Effects of lifting frequency and technique on physical fatigue with special reference to psychophysical methodology and metabolic rate

Effects of Lifting Method, Safety Shoe Type, and Lifting Frequency on Maximum Acceptable Weight of Lift, Physiological Responses, and Safety Shoes Discomfort Rating

This study aimed to investigate the physical effects of precision lifting tasks on the maximal acceptable weight of a lift (i.e., psychophysiological lifting capacity where the workers adjust the lifting weight in order to work without any fatigue or strain at the end of the work while wearing common safety shoe types). Additionally, the physical difference between the precise and non-precise lifting conditions associated with wearing safety shoes were assessed by respiration responses and shoe discomfort ratings. To achieve the objective of the study, ten healthy male workers were selected by age (between 25 to 35 years old). Their anthropometric characteristics, including knuckle height, knee height, and body mass index (BMI), were measured. A three-way repeated measures design with three independent variables was used; the variables included-the (1) lifting method (precise and non-precise), (2) lifting frequency (1 and 4 lifts per min), and (3) safety shoe type (light-duty, medium-duty, and heavy-duty). The physiological response variables and one of the subjective factors of this study were-(1) respiration responses, and (2) shoe discomfort rating, respectively. The data were analyzed using the Mauchly's test of sphericity, Shapiro-Wilk normality test, and analysis of variance (ANOVA). The results showed that the use of heavy-duty safety shoes typically increased the shoe discomfort rating under precise lifting methods. Additionally, the lifting frequency was determined to be one of the main factors affecting respiratory responses and shoe discomfort rating. This study also found that respiration responses rose on four lifts per min as compared to 1 lift per min, regardless of the lifting method type. This study indicated that the replacement of some types of ordinary safety shoes used in some workplaces with those selected appropriately might significantly reduce the rating effort required to lift objects or tools. However, the benefits should be carefully evaluated before replacing the safety shoes.

Adding Handles to Optimize Manual Box Handling

The risk factors for developing musculoskeletal disorders in material handling tasks are well known. Among strategies for controlling risks, modifying boxes by adding handles is suggested. However, there are no clear recommendations regarding box modification as an approach to improve musculoskeletal health. In this study, we investigated the main literature databases to identify effects of box modification on reducing physical load. Electronic and manual searches were performed to identify studies that evaluated effects of boxes handles on physical exposure during handling tasks. The included studies were very heterogeneous (methods of assessment, types of handles used, and methodological quality), jeopardizing synthesis of evidence. Despite the mentioned limitations, we could suggest some features that could improve manual handling in practical settings, like the use of cylindrical handles forms with intermediate diameters (between 31 and 51 mm) and 30° inclination. Those characteristics demonstrated positive results on physical exposure. Regular cut-outs were indicated as a beneficial approach when boxes are handled in high surfaces. When handling occurs in medium heights or in the floor level, handles positioned on the top of the box might bring better results. Efforts to standardize methods are important to support both objective and subjective assessment of box handle design, as well to improve the internal validity of studies.

Comparison of Lifting Capabilities of Industrial and Non-Industrial Populations

The major objectives of the work presented here were to determine the differences between lifting capabilities of industrial and non-industrial populations by collecting experimental data under identical conditions and to quantify the patterns of such differences. Using the psychophysical approach, male and female industrial and non-industrial workers performed 36 variations of manual lifting task. The major findings of the study were: (i) generally the responses of non-industrial workers to task variables is very similar to the responses of industrial workers, (ii) overall, the lifting capability of non-industrial workers is significantly lower than the lifting capability of industrial workers.

The Effects of Lifting Frequency on the Dynamics of Lifting

The goal of this study was to quantify the effects of different lifting frequencies (3, 6 and 9 lifts/minute) at different lifting heights (30 and 60 cm) on the kinematics of the lumbar region. Each of these lifting tasks was performed for twenty minutes. The time dependent traces of the both the mean and standard deviation of sagittal acceleration showed subject dependent trends over time. Averaged across time, the results of this study reveal that there is a non-linear increase in the sagittal acceleration with greater frequency of lifting.

Symmetric and Asymmetric Stoop-Lifting Strength

The strength capability of workers is an essential design criterion. Though true job simulated strength is most appropriate, availability of strength values in many standardized postures will permit selection of most suitable or a series of suitable strength values. Six normal young male subjects performed a maximal lifting effort in a stooping posture in isometric (with hip angles 60° and 90° flexion) and isokinetic modes (50 cm per second linear velocity). The isokinetic lift was performed from floor to knuckle height. Each lift was performed at half, three-quarters, and full reaches by each subject in sagittal, 30° lateral and 60° lateral planes. The strength was measured using the Dynamic Strength Tester (DST) described by Kumar et al (1988). The strength was inversely related to reach distance (p<0.0001). The largest declines in strength were between half and three-quarters reach distances (45% to 50%). From three-quarters reach to full reach, the drop was between 10% to 30%. With increasing asymmetry, the strength declined significantly (p<0.0001). The extent of this decline decreased at longer reaches. Based on these findings, a case is built for a biomechanical standardization of strength measurements.

Safety of Back! What is the Margin?

Despite many efforts to control low-back pain problem it still continues to be a concern. Since the causation of low-back pain is under multifactorial control, it will always be the factor most vulnerable at a given time which will determine safety. The present study is an integrative inferential synthesis of the published work to discern the margin of safety. An attempt has been made to conclude, on the basis of objective evidence, an all encompassing criteria to ensure system safety. Though psychophysical approach integrates biomechanical and physiological variables the role of sensory conditioning needs to be addressed.

Comparison of static lifting capacity between experienced and novice Taiwanese female workers

BACKGROUND

Previous studies have analyzed the lifting capacity of either experienced workers or novices, but a systematic comparison of the lifting capacity of experienced female workers and novices has not been conducted.

OBJECTIVE

This study was conducted to identify differences in lifting strengths and postures between experienced and novice Taiwanese female workers.

METHODS

Twenty-three experienced female workers and 23 novices volunteered for this study. Their static lifting strengths (n = 46) and lifting postures (n = 22) were analyzed using statistical analysis software.

RESULTS

Experienced workers' vertical lifting strengths were approximately 5-7  kg lower than those of novices at lower heights (≤50  cm). However, at heights equal to or higher than 90  cm, the experienced workers generated approximately 2-3  kg higher toward-body lifting strengths than did novices. The experienced workers' strengths at all 15 lifting heights were relatively unchanged, compared with changes in the novices' strengths. Experienced workers tended to adopt a consistently deep squat at lower heights and a more flexed arm posture at higher heights.

CONCLUSIONS

Through daily work experience, experienced workers may have subconsciously learned to shift their postures to avoid overexertion. These techniques for safe lifting should be taught to new Taiwanese female workers.

Lifting strategies of expert and novice workers during a repetitive palletizing task

Thirty manual material handlers (15 experts and 15 novices) were invited to perform series of box transfers under conditions similar to those of large distribution centers. The objective of the present study was to verify whether multiple box transfers leading to fatigue would also lead to differences between expert and novice workers in joint motions and in back loading variables (L5/S1 moments). The task consisted in transferring 24 15-kg boxes from one pallet to another (4 layers of boxes; 6 boxes/layer: 3 in the front row, 3 in the back) at a self-determined pace and then at an imposed pace of 9 lifts/min for a total of 240 lifts. The underlying idea was to set a challenging task that would force the experts to use their skills. Full-body 3D kinematic data were collected as well as external foot forces. A dynamic 3D linked segment model was used to estimate the net moments at L5/S1. The results clearly show that the experts bent their lumbar spine less (10° less) and were closer (4 cm) to the box than novice workers. Knee flexions were similar in both groups except when the box was lifted from ground level (expert ≈ 71°, novice ≈ 48°). The peak resultant moment was not statistically different (expert = 168 Nm, novice = 184 Nm) although experts had lower values on average than novices when lifting heights (and deposit heights) of the boxes increased. Therefore, experts differed from novice workers mostly in the posture-related variables. These differences are especially important to consider when the box is located on the ground, as the back posture and back loading are then at their greatest magnitude and could have a major impact on the distribution of internal forces on the spine.

Efficacy of three interventions at mitigating the adverse effects of muscle fatigue on postural control

This study evaluated the efficacy of three interventions at reducing the adverse effects of muscle fatigue on postural control. The first provided rest breaks according to perceived decrements in postural stability, while the other two involved auditory stimulations (static pure tone and moving conversation). Sixteen participants performed repetitive box handling (lifting + lowering) over 1.5 h to induce muscle fatigue mainly in the lumbar extensors. Trials of quiet upright stance were completed at 10-min intervals, during which the interventions (or a control condition) were applied. Postural control was assessed using perceived stability (PS) and several measures derived from centre-of-pressure (COP) time series. Allowance of rest breaks did not significantly affect any of the objective measures, though a trend indicated an offset to fatigue-induced decreases in PS. Both the static pure tone and moving conversation led to significant changes in the dependent measures indicating a mitigation of fatigue-induced postural instability.

PRACTITIONER SUMMARY

We examined the effects of three control strategies on postural control in the presence of muscle fatigue induced by a simulated occupational task. The findings can facilitate the development of future strategies or practical interventions to reduce falling risk and prevent falls.

Performance-integrated self-report measurement of physical ability

BACKGROUND CONTEXT

The technology of self-report measures has advanced rapidly over the past few years. Recently, this technology was used to develop a performance-integrated self-report measure for use with patients with musculoskeletal impairments that may lead to work disability. Psychometric studies of the new measure in patient populations have been successful. A validation study of the measure with adults in good general health is necessary.

PURPOSE

The purpose of this study was to assess the concurrent validity of a new performance-integrated self-report measure, the multidimensional task ability profile (MTAP).

STUDY DESIGN/SETTING

A prospective validation study was conducted in which a self-report measure was administered online, and a physical performance test was administered at various clinics in North America.

PATIENT SAMPLE

One hundred ninety-six (34% male) adult volunteers in good general health participated in this study.

OUTCOME MEASURES

Self-report measure-MTAP. Physiologic measure-EPIC Lift Capacity test.

METHODS

The MTAP was administered online within 1 week of formal testing of lift capacity using a standardized lift capacity test, the EPIC Lift Capacity test. MTAP scores were compared with performance on the EPIC Lift Capacity test. Stepwise regression analysis was used to identify the strength of the relationship between the two measures and the relative explanation of lift capacity variance by the MTAP score, along with gender and age.

RESULTS

The combination of MTAP score, gender, and age demonstrated a regression coefficient of R=0.82, which accounts for 67.3% of the variance in lift capacity.

CONCLUSIONS

The MTAP displayed good concurrent validity compared with actual physical performance as assessed by the EPIC Lift Capacity test. Modern performance-integrated self-report measures, such as the MTAP, have the potential to provide information about functional capacity that is sufficiently useful to confirm status and help guide treatment algorithms.

Obesity does not reduce maximum acceptable weights of lift

The maximum acceptable weights of lift (MAWL) of obese and non-obese participants were empirically investigated. Three obesity levels were considered: non-obese (18.5 kg/m(2)< or= body mass index (BMI)<or=24.9 kg/m(2)), moderately obese (35 kg/m(2)<or=BMI<or=39.9 kg/m(2)) and extremely obese (BMI>or= 40 kg/m(2)). Ten male and 10 female participants were recruited for each obesity level. The participants determined their MAWL for 18 different lifting task conditions (six lifting frequencies x three lifting heights). An analysis of variance (ANOVA) was conducted to determine the effects of obesity level, gender, lifting height, lifting frequency and their interactions on MAWL. Overall, the ANOVA results indicated that obesity does not reduce MAWL, and thus, suggested that the existing MAWL data can be used to accommodate both general and obese workers. However, further studies based on the biomechanical and physiological approaches are required to provide more complete understanding of obesity effects on lifting tolerance limits.

The effect of fatigue on trunk muscle activation patterns and spine postures during simulated firefighting tasks

The purpose of this study was to determine the effect of a fatiguing task (3 min intense stair climbing) on the adopted spinal postures and trunk muscular activation patterns during three highly physically demanding simulated firefighting tasks. Following the fatigue protocol, it was observed that individuals adopted significantly greater spinal flexion (16.3 degrees maximum prior to fatigue as compared to 20.1 degrees post fatigue) and displayed reduced abdominal muscle activation as compared to before the fatigue protocol (mean ranging from 16.6% maximum voluntary contraction (MVC) to 30.6% MVC prior to fatigue as compared to ranging from 14.6% MVC to 25.2% MVC post fatigue). The reduced abdominal activation may be due to a reduction in co-contraction during these tasks, which may compromise spinal stability. Reduced co-contraction combined with the increased spinal flexion may increase the risk of sustaining an injury to the low back.

Spine loading as a function of lift frequency, exposure duration, and work experience

BACKGROUND

Physiological and psychophysical studies of the effects of lifting frequency have focused on whole-body measurements of fatigue or subjective acceptance of the task and have not considered how spine loads may change as a function of lift frequency or lift time exposure. Our understanding of biomechanical spine loading has been extrapolated from short lifting bouts to the entire work day and may have led us to incorrect assumptions. The objective of this project was to document how spine loading changes as a function of experience, lift frequency, and lift duration while repetitively lifting over the course of an 8-h workday.

METHODS

Twelve novice and twelve experienced manual materials handlers performed repetitive, asymmetric lifts at different load and lift frequency levels throughout an 8-h exposure period. Compression, anterior-posterior shear, and lateral shear were evaluated over the lifting period using an EMG-assisted biomechanical model.

RESULTS

Spinal loads increased after the first 2 h of lifting exposure regardless of the lift frequency. Loading was also greater for the inexperienced subjects compared to experienced lifters. The greatest spine loads occurred at those lift frequencies and weights to which the workers were unaccustomed.

INTERPRETATION

Increases in spine loading were tracked back to the changes in muscle recruitment patterns that typically involved increased muscle coactivation. The results emphasize the importance of previous motor programming in defining spine loads during repetitive lifting. These results indicate a very different influence of frequency and lift time exposure compared to physiologic and psychophysical assessments. This study has shown that it is not sufficient to extrapolate from short lift periods to extended exposure periods if the biomechanical loading implications of the task are of interest.

Psychophysical and psychosocial comparison of squat and stoop lifting by young females

Psychophysical and psychological criteria are accepted risk measures for manual handling task assessment, yet few reports have compared squat and stoop using these criteria. Seventeen university students participated in a within-subjects cross over design study to compare squat and stoop techniques using maximum acceptable weight (MAW), perceptions of exertion (RPE), discomfort and preference. Mean (SD) MAW for squat was lower than for stoop (7.0 (2.2) kg vs 8.5 (2.4) kg) and RPE for squat was greater than for stoop (15.2 (1.5) kg vs 13.3(1.5) kg). More subjects reported discomfort following squat and a preference for stoop. The results provide limited support for use of stoop rather than squat technique for lifting a medium sized box from floor to knuckle height.

Heart rate response to two lifting techniques

OBJECTIVE

To determine the heart rate response to repetitive lifting of light weights employing two techniques, stoop and squat.

DESIGN

Randomized cross-over trial.

PARTICIPANTS

Twenty healthy volunteers (11 women, 9 men), mean age of 25 years, recruited from the staff of a physical medicine and rehabilitation department.

METHODS

Subjects lifted and lowered a box weighing 0, 1, or 3 kg, using the stoop (bent at waist with knees straight) and squat (knees bent) techniques, 20 times in 1 minute.

MAIN OUTCOME MEASURES

The heart rate was recorded every 5 seconds during the effort aid 30 seconds and 1 minute after the test.

RESULTS

Significantly higher heart rates were found during both the test and recuperation phases of the squat technique with each of the three loads (p < .05). The heart rate increase was also higher with the 3 kg load than with the 1 kg or 0 kg load (p = .0001).

CONCLUSION

The heart rate is lower using the stoop technique for lifting. This may explain why individuals choose the more ergonomically risky stoop technique in everyday lifting tasks.

Feature extraction and quantification of the variability of dynamic performance profiles due to the different sagittal lift characteristics

Investigation of manual material handling (MMH) tasks, such as lifting, requires the quantification of the various kinematic and kinetic parameters of performance for assessment of the functional capacity and/or task demand profiles. Traditional statistical descriptive analyses usually involve computing the summary statistics (maximum, minimum, mean, and/or range) of the resulting performance parameters over the cycle duration (i.e., lifting/lowering cycle). Consequently, the significant information content of the time-varying signals is diminished, limiting the sensitivity of subsequent hypothesis testing procedures. The present study developed a methodology for representing and quantifying performance data variability of the kinematic and kinetic motion profiles due to the different lift characteristics (load, mode, and speed) during MMH tasks while capturing the temporal characteristics. Using a database of motion profiles from a manual lifting experiment, the Karhunen-Loeve Expansion (KLE) feature extraction technique was shown to be quite effective for representing the various motion profiles. The number of basis vectors (eigenvectors) and corresponding coefficients needed for accurate representation were substantially smaller than the original data set, resulting in data compression. Moreover, the effects of lift characteristics were investigated using analysis of variance techniques that recognize the vectorial constitution of the waveforms. The application of these techniques will enable the quantification of highly phasic profiles and enhance the ability to document the effect of intervening measures such as educational or physical training/exercise on the kinematic and kinetic patterns of performance. Additionally, the differential influence of lift characteristics on the variability of performance during different phases of lifting and lowering provides added resolution in the analysis of MMH tasks.

Effect of semi-rigid lumbosacral orthosis use on oxygen consumption during repetitive stoop and squat lifting

The use of back belts in industry has increased despite the lack of scientific evidence supporting their efficacy. The purpose of this study was to investigate the effect of a semi-rigid lumbosacral orthosis (SRLSO) on oxygen consumption during 6-min submaximal repetitive lifting bouts of 10 kg at a lifting frequency of 20 repetitions min-1. Fifteen healthy subjects (13 men, two women) participated in this study. Each subject performed squat and stoop lifting with and without an SRLSO for a total of four lifting bouts. Lifting bouts were performed in random order. Oxygen consumption during the final minute of each lifting bout was used for analysis. A two-way analysis of variance with repeated measures was used to analyse the effects of lift and belt conditions. The stoop and squat methods were significantly different, with the squat lift requiring 23% more oxygen on average than the stoop lift for equal bouts of work. No significant difference was found between the belt and no belt condition within the same lifting technique and no interaction was present. These data suggest that an SRLSO does not passively assist the paravertebral muscles (PVM) in stabilizing the spine during submaximal lifting bouts.

A critical review of biomechanical, epidemiological, physiological and psychophysical criteria for designing manual materials handling tasks

For over four decades, individuals from various disciplines have extensively researched the tasks comprising manual materials handling (MMH): lifting, lowering, pushing, pulling, carrying and holding. The primary motivation for these research efforts has been a desire to understand human capabilities so that tasks can be designed such that the demands of the task are at or below the capacities of the workers performing the task. Various criteria for defining acceptable task demands have been developed from the principles of biomechanics, physiology, and psychophysics. Although significant bodies of literature exist on each class of criteria, there are still areas that need to be examined. Additionally, the validity of several of the criteria is unknown, primarily due to a lack of epidemiological verification of the criteria. This paper presents a critical review of MMH criteria, the shortcomings of and conflicts between the various criteria, and the areas needing further examination. The critical need for epidemiological studies is also detailed.

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.

The influence of personal variables on work-related low-back disorders and implications for future research

Work-related low-back disorders (LBDs) continue to be one of the single largest sources of compensation costs. The relative contributions of personal, workplace, organizational, and environmental variables to the development and severity of LBDs are not completely understood. The inclusion of personal variables in epidemiologic studies of LBDs has been inconsistent, and different authors have different opinions concerning the importance of such variables. Personal variables either known or suspected to influence outcomes are discussed to elucidate the importance of these variables with respect to understanding LBDs and conducting epidemiological studies in industry. The authors suggest that age, gender, injury history, relative strength, smoking, and psychosocial variables be studied further, and that height, weight, pathologies, genetic factors, maximum oxygen uptake, and absolute strength are unlikely to produce significant effects in industrial populations.

An isometric predictor for maximum acceptable weight of lift for Chinese men

The aim of this study was to examine the practicality of the modified isometric strength tests to predict the maximum acceptable weight of lift (MAWL) of Chinese men. The modified strength tests allow the participant to pull on the load cell in front of the body and to apply force in a functional free posture. Both the modified and the standard strength data of each participant were used as predictors for the MAWLs. The prediction models were constructed and evaluated under task conditions of two lifting ranges, two box sizes, and three lifting frequencies. To realize the effect of modifications, testing posture was recorded and the joint angles were calculated. A stepwise multiple regression analysis indicated that modified composite strength (MCS), chest circumference, and acromial height accounted for 86% to 91% of the variance. Because the strength of the upper extremity body was also recruited in the test, the weak upper extremity strength of the Chinese participants would therefore be better reflected. Evidence for the existence of a close match between MAWL and MCS values, as well as the task conditions for its existence, suggest that a simple isometric strength measure is a good predictor for the MAWL.

Self-selected manual lifting technique: functional consequences of the interjoint coordination

The pattern of movement self-selected by 39 subjects to lift light loads from 9 cm above the ground is described in kinematic and electromyographic terms. Hamstring length changes were estimated from hip and knee angular kinematics. Subjects adopted a posture at the start of the lift intermediate between stoop and full-squat postures. A consistent coordination between knee, hip, and lumbar vertebral joints during lifting was described through calculation of the relative phase between adjacent joints and found to be exaggerated with increases in load mass. During the early phase of lifting, knee extension leads hip extension, which in turn leads extension of the lumbar vertebral joints. Early in the lifting movement, when load acceleration is greatest, the erectores spinae are thus relatively long and shortening slowly. Both of these factors produce greater back extensor strength. Rapid hamstring shortening is also delayed, which enhances their strength, and coactivation of the monoarticular knee extensors and biarticular hamstring observed early in the lifting movement suggested that the knee extensors contribute to hip extension through a tendinous action of the hamstrings.

The handling of objects other than boxes: univariate analysis of handling techniques in a large transport company

The purpose of this study, carried out on handlers in the distribution centre of a large transport company, was to identify the techniques used for handling objects other than boxes. Thirty-one workers, with experience varying from one month to 17 years, were each filmed during one shift. A grid with 36 variables allowed the successive operations to be described: the type of grip, the nature and the direction of the efforts by the upper limbs, the use of the back and the lower limbs, and the displacement of the object. Nine hundred and forty-four handlings carried out in 3217 distinct movements were analysed. These observations revealed the complexity of the handling techniques and the tendency to favour some of them. Seventy-nine per cent of the handlings observed included pre-transfer and/or post-placement phases in addition to the transfer itself. Essentially horizontal phases (pulling, pushing) are more frequent than essentially vertical ones (lifting, lowering); asymmetry is generalized, whether it involves the back position (torsion), the direction and the type of effort, the position of the hands on the object, etc. More than half of the efforts are used to move an object resting on a surface (sliding, pivoting, turning, rolling); resistive efforts downwards (e.g., lowering) are avoided in favour of 'throwing' or 'dropping'.

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.

Research to reality: a critical review of the validity of various criteria for the prevention of occupationally induced low back pain disability

Criteria have been suggested to reduce the incidence, or severity of low back pain disability. Five underlying theories for such criteria have been identified in the literature, and a critical review of the validity of these criteria has been carried out. Despite attributions elsewhere, peer-reviewed validation of the various criteria range from modest to nil. The need for a validation criterion for use in workplace design in order to reduce low back pain disability, or severity, is identified and the need for an international protocol to allow cross-study validation of present and future criteria is suggested.

Perception of heavy work operations by tank truck drivers

Seven male tank truck drivers were followed for about 3 h during ordinary work. They rated the perception of heavy work operations on Borg's CR-10 scale. The heart rates were monitored every minute. The hose was pulled out an average of 22.3 m and the corresponding mean rating was 3.2, a little above 'Moderate' on the CR-10 scale. The heart rate averaged 137 beats/min at the end of this operation. When transforming the physiological parameter heart rate to a subjective strain intensity, surprisingly high agreement was obtained between the results from the field study and the predicted ratings. By transforming ordinally scaled data from another study to a scale with ratio properties, the results could be compared with this study. Good agreement was found between the variables.

Physiological and subjective responses to maximal repetitive lifting employing stoop and squat technique

To establish safe levels for physical strain in occupational repetitive lifting, it is of interest to know the specific maximal working capacity. Power output, O2 consumption, heart rate and ventilation were measured in ten experienced forestry workers during maximal squat and stoop repetitive lifting. The two modes of repetitive lifting were also compared with maximal treadmill running. In addition, electromyogram (EMG) activity in four muscles was recorded and perceived central, local low-back and thigh exertion were assessed during the lifting modes. No significant difference was found in power output between the two lifting techniques. Despite this the mean O2 consumption was significantly greater during maximal squat lifting [38.7 (SD 5.8) ml.kg-1.min-1] than maximal stoop lifting [32.9 (SD 5.7) ml.kg-1.min-1] (P < 0.001). No significant correlation was found between O2 consumption (in millilitres per kilogram per minute) during maximal treadmill running and maximal stoop lifting, while O2 consumption during maximal squat lifting correlated highly with that of maximal treadmill running (r = 0.928, P < 0.001) and maximal stoop lifting (r = 0.808, P < 0.01). While maximal heart rates were significantly different among the three types of exercise, no such differences were found in the central rated perceived exertions. Perceived low-back exertion was rated significantly lower during squat lifting than during stoop lifting. The EMG recordings showed a higher activity for the vastus lateralis muscle and lower activity for the biceps femoris muscle during squat lifting than during stoop lifting. Related to the maximal voluntary contraction, the erector spinae muscle showed the highest activity irrespective of lifting technique.

Revised NIOSH equation for the design and evaluation of manual lifting tasks

In 1985, the National Institute for Occupational Safety and Health (NIOSH) convened an ad hoc committee of experts who reviewed the current literature on lifting, recommend criteria for defining lifting capacity, and in 1991 developed a revised lifting equation. Subsequently, NIOSH developed the documentation for the equation and played a prominent role in recommending methods for interpreting the results of the equation. The 1991 equation reflects new findings and provides methods for evaluating asymmetrical lifting tasks, lifts of objects with less than optimal hand-container couplings, and also provides guidelines for a larger range of work durations and lifting frequencies than the 1981 equation. This paper provides the basis for selecting the three criteria (biomechanical, physiological, and psychophysical) that were used to define the 1991 equation, and describes the derivation of the individual components (Putz-Anderson and Waters 1991). The paper also describes the lifting index (LI), an index of relative physical stress, that can be used to identify hazardous lifting tasks. Although the 1991 equation has not been fully validated, the recommended weight limits derived from the revised equation are consistent with or lower than those generally reported in the literature. NIOSH believes that the revised 1991 lifting equation is more likely than the 1981 equation to protect most workers.

An experimental evaluation of psychophysical criteria for repetitive lifting work

Two experiments were performed to test the reliability and validity of psychophysically determined maximum acceptable workloads for setting lifting standards. The perceived workload in a repetitive diagonal lifting task was found to be a positively accelerated function of the weight lifted and of the work pace respectively. A twofold increase in objective workload resulted in a four- to fivefold increase in perceived workload. This relation was independent of previous occupational experience of lifting work. The psychophysically assessed maximum acceptable workloads for this type of lifting task appeared to be satisfactorily reproducible when subjects had to adjust work pace or when they were left free to adjust both the weight and the work pace. However, the results raised several questions concerning the applicability of the psychophysical assessment of maximum acceptable lifting work. Slight changes in the instructions given to the subjects had a definite effect on the selection of workloads. Furthermore, the workloads selected by subjects with previous occupational experience of lifting work - i e, warehouse workers - were systematically lower than those selected by subjects without such previous experience - i e, office employees. At the same time, the warehouse workers rated perceived exertion higher than the office employees, indicating that previous occupational experience of lifting work enhanced the subjective assessment of physical effort. There were no consistent relations between the workloads found acceptable by the subjects and their physical characteristics and performance capacity.

Reliability of repetitive dynamic strengths as a screening tool for manual lifting tasks

Recent studies have shown that maximal dynamic strengths (MDSs) of individuals are superior and more reliable predictors of their maximum safe lifting capacity compared to maximal static strengths. The evidence, however, is based on studies that have investigated only infrequent lifting activities, i.e. lifting only a few times a day and with complete recovery from fatigue between successive lifts. It remains to be seen if MDSs are also as highly correlated to lifting capacity for frequent lifting tasks i.e. tasks that do not allow complete recovery from fatigue between successive lifts. The study reported here was conducted to determine the degree of affinity between MDSs and lifting capacities of individuals for frequent and infrequent lifting tasks. In addition, the hypothesis that lifting capacity for frequent manual lifting tasks will be more highly correlated to dynamic strength values which take into account the effect of repetition related fatigue, instead of MDSs, was tested. The results overwhelmingly supported the hypothesis. We therefore concluded that repetitive dynamic strength (RDS) is a more accurate measure of an individual's lifting capacity for frequently performed tasks, than maximal static or dynamic strengths, and deserves recognition as a reliable pre-employment screening tool for frequently performed manual lifting tasks. The experimental evidence also indicated that manual lifting tasks performed once every minute or less frequently are strength oriented while those per formed three times a minute or more frequently are not.

Biomechanical stresses as related to motion trajectory of lifting

A laboratory study was conducted to assess the effects of lift angle on biomechanical stresses to the musculoskeletal system, and in particular, on the lower back. Male subjects were required to lift four different tote boxes from the floor to a height of 0.81 m using two different lifting techniques. Maximum acceptable weights were determined using a psychophysical method. The motion trajectory of lift was studied by means of stroboscopic photography. Moments and forces at various body joints were computed using a biomechanical strength model.

Maximum acceptable weights were 16% (range = 9 to 23%) greater for the free-style lifting technique as compared with the squat method of lifting. There were large differences between the maximum acceptable weights and the "maximum permissible limits" recommended by NIOSH. The results showed that the subjects pulled the load toward the body when lifting psychophysically determined maximum weights. The lift angle increased with an increase in box width. The lift angle was greater for the free-style lifting method as compared with the squat lifting technique for lifting compact loads. Lifting at an angle resulted in a decrease in moment at the elbow, shoulder, L5IS1 disc, and hips, and an increase in moment at the knees and ankles. The estimated compressive force on the lumbar spine was, on the average, 11% lower for lifting at an angle as compared with a straight vertical lift.