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

Are pushing and pulling work-related risk factors for upper extremity symptoms? A systematic review of observational studies

Systematically review observational studies concerning the question whether workers that perform pushing/pulling activities have an increased risk for upper extremity symptoms as compared to workers that perform no pushing/pulling activities. A search in MEDLINE via PubMed and EMBASE was performed with work-related search terms combined with push/pushing/pull/pulling. Studies had to examine exposure to pushing/pulling in relation to upper extremity symptoms. Two authors performed the literature selection and assessment of the risk of bias in the studies independently. A best evidence synthesis was used to draw conclusions in terms of strong, moderate or conflicting/insufficient evidence. The search resulted in 4764 studies. Seven studies were included, with three of them of low risk of bias, in total including 8279 participants. A positive significant relationship with upper extremity symptoms was observed in all four prospective cohort studies with effect sizes varying between 1.5 and 4.9. Two out of the three remaining studies also reported a positive association with upper extremity symptoms. In addition, significant positive associations with neck/shoulder symptoms were found in two prospective cohort studies with effect sizes of 1.5 and 1.6, and with shoulder symptoms in one of two cross-sectional studies with an effect size of 2.1. There is strong evidence that pushing/pulling is related to upper extremity symptoms, specifically for shoulder symptoms. There is insufficient or conflicting evidence that pushing/pulling is related to (combinations of) upper arm, elbow, forearm, wrist or hand symptoms.

Lumbar bone mass predicts low back pain in males

STUDY DESIGN

Longitudinal study of lumbar bone mass as predictor of low back pain (LBP).

OBJECTIVE

To investigate whether low bone mineral content (BMC) and bone mineral density (BMD) values at the age of 36 years are associated with the prevalence of LBP at the age of 42 years among the study population of the Amsterdam Growth and Health Longitudinal Study.

SUMMARY OF BACKGROUND DATA

Results of epidemiological, clinical, and in vitro studies indicate that spinal injuries, caused by mechanical loading, might be a cause of LBP. BMC and BMD are determinants of spinal strength. We therefore hypothesized that BMC and BMD are associated with LBP.

METHODS

At the age of 36 years, the lumbar BMC and BMD were determined by dual-energy x-ray absorptiometry in 140 men and 152 women. At the age of 42 years, the participants were asked whether they had experienced LBP in the previous 12 months. Logistic regression analyses were performed to determine odds ratios (ORs)-adjusted for stature, body weight, physical activity, and smoking-for the relationship of BMC and BMD with LBP.

RESULTS

BMC and BMD at the age of 36 years were significantly associated with the reported 12-month prevalence of LBP at the age of 42 years. This association, however, was observed only for men and not for women. Men within the quartile with the lowest BMC or BMD values had higher odds for LBP with ORs of 4.78 (95% confidence interval, 1.52-15.00) and 3.48 (95% confidence interval, 1.23-9.85), respectively.

CONCLUSION

For a male population that is not characterized by osteoporosis or old age, lower lumbar BMC and BMD values at the age of 36 years are associated with an increased risk of reporting to have had LBP in the previous 12 months at the age of 42 years.

Evaluation of three ergonomic measures on productivity, physical work demands, and workload in gypsum bricklayers

BACKGROUND

This study evaluated the effects of a combination of three ergonomic measures designed to reduce the risk of low back complaints among gypsum bricklayers. The measures focused on optimizing working height and reducing carrying distances.

METHODS

A within-subjects (N = 10) controlled field study was used to compare the effects of working with the ergonomic measures with those of working with conventional working methods at the worksite during the course of a full working day. Productivity, work demands, and workload were assessed.

RESULTS

No effects were found on productivity, total work time, duration of tasks, duration of carrying, or energetic or biomechanical workload. However, the duration and frequency of working between knee and hip height during a working day increased by 25% and 15%, respectively, due to the ergonomic measures. During the finishing task, the duration and frequency of working below knee level decreased significantly by 4 min and 71 times, respectively.

CONCLUSION

The limited impact of the ergonomic measures argues for additional measures to reduce the risk of low back complaints.

Validity of estimates of spinal compression forces obtained from worksite measurements

Estimates of peak spinal compression in manual materials handling were compared between a state-of-the-art laboratory technique and a method applicable at the worksite. Nine experienced masons performed seven simulated tasks in a mock-up in the laboratory and nine matched masons were studied during actual performance of the same tasks at the worksite. From kinematic and kinetic data obtained in the laboratory, compression forces on the L5S1 joint were calculated. In addition, compression forces were estimated from the horizontal and vertical position of the blocks handled relative to the subject measured at the worksite. Comparison of group-averaged values showed that the worksite method underestimated peak compression by about 20%. Rank ordering of tasks for back load was, however, consistent between methods, supporting validity of the worksite method to compare different tasks or to determine the effects of ergonomic interventions with regard to mechanical back load. STATEMENT OF RELEVANCE: This study validated a method that can be used by ergonomists to determine the effects of (characteristics of) manual materials handling tasks on back load at the worksite.

Effect of lifting height and load mass on low back loading

The objective of this study was to quantify the effect of lifting height and mass lifted on the peak low back load in terms of net moments, compression forces and anterior-posterior shear forces. Ten participants had to lift a box using four handle heights. Low back loading was quantified using a dynamic 3-D linked segment model and a detailed electromyographic driven model of the trunk musculature. The effects of lifting height and lifting mass were quantified using a regression technique (GEE) for correlated data. Results indicate that an increase in lifting height and a decrease in lifting mass were related to a decrease in low back load. It is argued that trunk flexion is a major contributor to low back load. For ergonomic interventions it can be advised to prioritise optimisation of the vertical location of the load to be lifted rather than decreasing the mass of the load for handle heights between 32 cm and 155 cm, and for load masses between 7.5 and 15 kg. Lifting height and load mass are important determinants of low back load during manual materials handling. This paper provides the quantitative effect of lifting height and mass lifted, the results of which can be used by ergonomists at the workplace to evaluate interventions regarding lifting height and load mass.

Effect of block weight on work demands and physical workload during masonry work

The effect of block weight on work demands and physical workload was determined for masons who laid sandstone building blocks over the course of a full work day. Three groups of five sandstone block masons participated. Each group worked with a different block weight: 11 kg, 14 kg or 16 kg. Productivity and durations of tasks and activities were assessed through real time observations at the work site. Energetic workload was also assessed through monitoring the heart rate and oxygen consumption at the work site. Spinal load of the low back was estimated by calculating the cumulated elastic energy stored in the lumbar spine using durations of activities and previous data on corresponding compression forces. Block weight had no effect on productivity, duration or frequency of tasks and activities, energetic workload or cumulative spinal load. Working with any of the block weights exceeded exposure guidelines for work demands and physical workload. This implies that, regardless of block weight in the range of 11 to 16 kg, mechanical lifting equipment or devices to adjust work height should be implemented to substantially lower the risk of low back injuries.

Diversity and variation in biomechanical exposure: what is it, and why would we like to know?

Trends in global working life suggest that the occurrence of jobs characterized by long-lasting low-level loads or repetitive operations is increasing. More physical "variation" is commonly believed to be a remedy against musculoskeletal disorders in such jobs. One aim of the present paper was to shortly review the validity of this conviction. An examination of the available epidemiologic literature pointed out that the effectiveness of initiatives like job rotation or more breaks is weakly supported by empirical evidence, and only for short-term psychophysical outcomes. Only a limited number of studies have been devoted to physical variation, and concepts and metrics for variation in biomechanical exposure are not well developed. Thus, as a second objective, the paper proposes a framework for investigating and evaluating aspects of exposure variation, based on explicit definitions of variation as "the change in exposure across time" and diversity as "the extent that exposure entities differ". Operational methods for assessing these concepts are also discussed.

Effect of the number of two-wheeled containers at a gathering point on the energetic workload and work efficiency in refuse collecting

The effect of the number of two-wheeled containers at a gathering point on the energetic workload and the work efficiency in refuse collecting was studied in order to design an optimal gathering point for two-wheeled containers. Three sizes of gathering points were investigated, i.e. with 2, 16 and 32 two-wheeled containers at a gathering point. The collecting of two-wheeled containers was simulated in a test circuit. The energetic workload was quantified by the parameters oxygen uptake (1 min(-1)), heart rate (beats min(-1)) and perceived exertion. The work efficiency was quantified as the time it took to collect 32 two-wheeled containers per time period. The maximum acceptable amount of two-wheeled containers collected during an 8-h working day was estimated using the energetic criterion of a maximum oxygen uptake of 30% VO2max. The size of the gathering point had no effect on the oxygen uptake, heart rate or perceived exertion. However, the number of two-wheeled containers per collecting period (work efficiency) and the maximum acceptable amount during an 8-h working day were higher in the conditions with 16 and 32 two-wheeled containers at a gathering point compared to the condition with the 2 two-wheeled containers at a gathering point.

Stress distribution changes in bovine vertebrae just below the endplate after sustained loading

OBJECTIVE

To describe the pattern of stress distribution in the vertebral body just behind the endplate, and to document its changes due to sustained loading.

METHODS

Twelve fresh bovine coccygeal motion segments were dissected and tested. Each specimen was axially loaded with a sustained compressive force of 50% of its estimated compressive strength. Before loading, after 1.5 h and after 3 h of loading, the distribution of the axial pressure under the bottom vertebra (i.e., just below its top endplate) was recorded at three force levels (25%, 37.5% and 50% of the estimated compressive strength), using pressure-sensitive film.

RESULTS

Stress distribution over the endplate was found to be fairly uniform. At low compression forces, the stress was the highest centrally. With increased compression and after sustained compression the uniformity improved through a significant redistribution of stress to the periphery. No stress peaks were found to occur after sustained loading.

CONCLUSION

Stress peaks after sustained loading cannot explain the occurrence of endplate fractures in sustained cyclic compression in non-degenerated discs. Competing explanations, such as creep, and fatigue failure, would appear more likely candidates.

RELEVANCE

It has been hypothesised that compression induced fractures of the lumbar vertebral endplate constitute an important etiological factor for low back pain. Competing theories exist on the fracture mechanism in sustained loading and these would have different implications with respect to prevention. The present study evaluated one of these theories.

Within-subject variability in low back load in a repetitively performed, mildly constrained lifting task

STUDY DESIGN

A repeated-measures in vivo experiment.

OBJECTIVE

To describe within-subject variability of spinal compression in repetitive lifting.

SUMMARY OF BACKGROUND DATA

Epidemiology and failure mechanics suggest that peak loads may be more predictive of injury than average loads. Nevertheless, biomechanical studies usually focus on the latter.

METHODS

Ten healthy males performed 360 lifts in 1 hour of a 45-L crate, weighted with a stable 10-kg mass on 1 day and with an unstable mass (10 kg of water) on another day. The maximum compression force in each lift was estimated, using a simple inverse dynamics model and a single equivalent muscle model.

RESULTS

The individual distributions of maximum compression force were slightly skewed to the right (average skewness 0.67). Median and 95th percentile values were used to characterize the distribution. The median (50th percentile) compression ranged from 3375 to 6125 N, and from 3632 to 6298 N in the stable and unstable load conditions, respectively. The within-subjects peak (95th percentile) compression forces were from 405 to 1767 N and from 526 to 2216 N, respectively, higher than the median values. The peak values differed significantly between conditions, whereas the difference in medians did not reach significance. Only a limited trendwise (fatigue-related) variance could be demonstrated.

CONCLUSION

Peak spinal compression by far exceeds median compression in repetitive lifting and can be affected by task conditions independently from the median. Therefore, the variability of spinal loads needs to be taken into consideration when analyzing and redesigning tasks that can cause spinal injuries.

Fractures of the lumbar vertebral endplate in the etiology of low back pain: a hypothesis on the causative role of spinal compression in aspecific low back pain

It is hypothesized that, in a large number of cases of aspecific low back pain, the primary cause of the pain is a fracture of the vertebral endplate caused by compression forces. Clinical studies have shown that, in many low back pain patients, damage of the vertebral bodies and or the intervertebral disc is present. In vitro studies reveal that the most likely type of failure of this anterior part of the spine is a fracture of the endplate as a result of compression. The high incidence of aspecific low back pain concurs with the likeliness of compression fractures of the endplate to occur in everyday life. Furthermore, epidemiological findings and the natural history of low back pain appear to be in line with the proposed hypothesis.

Assessment of mechanical exposure in ergonomic epidemiology

In recent years several different methods have been developed to assess mechanical exposures, which are related to musculoskeletal disorders in ergonomic epidemiology. Each of these methods is capable of measuring one or more aspects of risk factors, but has drawbacks as well. Improper application of methods might result in biased exposure estimates, which has serious consequences for risk estimates arising from epidemiological studies. The aim of this paper was to systematically evaluate the usefulness of different measurement methods in terms of accuracy and applicability. Assessment of external exposure measures by subjective judgements (from experts or self reports from workers), observational methods (on site or afterwards from video recordings), and direct measurements methods (at work or during laboratory simulations) are discussed for each of the dimensions of exposure level, duration, and frequency. It is concluded that expert judgements and self reports give only limited insight into the occurrence of tasks and activities. Further information can be obtained from observations, which can best be combined with direct measurements of exposure to posture, movement, and exerted forces to achieve exposure profiles by occupational task. Internal exposures estimated by biomechanical modelling mostly consider the low back and require information on postures of the different body segments and exerted forces, completed with movement data in the case of dynamic models. Moreover, electromyography (EMG) and measurements of intra-abdominal pressure might be used for this purpose. Both biomechanical models and EMG are useful methods to assess internal exposure, but biomechanical models should not be restricted to the level of compressive forces on the lower back. Finally, current problems and future directions in measurement strategies and methods are discussed.