJ growth is a cause of AIS. This concurs with recent animal experiments in which NCJ growth was unilaterally restricted and no scoliosis, vertebral wedging, or rotation was noted.

Disc arthroplasty design influences intervertebral kinematics and facet forces

BACKGROUND CONTEXT

Total disc replacement is a novel approach for dynamically stabilizing a painful intervertebral segment. While this approach is gaining popularity, and several types of implants are used, the effect of disc arthroplasty on lumbar biomechanics has not been widely reported. Consequently, beneficial or adverse effects of this procedure may not be fully realized, and data for kinematic optimization are unavailable.

PURPOSE

To characterize kinematic and load transfer modifications at L5/S1 secondary to joint replacement.

STUDY DESIGN

A human cadaveric biomechanical study in which the facet forces and instant axes of rotation (IAR) were measured for different spinal positions under simulated weightbearing conditions before and after total disc replacement at L5/S1 using semiconstrained (3 degrees of freedom [DOF]; Prodisc) and unconstrained (5 DOF; Charité) articulated implants.

METHODS

Twelve radiographically normal human cadaveric L5/S1 joints (age range 45-64 years) were tested before and after disc replacement using Prodisc II implants (Spine Solutions, Paoli, PA) in six specimens and SB Charité III (Johnson & Johnson, New Brunswick, NJ) in six other specimens. Semiconstrained fixtures in combination with a servo-hydraulic materials testing system subjected the test specimens to a physiologic combination of compression and anterior shear. Multiple intervertebral positions were studied and included up to 6 degrees of flexion, extension, and lateral bending. The IAR was calculated for every 3-degree intervals, and the force through the facet joints was simultaneously measured using flexible intra-articular sensors. Data were analyzed using repeated-measures analysis of variance.

RESULTS

During flexion/extension, the average IAR positions and directions were not significantly modified by implantation with the exception that the IAR was higher relative to S1 end plate with the Charité (p=.028). The IAR had a vertically oriented centrode throughout flexion/extension with the Prodisc (p<.001) and the Charité (p<.016). The centrode tended to be greater with the Prodisc. There was a trend that the facet force was decreased throughout flexion/extension for the Prodisc; however, this was statistically significant only at 6 degrees extension (27%, p=.013). In lateral bending, the IAR was significantly modified by Prodisc replacement, with a decreased inclination relative to S1 end plate, (ie, increased coupled axial rotation). While the IAR moved in the horizontal plane toward the side of bending, this effect was more pronounced with the Prodisc. The ipsilateral facet force was significantly increased in 6 degrees lateral bending with the Charité (85%; p=.001).

CONCLUSIONS

The degree of constraint affects post-implantation kinematics and load transfer. With the Prodisc (3 DOF), the facets were partially unloaded, though the IAR did not match the fixed geometrical center of the UHMWPE. The latter observation suggests joint surface incongruence is developed during movement. With the Charité (5 DOF), the IAR was less variable, yet the facet forces tended to increase, particularly during lateral bending. These results highlight the important role the facets play in guiding movement, and that implant constraint influences facet/implant synergy. The long-term consequences of the differing kinematics on clinically important outcomes such as wear and facet arthritis have yet to be determined.

The instant axis of rotation influences facet forces at L5/S1 during flexion/extension and lateral bending

Because the disc and facets work together to constrain spinal kinematics, changes in the instant axis of rotation associated with disc degeneration or disc replacement may adversely influence risk for facet overloading and arthritis. The relationships between L5/S1 segmental kinematics and facet forces are not well defined, since previous studies have separated investigations of spinal motion and facet force. The goal of this cadaveric biomechanical study was to report and correlate a measure of intervertebral kinematics (the centrode, or the path of the instant axis of rotation) and the facet forces at the L5/S1 motion segment while under a physiologic combination of compression and anterior shear loading. Twelve fresh-frozen human cadaveric L5/S1 joints (age range 50-64 years) were tested biomechanically under semi-constrained conditions by applying compression plus shear forces in several postures: neutral, and 3 degrees and 6 degrees of flexion, extension and lateral bending. The experimental boundary conditions imposed compression and shear representative of in vivo conditions during upright stance. The 3-D instantaneous axis of rotation (IAR) was calculated between two consecutive postures. The facet joint force was simultaneously measured using thin-film sensors placed between both facet surfaces. Variations of IAR location and facet force during motion were analyzed. During flexion and extension, the IAR was oriented laterally. The IAR intersection with the mid-sagittal plane moved cephalad relative to S1 endplate during flexion (P=0.010), and posterior during extension (P=0.001). The facet force did not correlate with posture (P=0.844). However, changes in the facet force between postures did correlate with IAR position: higher IAR's during flexion correlated with lower facet forces and vice versa (P=0.04). During lateral bending, the IAR was oblique relative to the main plane of motion and translated parallel to S1 endplate, toward the side of the bending. Overall, the facet force was increased on the ipsilateral side of bending (P=0.002). The IAR positions demonstrate that the L5 vertebral body primarily rotates forward during flexion (IAR close to vertebral body center) and rotates/translates backward during extension (IAR at or below the L5/S1 intervertebral disc). In lateral bending, the IAR obliquity demonstrated coupling with axial torsion due to resistance of the ipsilateral facet.

The effect of insoles on the incidence and severity of low back pain among workers whose job involves long-distance walking

The prevalence and incidence of low back pain in general society is high. Workers whose job involves walking long distances have an even higher tendency to suffer from low back pain. A positive effect of insoles in reducing low back pain was found in professional sports players. This was not examined on people whose job involves walking long distances. In this double blind prospective study we examined the effectiveness of insoles constructed in a computerized method to placebo insoles in 58 employees whose work entailed extensive walking and who suffered from low back pain. The evaluation was performed by the MILLION questionnaire, which is considered as a valid questionnaire for evaluation of low back pain. We calculated the differences of the pain intensity before and after the intervention, in the employees using the insoles manufactured by computer in comparison to the users of the placebo insoles. In each group, the analysis was performed in comparison to the baseline. A total of 81% of the employees preferred the real insoles as effective and comfortable in comparison to 19% of the users of the placebo insoles (P<0.05). The results of this study indicate a substantial improvement in the low back pain after the use of the true insoles. The average pain intensity according to the MILLION questionnaire before the use of the insoles was 5.46. However, after the use of the real insoles and the placebo insoles, the average pain intensity decreased to 3.96 and 5.11, respectively. The difference of the average pain intensity at the start of the study and after the use of the real insoles was significant: -1.49 (P=0.0001), whereas this difference after the use of the placebo insoles was not significant: -0.31 (P=0.1189). The reported severity of pain also decreased significantly: a level 5 pain and above was reported by 77% of the subjects at the start of the study. After the use of the real insoles only 37.9% of the subjects reported a similar degree of pain severity, and 50% of the subjects did so after the use of the placebo insoles (P< 0.05). We did not find a link between low back pain and other variables such as gender, age, number of offspring, work seniority, smoking, previous use of insoles and previous medication. This study demonstrates that the low back pain decreased significantly after the use of real insoles compared to placebo ones.

Fracture patterns of the adolescent porcine spine: an experimental loading study in bending-compression

STUDY DESIGN

To expose functional spinal units from adolescent porcine to mechanical flexion-compression and extension-compression to failure. The biomechanical, radiologic, magnetic resonance imaging, and histologic characteristics are described.

OBJECTIVES

The aim of the present study was to investigate the fracture pattern of functional spinal units from adolescent porcine lumbar spines in in vitro compression loading and bending.

SUMMARY OF BACKGROUND DATA

In several studies, it has been shown that the adolescent spine, especially the vertebral growth zones, is vulnerable to trauma. A high frequency of abnormalities affecting the spine has been found among athletes participating in sports with high demands on the back. The etiology of these abnormalities is still a controversial issue.

METHODS

Sixteen functional spinal units obtained from eight adolescent male pigs were used. Eight functional spinal units were exposed to flexion-compression and eight functional spinal units to extension-compression loading to failure. They were examined with plain radiography and magnetic resonance imaging before and after the loading. The functional spinal units were finally examined macroscopically and histologically.

RESULTS

Fractures/separations were seen in the growth zone anteriorly and more frequently, posteriorly in functional spinal units exposed to flexion-compression. In the extension-compression group, such injuries occurred only anteriorly. Only large fractures could be seen on plain radiographs and on magnetic resonance imaging. Macroscopically, a fracture/separation could be seen in 15 cases and histologically in all 16 cases. The median angle at failure for the flexion group was 17 degrees (range, 12-19) and for the extension group 17 degrees (range, 13-19 degrees). The median ultimate compression load in the flexion-compression group was 1894 N (range, 1607-3138 N) and in the extension-compression group 1801 N (range, 1158-2368 N).

CONCLUSIONS

The weakest part of the growing porcine lumbar spine, when compressed into flexion- or extension-compression, was the growth zone. The injury was more extensive in extension loading than during flexion loading. Growth zone injuries of the adolescent spine may go undetected on plain radiographs and magnetic resonance imaging.

Posture-dependent trunk extensor EMG activity during maximum isometrics exertions in normal male and female subjects

Posture-dependent trunk function data are important for appropriate normalization of submaximal trunk exertions, and is also necessary to define a more precise and specific use for strength testing in the prevention and diagnosis of spinal disorders. The aim of the current study was to quantify maximal effort trunk muscle extensor activity and trunk isometric extension torque over a functional range of sagittal standing postures. Twenty healthy, young adult male and female subjects performed isometric extension tasks over a sagittal posture range of -20 degrees extension to +50 degrees flexion, in 10 degrees increments. Erector spinae muscle activity was recorded bilaterally at the level of L3 using surface EMG electrodes. Isometric trunk extension torque was measured using a trunk dynamometer. EMG and trunk torque differed significantly between genders, but there were no differences between male and female subjects when the data were normalized with respect to the upright posture. For the combined male and female population, upright posture normalized L3 EMG activity (EMGn) and trunk extension torque (Tn) increased 1.7-fold and 3.5-fold, respectively, over the 70 degrees range of sagittal postures examined. The ratio (Tn/EMGn) increased two-fold (0.83 to 1.67) from -20 degrees extension to +50 degrees flexion, indicating that the neuromuscular efficiency increases with flexion. Trunk extension torque normalized with respect to the upright posture was linearly and positively correlated (r = 0.59, P < 0.001) to similarly normalized L3 EMG activity. This relatively weak correlation suggests that trunk muscle synergism and/or intrinsic muscle length-tension relationships are also modulated by posture. This study provides data that can be used to estimate trunk extensor muscle function over a broad range of sagittal postures. Our findings indicate that appropriate postural normalization of trunk extensor EMG activity is necessary for studies where submaximal trunk exertions are performed over a range of upright postures.

Lumbar spine mechanical response to axial compression load in vivo

STUDY DESIGN

Lumbar spine kinematic response to a 1.0 body weight compressive load was measured in vivo by comparison of relaxed and loaded magnetic resonance image sets in the sagittal plane.

OBJECTIVES

To identify and measure acute response mechanisms of the lumbar spine during compression loading.

SUMMARY OF BACKGROUND DATA

The isolated ligamentous spine buckles under small loads (88 N); yet, the spine supports >10 times that load in daily activities. Mechanical function of the lumbar spine in vivo is not well understood, and only a few studies examined the spine during in vivo loading.

METHODS

Magnetic resonance imaging scans of subjects were taken while subjects were relaxed and while supporting a 1.0 body weight compressive load. Vertebral bodies and disc perimeters were digitized, and relative centroid positions were measured and compared between conditions. Lumbar rotation, bending, compression, and disc translation were determined. Two parameter ensembles were analyzed to describe mechanisms of "spine shrinkage" (decrease of projected spine length) and lumbosacral response.

RESULTS

All subjects underwent spine shrinkage (-3.9 +/- 1.2 mm) dominated by cumulative bending, except in three subjects where the rotation component dominated. Levels L2-L4 extended, while L5 flexed, and dL2 through dL4 translated anterior, while dL5 translated posterior. Significant segmental deformations were as follows: L3 extension (-3.3 +/- 3.1 degrees ), dL5 disc translation (-1.4 +/- 1.4 mm), and posterior sacral rotation (3.2 +/- 4.7 degrees ).

CONCLUSIONS

Spine shrinkage occurred mainly from spine bending and rotation, with only small contribution from spine compression (shortening along the spine curvature). Response pattern groupings indicated at least two unique subgroups, but the cause remains to be determined.

Muscle forces and spinal loads at C4/5 level during isometric voluntary efforts

PURPOSE

The goal of this study was to determine neck muscle forces and spinal loads that result from isometric muscle contractions.

METHODS

Electromyographic (EMG) activity of the neck musculature and a three-dimensional biomechanical model of the neck were used. The model was EMG-based and estimated muscle forces and spinal loads at the C4/5 level. EMG signals were collected from eight sites at the C4/5 level of the neck using Ag-AgCl surface electrodes from 10 adult male subjects. The subjects performed isometric contractions gradually developing to maximum efforts in flexion, extension, left lateral bending, and right lateral bending.

RESULTS

During maximum voluntary contraction (MVC) trials most muscles generated high levels of EMG signal during cervical rotation. The posterior surface of the neck (trapezius) was the only electrode site at which maximum activity EMG consistently occurred by the same method (rotation) in all subjects. Variations in the EMG patterns were observed in different experiments that produced overall neck moments of equal magnitudes. With these data the model computed variations in load distribution among the agonist muscles. Consistent also with EMG distributions, the model also computed co-contractions of antagonist muscles. The average (+/- SD) magnitudes of peak moments were 28.3 (+/- 3.3) Nm in extension, 17.7 (+/- 3.1) Nm in flexion, 16.9 (+/- 2.8) Nm in left lateral bending, and 17.0 (+/- 2.9) Nm in right lateral bending. The model predicted C4/5 joint compressive forces during peak moments were 1372 (+/- 140) N in extension, 1654 (+/- 308) N in flexion, 956 (+/- 169) N in left lateral bending, and 1065 (+/- 207) N in right lateral bending.

CONCLUSIONS

Results suggest that higher C4/5 joint loads than previously reported are possible during maximum isometric muscle contractions.

Injuries in the adolescent porcine spine exposed to mechanical compression

STUDY DESIGN

An experimental porcine study in which functional lumbar spinal units were tested in compression to failure. Biomechanical, radiographic, magnetic resonance imaging, and histological characteristics are described.

OBJECTIVES

To explain the different patterns of injury seen in adults and adolescents resulting from traumatic injury to the vertebrae and to explain the mechanism behind traumatic displacement of the ring apophysis seen in athletes.

SUMMARY OF BACKGROUND DATA

Recent investigations of the spine in adolescent who have sustained trauma have shown injuries to the growth zone, whereas studies of adults have shown injuries to the vertebral body. A higher frequency of abnormalities in the discs, the vertebral bodies, the endplates, and the ring apophyses has been demonstrated in athletes with high loads on the spine. There is controversy over the etiology of these changes.

METHODS

Twelve functional lumbar spinal units (vertebra-disc-vertebra) obtained from six young male pigs were tested in compression to failure. All units were examined with plain radiography and magnetic resonance imaging before and after compression. After the compression, histologic samples were taken from the injury site.

RESULTS

Identical traumatic changes were seen in all functional lumbar spinal units, i.e., fracture in the endplate through the growth zone posteriorly and displacement of the anulus fibrosus with a bony fragment at the point of insertion of the vertebra. The nucleus pulposus was ruptured and displaced through the fracture line in all cases. The injuries were not seen on radiographs but were detected on magnetic resonance images, as confirmed on microscopic and histologic examination.

CONCLUSION

This study shows that the weakest part of the lumbar spine of the juvenile pig, when compressed, is the growth zone and the junction between the point of insertion of the anulus fibrosus and the vertebra. This location of weakness may explain the high frequency of disc degeneration and "persisting apophysis" seen in the spine of athletes.

Torso loading via a harness method activates trunk muscles less than a hand loading method

There is a need for data comparing muscle activity under equivalent moments at the torso using different loading methods. In torso biomechanics, it is often necessary to apply moments to people and then measure or predict the muscle activity in response to the external load. Alternative methods exist to generate moments including applying forces by a chest harness and loads held in the hands. The limitation of previous research has been to use only one of the loading methods, generally harness loading, assuming it produces muscle activity equal to when loads are held in the hands. The objective of the this study was to test if equivalent moments at L3/L4 applied to subjects through two loading methods resulted in the same muscle activity. Subjects maintained a static posture while attempting a combination of flexion/extension, lateral bending, and torsion to counter the external loads applied via hand and harness loading methods. Hand and harness loading did not result in equal muscle activity as measured by electromyographic techniques. The left and right latissimus dorsi had increases in activity (16-25%) when loads were held with the hands instead of applied via the harness. Increased activity for hand loading was also found for the left and right rectus abdominis (13-17%) and left external oblique (24%). The left and right erector spinae and right external oblique exhibited the same activity for both loading methods. The current findings indicate that studies of torso loading using a shoulder harness to create torso moments may be lowering activity of some torso muscles.

A new magnetic resonance imaging analysis method for the measurement of disc height variations

STUDY DESIGN

A new magnetic resonance image analysis method is proposed which is based on the definition of the borders of the vertebral bodies adjacent to the intervertebral disc and their varying relationships. The reproducibility of this method (the so-called "centroid" method) was assessed by consecutive measurements. Its potential to depict diurnal disc height variations was studied using randomized groups of volunteers.

OBJECTIVE

To determine if magnetic resonance imaging can reliably measure disc height variations in the lumbar spine in vivo.

SUMMARY OF BACKGROUND DATA

A review of the literature indicates that noninvasive, accurate methods to study the effect of load on intervertebral discs in vivo are needed.

METHODS

The reproducibility of the centroid method was assessed in 10 healthy volunteers in 2 consecutive measurements and compared to a conventional method (mean anterior and posterior disc height). To investigate the potential for the depiction of diurnal disc height variations, 10 volunteers were randomized in a study group (1 measurement in the morning, 1 measurement in the evening) and a control group (2 consecutive measurements in the morning).

RESULTS

The centroid method allows the depiction of disc height variations as small as 0.85 mm with a 95% confidence (tolerance limits), whereas a conventional method needs variations of at least 1.66 mm. In the study (diurnal) group, the disc height decreased significantly (P < 0.0001) during the day (mean, -0.9 mm), while no variation (P < 0.8) was found in the control group.

CONCLUSIONS

These results indicate that the centroid method can reliably detect disc height variations in an experimental setting. The centroid method provides the potential for evaluations of the effects of various work places, work equipment, work tasks, and postures.

Loads in the spinal structures during lifting: development of a three-dimensional comprehensive biomechanical model

Epidemiological studies have shown that loads imposed on the human spine during daily living play a significant role in the onset of low back pain. The loads applied to the lumbar spine are shared by a number of structures: muscles; posterior elements, including facets and ligaments; and the disc of a ligamentous motion segment. In vivo, it is not practical to determine forces in these structures using experimental techniques. Biomechanical models, based on an optimization technique of electromyographic activities of the trunk muscles, have been proposed to predict forces in the load transmitting structures. The mathematical models reported in the literature are based on information collected from a wide variety of sources, of which the subject that takes part in the experiment is only one. The present study describes techniques developed in our laboratory to collect from the subjects themselves all the data needed for the formulation of a biomechanical model. The results demonstrated that back lifting with 0 N (no load), 90 N, and 180 N in the hands created maximum external flexion moments respectively of 109.6 Nm, 137.9 Nm, and 161.7 Nm, at the L3-4 disc level. The corresponding external axial compression forces on the disc were 469.5 N, 511.8 N, and 601.5 N. The predicted disc compression varied from 3.4 to 5.0 times the body weight. In comparison to the static lifting mode, the dynamic lifting task caused an increase in the disc compression force ranging from 15.8% to 39.4% depending on the load being lifted (e.g., 3256 N for the dynamic mode vs. 2516 N for the static mode when the subject lifted 90 N). The salient features of the entire protocol developed by the authors and the need for further improvements are also presented.

Flexion relaxation during lifting: implications for torque production by muscle activity and tissue strain at the lumbo-sacral joint

During the full flexion phase of the back lift movement the lumbar part of the erector spinae muscle exhibits a reduced activity level (flexion relaxation). This study addresses the question how the required extension torque in the lumbo-sacral joint (L5/S1 joint) is balanced during the period in which apparently the lumbar erector spinae ceases to take its share. Six subjects participated in the experiment in which they performed seven lifting tasks. The load, the range of movement, and the phase in which the load was handled (lifting or lowering) were varied. A dynamic linked segment model was applied to determine the momentary torques acting at the L5/S1 joint, while the EMGs of the lumbar and thoracic part of the erector spinae muscle were measured. Furthermore, the lengths between markers on the lumbar and thoracic part of the trunk were determined to reveal changes in length during the movement. The dynamic EMGs were normalized to trunk angle-dependent maximal levels. The L5/S1 joint torques were analysed and combined with the normalized EMG data and the kinematics of the trunk, which are assumed to indicate the elongation of passive tissues. Although in the normalization procedure the change of the length-force relationship of the erector spinae was taken into account, the dynamic lumbar EMG activity decreased to a low-activity level (the phenomenon of flexion relaxation). This coincided with a 25% increase in lumbar length suggesting that passive tissue strain provided part of the required extension torque. In the tasks where a barbell was handled a significant increase in EMG level of the thoracic part of the erector spinae occurred just before the flexion relaxation at the lumbar level. Apparently, the extensor function of the lumbar part is then taken over by the thoracic part of the erector spinae muscle. This suggests that an intricate coordinating mechanism is operative that apportions the load to be balanced over active--(lumbar and thoracic part of the erector spinae) and passive structures (post vertebral ligaments).

Trunk muscle cocontraction: the effects of moment direction and moment magnitude

This study investigated the cocontraction of eight trunk muscles during the application of asymmetric loads to the torso. External moments of 10, 20, 30, 40, and 50 Nm were applied to the torso via a harness system. The direction of the applied moment was varied by 30 degrees increments to the subjects' right side between the sagittally symmetric orientations front and rear. Electromyographic (EMG) data from the left and right latissimus dorsi, erector spinae, external oblique, and rectus abdominus were collected from 10 subjects. The normalized EMG data were tested using multivariate and univariate analyses of variance procedures. These analyses showed significant interactions between the moment magnitude and the moment direction for seven of the eight muscles. Most of the interactions could be characterized as due to changes in muscle recruitment with changes in the direction of the external moment. Analysis of the relative activation levels, which were computed for each combination of moment magnitude and direction, indicated large changes in muscle recruitment due to asymmetry, but only small adjustments in the relative activation levels due to increased moment magnitude.

Value of biomechanical macromodels as suitable tools for the prevention of work-related low back problems

Biomechanical macromodels are evaluated with respect to their possible usefulness for health professionals and ergonomists, as well as for applied research on the prevention of low back problems. It is concluded that in the context stated geometrically simple models, in particular the model by Schultz and co-workers, are to be favoured over more complex models. However, load predictions in extreme trunk postures should be dealt with carefully. It is recommended that the model load predictions should be used only in the comparison of work situations and not for an assessment of the absolute acceptability of a work situation. Low back problems are related to mechanical (over)load at work. This study shows the pros and cons of various biomechanical macromodels as tools for health professionals and ergonomists, as well as for applied research on the prevention of work-related low back problems.

Isokinetic trunk strength and lumbosacral range of motion in elite female field hockey players reporting low back pain

This study was funded, in part, by a Small Faculties Grant from the University of Alberta. Physical therapists have reported an increased incidence of low back pain (LBP) in female field hockey (FH) players, commonly accompanied by decreased trunk range of motion (ROM) and strength. The purpose of this study was to compare lumbosacral sagittal ROM and isokinetic trunk strength in three groups of women: 1) FH athletes with a history of chronic LBP, 2) pain-free FH athletes, and 3) an age-matched, healthy nonathletic control group. Photographs (35 mm) of subjects wearing spinal motion markers were used to determine the limits of lumbosacral saggital ROM in standing. Eccentric and concentric isokinetic trunk flexion and extension torques were measured in sitting through 60 degrees of trunk movement using a Kin-Com dynamometer set at 60 degrees /sec. The ANOVA showed that the pain group had 12 degrees and 18 degrees less extension (p < .01), as well as 18 degrees and 24 degrees less total ROM (p < .001) than the pain-free and control groups, respectively. Only peak (p < .001) and average (p < .03) eccentric extension torques were weaker in the pain group than in the nonathletic control group. These results suggest that physical therapists should perform preseason screening of trunk strength and lumbosacral ROM. In-season trunk extension stretching and strengthening is needed in the training regimes of these athletes. J Orthop Sports Phys Ther 1992;16(3):129-135.

Influence of posture on the relation between surface electromyogram amplitude and back muscle moment: consequences for the use of surface electromyogram to measure back load

The aim of the study was to analyse the effect of posture on the relation between EMG amplitude and moment of the back muscles in different subjects, in order to gain a better insight into the possibilities of EMG as a means of measuring individual back load. Eight healthy subjects participated in the experiments. Isometric back extensions were performed in three postures: upright standing, standing with the spine flexed, and upright sitting. In each posture the lumbar moments of three maximal voluntary contractions were measured and then exertions of 10 to 90% of maximal voluntary contraction (increments of 10) were performed. EMG signals from the back muscles were recorded with four pairs of surface electrodes located 3 cm and 6 cm lateral to the midline of the spine at the L3 level. EMG signals were full-wave rectified and averaged over 2 s intervals of constant moment. The results show that posture has a considerable influence on the relationship between EMG and the lumbar moment. Besides this, large individual differences and an influence of electrode position on the relationship were found. Therefore it is concluded that whenever EMG is to be used as a means to measure individual back load, a calibration of the EMG amplitude to lumbar moment ratio is necessary for each subject, each electrode position, and each posture. Interpretation of EMG amplitudes has to be done on an individual basis and taking influences of posture and electrode location on the EMG-lumbar moment relationship into account.

Mineral content and strength of lumbar vertebrae. A cadaver study

Fifty-two cadaveric spine-motion segments were tested in compression alone and in combined compression-flexion to determine whether the compressive strength of lumbar vertebrae varied with the direction of the applied load, that is, whether similar relationships existed between the compressive strength and the amount of bone mineral depending on the direction of the loading. The bone mineral content (BMC) ranged between 1.6 and 5.8 g/cm and the ultimate strength between 810 and 10,090 N. The BMC of the motion segments was correlated with their strength irrespective of degree of flexion during testing (0-15 degrees). For compression-flexion within physiologic limits, the first part of the motion segment to fail was, with few exceptions, the end plate and the adjacent spongy bone.

Loads on the lumbar trunk during level walking

The goal of this study was to estimate the loads internal to the lumbar trunk that arise during level walking. To do this, (a) trunk muscle activities were calibrated in terms of muscle contraction force levels in a set of isometric exertions; (b) trunk muscle myoelectric activities were measured during level walking; and then (c) the muscle contraction forces that arose during walking were calculated from these measurements and calibrations. Lumbar trunk muscle myoelectric activities were quantified in 10 healthy young males. Myoelectric activities were monitored using eight bipolar surface electrode pairs placed around the trunk at the level of the third lumbar vertebrae. The subjects first performed four static weight-resisting tasks to calibrate muscle force/activity relationships. They then traversed a 8.25 m walkway three times each at cadences of 72 and 120 steps/min. A biomechanical model incorporating 22 lumbar trunk muscles was used to predict muscle contraction forces for the calibration tasks. Predicted forces were linearly correlated with the measured myoelectric activities for these tasks. The regression equations were then interpolated to estimate the muscle contraction forces from the myoelectric data during gait. Peak contraction forces for the iliocostalis muscles were calculated to be approximately 55 N per side, corresponding to total erector spinae peak contractions on the order of 140 N per side. For the other six muscles that were monitored, contraction forces were less than 15 N per side. This suggests that peak net reaction moments and peak spine compressions on the lumbar trunk during these walking tasks were on the order of 15 Nm and 1.2 times body weight, respectively.

Analysis of human torso motion with muscle actuators

A biomechanical model is devised to analyze the motion of the human torso and estimate the load on the lumbar spine, the contraction forces in the trunk muscles, and the sensory signals between the muscle and the nervous system. A state space formulation of three-dimensional (3D) equations of motion of the human torso is presented with muscle forces as input to the system. At least 3 pairs of skeletal muscles are considered to be necessary for 3D motion of human torso. Functional anatomy of these major muscles of the human trunk are discussed. These muscles as well as their feedback and feedforward sensory paths are modeled by linear viscoelastic components and force generators. Stability of the torso with three pairs of muscles is studied both with muscle spindles inactive (open loop) and active (closed loop). Voluntary point-to-point motion of the torso in three-dimensional space is simulated on a digital computer, employing a dynamic controller, where feedback gains are tuned (programmed) by higher centers of nervous system.

Relaxation phenomenon in lumbar trunk muscles during lateral bending

This paper reports myoelectric activity measurements in the lumbar trunk muscles when subjects performed tasks involving various degrees of lateral bending. Biomechanical model analyses were made to estimate the tensions in the lumbar trunk muscles required to perform those tasks. The tensions and the activity measurements were compared to see if a muscle relaxation phenomenon occurred. A relaxation phenomenon in the erector spinae muscles was observed to occur in quiet standing in a laterally-bent position of the trunk, qualitatively similar to the flexion-relaxation phenomenon reported by Schultz et al. in 1985(13). However, no relaxation was observed to occur in the lateral oblique abdominal muscles in laterally-bent postures of the trunk.

Comparison of acoustic and electrical signals from erectores spinae muscles

Comparison was made between simultaneous recordings of the electromyogram (EMG) and acoustic myogram (AMG) signals and the torque produced by the lumbar erectores spinae muscles of three healthy subjects while they performed isometric contractions. Repeatability of each signal was studied. The AMG had a relationship to the torque which was more quadratic than linear, whereas that of the EMG was more linear. The coefficient of variability of residuals about a "best-fit" line was 8.55% for torque-increasing EMG recordings and 19.2% for torque-increasing AMG recordings. At the same torque, the EMG was on average about 10% greater for increasing-torque recordings than for decreasing-torque parts of the recordings, but the magnitude of the AMG signal was about 10% less. With repeated testing on the same day, the coefficient of variability of the signal/torque relationship (torque-increasing) was 9% for EMG and 23% for AMG signals. With long-term testing over 2 weeks these coefficients were 23 and 39%, respectively. Correlation analysis showed that the two signal/torque relationships varied independently of each other.

How to start a back school

Back schools have been shown to be a care efficient and cost effective approach to the management of patients with low back pain, decreasing back symptoms, restricted working capacity, and additional medical care pursuits. This paper presents the indications, goals, and contents of a back school and instructs the reader in how to implement such a program in a physical therapy setting.J Orthop Sports Phys Ther 1988;10(1):1-7.

Influence of physical activity on the regulation of bone density

Using a mathematical model which relates bone density to daily stress histories, the influence of physical activities on the apparent density of the calcaneal cancellous bone was investigated. Assuming that the mechanical bone maintenance stimulus is constant for all bone tissue, bone apparent density was calculated by a linear superposition of the mechanical stimulus provided by different daily physical activities. An empirical weighting factor, m, accounted for possible differences in the relative importance of load magnitude and number of cycles in each activity. By considering hypothetical variations in body weight and occupational activity levels, the range of probable m values was established. The model was then applied to the results of two previous running studies in which calcaneal density was measured to obtain an estimate of the stress exponent parameter, m. The results indicate that stress magnitudes (or joint forces) have a greater influence on bone mass than the number of loading cycles. We demonstrate that by carefully considering the magnitudes of imposed skeletal forces and the number of loading cycles, it may be possible to design exercise programs to achieve predictable changes in bone mass.

Lumbar trunk muscle use in standing isometric heavy exertions

A study was conducted to see whether a biomechanical model previously validated for predicting the lumbar spine internal loads imposed by the performance of easy and moderately strenuous physical tasks was also adequate for predicting loads imposed by heavy exertions. Lumbar trunk muscle myoelectric activities were measured in 10 healthy young adult men performing a variety of less strenuous and more strenuous tasks while standing upright, and these were compared to the lumbar muscle contraction magnitudes predicted by the model. For the less strenuous tasks, measured activities and predicted forces showed strong linear correlations, confirming the validity of the model at those load levels. Model predictions for the more strenuous tasks were often found to be inadequate. Contrary to model-incorporated assumptions, substantial antagonistic muscle contractions sometimes occurred, intraabdominal pressurization may sometimes have contributed substantially to the maintenance of structural equilibrium, and the ligamentous tissues of the trunk seemed sometimes to develop substantial passive resistances to bending and twisting moments.

Spinal strain from shopping bags with and without handles

Electromyography has been used to study the effects on right and left lumbar and suprascapular muscle activity resulting from carrying groceries in shopping bags with handles (British) and without handles (American). The results showed that the load on these structures is lower and significantly more evenly distributed with the American bag ( [Formula: see text] ), but there is a group of carriers whose lumbar muscle activity is not imbalanced by the British bag. It is suggested that the design of the British shopping bag be reconsidered to enable it to be carried clutched to the body.