Robert W. Norman

Disability resulting from occupational low back pain. Part II : What do we know about secondary prevention? A review of the scientific evidence on prevention after disability begins

This is the second of two papers that systematically review available scientific evidence on the causes of disability from occupational low back pain, and the effectiveness of interventions to prevent it after its onset (secondary prevention). This paper reviews the national history of how back pain and the risk factors for its extension into chronic disability, followed by a critical summary of intervention studies attempting to reduce the duration of this disability, and to evaluate the results.

DYNAMICALLY AND STATICALLY DETERMINED LOW BACK MOMENTS DURING LIFTING

Assessment of the effects of lifting on the low back has most frequently been done with the aid of static models. Many lifting movements appear to have substantial inertial components. It was of interest, therefore, to determine the size of the difference between statically and dynamically calculated lumbar moments during a demanding but not unusual manual lift observed in a metal fabrication industry. The results of several trials by four young men showed that the dynamic model resulted in peak L4/L5 moments 19% higher on average, with a maximum difference of 52%, than those determined from the static model. The technique adopted in the lift could minimize the difference. When the inertial forces of the load itself and the load weight were incorporated into an otherwise static model (quasi-dynamic) then the resulting L4/L5 moments exceeded those of the fully dynamic model by 25%. In many industrial tasks static analyses may severely underestimate the demands of dynamic lifts. These results show that a reasonably inexpensive approach in lifting task analysis is to measure the dynamic forces of the load on the hands and to use these in an otherwise static model. This results in a conservative assessment of the injury risk of lifts at least of the type reported in this study.

Trunk muscle and lumbar ligament contributions to dynamic lifts with varying degrees of trunk flexion

This study was done to assess the interplay between muscular and ligamentous sources of extensor moment during dynamic lifting with various loads and flexion angles of the trunk segment for 15 subjects lifting a total of 150 loads. Ligament forces predicted from an anatomically detailed biomechanical model did not generally contribute more than 60 Nm for most of the lifts because the lumbar spine was only flexed to a moderate and constant degree for each load condition. In contrast, additional moment demands associated with increases in hand load were supported by muscle. Although the compression forces on the L4-5 intervertebral disc were fairly insensitive to the interplay between the recruitment of muscle and ligament, the shear force was significantly higher with a greater degree of lumbar flexion. The risk of injury may be influenced more by the degree of lumbar flexion than the choice of stoop or squat technique.

Biomechanical analysis of peak and cumulative spinal loads during simulated patient-handling activities: a substudy of a randomized controlled trial to prevent lift and transfer injury of health care workers

Back injuries are a serious problem for nursing personnel who perform frequent patient-handling activities. Common prevention strategies include body mechanics education, technique training, and ergonomic interventions such as the introduction of assistive equipment. This investigation assessed and compared the effectiveness of two patient-handling approaches to reducing injury risk. One strategy involved using improved patient-handling technique with existing equipment, and the other approach aimed at eliminating manual patient handling through the use of additional mechanical and other assistive equipment. Both intervention arms received training in back care, patient assessment, and use of the equipment available on their particular wards. An analysis of compliance with interventions and the effects of patient-handling methods on both peak and cumulative spinal compression and shear during various tasks was conducted. Results showed greater compliance with interventions that incorporated new assistive patient-handling equipment, as opposed to those consisting of education and technique training alone. In several tasks, subjects who were untrained or non-compliant with interventions experienced significantly higher peak spinal loading. However, patient-handling tasks conducted with the aid of assistive equipment took substantially longer than those performed manually. This, along with variations in techniques, led to increases in cumulative spinal loading with the use of patient-handling equipment on some tasks. Thus, the use of mechanical assistive devices may not always be the best approach to reducing back injuries in all situations. No single intervention can be recommended; instead all patient-handling tasks should be examined separately to determine which methods maximize reductions in both peak and cumulative lumbar forces during a manoeuver.

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.

Effects of load placement on back muscle activity in load carriage.

SummaryThe effect of two different load placements (just below mid-back or just above shoulder level) on erector spinae EMG, trapezius EMG, and heart rate were investigated during load carriage. The EMG and heart rates were telemetered from 11 subjects while they walked on a smooth level surface at an average velocity of 5.6 km·h−1 carrying a load of 19.5 kg in a specially designed backpack. The average rectified EMG amplitude was calculated digitally for both load placements. The high load placement resulted in significantly higher levels of muscle activity than did the lower placement. Heart rate was not significantly different between the two placements. A qualitative biomechanical analysis suggests that the EMG differences are primarily due to differences in the moments and forces arising from the angular and linear accelerations of the load and trunk. The results indicate that metabolic measures alone are not sufficient to adequately assess tasks which evoke primarily local muscle demands.

An assessment of derivative determining techniques used for motion analysis

Abstract All kinematic studies of human motion employ measurement techniques which introduce noise into displacement data. Commonly, the data, as time related functions, are differentiated to produce velocity and acceleration information. Unfortunately, differentiation amplifies the noise present to such an extent that additional signal treatment is essential. The study was conducted to compare film generated acceleration curves with the analog acceleration curves of single segment movements. An instrumented segment was manually manipulated by the hand to produce analog records of the angular displacement and acceleration of the segment. Simultaneous filming of the segment produced synchronized displacement data. Acceleration functions were determined from these data using “finite difference”, “Chebyshev least squares polynomials” and “digital filtering followed by finite difference” techniques. Digital filtering combined with a first order finite difference technique produced acceleration data very closely approximating the analog signals. The other two techniques were clearly inferior. Data are provided to enable the reader to evaluate his own differentiation procedures.

The effect of an abdominal belt on trunk muscle activity and intra-abdominal pressure during squat lifts.

The purpose of this study was to determine whether abdominal belts such as those prescribed to industrial workers reduced trunk muscle activity and/or increased intra-abdominal pressure (IAP). In this study, six subjects lifted loads (72.7 to 90.9 kg) both with and without wearing a weightlifter belt. In addition, further trial conditions required that subjects lifted both with the breath held or continuously expiring on lifting effort. Dynamic hand loads were recorded together with intra-abdominal pressure (IAP) and abdominal, intercostal and low back EMG. Every subject demonstrated an increase in IAP when wearing the belt during both breathing conditions: 99 mmHg with no belt; 120 mmHg wearing belt (p less than 0.0001). However, it was also found that significant increases in IAP occurred (p less than 0.017) when the breath was held versus exhaling with or without the belt. One would expect that if the belt relieved either the direct compressive load on the spine or assisted IAP to produce an extensor moment then this would be reflected in diminished extensor muscle activity. Erector spinae activity tended to be lower with the breath held suggesting a reduced load on the lumbar spine although wearing a belt did not augment this reduction. In the case studies with subjects wearing an ergogenic corset designed for use by industrial manual materials handlers, perceptions of improved trunk stability were reported. However, the muscle activity and IAP results of this study during short duration lifting tasks make it difficult to justify the prescription of abdominal belts to workers.

Lumbar spine loads during the lifting of extremely heavy weights.

The reaction moments at the knee, hip, and L4/L5 joints, and the compressive and shearing forces on L4/L5 are documented in powerlifters competing in a national powerlifting championship. Analyses were made of 13 female and 44 male competitors. The joint moments and forces were estimated from a linked segment model (WATBAK) that incorporated functional low back extensor musculature with a moment arm of 6 cm and a line action that was oriented 5 degrees posteriorly to the L4/L5 compression axis. This oblique orientation of the extensor muscles reduced the anterior shearing load on the vertebral motion unit. Average compressive loads on L4/L5 were estimated up to 17,192 N while the highest average L4/L5 and hip moments were 988 and 1047 N.m, respectively. The sumo deadlift style resulted in a 10% reduction in the joint moment and 8% reduction in the load shear force at the L4/L5 level when compared with the conventional lifting style. Formulation of linear regression equations to predict the load lifted using reaction joint moments yielded substantial unexplained variability, though significant relationships were found. This analysis suggested that there is large variability in the pattern of loading joints among national class powerlifters.

Reassessment of the role of intra-abdominal pressure in spinal compression

Biomechanical models used to estimate loads on the lumbar spine often predict internal low back forces for heavy lifts that exceed known tissue tolerances, yet the particular lift caused no apparent damage to the lifter. To deal with this paradox, many researchers have incorporated some form of spinal compression alleviation from intra-abdominal pressure (IAP). The purpose of this work was to re-examine some of the issues involved in the feasibility of IAP to reduce spinal loads during stressful lifts. Questions remain over the trade-off between the beneficial tensile force on the spine, exerted via the diaphragm and pelvic floor when IAP is produced, and the undesirable compressive effects of abdominal muscular force required to maintain the pressure within the abdomen. Various strategies of modelling IAP and its effects on low back loading were employed, Three major differences between this and most previous models of IAP effects were the attempt to quantify the size of abdominal muscle forces and the u...