Recent advances in lumbar spinal mechanics and their clinical significance

Biomechanical analysis of reducing sacroiliac joint shear load by optimization of pelvic muscle and ligament forces

Effective stabilization of the sacroiliac joints (SIJ) is essential, since spinal loading is transferred via the SIJ to the coxal bones, and further to the legs. We performed a biomechanical analysis of SIJ stability in terms of reduced SIJ shear force in standing posture using a validated static 3-D simulation model. This model contained 100 muscle elements, 8 ligaments, and 8 joints in trunk, pelvis, and upper legs. Initially, the model was set up to minimize the maximum muscle stress. In this situation, the trunk load was mainly balanced between the coxal bones by vertical SIJ shear force. An imposed reduction of the vertical SIJ shear by 20% resulted in 70% increase of SIJ compression force due to activation of hip flexors and counteracting hip extensors. Another 20% reduction of the vertical SIJ shear force resulted in further increase of SIJ compression force by 400%, due to activation of the transversely oriented M. transversus abdominis and pelvic floor muscles. The M. transversus abdominis crosses the SIJ and clamps the sacrum between the coxal bones. Moreover, the pelvic floor muscles oppose lateral movement of the coxal bones, which stabilizes the position of the sacrum between the coxal bones (the pelvic arc). Our results suggest that training of the M. transversus abdominis and the pelvic floor muscles could help to relieve SI-joint related pelvic pain.

Spinal unloading after abdominal exercises

BACKGROUND

Intervertebral discs are exposed to compressive forces, which produces fluid loss. This loss decreases disc height, spinal length and consequently overall stature. The loss of stature has been associated with spine loading and low back pain. Abdominal exercises increase intra-abdominal pressure and unload the spine. The purpose of this study was to identify if abdominal exercises may produce acute spinal unloading compared to a known unloading position.

METHODS

Nine subjects performed a loading protocol and an unloading protocol on three experimental sessions. The loading protocol consisted of three sets of military press, while three unloading protocols were: three sets of regular abdominal exercises, three sets of abdominal exercises performed in an inclined board or an unloading resting posture.

FINDINGS

Abdominal exercises showed a greater recovery (mean (SD)) (regular=87.8 (20.4)%; inclined=70.1 (14.5)%) in comparison to the resting position (Fowler=33.6 (14.1)%), although no significant differences were found between abdominal exercises in stature recovery (P=0.07).

INTERPRETATION

Abdominal exercises may be introduced between sets of resistance training to decrease the negative effect of compressive forces imposed during such highly stressing activities. Further studies are required to confirm the proposed underlying mechanisms.

Further examination of modifying patient-preferred movement and alignment strategies in patients with low back pain during symptomatic tests

Our purpose was to examine the effect of modifying symptomatic movement and alignment tests in a sample of people with LBP referred to physical therapy. Fifty-one patients (19 males, 32 females; mean age 37+/-10.59 yr) with LBP and a mean Oswestry Disability Index score of 34+/-18% were examined. The examination included 28 primary tests in which patients used their preferred movement or alignment strategy and reported symptoms. Symptomatic tests were followed by a secondary test in which the patient's strategy was standardly modified to correct the spinal alignment or movement that occurred with the primary test. Symptoms and directions of movement or alignment modified were recorded. For 82% of the secondary tests, the majority of the patients' symptoms improved. For 54% of the secondary tests, some patients required modification of more than one direction of movement or alignment to eliminate symptoms. The findings suggest that the modifications described are generalizable across a number of tests with a moderately involved group of patients, and for individual tests there is variability in the numbers and directions of movements or alignments that appear to contribute to symptoms. Information obtained from the modifications is important because it can be used to confirm the patient's LBP classification and, within the context of the examination, immediately be used to teach the patient strategies to change movements and positions that appear to be contributing to his LBP.

Regional changes in spine posture at lift onset with changes in lift distance and lift style

STUDY DESIGN

Repeated measures experiment.

OBJECTIVE

To determine the effect of changes in horizontal lift distance on the amount of flexion, at lift onset, in different spine regions when using different lift styles.

SUMMARY OF BACKGROUND DATA

By approximating spine bending during lifting as a pure rotation about a single revolute joint, the differential effects of task constraints and instructions on motions of different spine levels will be obscured.

METHODS

Eight participants lifted a 10-kg crate from the floor, 10 times at each of five distances. Participants were instructed to use freestyle (a participant's preferred lift style), squat, or stoop lift styles. Kinematic data were collected from the mid thoracic spine, lower thoracic/upper lumbar spine, mid lumbar spine, and the lower lumbar spine at lift onset. A whole spine angle was also calculated.

RESULTS

Flexion of the lower lumbar spine was not affected by lift distance and style. Differences between lift styles occurred mainly in the mid thoracic and the lower thoracic/upper lumbar regions. With increasing horizontal distance, changes in lift style occurred in the upper three spine regions.

CONCLUSIONS

These results suggest that the tensile strain on tissues in the lower lumbar spine, which can be a cause of injury in lifting, was not affected by lift style or horizontal lift distance when lifting from floor level.

Three-dimensional motion analysis of the lumbar spine during "free squat" weight lift training

BACKGROUND

Heavy weight lifting using a squat bar is a commonly used athletic training exercise. Previous in vivo motion studies have concentrated on lifting of everyday objects and not on the vastly increased loads that athletes subject themselves to when performing this exercise.

HYPOTHESIS

Athletes significantly alter their lumbar spinal motion when performing squat lifting at heavy weights.

STUDY DESIGN

Controlled laboratory study.

METHODS

Forty-eight athletes (28 men, 20 women) performed 6 lifts at 40% maximum, 4 lifts at 60% maximum, and 2 lifts at 80% maximum. The Zebris 3D motion analysis system was used to measure lumbar spine motion. Exercise was performed as a "free" squat and repeated with a weight lifting support belt. Data obtained were analyzed using SAS.

RESULTS

A significant decrease (P < .05) was seen in flexion in all groups studied when lifting at 40% maximum compared with lifting at 60% and 80% of maximum lift. Flexion from calibrated 0 point ranged from 24.7 degrees (40% group) to 6.8 degrees (80% group). A significant increase (P < .05) was seen in extension when lifting at 40% maximum was compared with lifting at 60% and 80% maximum lift. Extension from calibrated 0 point ranged from -1.5 degrees (40% group) to -20.3 degrees (80% group). No statistically significant difference was found between motion seen when exercise was performed as a free squat or when lifting using a support belt in any of the groups studied.

CONCLUSION

Weight lifting using a squat bar causes athletes to significantly hyperextend their lumbar spines at heavier weights. The use of a weight lifting support belt does not significantly alter spinal motion during lifting.

Three-dimensional kinetics of the lumbar spine and hips in low back pain patients during sit-to-stand and stand-to-sit

STUDY DESIGN

Experimental, biomechanical study to determine the kinetics of the lumbar spine and hips during sit-to-stand and stand-to-sit.

OBJECTIVE

To investigate the effects of back pain, with and without limitation in straight leg raise, on the joint moment and power of the lumbar and hips during sit-to-stand and stand-to-sit.

SUMMARY OF BACKGROUND DATA

Movements of the lumbar spine and hips, and their coordination have been reported to be affected by the presence of low back pain (LBP), especially in those with a positive straight leg raise. However, the literature has no information concerning moment and power characteristics of the lumbar spine and hips during sit-to-stand and stand-to-sit in such patients.

METHODS

Twenty asymptomatic subjects, 20 LBP patients, and 20 patients with LBP and a positive straight leg raise sign were requested to perform the sit-to-stand and stand-to-sit activities. Electromagnetic sensors were attached to the body segments to measure their kinematics while 2 nonconductive force plates gathered ground reaction force data. Biomechanical models were used to determine the muscle moments and power at the lumbosacral (L5/S1) joint and hips.

RESULTS

Muscle moments acting at the lumbar spine and hip in the sagittal plane were found to decrease in subjects with LBP, but there were significant increases in moments in other planes of motion. The power patterns of the spine and hips were also significantly altered, particularly in subjects with a positive straight leg raise sign.

CONCLUSIONS

Back pain subjects exhibit compensatory movements and altered load sharing strategies during the sit-to-stand and stand-to-sit activities. Exercise therapy should take account of these changes so that the normal kinematic and kinetic characteristics of the spine and hips can be restored.

A new framework for evaluating potential risk of back disorders due to whole body vibration and repeated mechanical shock

A number of studies have examined the potential relationship between exposure to occupational vibration and low back pain associated with operation of vehicles. Only a handful of studies, however, have attempted to differentiate between the relative contributions of the steady state and transient mechanical shock components (the latter also being known as 'jarring and jolting', 'high acceleration event', 'multiple shocks' and 'impact') of the vibration exposure. The primary objective of this paper is to present a review of current studies that examine mechanical shock, present a case for the importance of evaluating both steady state and mechanical shock components and propose a new framework for evaluating the health effects due to occupational vibration exposure. A computerized bibliographical search of several databases was performed with special reference to the health effects of mechanical shock in relation to lower back disorders. Based on the analysis, eight experimental studies and nine epidemiological studies with relevance to exposure to 'mechanical shock' were identified. These studies suggested that rough vehicle rides are prevalent and that repeated exposure to mechanical shock may increase the risk of lower back pain. There is an urgent need for assessing the health effects of mechanical shocks in epidemiological studies. In particular, the new ISO 2631-5: International Organization for Standardization 2004 standard for shock exposure assessment should be evaluated with regard to musculoskeletal health effects.

Physical capacity in relation to low back, neck, or shoulder pain in a working population

AIMS

To investigate the longitudinal relation between physical capacity (isokinetic lifting strength, static endurance of the back, neck, and shoulder muscles, and mobility of the spine) and low back, neck, and shoulder pain.

METHODS

In this prospective cohort study, 1789 Dutch workers participated. At baseline, isokinetic lifting strength, static endurance of the back, neck, and shoulder muscles, and mobility of the spine were measured in the pain free workers, as well as potential confounders, including physical workload. Low back, neck, and shoulder pain were self-reported annually at baseline and three times during follow up.

RESULTS

After adjustment for confounders, Poisson generalised estimation equations showed an increased risk of low back pain among workers in the lowest sex specific tertile of performance in the static back endurance tests compared to workers in the reference category (RR = 1.42; 95% CI 1.19 to 1.71), but this was not found for isokinetic trunk lifting strength or mobility of the spine. An increased risk of neck pain was shown for workers with low performance in tests of isokinetic neck/shoulder lifting strength (RR = 1.31; 95% CI 1.03 to 1.67) and static neck endurance (RR = 1.22; 95% CI 1.00 to 1.49). Among workers in the lowest tertiles of isokinetic neck/shoulder lifting strength or endurance of the shoulder muscles, no increased risk of shoulder pain was found.

CONCLUSIONS

The findings of this study suggest that low back or neck endurance were independent predictors of low back or neck pain, respectively, and that low lifting neck/shoulder strength was an independent predictor of neck pain. No association was found between lifting trunk strength, or mobility of the spine and the risk of low back pain, nor between lifting neck/shoulder strength or endurance of the shoulder muscles and the risk of shoulder pain.

Sitting on a chair or an exercise ball: various perspectives to guide decision making

BACKGROUND

Prolonged sitting is recognized as a risk factor for the reporting of low back troubles. Despite the use of exercise balls in replacement of the office chair, little quantitative evidence exists to support this practice and hence motivated this research. Given the potential for several biological effects and mechanisms this study was approached with several layers of instrumentation to quantify differences in muscle activation, spine posture, spine compression and stability while sitting on an exercise ball versus a stable seat surface. Also, differences in the pressure distribution at the seat-user interface were quantified for the different seat surfaces to provide an objective perspective on the mechanism influencing perceived comfort levels.

METHODS

Eight male subjects volunteered to sit for 30 min on an exercise ball and on a wooden stool. Muscle activity and spine position were used to model spine load and stability. An additional seven sat on an exercise ball and chair to examine pressure distribution over the contact area.

FINDINGS

There was no difference in muscle activation profiles of each of the 14 muscles between sitting on the stool and ball. Calculated stability and compression values showed sitting on the ball made no difference in mean response values. The contact area of the seat-user interface was greatest on the exercise ball.

INTERPRETATION

The results of this study suggest that prolonged sitting on a dynamic, unstable seat surface does not significantly affect the magnitudes of muscle activation, spine posture, spine loads or overall spine stability. Sitting on a ball appears to spread out the contact area possibly resulting in uncomfortable soft tissue compression perhaps explaining the reported discomfort.

An exploratory study of loading and morphometric factors associated with specific failure modes in fatigue testing of lumbar motion segments

BACKGROUND

There is currently little information regarding factors associated with specific modes of motion segment failure using a fatigue failure model.

METHODS

Thirty-six human lumbar motion segments were fatigue tested using spinal compressive and shear loads that simulated lifting a 9 kg weight in three torso flexion angles (0 degrees, 22.5 degrees, and 45 degrees). Twenty-five segments failed via fatigue prior to the 10,000 cycle maximum. These specimens were visually inspected and dissected so that the mode(s) of failure could be determined. Failure modes included endplate fractures (classified into nine varieties), vertebral body fractures, and/or zygapophysial joint disruption. Logistic regression analyses were performed to determine whether certain morphometric variables, amount of motion segment flexion, disk degeneration scores, and/or loading characteristics were associated with the occurrence of specific failure modes.

FINDINGS

Results indicated that stellate endplate fractures were associated with increased posterior shear forces (P < 0.05) and less degenerated discs (P < 0.01). Fractures running laterally across the endplate were associated with motion segments having larger volumes (P < 0.01). Endplate depression was more common in smaller specimens (P < 0.01), as well as those experiencing increased posterior shear force (P < 0.05). Zygapophysial joint damage was more likely to occur in a neutral posture (P < 0.01).

INTERPRETATION

These results suggest that prediction of failure modes (e.g., specific endplate fracture patterns) may be possible (at least for older specimens) given knowledge of the spinal loads along with certain characteristics of the lumbar spine.

Changes in stature and spine kinematics during a loaded walking task

The aim of the present study was to quantify the kinematics of the spine and stature loss induced by the asymmetric load carriage. Six healthy males with no history of low-back disorders walked at their self-selected pace for 8,500 m with and without a standard Royal Mail bag (model MB36) containing 17.5% of the participant's body mass. The load was reduced gradually during the task. The loaded condition produced a stature loss double that observed in the unloaded condition. Increased forward leaning (up to 6 degrees ) and lateral bending of the spine (up to 12 degrees ) was observed with load. Thoracic adjustments occurred in the sagittal plane, changes in the lumbar area occurred in the frontal plane. The data provided evidence against mailbags designs in which the workers cannot alternate the side of the mailbag.

Movements of the pelvis and lumbar spine during walking in people with acute low back pain

BACKGROUND AND PURPOSE

Little is known about how acute low back pain affects pelvic and lumbar movements during walking. The aim of the present study was to determine if measurement of the amplitude of the angular movements of the pelvis and lumbar spine during walking is useful in the evaluation of people with acute low back pain.

METHOD

The study used a repeated-measures and correlational design; 11 individuals with low back pain (tested in the acute phase and six weeks later when symptoms had resolved) and matched control subjects were tested during treadmill walking. A video-analysis system was used to measure the amplitudes of movements of the pelvis and lumbar spine during walking. Pain level was measured with a visual analogue scale (VAS).

RESULTS

During walking, movements of the pelvis (axial rotation) and lumbar spine (lateral flexion) were reduced in people with acute back pain compared with the resolved condition, but were not different from a group without a history of back pain. The amplitudes of the frontal plane movements of the pelvis and lumbar spine were negatively correlated with the intensity of pain (r(s) = -0.74).

CONCLUSIONS

Measurement of pelvic and lumbar movements during walking is unlikely to have useful clinical applications for individuals, or when discriminating between impaired and unimpaired people, but may be applied to groups for hypothesis testing in evaluating change in back pain over time. An hypothesized model to explain the observed movements has been proposed.

The influence of static axial torque in combined loading on intervertebral joint failure mechanics using a porcine model

BACKGROUND

The spine is routinely subjected to repetitive combined loading, including axial torque. Repetitive flexion-extension motions with low magnitude compressive forces have been shown to be an effective mechanism for causing disc herniations. The addition of axial torque to the efficacy of failure mechanisms, such as disc herniation, need to be quantified. The purpose of this study was to determine the role of static axial torque on the failure mechanics of the intervertebral joint under repetitive combined loading.

METHODS

Repetitive flexion-extension motions combined with 1472 N of compression were applied to two groups of nine porcine motion segments. Five Nm of axial torque was applied to one group. Load-displacement behaviour was quantified, and planar radiography was used to document tracking of the nucleus pulposus and to identify fractures.

FINDINGS

The occurrence of facet fractures was found to be higher (P=0.028) in the axial torque group (7/9), compared to the no axial torque group (2/9). More hysteresis energy was lost up to 3000 cycles of loading in the axial torque group (P<0.014). The flexion-extension cycle stiffness was not different between the two groups until 4000 cycles of loading, after which the axial torque group stiffness increased (P=0.016). The percentage of specimens that herniated after 3000 cycles of loading was significantly larger (P=0.049) for the axial torque group (71%) compared to the no axial torque group (29%).

INTERPRETATION

Small magnitudes of static axial torque alter the failure mechanics of the intervertebral disc and vertebrae in combined loading situations. Axial torque appears to accelerate the susceptibility for injury to the intervertebral joint complex. This suggests tasks involving axial torque with other types of loading, apart from axial twist motion, should be monitored to assess exposure and injury risk.

Torso flexion loads and the fatigue failure of human lumbosacral motion segments

STUDY DESIGN

Spine loads associated with lifting a 9-kg weight were estimated at three torso flexion angles (0 degrees, 22.5 degrees, and 45 degrees), and lumbosacral motion segments were cyclically loaded using these loads until failure or to a maximum of 10,020 cycles.

OBJECTIVES

To simulate the postures and loads experienced by the lumbar spine during repetitive lifting of moderate weights in different torso flexion postures, and to analyze the fatigue failure response of lumbosacral motion segments.

SUMMARY OF BACKGROUND DATA

Previous fatigue failure studies of lumbar motion segments have not reproduced the combination of spinal postures, loads, and load rates anticipated in different torso flexion postures during lifting tasks characteristic of those in occupational settings.

METHODS

Twelve fresh human lumbosacral spines were dissected into three motion segments each (L1-L2, L3-L4, and L5-S1). Motion segments within each spine were randomly assigned to a simulated torso flexion angle (0 degrees, 22.5 degrees, or 45 degrees) using a partially balanced incomplete block experimental design. Spinal load and load rate were determined for each torso flexion angle using previously collected data from an EMG-assisted biomechanical model. Motion segments were creep loaded for 15 minutes, then cyclically loaded at 0.33 Hz. Fatigue life was taken as the number of cycles to failure (10 mm displacement after creep loading). Specimens were inspected to determine failure mechanisms.

RESULTS

The degree of torso flexion had a dramatic impact on cycles to failure. Motion segments experiencing the 0 degrees torso flexion condition averaged 8,253 cycles to failure (+/-2,895), while the 22.5 degrees torso flexion angle averaged 3,257 (+/-4,443) cycles to failure, and motion segments at the 45 degrees torso flexion angle lasted only 263 cycles (+/-646), on average. The difference was significant at P < 0.0001, and torso flexion accounted for 50% of the total variance in cycles to failure.

CONCLUSIONS

Fatigue failure of spinal tissues can occur rapidly when the torso is fully flexed during occupational lifting tasks; however, many thousands of cycles can be tolerated in a neutral posture. Future lifting recommendations should be sensitive to rapid development of fatigue failure in torso flexion.

Body mass as a factor in stature change

BACKGROUND

Back pain is a common condition which has been described as a serious public health problem. Spinal shrinkage has been used as an index of spinal loading in a range of tasks. Epidemiological evidence shows that body mass index (BMI: 30 kg/m(2)) is related to the development of low back pain however, no studies have described the stature change patterns of obese individuals. This study aimed to compare changes in stature after an exercise task in obese and non-obese individuals.

METHODS

Twenty volunteers were divided into two equal groups; obese: BMI>30 kg/m(2), non-obese: BMI<25 kg/m(2). Stature was measured at 3 min intervals during a 30 min walking task and a 30 min standing recovery period. Tests were performed on two occasions, once with participants loaded during the walking task (10% body mass) and once unloaded. The influence of obesity and load condition on the magnitude and rate of stature change were compared by a two-way ANOVA:

FINDINGS

In both groups the stature loss was greater in the loaded than unloaded condition (mean (SD)) (6.52 (1.45)mm and 3.55 (0.93)mm non-obese; 8.49 (1.75)mm and 7.02 (1.32)mm obese: P=0.016). The obese presented a greater reduction in stature in both task conditions. The obese group were unable to recover stature regardless of the task condition during the recovery period (loaded: 0.06 (0.3)mm; unloaded: 0.32 (0.6)mm; P=0.013).

INTERPRETATION

It was concluded that the acute response of the spine to loading may represent a risk factor for low back pain in the obese, in addition to the chronic adaptations previously reported. A greater period of recovery may be necessary for obese individuals to re-establish intervertebral disc height. These findings may help to explain the high incidence of back disorders in obese individuals.

Repeatability of stature measurements in individuals with and without chronic low-back pain

Measurements of reduction in stature have been used to compare spinal loading in chronic low-back pain (CLBP) and asymptomatic populations. Whether there are any differences in the repeatability of stature measurements, between those with and without CLBP, is not known. This investigation aimed to determine the repeatability of stature measurements in those with (n = 12) and without (n = 12) CLBP, and to establish if the ability to produce repeatable measurements is retained after a specific timeframe. Stature measurements were taken on two separate sessions that were 2 weeks apart, using a stadiometer accurate to 0.01 mm. All participants attained a mean SD of < or = 0.5 mm by the third measurement set taken on the first session of testing and no significant difference in mean SD was found between those with (0.37 mm) and without (0.40 mm) CLBP (p > 0.05). Intraclass correlation coefficients (ICC) demonstrated good levels of repeatability for all stature measurements obtained from the participants and the values for Standard error of the measurement (SEM) improved as the mean SD decreased with each measurement set. Investigators should have confidence in the ability of those with and without CLBP to produce equally repeatable stature measurements with appropriate prior practice. The second session of testing demonstrated that both groups had retained the ability to achieve the desired level of repeatability (SD < or = 0.5 mm) 2 weeks later without further practice.

Flow-related mechanics of the intervertebral disc: the validity of an in vitro model

STUDY DESIGN

An in vitro mechanical study on porcine motion segments.

OBJECTIVES

To test the validity of in vitro studies of the flow-related mechanics of the intervertebral disc and, in particular, to investigate whether fluid flows back into the disc during unloading after a loading cycle.

SUMMARY OF BACKGROUND DATA

In vivo studies show both the inflow and outflow of fluid in the intervertebral disc. The resistance to flow out of the disc is higher than to inflow. The fluid flow is regulated via unbalance between the external load and the osmotic pressure of the nucleus pulposus.

MATERIALS

There were 8 porcine lumbar motion segments (without posterior elements) and 8 isolated discs tested in a physiologic saline bath (39 degrees C). The specimens were preloaded at 0.025 MPa for 15 minutes. Three 15-minute loading periods at 2.0 MPa were applied, each followed by an unloading period of 30 minutes. Loads, axial displacements, and nucleus pressure were recorded online.

RESULTS

Over the 3 loading and unloading periods, all specimens showed a net loss of height and mass. The time series of specimen height during the 3 unloading periods showed virtually identical responses. The pressure in the nucleus decreased in the subsequent loading periods and showed no increase during unloading.

CONCLUSION

The data show the limitations of an in vitro model for studying fluid flow-related intervertebral disc mechanics. During loading, outflow of fluid occurred, but inflow appears to be virtually absent during unloading. Poro-elastic behavior cannot be reproduced in an in vitro model.

Serum markers of collagen metabolism: construction workers compared to sedentary workers

BACKGROUND

Evaluation of causal relations between physical load and musculoskeletal disorders is hampered by the lack of knowledge as to the biological relevance of different loading parameters and the large variability between individuals. As indicators of molecular changes in the extracellular matrices of structures of the musculoskeletal system, biomarkers of collagen metabolism may provide important information on biological effects of physical load. The carboxyterminal propeptide of type I collagen (PICP) is a serum marker of synthesis and the carboxyterminal telopeptide region of type I collagen (CTx) reflects degradation of type I collagen.

AIMS

To explore the feasibility of biomarkers of type I collagen metabolism as measures of the effects of physical load at tissue level.

METHODS

Serum concentrations of PICP and CTx were assessed in a group of male construction workers involved in heavy manual materials handling (n = 47) and in a group of male sedentary workers (n = 49).

RESULTS

Serum concentrations of both PICP and CTx seemed to be related to heavy physical work. The ratio PICP/CTx, illustrative of the effective metabolic changes, did not differ between the two groups.

CONCLUSIONS

The higher turnover rate but similar effective synthesis may be indicative of an increased type I collagen content in the connective tissues as a result of adaptive remodelling in response to years of exposure to physical load. Further validation of these biomarkers is required with respect to dose-response relations and temporal associations between exposure to back load and biomarker concentrations.

Raised paraspinal muscle activity reduces rate of stature recovery after loaded exercise in individuals with chronic low back pain

OBJECTIVES

To further the understanding of stature recovery in subjects with and without chronic low back pain (CLBP) and to determine the relations among stature recovery, paraspinal muscle activity, and perceived pain and disability.

DESIGN

A case-control study in which stature loss and recovery were assessed in subjects with and without CLBP after a 20-minute loaded walking task (10% of body mass). Group differences in pain, disability, and paraspinal muscle activity were also assessed.

SETTING

A university laboratory.

PARTICIPANTS

Twenty subjects with CLBP were matched (age, sex, body mass, physical activity level) with 20 controls who were recruited through notices in the university, general community, and local Primary Care Trust.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Stature changes and integrated electromyograms of the paraspinal muscles during a loading and unloading period were assessed.

RESULTS

Stature changes after loading did not differ between groups ( P <.05). Subjects with CLBP recovered significantly less stature during unloading than did the controls ( P <.05). Paraspinal muscle activity correlated negatively with stature recovery ( P <.05). Relations among stature recovery, pain, and disability were shown ( P <.05).

CONCLUSIONS

The elevated paraspinal muscle activity exhibited by the CLBP group increased compression on the intervertebral disks and diminished their ability to recover the height lost through loaded exercise. Further research is required to establish whether a change in paraspinal muscle activity is associated with corresponding changes in stature recovery, and if this has any clinical implications by reducing pain and disability.

A diffusion and T2 relaxation MRI study of the ovine lumbar intervertebral disc under compression in vitro

The ovine lumbar intervertebral disc is a useful model for the human lumbar disc. We present preliminary estimates of diffusion coefficients and T2 relaxation times in a pilot MRI study of the ovine lumbar intervertebral disc during uniaxial compression in vitro, and identify factors that hamper the ability to accurately monitor the temporal evolution of the effective diffusion tensor at high spatial resolution.

Can acute low back pain result from segmental spinal buckling during sub-maximal activities? A review of the current literature

This paper provides a review of the current literature supporting the hypothesis that segmental spine buckling resulting in tissue damage may be a primary cause of sudden onset low back pain, even during activities that are sub-maximal with respect to loading and muscle activation. While a temporal link exists, it is supported primarily by anecdotal and clinical reports. More pertinent to this review is the biological plausibility of segmental spine buckling as a mechanism of acute injury, supported by modelling studies as well as current knowledge of tissue mechanics and neurophysiology. One antithesis, however, is the low incidence of low back injuries reported during sub-maximal tasks. In order to account for this discrepancy, several predisposing factors are addressed, both constant and situation-dependent, which may contribute to the occurrence of segmental spinal buckling during sub-maximal activities.

A finite element model of the L4–L5 spinal motion segment: biomechanical compatibility of an interspinous device

The biomechanical compatibility of an interspinous device, used for the "dynamic stabilization" of a diseased spinal motion segment, was investigated. The behaviour of an implant made of titanium based alloy (Ti6Al4V) and that of an implant made of a super-elastic alloy (Ni-Ti) have been compared. The assessment of the biomechanical compatibility was achieved by means of the finite element method, in which suitable constitutive laws have been adopted for the annulus fibrosus and for the metal alloys. The model was aimed at simulating the healthy, the nucleotomized and the treated L4-L5 lumbar segment, subjected to compressive force and flexion-extension as well as lateral flexion moments. The computational model has shown that both the implants were able to achieve their main design purpose, which is to diminish the forces acting on the apophyseal joints. Nevertheless, the Ni-Ti implant has shown a more physiological flexural stiffness with respect to the Ti6Al4V implant, which exhibited an excessive stiffness and permanent strains (plastic strains), even under physiological loads. The computational models presented in this paper seems to be a promising tool able to predict the effectiveness of a biomedical device and to select the materials to be used for the implant manufacturing, within an engineering approach to the clinical problem of the spinal diseases.

Stochastic lattice model for bone remodeling and aging

We investigate the remodeling process of trabecular bone inside a human vertebral body using a stochastic lattice model, in which the ability of living bone to adapt to mechanical stimuli is incorporated. Our simulations show the emergence of a networklike structure similar to real trabecular bone. With time, the bone volume fraction reaches a steady state. The microstructure, however, coarsens with a typical length in the system following a power law. The simulation results suggest that a coarsening of the trabecular structure should occur as a natural aging phenomenon, not related to disease.

The influence of a back support harness on spinal forces during sheep shearing

Previous research has classified the occupation of sheep shearing as heavy work where shearers flex their spine and hips for long periods of time, handle awkward loads and expend high amounts of energy. The aim of this research was to investigate the magnitude of spinal forces produced during the shearing phase of the work and to determine whether the use of a commercially available back support harness would reduce these forces. Following discussion on task complexity and risk of back injury with senior shearing instructors, three component tasks of the shearing phase were identified as posing high risk of injury and were prioritized for primary analysis. Although the dragging out of a sheep in preparation for shearing and an unexpected loss of animal control were also identified as being of high risk, technological and instrumentation difficulties precluded their analysis. Twelve experienced shearers were videotaped while shearing with and without the use of a back harness. Surface mounted retro-reflective markers placed on the trunk defined three linked segments: Pelvis, Lumbar and Head, Arms, and Upper Trunk (HAUT). A 3D, link segment, top down, inverse dynamics approach was used to describe the motion and to estimate forces involved during the identified tasks of shearing. The spinal force/time profiles of this sample of shearers demonstrated large compressive and shear forces for all three tasks that are close to the NIOSH and University of Waterloo action limits for compressive and shear forces respectively (McGill 1997, Yingling and McGill 1999, Marras 2000). The use of the back support harness reduced these forces by substantial and statistically significant amounts. This effect was consistent across all three tasks. The results of this study demonstrate the production of high levels of compressive and shear forces within the spine of shearers during the three shearing tasks studied and that the use of a back support harness can substantially reduce these forces. Therefore the use of a back harness may reduce the cumulative load on the spine during shearing thereby moderating damage to the articular structures. However it is not known whether the harness would protect the spine from a sudden or unexpected force.

Mechanically induced disruption of the healthy bovine intervertebral disc

STUDY DESIGN

An intact bovine caudal disc model was used to investigate how combinations of biomechanical parameters influence the severity of disruption during compressive loading.

OBJECTIVES

To quantify the combined influence of flex-ion, hydration level, and compressive loading rate on nuclear disruption.

SUMMARY OF BACKGROUND DATA

The risk of disc pro-lapse is known to increase when the disc is loaded flexed. However, there are few experimental data available quantifying the extent to which loading parameters might interact to produce disruption in the healthy disc.

METHODS

Reproducible states of full and partial hydration were established for 96 isolated caudal discs. These discs were then subjected to compression under combined conditions of high or low hydration, zero or full flexion, and moderate or low loading rate. The extent of disc disruption was assessed macroscopically using a damage weighting procedure.

RESULTS

Maximum disruption of the intact, healthy disc occurred under combined conditions of full hydration and flexion. Loading rate, whether at 0.004 MPa/sec or 4 MPa/sec, had little influence when applied without flexion, but there was an increased risk of disruption when the moderate rate was combined with full flexion.

CONCLUSIONS

The investigation demonstrates that significant levels of disruption can be induced by mechanical loading in intact discs free of degenerative change. Our findings support the hypothesis that mechanical injury to a healthy disc might initiate a process of degeneration.

The effects of repetitive motion on lumbar flexion and erector spinae muscle activity in rowers

OBJECTIVE

The purpose of this study was to investigate changes in lumbar flexion together with the pattern and level of muscle activity of selected erector spinae during a rowing trial.Design. Cross-sectional repeated measures design.

BACKGROUND

Low back pain is a common problem in rowers. The amount of lumbar flexion occurring during rowing might influence the possibility of injury.

METHODS

Sixteen young adult school rowers participated in the study. Changes in lumbar flexion and muscle activity were recorded across the drive phase, at three stages of an ergometer based rowing trial. Lumbar flexion was calculated by computerised motion analysis of surface markers attached to the spinous processes of L1 and S1. Surface electromyography techniques were used to examine the magnitude of activity from three erector spinae muscles. The median frequency of the electromyographic signal was examined to quantify fatigue in the erector spinae muscles during isometric maximal effort muscle activation prior to and after the rowing trial.

RESULTS

Lumbar flexion increased significantly (P<0.05) during the rowing trial, as did the magnitude of electromyographic activity from sites over the lumbar multifidus, iliocostalis lumborum and longissimus thoracis muscles. The median frequency decreased significantly (P<0.05) in each muscle examined.

CONCLUSIONS

The findings showed that rowers attain relatively high levels of lumbar flexion during the rowing stroke, and these levels are increased during the course of the rowing trial. Indirect evidence of muscle fatigue in erector spinae muscles was also apparent, and this observation may in part be responsible for the increased levels of lumbar flexion observed.

RELEVANCE

Excessive lumbar flexion may influence the potential for injury to spinal structures. An awareness of increased lumbar flexion and muscle fatigue in the erector spinae muscles may be important for injury prevention programs for rowers.

Muscle fatigue and fatigue-related biomechanical changes during a cyclic lifting task

STUDY DESIGN

Electromyographic and biomechanical methods were utilized to investigate correlations between indexes of localized muscle fatigue and changes in the kinematics and kinetics of motion during a cyclic lifting task.

SUMMARY OF BACKGROUND DATA

Recent advances in time-frequency analysis procedures for electromyographicic signal processing provide a new way of studying localized muscle fatigue during dynamic contractions. These methods provide a means to investigate fatigue-related functional impairments in patients with low back pain.

OBJECTIVES

To study the relationship between localized muscle fatigue and the biomechanics of lifting and lowering a weighted box. Fatigue-related changes in the electromyographicic signal of trunk and limb muscles were evaluated and compared to kinematic and kinetic measures in order to determine whether lifting strategy is modified with fatigue.

METHODS

A total of 14 healthy male subjects (26 +/- 5 years) cyclically lifted and lowered a 13 kg box (12 lifts/min) for 4.5 minutes. A 5-second static maximum lifting task was included immediately before and after the cyclic lifting task to measure changes in lifting strength and static electromyographicic fatigue indexes. Electromyographic signals from 14 muscle sites (including paravertebral and limb muscles) were measured. Changes in the electromyographicic Instantaneous Median Frequency, a fatigue index, were computed using time-frequency analysis methods. This index was compared with more standardized measures of fatigue, such as those based on electromyographicic median frequency acquired during a static trunk extension test, subjective fatigue measures, and maximal static lifting strength. Biomechanical measures were gathered using a motion analysis system to study kinematic and kinetic changes during the lifting task.

RESULTS

During the cyclic lifting task, the electromyographic Instantaneous Median Frequency significantly decreased over time in the paravertebral muscles, but not in the limb muscles. Paravertebral electromyographicic Instantaneous Median Frequency changes were consistent with self-reports of fatigue as well as decreases in trunk extension strength. The magnitude of muscle-specific changes in electromyographicic Instantaneous Median Frequency was not significantly correlated with electromyographicic median frequency changes from the static trunk extension task. The load of the box relative to the maximal static lifting strength significantly affected the electromyographicic Instantaneous Median Frequency changes of paravertebral back muscles. Significant changes with fatigue during the task were found in the angular displacements at the knee, hip, trunk, and elbow. These biomechanical changes were associated with increased peak torque and forces at the L4-L5 vertebral segment.

CONCLUSIONS

Our results demonstrate correlation between localized muscle fatigue and biomechanical adaptations that occur during a cyclic lifting task. This new technique may provide researchers and clinicians with a means to investigate fatigue-related effects of repetitive work tasks or assessment procedures that might be useful in improving education, lifting ergonomy, and back school programs. Although both the dynamic and static tasks resulted in spectral shifts in the electromyographicic data, the fact that these methods led to different muscle-specific findings indicates that they should not be considered as equivalent assessment procedures.

Spine ergonomics

Occupational low back pain (LBP) is an immense burden for both industry and medicine. Ergonomic and personal risk factors result in LBP, but psychosocial factors can influence LBP disability. Epidemiologic studies clearly indicate the role of mechanical loads on the etiology of occupational LBP. Occupational exposures such as lifting, particularly in awkward postures; heavy lifting; or repetitive lifting are related to LBP. Fixed postures and prolonged seating are also risk factors. LBP is found in both sedentary occupations and in drivers as well as those involved in manual materials handling. Any prolonged posture will lead to static loading of the soft tissues and cause discomfort. Standing and sitting have specific advantages and disadvantages for mobility, exertion of force, energy consumption, circulatory demands, coordination, and motion control. The seated posture leads to inactivity causing an accumulation of metabolites, accelerating disk degeneration and leading to disk herniation. Driver's postures can also lead to musculoskeletal problems. Workers in a driving environment are often subjected to postural stress leading to back, neck, and upper extremity pain. This exacerbates the problems due to the vibration. Prevention is by far the treatment of choice. Improved muscle function can be preventative. Poor coordination and motor control systems are as important as endurance and strength. Fixed postures should be avoided. Seats offering good lumbar support should be used in the office. A suspension seat should be used in vehicles whenever possible. Heavy and awkward lifting should be avoided and lifting aids should be made available. Workers should report LBP as early as possible and seek medical advice if they think occupational exposure is harming them. The combined effects of the medical community, labor, and management are required to cause some impact on this problem.

Spatial and temporal localization of transforming growth factor-beta, fibroblast growth factor-2, and osteonectin, and identification of cells expressing alpha-smooth muscle actin in the injured anulus fibrosus: implications for extracellular matrix repair

STUDY DESIGN

The spatial and temporal localization of fibroblast growth factor-2, transforming growth factor-beta, osteonectin, and alpha-smooth muscle cell actin in the injured anulus fibrosus was investigated.

OBJECTIVE

To assess the involvement of fibroblast growth factor-2, transforming growth factor-beta, osteonectin, and alpha-smooth muscle cell actin in anulus fibrosus repair.

SUMMARY OF BACKGROUND DATA

Fibroblast growth factor-2 and transforming growth factor-beta have been localized to disc herniation tissue, and alpha-smooth muscle cell actin has been identified in a number of mesenchymal cell types, but their roles have not been evaluated in repair processes in the experimentally injured anulus fibrosus.

METHODS

For this study, 32 two adult merinos received a 4-mm deep standard annular incision in their L1L2 and L3L4 discs (lesion group). A similar number of sham-surgery animals served as control subjects. Osteonectin, fibroblast growth factor-2, transforming growth factor-beta, and alpha-smooth muscle cell actin were immunolocalized in sagittal disc sections 3, 6, 12, and 26 months after the operation. Selected specimens also were stained with hematoxylin and eosin, Masson-trichrome, toluidine blue, and picrosirius red.

RESULTS

Early focal depletion of proteoglycan was evident in the anulus fibrosus and reorganization of outer annular lamellas 3 to 6 months after the operation. Blood vessel ingrowth and fibroblast infiltration from the outer anulus fibrosus along the plane of the annular defect were maximal 12 months after the operation. Focal upregulation in alpha-smooth muscle cell actin expression was evident with maximal staining in the 12-month lesion samples near infiltrating blood vessels at the lesion site, and also in cells well away from these vessels. Some of the anulus fibrosus cells of the sham sections also stained positively for alpha-smooth muscle cell actin, but this staining was significantly less than in the lesion samples. Staining for fibroblast growth factor-2, transforming growth factor-beta, and osteonectin was strongly localized to blood vessels and cells in the vicinity of the annular lesion. It was maximal 12 months after the operation and diminished by 26 months after the operation. Osteonectin expression also was significantly elevated in outer anulus fibrosus cells distant from the lesion site and its associated blood vessels. In the sham discs, immunoreactivity to fibroblast growth factor-2, transforming growth factor-beta, osteonectin, and alpha-smooth muscle cell actin was confined to sparsely distributed cells in the anulus fibrosus. No matrix staining was observed.

CONCLUSIONS

Immunoreactivity for the noted agents was strongly associated with regions of the annular lesions undergoing matrix reorganization consistent with an active repair response. This response extended as far as the middle third of the anulus fibrosus, which also demarcated the extent of blood vessel ingrowth and cellular infiltration in this model. The alpha-smooth muscle cell actin expression suggested an active involvement of myofibroblasts in the anulus fibrosus repair processes.

A biomechanical comparison of lifting techniques between subjects with and without chronic low back pain during freestyle lifting and lowering tasks

OBJECTIVE

To evaluate if chronic low back pain patients perform manual material handling tasks differently from control subjects.

DESIGN

Comparative study using a repeated measures design.

BACKGROUND

No study evaluated the lifting technique of back pain patients relative to control subjects during free style lifting and lowering tasks. Previous findings suggest that lowering would be more hazardous than lifting to the low back. It would be interesting to evaluate if chronic low back pain patients behave differently than controls when lifting and lowering.

METHODS

Thirty-three male subjects (18 controls, 15 suffering from non-specific chronic low back pain) participated. A 12-kg box was lifted (freestyle) from the floor to the hips (1) in front (symmetric task) or (2) to a shelf located at 90 degree on the right (asymmetric task) and was lowered back to the floor. A 3D biomechanical analysis involving the assessment of L5/S1 loading, posture of segments, inertial parameters, and EMG was performed.

RESULTS

There was no difference between the groups for postural (trunk and lower limb angles), inertial (trunk velocity and acceleration), and L5/S1 loading (moments and compression) variables. The patients showed abnormally low left lumbar erector spinae (symmetric task, lowering) or high left thoracic erector spinae (all tasks) EMG activation. Significant Group x Action (lifting vs. lowering) interactions were also observed for some inertial and L5/S1 loading variables suggesting that the biomechanical differences detected between lifting and lowering may have a differential influence on the technique used by back pain patients and control subjects.

CONCLUSIONS

The gross lifting technique of back pain patients was unaltered relative to controls but the activation of paraspinal muscles differed, suggesting that a more detailed biomechanical analysis, such as the use of EMG driven models, might be required to reveal lumbar impairments during lifting.

RELEVANCE

To evaluate if chronic low back pain patients use naturally different lifting techniques to prevent pain exacerbation and damaged lumbar tissue overloading.

Balance loss when lifting a heavier-than-expected load: effects of lifting technique

OBJECTIVES

To compare the lifting techniques of subjects who did and did not maintain their balance with an unexpectedly heavy load, and to examine whether the balance loss increased low back loading.

DESIGN

Repeated-measures design.

SETTING

A research laboratory.

PARTICIPANTS

Fourteen healthy volunteers were assigned to 2 groups in the post hoc analysis. Group 1 (7 men; mean age, 25.6 +/- 4.2yr; height, 1.78 +/-.08m; weight, 83.0 +/- 8.5kg; lifting capacity, 63.2 +/- 8.0kg) maintained balance and was matched to group 2 (7 men; mean age, 26.3 +/- 4.1yr; height, 1.75 +/-.06m; weight, 78.2 +/- 5.3kg; lifting capacity, 64.7 +/- 4.9kg) who lost balance.

INTERVENTIONS

Subjects lifted boxes of 5%, 20%, and 35% of their lifting capacity. Load magnitude was expected or unexpected.

MAIN OUTCOME MEASURES

Center of mass (COM), lower body mechanics, ground reaction forces, and angular and horizontal momentum.

RESULTS

Group 1 consistently showed greater lumbar flexion, less knee flexion, and a higher COM just before and after load liftoff. During the heavier-than-expected 35% lift, the trunk angular velocities lifts indicated that both groups experienced eccentric trunk extensor muscle contractions.

CONCLUSIONS

The semisquat technique may protect against balance loss when lifting unexpectedly heavy loads. Eccentric muscle contractions and rapid increases in lumbar joint reaction moments may increase the risk of low back injury when there is a large, unexpected increase in the weight of the lifted load.

Sensory-motor control of the lower back: implications for rehabilitation

Although low back pain (LBP) is a widespread and disabling health problem, there is a lack of evidence based medicine with respect to its treatment and rehabilitation. A major reason for this is the poor understanding of the underlying mechanisms of the LBP syndromes. In an attempt to fill this gap, the present review article provides an overview of the sensory-motor control aspects of trunk stabilization and postural control of the trunk, and how they may relate to the evolution of LBP. In particular, the anatomy and physiology of the sensory-motor control mechanisms of the trunk muscles that contribute to general and segmental stability of the lumbar spine will be elucidated. Furthermore, a brief overview of current theories of postural control will be provided with respect to spinal stabilization. Finally, a concept of the pathophysiological changes within the sensory-motor control mechanisms of the lumbar spine in the presence of muscle injury and pain will be presented. The impact of pain and muscle injury on the muscular support for the lumbar motion segment will be discussed along with the deficits in neuromuscular control in LBP patients with decreased segmental lumbar stability.

Technical note: repeatability of measurement in determining stature in sitting and standing postures

The aim of this study was to determine the effect of sitting and standing postures on the repeatability of a stadiometer designed to detect small variations in spinal length. Two groups of ten healthy subjects, with no previous or known history of back problems, participated in this study. One group was measured in the standing posture, while the other group was measured in a sitting posture. All subjects gave informed consent to participate in this study. Subjects had a set of landmarks defining the spinal contour marked on their backs and then stood in the stadiometer for three series of ten measurements to be performed. At the end of each measurement, the subjects were requested to move away from and then be repositioned in the stadiometer. Subjects improved the repeatability across the measurement series. At the end of the second measurement series, all subjects presented mean standard deviations of 0.43 +/- 0.08 mm (range 0.30-0.50 mm) in the standing posture. In the sitting posture, deviations of less than 0.05 mm were obtained only at the end of the third measurement series (0.48 +/- 0.08 mm; range 0.34-0.62 mm), suggesting that this posture required three measurement series before repeatable measurements could be assured rather than two in the standing posture.

Effect of active limb movements on symptoms in patients with low back pain

STUDY DESIGN

A descriptive, correlational study of patients with mechanical low back pain (LBP).

OBJECTIVES

To assess the effect of active limb movements on symptoms in patients with LBP and to examine the relationship between symptoms with limb movements and select patient characteristics.

BACKGROUND

Limb movements result in forces applied to the spine and, thus, may be important in the examination and treatment of patients with LBP.

METHODS AND MEASURES

A total of 188 people with LBP, 84 men and 104 women, participated in a standardized examination. Six of the items required patients to move their limbs and note LBP symptoms as increased, remained the same, or decreased. The prevalence of various symptom responses with each limb movement test was calculated. Relationships between patient characteristics and reports of increased symptoms were examined with Cochran's linear trend statistic and the Spearman and Pearson correlation coefficients. Differences in characteristics of patients with and without increased symptoms were examined with chi2 test, Mann-Whitney U test, or Student's t test for independent groups.

RESULTS

An increase in symptoms was reported by 149 patients with at least 1 of the limb movement tests, and 3 of the patients reported a decrease in symptoms. Across the patient sample, the mean number of limb movement tests for which symptoms were reported as increased was 2.30 +/- 1.64. Patients with an increase in symptoms reported higher average pain intensity the week prior to the examination (median = 2; range: 1-5) and higher functional disability (mean = 0.25; SD = 0.15) than those without a change in symptoms (pain intensity: median = 1; range: 0-2 and functional disability: mean = 0.16; SD = 0.12). The correlation between the number of increased symptoms and the person's average pain intensity was r = 0.23; the correlation with the functional disability score was r = 0.36. Patients with a history of LBP tended to report an increase in symptoms with more of the limb movement tests (mean = 3.5; SD = 1.40) than those without a previous history of LBP (mean = 2.0; SD = 1.11).

CONCLUSIONS

Active limb movements performed during the examination primarily resulted in increased LBP symptoms. The presence and number of increased symptoms with the active limb movements was related to the patient's report of average pain intensity and functional disability. Tests of symptoms with active limb movements may provide insight into factors contributing to a LBP problem, as well as information to guide the treatment of patients with LBP.

Biomechanical and psychosocial risk factors for low back pain at work

OBJECTIVES

This study determined whether the physical and psychosocial demands of work are associated with low back pain.

METHODS

A case-control approach was used. Case subjects (n = 137) reported a new episode of low back pain to their employer, a large automobile manufacturing complex. Control subjects were randomly selected from the study base as cases accrued (n = 179) or were matched to cases by exact job (n = 65). Individual, clinical, and psychosocial variables were assessed by interview. Physical demands were assessed with direct workplace measurements of subjects at their usual jobs. The analysis used multiple logistic regression adjusted for individual characteristics.

RESULTS

Self-reported risk factors included a physically demanding job, a poor workplace social environment, inconsistency between job and education level, better job satisfaction, and better coworker support. Low job control showed a borderline association. Physical-measure risk factors included peak lumbar shear force, peak load handled, and cumulative lumbar disc compression. Low body mass index and prior low back pain compensation claims were the only significant individual characteristics.

CONCLUSIONS

This study identified specific physical and psychosocial demands of work as independent risk factors for low back pain.

Computational studies of 'whiplash' injuries

The term 'whiplash' was initially used to describe injuries to the neck caused by the head being forced backwards during a rear-end collision in cars without head restraints. The addition of head restraints in the 1970s was expected to solve this problem by preventing excessive extension of the neck but experience suggests the problem still exists. This paper reviews available experimental studies of whiplash and uses the data to construct a finite element model which is capable of dynamically simulating whiplash collisions and predicting the forces in all the relevant neck ligaments. For the first time, it is shown that trauma occurs long before the head hits the head restraint as a result of displacement between the head and the torso caused by the head's inertia leading to markedly different acceleration histories. It is concluded that experimental and computational studies must be used together to produce progress in biomechanical studies.

Biomechanical factors influencing nuclear disruption of the intervertebral disc

STUDY DESIGN

A disc model with full anular division was used to investigate how different biomechanical parameters influence the severity of nuclear disruption during compressive loading.

OBJECTIVE

To quantify the manner in which flexion, hydration, and loading rate contribute to the breakdown in the intrinsic cohesive structure of the nucleus pulposus.

SUMMARY OF BACKGROUND DATA

The risk of disc herniation is known to increase when the disc is loaded in flexed positions. However, there is a lack of experimental data showing how a combination of flexion with different loading rates and hydration levels affects the extent of nuclear disruption.

METHODS

A reproducible state of full hydration was established for isolated bovine caudal discs. A period of static preloading at an applied stress of 1 MPa was used to obtain a consistent state of partial hydration. Then 96 discs were subjected to a full-thickness division of the anulus fibrosus and compressed while hydration level, degree of flexion, and rate of loading were varied systematically.

RESULTS

A full spectrum of nuclear damage was observed in the tests, ranging from no detectable disruption to sudden sequestration of the entire nucleus. These results were quantified, and a general correlation was established between the severity of disruption and the different loading parameters.

CONCLUSIONS

The degree of flexion and the level of hydration were shown to play an important role in influencing the tendency of the nucleus to break loose and extrude through a preexisting anular division. Interestingly, the rate of loading appeared to have only a minor effect on the severity of damage induced in discs that incorporated a full depth anular division.

Spinal biomechanics and aging are major determinants of the proteoglycan metabolism of intervertebral disc cells

STUDY DESIGN

The proteoglycan metabolism of ovine disc nucleus pulposus and anulus fibrosus cells was investigated in relation to age, spinal level, and intrinsic spinal biomechanical properties.

OBJECTIVE

To evaluate the hypothesis that with aging loss of proteoglycans from the lumbosacral disc exceeds that from upper lumbar discs because of its proximity to a rigid segment, the sacrum.

SUMMARY OF BACKGROUND DATA

The proteoglycan and associated water of the disc decreases with aging.

METHODS

Proteoglycans were extracted directly from the disc tissues using 4 M GuHCl and examined by composite agarose polyacrylamide gel electrophoresis. Disc cells were cultured in alginate beads, and their metabolic activity was assessed by 3H-thymidine incorporation into DNA and by bioreduction of a cell proliferation dye. Newly synthesized proteoglycans were radiolabeled with 35S, and their molecular weight distributions and ability to aggregate with hyaluronan were determined by Sephacryl S1000 gel chromatography. Resident proteoglycans extracted from disc tissues with 4 M GuHCl were similarly evaluated. A group of adult animals also were studied biomechanically to evaluate the range of spinal motion (L4/L5 to L7/S1).

RESULTS

In contrast to the neonatal proteoglycan samples, the biosynthesis of proteoglycans by nucleus pulposus cells of adult discs increased progressively toward the sacrum. This correlated with increased metabolic activity. Analysis of the resident proteoglycans by composite agarose polyacrylamide gel electrophoresis indicated that although the aggrecan-1 population was present almost exclusively in the neonatal group, it was the aggrecan-2 population that predominated in the adult discs, and it became progressively more heterogeneous with aging and proximity of the disc to the sacrum.

CONCLUSIONS

The proteoglycans of the lumbosacral disc of adult animals turned over faster than proteoglycans of adjacent lumbar discs. The reduced proteoglycan content and ability to aggregate, particularly in the nucleus pulposus of lumbosacral discs, indicated that proteoglycan catabolism exceeded the rate of biosynthesis. These events in the lumbosacral disc are thought to be determined mechanically by its proximity to the sacrum.

Occupational low back disorder causation and control

Low back disorders (LBDs) continue to be the most common musculoskeletal problem in the workplace. It affects many workers, is associated with high costs to industry and the individual, and can negatively influence the quality of life for the workers. Currently there is significant controversy about the work-relatedness of LBD and the ability of ergonomics interventions to control the problem. This paper systematically examines the body of knowledge associated with LBDs and considers how information from different disciplines of study collectively might be used to assess the causality and control of LBD due to physical factors associated with work.

Sudden and unexpected loading generates high forces on the lumbar spine

STUDY DESIGN

A cross-sectional study of spinal loading in healthy volunteers.

OBJECTIVES

To measure the bending and compressive forces acting on the lumbar spine, in a range of postures, when unknown loads are delivered unexpectedly to the hands.

SUMMARY OF BACKGROUND DATA

Epidemiologic studies suggest that sudden and unexpected loading events often lead to back injuries. Such incidents have been shown to increase back muscle activity, but their effects on the compressive force and bending moment acting on the spine have not been fully quantified. Furthermore, previous investigations have focused on the upright posture only.

METHODS

In this study, 12 volunteers each stood on a force plate while weights of 0, 2, 4, and 6 kg (for men, 40% less for women) were delivered into their hands in one of three ways: 1) by the volunteer holding an empty box with handles, into which an unknown weight was dropped; 2) by the same way as in 1, but with volunteer wearing a blindfold and earphones to eliminate sensory cues; or 3) by the volunteer sliding a box of unknown weight off a smooth table. Experiments were carried out with participants standing in upright, partially flexed, and moderately flexed postures. Spinal compression resulting from muscular activity was quantified using electromyographic signals recorded from the back and abdominal muscles. The axial inertial force acting up the long axis of the spine was calculated from the vertical ground reaction force. The bending moment acting on the osteoligamentous spine was quantified by comparing measurements of lumbar curvature with the bending stiffness properties of cadaveric lumbar spines.

RESULTS

The contribution from abdominal muscle contraction to overall spinal compression was small (average, 8%), as was the axial inertial force (average, 2.5%), and both were highest in the upright posture. Peak bending moments were higher in flexed postures, but did not increase much at the moment of load delivery in any posture. Peak spinal compressive forces were increased by 30% to 70% when loads were suddenly and unexpectedly dropped into the box, and by 20% to 30% when they were slid off the table, as compared with loads simply held statically in the same posture (P < 0.001). The removal of audiovisual cues had little effect.

CONCLUSIONS

Sudden and alarming events associated with manual handling cause a reflex overreaction of the back muscles, which substantially increases spine compressive loading. Manual handling regulations should aim to prevent such events and limit the weight of objects to be lifted.