1988 Volvo award in biomechanics. The triaxial coupling of torque generation of trunk muscles during isometric exertions and the effect of fatiguing isoinertial movements on the motor output and movement patterns

The effect of lumbar fatigue on the ability to sense a change in lumbar position. A controlled study

STUDY DESIGN

A cross-sectional study in patients with recurrent/chronic low back trouble and healthy control subjects.

OBJECTIVE

To evaluate the effect of paraspinal muscle fatigue on the ability to sense a change in lumbar position.

SUMMARY OF BACKGROUND DATA

Protection against spinal injury requires proper anticipation of events, appropriate sensation of body position, and reasonable muscular responses. Lumbar fatigue is known to delay lumbar muscle responses to sudden loads. It is not known whether the delay is because of failure in the sensation of position, output of the response, or both.

METHODS

Altogether, 106 subjects (57 patients with low back trouble [27 men and 30 women] and 49 healthy control subjects [28 men and 21 women]) participated in the study. Their ability to sense a change in lumbar position while seated on a special trunk rotation unit was assessed. A motor rotated the seat with an angular velocity of 1 degree per second. The task in the test involved reacting to the perception of lumbar movement (rotation) by releasing a button with a finger movement. The test was performed twice, before and immediately after a fatiguing procedure. During the endurance task, the participants performed upper trunk repetitive extensions against a resistance, with a movement amplitude adjusted between 25 degrees flexion and 5 degrees extension, until exhaustion.

RESULTS

Patients with chronic low back trouble had significantly poorer ability than control subjects on the average to sense a change in lumbar position (P = 0.007), which was noticed before and after the fatiguing procedure. Lumbar fatigue induced significant impairment in the sensation of position change (P < 0.000001).

CONCLUSIONS

Lumbar fatigue impairs the ability to sense a change in lumbar position. This feature was found in patients and control subjects, but patients with low back trouble had poorer ability to sense a change in lumbar position than control subjects even when they were not fatigued. There seems to be a period after a fatiguing task during which the available information on lumbar position and its changes is inaccurate.

Back and hip extensor muscle function during therapeutic exercises

BACKGROUND

Therapeutic exercises are widely used in the treatment of low back problems. Clinical knowledge about targeting the load in these exercises, however, is insufficient. This study assessed the L2 and L5 level paraspinal and gluteus maximus muscle activities in different therapeutic exercises. Intramuscular and surface electromyography (EMG) measurements were obtained to study whether surface EMG measurements can be used in the assessment of multifidus muscle function.

METHODS

Eleven healthy subjects (5 men, 6 women) 21 to 38 years of age volunteered for the study. The subjects performed 18 different therapeutic exercises. During the exercises paraspinal EMG was recorded using fine wire and surface electrodes. The normalized peak and average muscle EMG activities (percentage of amplitude in maximal voluntary contraction [MVC]) during each task were determined.

RESULTS

The correlations between the average intramuscular and surface activities of the normalized EMG (% of MVC) at the L2 and L5 levels were .928 and .950, respectively. The peak and average EMG amplitudes of the exercises were below 50% and 25% of MVC, respectively. At the L5 level, the multifidus peak and average EMG amplitudes (% MVC) were higher in women than in men, whereas no significant difference was found at the L2 level. In women, the normalized multifidus EMG amplitude was higher at the L5 level than at the L2 level, whereas no significant difference was found in men. In both sexes, the normalized EMG amplitude was higher in the multifidus than in the longissimus muscle.

CONCLUSION

Surface EMG measurements may be used in the assessment of multifidus muscle function. Simple therapeutic exercises are effective in activating the lumbar paraspinal muscles.

The efficacy of active rehabilitation in chronic low back pain. Effect on pain intensity, self-experienced disability, and lumbar fatigability

STUDY DESIGN

A randomized study comparing the results of active rehabilitation and passive control treatment in patients with chronic low back pain with follow-up at 6 months and 1 year.

OBJECTIVES

To study the efficacy of active rehabilitation on pain, self-experienced disability, and lumbar fatigability.

SUMMARY OF BACKGROUND DATA

Exercises in an outpatient setting are widely used for the treatment of chronic low back pain. The efficacy of the active rehabilitation approach has been documented in randomized control studies, but these studies have seldom been focused on lumbar fatigability, which is now recognized as a frequent problem among patients with chronic low back pain.

METHODS

Fifty-nine middle-aged patients (37 men and 22 women) with nonspecific chronic low back pain were randomly assigned to 12 weeks' active rehabilitation or to a passive control treatment (massage, thermal therapy). Pain and disability index, low back pain intensity (visual analog scale, 100 mm), and the objectively assessed lumbar muscle fatigability (spectral electromyogram, mean power frequency slope [MPFSLOPE]) in a new 90-second submaximal isoinertial back endurance test were recorded before and after the interventions and at 6-month and 1-year follow-up visits.

RESULTS

Results of repeated measures multivariate analysis of variance indicated that back pain intensity (visual analog scale) and functional disability (pain and disability index score) decreased, and lumbar endurance (MPFSLOPE) improved significantly more (P < 0.05) in the active rehabilitation group than in the passive control treatment group, when measured at a 1-year follow-up examination. The group difference in visual analog scale and pain and disability index changes became even more significant at the end of 1 year. The change in lumbar endurance was significantly greater in the active rehabilitation group than in the passive control treatment group at the 6-month follow-up, but not at the 1-year follow-up.

CONCLUSIONS

The active progressive treatment program was more successful in reducing pain and self-experienced disability and also in improving lumbar endurance than was the passive control treatment. However, the group difference in lumbar endurance tended to diminish at the 1-year follow-up.

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

STUDY DESIGN

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

OBJECTIVE

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSION

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

Range of motion and motion patterns in patients with low back pain before and after rehabilitation

STUDY DESIGN

Data were collected from 27 patients who were participating in a rehabilitation program for chronic low back pain. The patients were tested on day 2 and day 11 of a 2-week rehabilitation program.

OBJECTIVES

To determine specific characteristics of trunk motion associated with long-term dysfunction caused by low back pain of various origin, to determine if a neural network analysis system can be effective in distinguishing between patterns, and to determine if the rehabilitation has an effect on range and pattern of motion.

SUMMARY OF BACKGROUND DATA

There is a lack of objective measures for evaluating the efficacy of rehabilitation programs. Numerous studies have established the difficulty of evaluating low back pain. Existing techniques, such as imaging methods, are in many cases either very rough and inaccurate or expensive and ineffective. A technique for evaluation of motion patterns in low back pain was developed based on analysis of dynamic motion features such as shape, velocity, and symmetry of movements.

METHODS

Dynamic motion data were collected before and after rehabilitation from 27 patients with low back pain by using a triaxial goniometer. Range of motion and features of the movement, such as shape, velocity, and repetitiveness, were extracted for analysis.

RESULTS

Motion features showed significant improvement after the rehabilitation program.

CONCLUSIONS

A neural network based on kinematic data is an excellent model for classification of low back pain dysfunction. Such a system could markedly improve the management of low back pain for an individual patient.

Importance of the intersegmental trunk muscles for the stability of the lumbar spine. A biomechanical study in vitro

STUDY DESIGN

A biomechanical study was performed to determine the consequences of a simulation of muscle forces on the loads imposed on the functional spinal units.

OBJECTIVES

No biomechanical study has investigated the effect of incorporation of agonist and antagonist muscle forces on the loading of functional spinal units.

SUMMARY OF BACKGROUND DATA

Spinal disorders and low back pain are increasingly becoming a worldwide problem. Traditional conservative therapies are intended to strengthen the muscles of the trunk using a judicious regimen of physical exercises.

METHODS

Eighteen whole, fresh-frozen human cadaveric lumbar spine specimens (L2-S2; average age, 53.4 years) were tested in a spine tester using pure flexion-extension, lateral bending, and axial moments. The effects of coactivation of psoas and multifidus muscles on L4-L5 mobility were simulated in vitro by applying two pairs of corresponding force vectors to L4. The segmental stability was defined by the correlation of an applied moment to the resultant deformation as shown in load-displacement curves, and the range of motion was defined as the angular deformation at maximum load.

RESULTS

The coactivation of muscles was accompanied by a 20% decrease in the range of motion (i.e., a significant increase in stability) during lateral bending and axial moments. Application of flexion-extension moments and muscle coactivation resulted in a 13% increase in the sagittal range of motion.

CONCLUSIONS

The action of the intersegmental agonist and antagonist muscles biomechanically increases the overall stiffness (stability) of the intervertebral joints in axial torque and lateral bending, whereas it may destabilize the segment in flexion.

Age, sex, and body mass index as determinants of back and hip extensor fatigue in the isometric Sørensen back endurance test

OBJECTIVE

To study the ability of a widely used isometric back endurance test to measure lumbar back erector muscle fatigue and to assess the influence of age, sex, and body mass index (BMI) on back and hip extensor muscle fatigability (EMG spectral indices).

DESIGN

Cross-sectional study of men and women without back problems.

SETTING

Occupational health center and rehabilitation clinic in Finland.

SUBJECTS

Experiment 1 consisted of 233 consecutive occupational health center customers (133 women, 100 men) without back problems. Experiment 2 consisted of 20 healthy women.

INTERVENTION

Subjects performed the isometric Sørensen back endurance test up to 240sec in experiment 1 and to the limit of endurance in experiment 2.

OUTCOME MEASURES

Raw surface EMG was recorded bilaterally over the belly of lumbar erector spinae muscles at L1-L2 and L4-L5 levels in experiment 1, and bilaterally over the medial paraspinal muscles at L1-L2, L3-L4, and L5-S1 levels and over the major hip extensor muscles (gluteus maximus and biceps femoris) in experiment 2. In both experiments, time to endurance was recorded (in experiment 1 up to 240sec). The EMG spectral median frequency (MF) decrease over time was used for the assessment of back and hip extensor fatigability.

RESULTS

In experiment 1, the rate of change in paraspinal MF was greater in men than in women, indicating greater paraspinal fatigability in men. Multiple regression analysis indicated that the rate of MF decrease (fatigue) during the test was dependent on age and BMI in both sexes and that the effects of age and BMI were more pronounced in women than in men. Correlation analysis revealed that the rate of paraspinal muscle MF decrease was associated with endurance time and BMI in women and with endurance time and age in men. In experiment 2, the paraspinal muscles, as well as the hip extensor muscles, biceps femoris, and gluteus maximus, showed clear decreases in MF during the isometric endurance test in women. MF decrease was highly related to endurance time and BMI in women.

CONCLUSIONS

Lumbar paraspinal muscle fatigability during the Sørensen test is influenced by subject characteristics. Further, the hip extensor muscles also significantly fatigue, indicating load sharing between back and hip extensor muscles during the test. According to these results, the validity of this widely used back endurance test in specifically measuring lumbar paraspinal muscle endurance is questionable, as is the direct comparison of test results between women and men.

Trunk muscle activities during asymmetric twisting motions

Axial twisting of the torso has been identified via epidemiologic studies as a significant risk factor for occupationally-related low back disorders. However, only recently have biomechanical studies been able to describe how twisting is accomplished through the use of the trunk musculature. These studies have been performed on subjects whose torso twists were performed in an upright posture. In this study, the electromyographic activity of ten trunk muscles was observed while 12 subjects performed twisting exertions in three different trunk postures. These postures included upright twisting, twisting while the trunk was flexed in the sagittal plane, and twisting while the trunk was flexed and rotated asymmetrically. In addition, twisting velocity and direction of motion were changed under the experimental conditions. Under upright twisting conditions, the twisting torque was generated easily and relatively efficiently through the employment of the oblique (internal and external) and latissimus dorsi muscles. When the trunk was flexed the activity of erector spinae muscles increased (about 10-15%) while the external oblique activity decreased (about 3-5%). Twisting while in asymmetric bent postures was accomplished with a reduction in oblique and latissimus dorsi muscle activities (approximately 5%) while the erector spinae muscle activity remained elevated. The change in muscle activity needed to balance the torso during twisting while bending also increased the amount of lateral torque that was produced by the trunk. These findings suggest that studies observing trunk muscle activities and trunk loading while subjects were in upright postures should be interpreted with caution when evaluating the activity of the trunk during occupational activities. Since many occupational twisting tasks are performed in awkward, asymmetric postures, application of results from upright twisting studies might underestimate the risk of these activities.

Relationship between muscle fiber composition and functional capacity of back muscles in healthy subjects and patients with back pain

Back muscles are important to the stability of the lumbar spine. Muscle fiber composition may give some indication of the functional capacity of these muscles. This review explores the relationship between muscle fiber composition and functional capacity of back muscles. The reference values for the type and size of the muscle fibers found in the back musculature of healthy subjects and patients with back pain are also presented. A high percentage of type I fibers, which are larger in size than type II fibers, has been found in back muscles at the thoracic and lumbar levels. This is in accordance with the postural function of these muscles. The diameter of type II fibers is smaller in females than males, which may partly explain the lesser strength and greater endurance capacity of back muscles in females. Due to the limited amount of pertinent data, no conclusive evidence is available regarding age-related changes in muscle fiber composition in the musculature of the back. In patients with lumbar disorders, pathological changes and selective atrophy of type II fibers are seen, and these can be changed with adequate exercises. Further research is suggested to address issues related to gender, age, back pain, and exercise and their effects on the apparent back muscle fiber composition.

Trunk strength in combined motions of rotation and flexion/extension in normal young adults

Thirty-eight normal healthy young subjects (14 males, 24 females) with mean ages of 23 years (males) and 21 years (females), performed 36 functional rotational tasks of the trunk. The subject's lower extremities were stabilized in a stabilizing platform, allowing the entire motion of flexion-rotation and extension-rotation to take place in the trunk. Of these tasks, 18 were isometric and the other 18 were isokinetic. The isometric tasks consisted of flexion-rotation and extension-rotation from a 20 degrees, 40 degrees and 60 degrees flexed trunk in 20 degrees, 40 degrees and 60 degrees of axial rotation. The isokinetic activity consisted of flexion-rotation and extension-rotation from upright and flexed postures respectively in 20 degrees, 40 degrees and 60 degrees rotation planes at 15 degrees, 30 degrees and 60 degrees/s angular velocities. The results revealed that the males were significantly stronger than females (p < 0.01) and isometric activities produced significantly greater torque compared to isokinetic efforts (p < 0.01). The degree of trunk flexion was not significant; the angle of rotation, although significant, had only a small effect. The 60 degrees trunk rotation was significantly different from 20 degrees and 40 degrees of trunk rotation. The multiple regressions were all significant (p < 0.01); however, they predicted only 40 to 60% of the variations. Based on the results and analysis, it is suggested that the motion involved rather than the torque may have a consequential effect in the precipitation of back injuries.

Asymmetric low back loading in asymmetric lifting movements is not prevented by pelvic twist

Asymmetric lifting is associated with an increased risk of low back disorders. Especially in lifting movements, characterized by a small amount of asymmetry, it is still the question if asymmetric lumbosacral torques occur, or if subjects try to avoid asymmetric back loading by twisting their pelvis with respect to the feet. An increase of the lifting speed or the box weight might amplify the lumbar torques but might also result in an attempt to limit further increase of asymmetric torques by increasing pelvic twist. In the current study, asymmetrical lifting movements were analyzed with the aid of a 3D linked segment model, using cuffs mounted to the body segments. Eight subjects performed lifting movements with five different asymmetry conditions, ranging from 0 to 90 degrees lifting asymmetry with respect to the sagittal plane, using two lifting speeds and two box weights. A significant increase in lateral flexing and twisting low back torque was found for each increase in asymmetry of the lifting movement. Pelvic twist accounted more or less constantly for about 25% of the lifting asymmetry and was hardly influenced by lifting speed or box weight. Even for 10 or 30 degrees of lifting asymmetry, subjects did not twist their pelvis far enough to avoid asymmetric loading of the low back. Assuming that asymmetric loading of the low back is more strenuous to the spine than symmetric loading, the current results indicate that even small deviations of a lifting movement from the sagittal plane can explain an increased risk of low back disorders.

The role of complex, simultaneous trunk motions in the risk of occupation-related low back disorders

STUDY DESIGN

Simultaneous trunk kinematic variables of industrial workers performing jobs with varying degrees of low back disorder risk were quantified, by using a three-dimensional electrogoniometer.

OBJECTIVES

To assess the distinguishing patterns of simultaneous multidimensional (complex) motion parameters of workers performing manual material handling jobs with varying degrees of low back disorder risk.

SUMMARY OF BACKGROUND DATA

There is significant epidemiologic and biomechanical evidence that implicates simultaneously occurring or combined motions and loading as important risk factors follow back disorder. However, the specific levels or magnitudes and patterns of these complex motions at which risk of low back disorder is increased are still unknown.

METHODS

An industrial database of 126 workers and jobs was used to quantify the complex trunk motions of groups with varying degrees of low back disorder risk. Three groups, low-, medium-, and high-risk, were defined on the basis of retrospective injury records of the corresponding jobs. The jobs were further classified into five cells of weight-lift rate combinations. Within each weight-lift rate cell, the three-dimensional trunk motion patterns of workers were analyzed. Bivariate distributions and cumulative distribution functions were used to compare the simultaneous occurrence of complex dynamic motions among risk groups.

RESULTS

High- and medium-risk groups exhibited complex trunk motion patterns involving high magnitudes of combined velocities, especially at extreme sagittal flexion; whereas the low-risk group did not. Postural trunk information alone did not provide a consistent pattern of distinguishing among risk groups.

CONCLUSIONS

Elevated levels of complex simultaneous velocity patterns were unique to groups with increased low back disorder risk. Knowledge of these complex trunk velocity patterns in combination with key workplace factors provides a more sensitive means for identifying low back disorder occupational risk factors than does mere postural information.

Trunk muscle co-contraction increases during fatiguing, isometric, lateral bend exertions. Possible implications for spine stability

STUDY DESIGN

Surface electromyographic activities were recorded from bilateral trunk muscles as test participants maintained a 50% maximum, voluntary, isometric, lateral bend contraction to volitional exhaustion.

OBJECTIVES

To challenge the trunk with a prolonged lateral bend task and observe the responses of the agonist and antagonist muscles to the resulting fatigue.

SUMMARY OF BACKGROUND DATA

Lateral bend exertions of the trunk have been identified as a risk factor for injury to spine tissues. However, little is known about the response of spine muscles to fatigue and the subsequent implications for spine stability, joint loading, and tissue injury. Surface electromyography provides a window on muscle loading and progressive fatigue.

METHODS

Eleven male participants performed a set of maximum lateral bend exertions at the start of the trial, then maintained an upright standing posture while resisting 50% of the maximum moment until volitional exhaustion, then performed another set of maximum contractions. All contractions were isometric. Measurements were made, throughout each contraction, of the lateral bend moment and abdominal and trunk extensor electromyographic activities at six bilateral surface electromyography sites. Electromyographic amplitude and mean power frequency were calculated with 500-millisecond segments recorded serially every 800 milliseconds. Mean values were calculated with data from the first and last 5% of the trial durations. Statistics determined if fatigue had a significant effect on maximum moment and the electromyographic statistics for each muscle site.

RESULTS

Fatigue resulted in a significant decrease in maximum lateral bend moment and increase in moment variability. Decreases in mean power frequency, indicating fatigue, were observed in three agonists (the trunk extensors) and one antagonist muscle. Unfatigued agonist electromyographic amplitudes correlated well with the mechanical advantage of muscles to generate lateral bend moments. Unfatigued antagonist activities were low except for the thoracic erector spinae. The agonists and antagonists demonstrated average increases of 17% and 8%, respectively, when pooled across muscles. Much of this change was predicted to have been associated with increases in co-contracting muscle forces.

CONCLUSIONS

The trunk responded to a prolonged, lateral bend contraction by increasing co-contraction as agonist trunk muscles fatigued. It was proposed that the fatigue compromised neural coordination and that the co-contraction served to maintain spine stability.

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

STUDY DESIGN

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

OBJECTIVE

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSION

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

Extrapolation of time series of EMG power spectrum parameters in isometric endurance tests of trunk extensor muscles

The aim of the present study was to test the viability of using short isometric contractions of trunk extensor muscles to perform an assessment of their endurance capacity. To this aim two types of analysis were performed. First, electromyographic (EMG) mean power frequency (MPF) slopes with respect to time as estimated over shorter fixed periods were compared to slopes estimated over the full contraction period of a contraction sustained until the endurance time. Second, the relationship between MPF slope estimates as estimated over various periods and the endurance time of the muscle group was evaluated. Five subjects performed three isometric trunk endurance tests at 25%, 50% and 75% of their maximum voluntary contraction (MVC), respectively. EMG signals of the left and right multifidus, iliocostalis and longissimus muscles were continuously recorded and spectral parameters were calculated. The MPF appeared to decrease consistently during all endurance tests. The extrapolation from a MPF time series of half the estimated contraction period to the time series of the complete contraction period gave reasonable results at all force levels, when data from several electrode locations were incorporated in a single slope estimate (mean or steepest slope). The accuracy of the prediction of trunk extensor endurance on the basis of these parameters describing the MPF time series over half the estimated contraction period was satisfactory. Endurance time predictions from yet shorter periods were unreliable.

The measurement of lumbar proprioception in individuals with and without low back pain

STUDY DESIGN

A clinical trial comparing a back pain group with a pain-free group.

OBJECTIVES

To investigate whether proprioceptive deficits existed in a group of individuals reporting low back pain.

SUMMARY OF BACKGROUND DATA

Little work has so far been conducted on the measurement of proprioception in the spine. Those studies that have been carried out, however, have failed to identify proprioceptive deficits in individuals with back pain. Previous work on peripheral joints has revealed that proprioception is affected with muscular or joint injury or degeneration.

METHODS

Forty individuals took part in the study, 20 with back pain and 20 with no pain. Participants were required to reproduce a predetermined target position, in standing and four-point kneeling, 10 times in 30 seconds. A computer screen was used to provide visual feedback on position. A mean deviation from the target position was obtained for each individual. A measurement of left elbow position sense was conducted in five individuals from each group to establish differences in short-term motor memory between the groups.

RESULTS

There were no differences between the subject groups in terms of short-term motor memory (P > 0.05). A two-way analysis of variance between subject groups and position to identify differences in accuracy (deviation from the target) found that there were differences between subject groups in either position (P < 0.05). There was no significant difference in accuracy between the positions used (P > 0.05).

CONCLUSIONS

Differences in proprioception do exist between individuals with back pain and those free from back pain. Further research needs to be undertaken on proprioceptive exercise programs and their effect on back pain.

Relation between functional characteristics of the trunk and the occurrence of low back pain. Associated risk factors

STUDY DESIGN

A 2-year prospective study relating a set of subject characteristics (professional and extraprofessional constraints, personal and anthropometric characteristics, and static and dynamic performance of the trunk) and incidence of low back pain in a population of male Belgian steel workers.

OBJECTIVES

To investigate the relation between occurrence of new cases of low back pain and prior personal, anthropometric, or functional characteristics.

SUMMARY OF BACKGROUND DATA

Some functional characteristics--in particular, dynamic parameters of the trunk such as the velocity--have been shown to be significantly reduced for low back pain in workers in cross-sectional studies. The question remained as to whether modifications of these parameters preceded the development of low back pain.

METHODS

The protocol included a questionnaire, a clinical examination, anthropometric measurements, and a set of functional tests on an isoinertial dynamometer. It was carried out twice, after a 1-year interval, on a sample of 215 workers without any history of low back pain. The reports of low back pain were again investigated 1 year later.

RESULTS

The development of low back pain is related to frontal plane imbalance of the trunk, lower body weight, and perception of heavy lifting efforts at the workplace. Individuals performing dynamic tests at higher velocities appear also to be at a greater risk of low back pain.

CONCLUSIONS

Although workers with a history of low back pain performed dynamic tests at significantly lower velocities, the probability for development of low back pain in the following year is greater for workers performing such tests at greater velocities.

Discrimination between maximal and submaximal effort in lumbar dynamometry

OBJECTIVE: The purpose of this study was to investigate differences in consistency between maximum, preferred and a low level trunk performance. It was hypothesized that if maximum effort yielded consistent performances while submaximal efforts were less consistent, a consistency parameter could be used to detect lack of effort. DESIGN: Survey study. METHODS: Eleven healthy subjects performed lumbar dynamometry measurements (Isostation-B200) with maximum, preferred and low effort. A standard protocol was used and velocity and static torque were measured. Five subjects performed three measurements with low effort and trained the low effort level in between to determine the impact of training on consistency of submaximal performances. Coefficient of variations (CV), percentage differences and onset slopes were used to determine consistency. RESULTS: Results show that maximum and preferred effort are equally consistent. Maximum and low effort and preferred and low effort show small significant differences in CV for some velocity and isometric torque parameters. No significant differences between the three performance levels were indicated for percentage differences and onset slopes. Training did not improve the consistency of the measurements. CONCLUSIONS: Small significant differences in CV were found between maximal and submaximal effort. However the combination with the relatively large standard deviations make it unlikely that this parameter can be used for a reliable discrimination.

Classification of low back pain from dynamic motion characteristics using an artificial neural network

STUDY DESIGN

Data were collected from 183 subjects who were randomly assigned to the training and test groups. During testing of the classification system, knowledge of the low back pain condition or motion characteristics of the patients in the test group was not made available to the system.

OBJECTIVES

To determine specific characteristics of trunk motion associated with different categories of spinal disorders and to determine whether a neural network analysis system can be effective in distinguishing patterns.

SUMMARY OF BACKGROUND DATA

Numerous studies have established the difficulty of evaluating lower back pain. Imaging techniques are expensive and ineffective in many cases. A technique for evaluation of lower back pain was developed on the basis of analysis of such dynamic motion features as shape, velocity, and symmetry of movements, using a neural network classification system.

METHODS

Dynamic motion data were collected from 183 subjects using a triaxial goniometer. Features of the movement were extracted and provided as input to a two-stage neural network classifier governed by a radial basis function architecture. After training, the output of the classifier was compared with Québec Task Force pain classifications obtained for the patients. Linear and nonlinear classification techniques were compared.

RESULTS

The system could determine low back pain classification from motion characteristics. The neural network classifier produced the best results with up to 85% accuracy on novel "validation" data.

CONCLUSIONS

A neural network based on kinematic data is an excellent predictive model for classification of lower back pain. Such a system could markedly improve the management of lower back pain in the individual patient.

The effect of fatigue on multijoint kinematics and load sharing during a repetitive lifting test

STUDY DESIGN

A repetitive lifting test in the sagittal plane was performed with a submaximal load at a maximal lifting rate to understand the effects of fatigue on kinematic and kinetic measures of performance.

OBJECTIVES

To quantify the effect of fatigue during a highly repetitive lifting task, in terms of lifting force transmitted to the load, joint motion patterns, and internal joint load sharing.

SUMMARY OF BACKGROUND DATA

Industrial surveillance and epidemiologic data suggest that repetitive lifting is a risk factor for low back pain. Previous studies examining the effect of fatigue have either been constrained to isolated trunk movement, or have not explored the internal load distribution and potential alteration in the loading patterns.

METHODS

Sixteen healthy male subjects performed repetitive lifting in the sagittal plane with a load equal to 25% of their maximal lifting capacity, at a maximal lifting rate. Changes in lifting performance were determined from the power transferred to the box, joint kinematics, and joint kinetics. Data from three cycles at the start and end of the exercise were tested for the effect of fatigue using repeated-measures analysis of variance.

RESULTS

Fatigue was documented by a reduction in average lifting force and hip and spine torque generation, whereas internal joint load sharing was relatively unchanged. The fatigue was associated with decreased knee and hip motion, and increased lumbar flexion. Decreased postural stability also was evident.

CONCLUSIONS

The significant decrease in postural stability and force generation capability because of the repetitive lifting task indicated a higher risk of injury in the presence of unexpected perturbation. Multijoint coordinated lifting tasks provide a more realistic protocol to study neuromuscular fatigue.

Fibre type characteristics of the lumbar paraspinal muscles in normal healthy subjects and in patients with low back pain

A knowledge of the alteration in the fibre type profile of paraspinal muscle associated with low back pain is essential for the design of successful rehabilitation programmes. In attempting to compare the muscles of patients with low back pain with those of controls, few previous studies have considered factors such as gender, age, and size of the subjects, each of which can potentially confound interpretation of the results. We obtained samples of lumbar paraspinal muscle during spinal surgery from 21 patients with low back pain and, using the percutaneous biopsy technique, from 21 control volunteers matched for gender, age, and body mass. The samples were subject to routine histochemical analysis to determine characteristics of muscle fibre type. Compared with controls, the muscle of the patients had a significantly higher proportion of type-IIB (fast-twitch glycolytic) fibres than type-I (slow oxidative) fibres. The mean size of a given fibre type did not differ between the patients and the controls. Consequently, the relative area of the muscle occupied by type-IIB fibres was higher and that by type-I fibres was lower in the patients. The patients had a greater number of muscle samples with more than 1% type-IIC fibres, and abnormalities that could be described as pathological were more marked in the patients than in the controls. In conclusion, the paraspinal muscles of patients who have low back pain display a more glycolytic (faster) profile; this can be expected to render them less resistant to fatigue.

Stabilizing function of trunk flexor-extensor muscles around a neutral spine posture

STUDY DESIGN

This study examined the coactivation of trunk flexor and extensor muscles in healthy individuals. The experimental electromyographic data and the theoretical calculations were analyzed in the context of mechanical stability of the lumbar spine.

OBJECTIVES

To test a set of hypotheses pertaining to healthy individuals: 1) that the trunk flexor-extensor muscle coactivation is present around a neutral spine posture, 2) that the coactivation is increased when the subject carries a load; and 3) that the coactivation provides the needed mechanical stability to the lumbar spine.

SUMMARY OF BACKGROUND DATA

Theoretically, antagonistic trunk muscle coactivation is necessary to provide mechanical stability to the human lumbar spine around its neutral posture. No experimental evidence exists, however, to support this hypothesis.

METHODS

Ten individuals executed slow trunk flexion-extension tasks, while six muscles on the right side were monitored with surface electromyography: external oblique, internal oblique, rectus abdominis, multifidus, lumbar erector spinae, and thoracic erector spinae. Simple, but realistic, calculations of spine stability also were performed and compared with experimental results.

RESULTS

Average antagonistic flexor-extensor muscle coactivation levels around the neutral spine posture as detected with electromyography were 1.7 +/- 0.8% of maximum voluntary contraction for no external load trials and 2.9 +/- 1.4% of maximum voluntary contraction for the trials with added 32-kg mass to the torso. The inverted pendulum model based on static moment equilibrium criteria predicted no antagonistic coactivation. The same model based on the mechanical stability criteria predicted 1.0% of maximum voluntary contraction coactivation of flexors and extensors with zero load and 3.1% of maximum voluntary contraction with a 32-kg mass. The stability model also was run with zero passive spine stiffness to simulate an injury. Under such conditions, the model predicted 3.4% and 5.5% of maximum voluntary contraction of antagonistic muscle coactivation for no extra load and the added 32 kg, respectively.

CONCLUSIONS

This study demonstrated that antagonistic trunk flexor-extensor muscle coactivation was present around the neutral spine posture in healthy individuals. This coactivation increased with added mass to the torso. Using a biomechanical model, the coactivation was explained entirely on the basis of the need for the neuromuscular system to provide the mechanical stability to the lumbar spine.

Evaluation of the probability of spinal damage caused by sustained cyclic compression loading

A sensitivity analysis of two alternative models predicting damage to vertebral motion segments (VMS) in cyclic compression was performed to evaluate the relative probability of damage occurring when peak compression force, loading frequency, or duration in a lifting task is changed. The first model is based on the assumption that fatigue failure is the mechanism underlying damage to the VMS in cyclic compression. The second model is based on the assumption that the VMS damage in cyclic compression is determined by the viscoelastic deformation of the segment and that the instant of failure can be predicted on the basis of the energy stored in this process. With both models, we estimated the percentage of the population likely to incur a VMS injury when performing a repetitive lifting task with peak compression forces ranging from 1500 to 4100 N, frequencies from 2 to 12 min-1, and durations between 30 and 120 min. The results indicate a dominant influence of the peak compression force on this outcome over the domain studied. This conclusion holds qualitatively for both models, suggesting that for a comparative analysis they can be considered interchangeable. However, a considerable quantitative difference in the absolute outcomes of the two models was found, which stresses the importance of further study on the validity of these models.

Trunk extension strength and muscle activity in standing and kneeling postures

STUDY DESIGN

A split-plot experimental design was used to evaluate the influence of posture, trunk angle, and rotational velocity on peak torque output and myoelectric activity during maximal trunk extension maneuvers.

OBJECTIVES

To determine whether the kneeling posture alters extension torque capabilities in isometric and isokinetic exercises as compared with standing. Also, to ascertain whether recruitment of trunk muscles is modified by such a postural change.

SUMMARY OF BACKGROUND DATA

Factors such as workplace geometry may force workers to adopt awkward or unusual postures in the performance of manual tasks. An understanding of the limitations placed on strength in unconventional working postures is crucial to the proper design of jobs.

METHODS

Twenty-one healthy male subjects (mean age = 36 years +/- 7 SD) performed 12 trunk extension exertions in standing and kneeling postures. Isometric tests were performed at 22.5 degrees, 45 degrees, and 67.5 degrees of trunk flexion. Isokinetic tests were done at three velocities: 30 degrees/sec, 60 degrees/sec, and 90 degrees/sec. Electromyographic data were collected from eight trunk muscles to assess muscle recruitment under each condition. A priori orthogonal contrasts were specified for analysis of both torque and electromyographic data.

RESULTS

The kneeling posture was associated with a 15% decrease in peak torque output when contrasted with standing: however, no concomitant change in trunk muscle activity was evident. Trunk hyperflexion (isometric tests) and increasing rotational velocity (isokinetic tests) were associated with reduced torque in both postures. Trunk muscle activity was primarily affected by changes in trunk angle and velocity of contraction.

CONCLUSIONS

A reduced extensor capability exists in the kneeling posture, despite equivalent trunk muscle activity. The similar activation patterns in both postures suggest that the strength deficit does not result from alterations in trunk muscle function. Rather, it may be the consequence of a reduced capability to rotate the pelvis in the kneeling posture, due to a disruption of the biomechanical linkage of the leg structures.

Sagittal back motion of college football athletes and nonathletes

STUDY DESIGN

The study was designed as an ex post facto study using volunteers.

OBJECTIVES

To compare sagittal back motion of male college athletes with that of nonathletes and to compare data from both groups with normative data.

SUMMARY OF BACKGROUND DATA

Few studies have evaluated athletic demands on the spine. Much of the information on athletic demands comes from electromyographic studies, flexibility comparisons, and lift task studies. Although these studies provide a basis for back testing and evaluation, they do not present direct evidence of athletic low back performance.

METHODS

Fifteen male college football athletes and 15 male college nonathletes volunteered for testing using the IsoStation B-200 BSCAN 2.0 protocol (Isotechnologies, Inc., Hillsborough, NC). Measures were recorded for range of motion, isometric flexion and extension, and moderate and high dynamic flexion and extension. Data were analyzed using multivariate analysis of variance.

RESULTS

The results of Hotelling's multivariate test were significant. Univariate follow-up analysis showed that athletes had significantly better isometric flexion, isometric extension, moderate dynamic flexion, high dynamic flexion, and high dynamic extension. Athletic data were compared with the BSCAN population data at the 50th and 80th percentile. Athletes were significantly better (P < 0.007) for all variables at the 50th percentile and for all dynamic variables at the 80th percentile.

CONCLUSIONS

Within the limitations of the study, college football athletes had better sagittal back motion strength and speed as tested with the B-200 than nonathletes. Population data for the B-200 were representative for nonathletes but nonrepresentative for football players.

Spine loading during trunk lateral bending motions

Increases in lateral trunk velocities have been identified as a mechanism for increasing the risk of low-back disorder. Previous studies have identified an increase in coactivation of the trunk musculature during lateral bends, but no studies have evaluated how spine loading changes as lateral trunk velocity increases. Twelve subjects were asked to lift loads laterally at one static and three dynamic velocities. Ten trunk muscle activities and trunk kinematics were documented and used as input parameters to an EMG-assisted model to evaluate spine loading. Muscle coactivation was observed in all lateral bends. Coactivation significantly increased during dynamic trials compared to the static trials. Coactivity increased spinal loads by as much as 25% compared to values predicted by models that did not consider coactivity. Movements to the right significantly increased spine loadings (252 N increase in compression) compared to movements to the left. Spine compression, A-P shear, and lateral shear all increased in the dynamic trials compared to the static conditions. Peak compression increased by an average of 525 N at 45 degrees s-1 compared to static loading. Compression and lateral shear increased monotonically as trunk velocity increased. It is expected that this combined (compression and lateral shear) loading is the mechanism for increased risk observed in industry.

Fatigue-related EMG responses of trunk muscles to a prolonged, isometric twist exertion

OBJECTIVE: To determine the responses of the trunk muscles to fatigue during sustained, isometric axial torque exertions. DESIGN: Electromyographic (EMG) amplitude and frequency parameters were used to investigate the effects of prolonged contractions on the recruitment patterns of trunk muscles, with special emphasis on those antagonistic and stabilizing muscles not contributing directly to the required axial torque. BACKGROUND: High levels of muscle coactivity have been observed during axial torque generation in the trunk. It has been suggested that this serves to enhance postural stability of the spine during potentially injurious twist exertions. Muscle fatigue decreases force-generating capacity but the resulting effects on muscle recruitment, and the subsequent consequences for postural stability, have not been explored previously during the production of axial torques in the spine. METHODS: Eleven male and 11 female subjects maintained an isometric axial torque exertion to the left (40% of maximum) until volitional exhaustion in an upright standing posture. Maximum axial torques were measured before and after the trial. The average EMG amplitude (AEMG) and mean power frequency (MPF) of seven bilateral trunk muscles (representing agonists, antagonists and trunk stabilizers) were continuously monitored throughout the trial. RESULTS: The average decrease in maximum torque was 18.5% and the average endurance time was 163 +/- 52 s. Evidence of muscle fatigue was provided by significant MPF decreases in 12 of the 14 muscles monitored (P < 0.05) with a overall average decrease of 20.4%. There was a significant increase in AEMG with time for 11 of the 14 muscles monitored (P < 0.05). These increases in activation were linked to increases in muscle force for most of the antagonistic and stabilizing muscles. Gender effects were rarely observed. CONCLUSIONS: Fatigue results in an change in the recruitment patterns of trunk muscles. Muscles serving as antagonists and trunk stabilizers during prolonged axial twist exertions increased their force levels in response to fatigue.

Development and application of predictive equations of maximal static moments generated by the trunk musculature

OBJECTIVE: To develop and apply regression equations that predict the maximal static moments generated by the extensor, axial rotator and lateral flexor muscles of the trunk as a function of forward bending angles and asymmetrical positions of the torso in human subjects. DESIGN: Using experimental data, polynomial predictive equations were developed. The equations were applied in an algorithm to estimate the maximal moment generated by individual subjects. METHODS: Three groups of male subjects participated in the evaluation of the effect of forward bending and trunk asymmetry on the static moment of the extensor, right axial rotator, and right lateral flexor muscles of the trunk, respectively. Two other groups of male subjects were evaluated to test the validity of the algorithm. During the dynamometric tests, the three-dimensional position of the trunk was determined using cinematography techniques. RESULTS: For the extensor muscles, a curvilinear increase of the maximal moment was observed with forward bending, with a slight decrease of the moment on each side of the neutral position. For the right axial rotator muscles, both forward bending and left asymmetrical position of the trunk increased the maximal moments generated by these muscles. The right lateral flexors were stronger in the standing and mid-flexion positions than in other positions. First-order, second-order and interaction terms appear in the regression equations to account for the effect of forward bending and asymmetrical positions. The regression equation for the extensors and right axial rotator muscles have R(2) coefficients (range 0.56-0.76) higher than those of the lateral flexor regression equations (range 0.09-0.12). The algorithm results in an average error of 13% in predicting the strength of extensor and axial rotator muscles of individual subjects. CONCLUSION: The results have shown that strength of the extensor and axial rotator muscles is affected by forward bending and the asymmetry of the trunk, while strength of the lateral flexor muscles is only dependent on the forward bending position. The regression equations and the corresponding algorithm are applicable for the range of positions used in the study.

Abdominal-wall recovery following TRAM flap: a functional outcome study

Abdominal-wall function was evaluated preoperatively and at intervals postoperatively in 25 consecutive patients undergoing breast reconstruction with transverse rectus abdominis myocutaneous (TRAM) flaps (single-pedicled TRAM flap, 14 patients; free TRAM flap, 9 patients; and bilateral free TRAM flaps, 2 patients). Objective measures of abdominal-wall function were performed with the B200 Isostation, a triaxial dynamometer. In addition, the patients were assessed by a physical therapist and filled out an activity questionnaire at each postoperative examination. Tests of abdominal-wall function demonstrated the greatest decrease in performance at the 6-week postoperative tests of flexion. The maximum isometric flexion torque of the pedicled TRAM flap group decreased to 58 +/- 10 percent, while the unilateral free TRAM flap group average was 87 +/- 11 percent of baseline. For the pedicled TRAM flap group this difference was significant (p = 0.004). By the 6-month evaluation, the maximum isometric flexion torque increased for both the pedicled and the free TRAM flap groups to 89 +/- 13 percent and 93 +/- 8 percent of baseline, respectively. The physical therapist evaluation of abdominal-wall strength and the activity questionnaire data showed no statistically significant differences between groups or over time. Rectus abdominis muscle harvest for pedicled TRAM flaps causes a greater insult to the abdominal wall than does free TRAM flap harvest. The ultimate clinical effect of the sacrifice of even an entire rectus abdominis muscle appears to be well tolerated by most patients. This is the first prospective outcome study of abdominal-wall function in TRAM flap patients. The clinical implications of this information will be discussed.

Isometric torso rotation strength: effect of training frequency on its development

OBJECTIVE

To examine training frequency's effect on torso rotation muscle strength.

DESIGN

The study followed a pretest-posttest randomized-group design.

SETTING

University laboratory.

PATIENTS

Subjects, 33 men (age 30 +/- 11yr) and 25 women (age 28 +/- 10yr) with no history of low back pain, volunteered to participate in the study and were tested for isometric (IM) torso rotation strength before (T1) and after (T2) 12 weeks of training. Measurements of maximal voluntary IM torso rotation torque (N.m) were made through a 108 degrees range of motion (54 degrees, 36 degrees, 18 degrees, 0 degree, -18 degrees, -36 degrees, -54 degrees). Subjects were stratified by peak torque at T1, and randomized to a nonexercising control group (C, n = 10), or groups that trained once a week (1x/wk, n = 16), twice a week (2x/wk, n = 17), or three times a week (3x/wk, n = 15); and all groups were similar in strength.

INTERVENTIONS

Training consisted of 8 to 12 repetitions of full range dynamic variable resistance exercise to volitional fatigue, for both left and right rotation.

MAIN OUTCOME

To determine the best training frequency for the development of torso rotation strength.

RESULTS

Relative improvements (average increase in strength gained at each angle) for the training groups were 4.9%, 16.3%, and 11.9% for the 1, 2, and 3x/wk groups, respectively. The 1x/wk group did not increase in IM torso rotation strength compared to the control group at any angle. Both the 2 and 3x/ wk groups increased their IM torso rotation strength compared to the control group at all but one angle. There were no significant differences in IM torso rotation strength between the groups that trained 2 or 3x/wk. During the training period, the 2 and 3x/wk groups increased their dynamic training load significantly more than the 1x/wk group.

CONCLUSIONS

Posttraining dynamic strength was not different between training frequencies of 2 and 3x/wk. Therefore, training the rotary torso muscles 2x/wk is recommended.

The effect of fatigue on multijoint kinematics, coordination, and postural stability during a repetitive lifting test

Because of the inability of strength tests to accurately discriminate between low back pain patients and healthy subjects, a multifactorial evaluation of low back pain patients is warranted. It is postulated that measurements of endurance, kinematics, postural stability, and coordination, in addition to strength, are necessary to fully document the patients' functional capabilities. This research study was conducted in order to understand the effects of fatigue on the above factors. Twelve healthy male subjects performed a repetitive lifting test in which a submaximal load was lifted at a maximal rate. Knee, hip, and trunk motion was measured using videography and electrogoniometry, postural stability was measured using a forceplate, and coordination parameters were determined using phase-plane analysis. Fatigue was documented by a 31% reduction in lifting power. At the end of the endurance test, there was less knee and hip range of motion and greater spine peak flexion, while the coordination measures demonstrated that there was greater hip and lumbar spine extension earlier in the lifting phase. The postural stability declined as the test endured. Utilization of these measures may guide physical therapists in their rehabilitation of low back pain patients.

Muscular response to sudden load. A tool to evaluate fatigue and rehabilitation

STUDY DESIGN

Subjects were exposed to fatiguing and restorative interventions to assess their response to sudden loads.

OBJECTIVES

To investigate the erector spinae and rectus abdominis response characteristics to "sudden load" and the effect of fatigue and rehabilitation.

SUMMARY OF BACKGROUND DATA

Unexpected loads which people often experience, can lead to high forces in the spine and may be a cause of low back injury.

METHODS

Muscle responses to sudden load were mediated by fatigue, walking, expectation, method of load application, exposure to vibration, and cognitive-behavioral rehabilitation in patients with chronic low back pain. A novel technique, perfected in this work, called wavelet analysis, was used to analyze these data.

RESULTS

Reaction time was affected by fatigue and expectation. Vibration exposure significantly increased the muscle response time. Walking was able to ameliorate that effect. Back muscles responded differently, depending on whether loads were applied to the back through the hands or through the trunk. Electromyographic reaction time and magnitude decreased in patients after a 2-week rehabilitation program.

CONCLUSIONS

Sudden loads can exacerbate fatigue effects. Walking after driving reduces the risk to the back caused by handling unpredictable loads. Vibration exposure guidelines should be more conservative. Patients have longer response times than healthy subjects, but patients can improve their response to sudden loads via rehabilitation. Patients exhibit a flexion-extension oscillation at 5 Hz in response to a sudden load, suggesting that the 5-Hz, seated, natural frequency observed during whole-body vibration may result from neurophysiologic control limits.

Asymmetry of erector spinae muscle activity in twisted postures and consistency of muscle activation patterns across subjects

STUDY DESIGN

An experimental study involving a repeated measures design was performed.

OBJECTIVES

To describe the muscle activation pattern in extending from a twisted posture; to evaluate interindividual variation of this pattern; and to study the effects of fatigue development.

SUMMARY OF BACKGROUND DATA

Extending the trunk from a twisted posture is a risk factor for low back pain.

METHODS

Twelve male subjects performed contractions at 20%, 40%, 60%, and 100% of maximum in a neutral position and 15 degrees, 30 degrees, and 45 degrees twisted posture. Surface electromyography of six bilateral back muscle tracts was recorded during these 5-second contractions and during sustained contractions at 0 degree and 30 degrees and 40% of maximum.

RESULTS

The data show an asymmetry in muscle activation, increasing with angle of twist. Activity was higher contralateral to the direction of twisting. Linear regression analysis with angle and torque level against electromyographic amplitude yielded a significant fit in 92% of the data sets. In 75% of the subjects, the regression lines were similar. The divergence of the activation pattern in the other subjects appears to be explained by a higher co-contraction level of abdominal muscles. Fatigue caused a shift in activity to the more laterally situated muscle tracts.

CONCLUSIONS

Twisting causes an asymmetry in back muscle activation. Resulting stress concentrations in spinal motion segments, and a reduced spinal stability could underlie the observed relationship between the task investigated and low back pain. Fatigue may reduce further spinal stability. Some subjects display a divergent activation pattern, which probably involves higher spinal compression forces.

Effects of posture and structure on three-dimensional coupled rotations in the lumbar spine. A biomechanical analysis

STUDY DESIGN

A biomechanical lumbar spine model was constructed to simulate three-dimensional spinal kinematics under the application of pure moments. Parametric analysis of the model allowed for the estimation of how much of the coupled motions could be predicted by the lumbar lordosis and the intrinsic mechanical properties of the spine.

OBJECTIVES

To evaluate the relative effects of lordosis and intrinsic mechanical spine properties on the magnitude and direction of coupled rotations.

SUMMARY OF BACKGROUND DATA

Clinical evidence suggests that abnormal coupled motion in the lumbar spine may be an indicator of low back disorders.

METHODS

The biomechanical lumbar spine model consisted of five vertebrae separated by intervertebral joints that provided three rotational degrees of freedom. In vitro experimental data, obtained from nine fresh-frozen (L1-S1) cadaveric specimens, were used to establish the mechanical properties of the intervertebral joints. Two different submodels were considered in simulating the three-dimensional intervertebral rotations in response to the applied moments. In the first, it was assumed that the coupled motions were generated solely as a result of the vertebral orientation caused by lordosis. In the second, additional intrinsic motion coupling was assumed.

RESULTS

Intervertebral coupling was partially predicted by lumbar lordosis; however, the inclusion of intrinsic mechanical coupling dramatically improved the simulation of the intervertebral rotations (root mean square error < 1 degree). Comparison of the results from the two models demonstrated that the lumbar lordosis and intrinsic mechanical properties of the spine had about an equal effect in predicting the coupling between axial rotation and lateral bending. In contrast, coupled flexion, associated with lateral bending, was almost fully accounted for by the presence of lumbar lordosis.

CONCLUSIONS

The lumbar lordosis and intrinsic mechanical properties of the spine were equally important in predicting the magnitude and direction of the coupled rotations.

Determination of trunk motion patterns associated with permanent or transient stenosis of the lumbar spine

Dynamometric devices used to assess back function are becoming increasingly used in research as well as in clinical practice. These devices provide values for a variety of movement variables such as torque, displacement, and velocity, but they also enable the study of movement patterns. The purpose of this study was to determine the movement patterns of patients presenting with specific spinal pathologies. Thirteen patients with spinal stenosis and 14 patients with degenerative changes and a borderline spinal canal diameter, as well as a control group of 30 symptom-free subjects, were studied. An isoinertial trunk-testing dynamometric device was used. Sagittal velocity and position were measured against a resistance set to 50% of the subject's maximum isometric torque. A phase plane analysis (velocity against position) was performed for each subject. An ensemble averaging technique was used to average the repetitions of every subject and a normalization technique was employed to draw an average graph for each group. This graph showed distinct patterns of movement for the three groups. The differences appeared mostly during the extension phase of movement, as confirmed by statistical analysis. The spinal stenosis group showed lower velocities than the controls during the entire extension phase (P < 0.05), while the borderline spinal canal group demonstrated a significant decrease in velocity at the end of the extension phase (P < 0.01). These findings can be related to the increase in disk bulge and decrease in the diameter of the spinal canal during extension. This study shows the relation between some spinal pathologies and movement patterns. This type of functional assessment may be a valuable tool for assessing the relation between anatomical lesions and function and for determining the responsibility of findings such as bulging disk and/or facet syndrome in causing a patient's complaints.

Mechanically corrected EMG for the continuous estimation of erector spinae muscle loading during repetitive lifting

Few studies have been carried out on the changes in biomechanical loading on low-back tissues during prolonged lifting. The purpose of this paper was to develop a model for continuously estimating erector spinae muscle loads during repetitive lifting and lowering tasks. The model was based on spine kinematics and bilateral lumbar and thoracic erector spinae electromyogram (EMG) signals and was developed with the data from eight male subjects. Each subject performed a series of isometric contractions to develop extensor moments about the low back. Maximum voluntary contractions (MVCs) were used to normalize all recorded EMG and moment time-histories. Ramp contractions were used to determine the non-linear relationship between extensor moments and EMG amplitudes. In addition, the most appropriate low-pass filter cut-off frequencies were calculated for matching the rectified EMG signals with the moment patterns. The mean low-pass cut-off frequency was 2.7 (0.4) Hz. The accuracy of the non-linear EMG-based estimates of isometric extensor moment were tested with data from a series of six rapid contractions by each subject. The mean error over the duration of these contractions was 9.2 (2.6)% MVC. During prolonged lifting sessions of 20 min and of 2 h, a model was used to calculate changes in muscle length based on monitored spine kinematics. EMG signals were first processed according to the parameters determined from the isometric contractions and then further processed to account for the effects of instantaneous muscle length and velocity. Simple EMG estimates were found to underestimate peak loading by 9.1 (4.0) and 25.7 (11.6)% MVC for eccentric and concentric phases of lifting respectively, when compared to load estimates based on the mechanically corrected EMG. To date, the model has been used to analyze over 5300 lifts.

Low back pain and other overuse injuries in a group of Japanese triathletes

OBJECTIVE

To document the incidence of low back pain and other overuse injuries in a group of triathletes, and to investigate any associations with various physical and triathlon related factors.

METHODS

By means of a questionnaire, the physical characteristics, training habits, and the incidences of overuse injuries of 92 Japanese triathletes (70 males, 22 females) were documented. Student's t and chi 2 tests were used to determine the significance of any associations with injury incidence, as well as differences between subjects experiencing or not experiencing low back pain in the previous year.

RESULTS

Low back pain was experienced by 32% of subjects in the previous year. The majority (54%) of low back pain episodes lasted under seven days, suggesting mainly soft tissue involvement, and 19% lasted over three months, suggesting involvement of the intervertebral discs. Weekly trunk flexor muscle training frequency was significantly greater (P = 0.035) for the low back pain subjects. Close to significant differences for average weekly cycling time, trunk flexor muscle training time, and low intensity aerobic training, as well near significant associations for weight training and average weekly triathlon training load, were also found. No other factors were significantly associated with low back pain. Low back injuries accounted for 28% of all injuries. Only the knee was a more common single site of injury (33%).

CONCLUSIONS

The three most common injuries suffered by the triathletes were of the knee, back and shoulder. The low back pain suffered by many triathletes could be of a potentially serious nature. It is suggested that cycling is a major risk factor for low back pain in triathletes.

Diagnostic accuracy of technologies used in low back pain assessment. Thermography, triaxial dynamometry, spinoscopy, and clinical examination

STUDY DESIGN

A prospective blind study compared three new technologies to assess back pain.

OBJECTIVE

To assess the diagnostic accuracy and comparability of thermography, triaxial dynamometry, and spinoscopy in the assessment of recent onset work-related low back pain.

SUMMARY OF BACKGROUND DATA

The role of these technologies in assessing patients with low back pain is unproved.

METHODS

Forty-one patients with low back pain and 46 control subjects were assessed by each technology and by two clinical examiners blind to clinical status. Twenty patients were trained to simulate a healthy back without low back pain, and 50% of the control subjects were trained to simulate the presence of a low back pain disorder. Each technology was interpreted on two occasions by each of two readers.

RESULTS

Thermography performed significantly worse than did triaxial dynamometry, spinoscopy, and clinical examination. The diagnostic accuracy of the last three was similar, and inter-rater comparability did not differ significantly. Among simulators, the diagnostic accuracy of triaxial dynamometry and spinoscopy was significantly higher than that of clinical examination, although considerable inaccuracy remained in assessing individual subjects.

CONCLUSIONS

The diagnostic accuracy of thermography in recent onset low back pain does not support its use. Among those simulating normality or low back pain, triaxial dynamometry and spinoscopy have greater diagnostic accuracy than does a single clinical evaluation. However, for an individual, the inaccuracy that remains limits the use of triaxial dynamometry or spinoscopy for diagnosis in recent onset low back pain.

Physical exercise and low back pain

Health care providers often prescribe exercises as treatment for nonspecific low back pain. However, the effectiveness of this treatment is poorly documented in the literature. While the evidence suggests that exercise in general is beneficial, there is a lack of knowledge about the types, frequency and duration of exercises that should be prescribed and at what stage of injury they are most helpful. In addition, few studies have dealt with exercise treatment alone rather than in combination with other treatments, making it hard to decipher the unique contribution of exercise. Inadequate study designs also make conclusions difficult. Conversely, the literature clearly shows that inactivity has detrimental effects (i.e. delayed return to normal activity, and negative physiological and psychological effects) for low back pain patients.

Mechanical stress reduction during seated jolt/vibration exposure

The risk of experiencing low back pain is associated with mechanical factors. Anatomic factors, such as advancing pregnancy, can also place extra mechanical stress on the lower back. Mechanical factors, such as those related to the workplace, can be minimized by ergonomic interventions. A constrained, seated posture, in combination with exposure to whole-body, jolt/vibration can impose significant stresses on the posterior intervertebral disc and can lead to back muscle fatigue. Interventions that reduce the jolt/vibration magnitude and duration of exposure will decrease the mechanical work performed on the intervertebral disc. Such interventions range from jolt/vibration isolating seats and vehicle cabs, to decreasing exposure time and maintaining simple supported postures during ingress and egress. Improvements in seat configuration can reduce the intervertebral disc pressure and the strain on the posterior disc.

Effects of various isoresistive training programmes on trunk muscle performance

Isoresistive movements provide a functional method of testing muscle performance and of providing exercise; the resistance applied is usually a percentage of the maximal isometric torque which can be developed by individual subjects. The aim of the current study was to compare the effectiveness of dynamic isoresistive exercise programmes using 25, 50 or 75% of the maximum isometric flexion torque and static isometric exercises for improving the performance of the trunk flexors and extensors. Subjects comprised 42 asymptomatic females aged 18-25 years who were randomly assigned to control or exercise groups. The exercise groups undertook 2 min of exercise 3 times per week. The control subjects showed significant improvement between tests indicating a strong learning effect. The study showed that the subjects who exercised at 50% of maximum flexion isometric torque achieved the greatest increases in muscle performance on most measures. maximal isometric flexion and extension torques did not significantly change in any of the groups. Flexion and extension velocity and power were the muscle performance characteristics which improved the most in response to the training programmes. RELEVANCE:--This paper attempts to determine which among several options is the most effective isoresistive protocol for conditioning trunk muscle performance, as improved strength and power of these muscles may contribute in preventing episodes of LBP.

Correlations of isokinetic and psychophysical back lift and static back extensor endurance tests in men

Isokinetic lift, psychophysical lift, and static back endurance tests are among the most widely used measurements of muscle function for determining risk for, and recovery from, back problems. After determining test repeatability we examined the degree of association between these test measures in 100 men selected to represent a variety of occupations and lifestyles. Isokinetic lifting had low correlations with test results for psychophysical lifting (r = 0.28) and static back endurance (r = 0.24). Static back endurance and psychophysical lift test results were not at all correlated (r = 0.03). RELEVANCE--:Muscle function measurements for back problems are sometimes arbitrarily selected due to health practioners' beliefs and preferences or practical reasons such as the availability of equipment. Yet little is known about the associations of these tests with one another or with anthropometric factors. The data presented in this study document that isokinetic lift, psychophysical lift, and static back endurance tests appear to measure very different aspects of muscle function, and comparing studies using these different test methods should be approached with these low inter-test correlations in mind.

A back-propagation neural network model of lumbar muscle recruitment during moderate static exertions

A model employing artificial neural networks (ANNs) is developed for the prediction of lumbar muscle activity in response to steady-state static external moment loads. The model is constructed using standard feedforward networks and trained with available data using the standard back-propagation algorithm. Training with a limited set of exemplars allowed accurate prediction of muscle activity for novel moment loads (generalization). Sensitivity analyses during training and testing phases showed that the choice of specific network parameters was not critical except at extreme values of those parameters. Model predictions were better correlated with experimental data than predictions made using two optimization-based methods (average r2 = 0.83 using ANNs and 0.65 using optimization). The results suggest that lumbar muscle response varies smoothly and consistently with respect to the magnitude and orientation of external moments, and they also imply an upper limit on the accuracy of muscle activity prediction using only moment loads as input. ANNs present a useful alternative to EMG- and optimization-based approaches by being both 'reality-based' and predictive.

Influence of weight and frequency on thigh and lower-trunk motion during repetitive lifting employing stoop and squat techniques

Changes in kinematics as a function of lifting weight and frequency was investigated in sagittal symmetric repetitive lifting. For every lift cycle (lowering and lifting) the motion range between the upright position (0 degrees ) and the maximum angular displacement of the thigh and lower-trunk body segments was recorded. Ten subjects performed five repetitive lifting bouts with different weight/frequency combinations, using both stoop and squat lifting techniques. In total, 6384 lifts were analysed. The lifting weight or frequency did not influence the motion ranges in stoop lifting. In squat lifting the weight lifted did not appear to have any influence on the motion ranges, while the thigh motion range was significantly smaller at lifting frequency of 20 lifts min(-1) than at a frequency of 10. A significant gradual decrease in the thigh motion range and corresponding increase in the lower-trunk motion range were seen for a majority of the subjects during squat lifting at frequency 20. These changes suggest that quadriceps muscle strength is the limiting factor in repetitive squat lifting. Also the variation in motion ranges was greater in squat lifting than in stoop lifting. RELEVANCE: Forestry work involves frequent lifting. However, compliance in using squat lifting technique, which is recommended for safe lifting, is sometimes poor. Fatigue may be one of the determinants for changes in kinematics and choice of technique in lifting tasks.

Comparison of muscle forces and joint load from an optimization and EMG assisted lumbar spine model: towards development of a hybrid approach

The purpose of this study was to determine whether the same estimates of individual muscle and L4/L5 lumbar joint compressive forces result from an optimization (OPT) compared to an electromyography (EMG) assisted approach for solving the inderminate moment equilibrium equations in the same anatomical model. Four male subjects performed near maximum, isometric, ramp efforts in trunk flexion, extension and lateral bending in a testing apparatus. The EMG approach was sensitive to subject and trial differences in the magnitudes of individual muscle forces needed to produce the same reaction moment. In contrast, the OPT method converged on a similar estimate of muscle forces for all subjects and trials producing the same moment. The OPT method predicted lower L4/L5 joint compression values, on average, by 32, 43 and 23% in trunk extension, flexion and lateral bending, respectively, because, unlike the EMG method, it could not predict co-contraction of anatomically antagonistic muscles. We incorporated the OPT method's advantage of forcing an equilibrium in the reaction moments into the EMG method in a new approach we have called 'EMG assisted optimization' (EMGAO). Muscle force estimates from the EMG and EMGAO methods differed from those from the OPT method, on average, by 123% (RMS) for flexion and extension and by 218% for lateral bends. Data from the two approaches result in different conclusions about spine mechanics. We have more confidence in the EMG assisted methods because they respond to variation in muscle synergy and co-contraction patterns commonly observed in different trials and subjects for the same reaction moments.