A cross-validation of the NIOSH limits for manual lifting

The psychophysical, biomechanical, and physiological criteria used in establishing the NIOSH limits for manual lifting were cross-validated against the data published by different researchers in the subject literature. Assessment of the 1991 NIOSH lifting equation indicated that: (1) NIOSH-based limits are significantly different from the psychophysical limits in the (i) low and high frequencies of lift, and (ii) small and large horizontal distances; (2) NIOSH limits are highly correlated with the data of Snook and Ciriello (1991) in the low frequency range, with the Recommended Weight Limit (RWL) protecting about 85% of the female population and 95% of the male population; (3) the 3·4 kN limit for compression on the lumbosacral joint cannot protect the majority of the worker population on the basis of damage load concept; and (4) energy expenditure limits used in development of the RWL index can be sustained by 57 to 99% of worker population when compared to the physiological limits based on previous fatigue studies. Results of the cross-validation for psychophysical criterion confirmed the validity of assumptions made in the 1991 NIOSH revised lifting equation. However, the results of cross-validation for the biomechanical and physiological criteria were not in total agreement with the 1991 NIOSH model     

New procedure for assessing sequential manual lifting jobs using the revised NIOSH lifting equation

A sequential manual lifting job is defined as a job where workers rotate between a series of manual lifting rotation slots or elements at specified time intervals during the course of a work shift. The original NIOSH lifting equation lacked a method for assessing the physical demands of these types of jobs. This paper presents the sequential lifting index (SLI), a new conceptual method for assessing the physical demands for sequential manual lifting jobs. The new method is similar to the composite lifting index (CLI) method that was provided by NIOSH for assessing multi-task jobs. The SLI method expands upon the methods originally provided by NIOSH by providing a simple method for estimating the relative magnitude of physical stress for sequential manual lifting jobs. It should also be useful in assisting safety and health specialists to prioritize or rank hazardous jobs within a plant.     

The relationship between maximum isometric strength and intramuscular circulatory occlusion

Twenty male college students served as subjects for lhe study which investigated the relationship between maximum isometric strength and the isometric tension necessary to produce total occlusion of intramuscular circulation. Subjects performed a scries of progressively increasing static contractions by squeezing a hand dynamometer and the subsequent blood flow responses were measured. Blood flow was occluded at a mean of 63-5% maximum handgrip strength for the total group tested. To determine whether individuals of different maximum strength occlude at different percentages of their MVC, subjects were divided into high and low strength groups. The high strength group was found to occlude at 51-5% of MVC while the low strength group occluded at 75-5% of MVC. A significant negative correlation (r= -0.58) was found between maximum isometric strength and the per cent of maximum strength necessary to produce intra-muscular occlusion. There was no significant difference between the absolute tension producing occlusion in the high strength group (34-8 kg) and that of the low strength group (34 3 kg).     

Accuracy of measurements for the revised NIOSH lifting equation

Twenty-seven non-ergonomists who participated in a one-day training session on the use of the NIOSH lifting equation (NLE) were subsequently tested on a simulated lifting task eight weeks later to determine their accuracy in measuring the variables. Analysis of the results indicate that (1) inter-observer variability was small, especially for the most important factor (i.e. horizontal distance); (2) individuals can be trained to make measurements with sufficient accuracy to provide consistent recommended weight limit and lifting index values; and (3) measurement of the coupling and asymmetric variables were the least accurate.     

Usability of the revised NIOSH lifting equation

Following the revision of the 1981 National Institute for Occupational Safety and Health (NIOSH) lifting equation, research needs related to the new equation were outlined. Aside from epidemiological studies, the need to evaluate the usability of the 1991 NIOSH equation in realistic work environments was expressed. This paper reports on extensive experiences with training users and application of the equation in varied work settings. Qualitative results from training sessions indicated that frequency, asymmetry and duration were the parameters that required relatively longer instruction periods and resulted in the most questions. Field applications indicated that the variable nature of lifting/lowering demands found in many jobs resulted in difficulty applying the equation. Approximately 35% of 1103 lifting and lowering tasks had at least one parameter outside of acceptable ranges, while a majority of workers (62.8%) reported other manual handling tasks that are counter to assumptions made in the development of the equation. The practical implications of the findings are discussed.     

Usability of the revised NIOSH lifting equation

Following the revision of the 1981 National Institute for Occupational Safety and Health (NIOSH) lifting equation, research needs related to the new equation were outlined. Aside from epidemiological studies, the need to evaluate the usability of the 1991 NIOSH equation in realistic work environments was expressed. This paper reports on extensive experiences with training users and application of the equation in varied work settings. Qualitative results from training sessions indicated that frequency, asymmetry and duration were the parameters that required relatively longer instruction periods and resulted in the most questions. Field applications indicated that the variable nature of lifting/lowering demands found in many jobs resulted in difficulty applying the equation. Approximately 35% of 1103 lifting and lowering tasks had at least one parameter outside of acceptable ranges, while a majority of workers (62.8%) reported other manual handling tasks that are counter to assumptions made in the development of the equation. The practical implications of the findings are discussed.     

The measurement and prediction of isometric lifting strength in symmetrical and asymmetrical postures.

Static lifting strengths of nine men and nine women were measured at six heights from just above the floor to just above the head, at two horizontal reaches from the mid-ankles (equal to the elbow to grip and acromium to grip distances), in the sagittal plane and also at 45 degrees and 90 degrees to the right for two-handed exertions and at 45 degrees and 90 degrees to each side for one-handed exertions, making a total of 96 postures. A second and different group of 18 subjects (nine men and nine women) were studied in 20 two-handed and 40 one-handed postures intermediate to those of the first group. A third group of 16 subjects (eight men and eight women), with six drawn from the other groups, were used to determine maximum possible reach (at which lifting strength is zero) at the same heights and planes as those for the first group. When strength was expressed as a fraction of body weight and height and reach were expressed as fractions of stature, predictive equations of static lifting strength were obtained which were gender free. The predictive equations may be used to generate isodyne contours for an individual in any chosen planes. Individuals exist whose strengths are consistently greater or less than the prediction. The possibility of identifying such persons in a process of worker selection is discussed.     

The role of lifting belts in manual lifting

The objectives of this research were to demonstrate the effects of belts on load lifting capabilities of individuals using the psychophysical method, subjective surveys, and the measurement of external pressure on the abdomen. Two types of belts were used in this study: (a) the Air belt, a registered trade mark and product of Pneumedic Corporation, which represented the type of support belt characterized by an inflatable bladder; and (b) the CompVest belt, a product and registered trade mark of the Comp Equipment Corporation, which represented a mechanical, elastic type of support. Twelve male college students were required to lift tote boxes containing steel shot from the floor to metacarpal III height (knuckle height) at a rate of three lifts per minute for a period of 45 min. Each subject performed three sets of lifts, i.e. once with no belt (control) and once with each of the two belts. The maximum acceptable weight of lift was recorded. Changes in the external abdominal belt pressures during the lifting were recorded using the Texas Interface Pressure Evaluator. A subjective survey consisting of eight questions was administered to each subject after each belt was tested to determine the subject's feelings. In addition to the above survey, a post-test question was given asking the subjects to rank the three treatments in order of preference. Statistical tests showed that the belts did increase the perceived maximum acceptable weight of lift with respect to the control (no belt); however, there was no significant difference between the two belts. Also, the external abdominal pressures generated by the two belts were essentially the same. Results of the subjective surveys tended to favor the CompVest belt over the Air belt.     

Isometric and isokinetic testing of lifting strength of males in teamwork

Society is becoming increasingly dynamic and integrated owing to the extensive use of information technology. This has several implications that pose new challenges to the human factors profession. In an integrated society, changes and disturbances propagate rapidly and widely and the increasing scale of operations requires also that rare events and circumstances are considered during systems design. In this situation, human factors contributions should be increasingly proactive, not only responding to observed problems, but also, they should be based on models of adaptive human behaviour in complex, dynamic systems. The paper suggests some methodological issues to consider for human factors analyses by designers, evaluators and teachers.     

Some considerations in the use of isometric,isoinertial and isokinetic strength models for predicting lifting capability

In order to predict lifting capacity of individuals, various mathematical models have been developed. These models have been based on several strength measurements of individuals including isometric, isokinetic and isoinertial strengths. A study was conducted to review and compare prediction models based on their performance. The results indicate that models based on dynamic (isokinetic and isoinertial) strengths are superior to models based on static (isometric) strength. Furthermore, different types of equipment used in measurement of strengths are discussed, and the advantages and disadvantages of each are cited. Their safety, accuracy, price and mobility are compared.     

Psychophysical modelling of lifting capacity of Chinese males using strength variables

This study used the psychophysical approach to determine the maximum acceptable weight of lift (MAWL) for 12 young Chinese male subjects, and used strength variables as predictors to develop prediction models. Each subject performed 12 different lifting tasks involving three lifting heights and four lifting frequencies. The results showed that both the dynamic and static models developed in this study could predict the MAWL with a reasonable degree of accuracy. However, a comparison of the models revealed that the use of the dynamic model resulted in less PRESS (PRediction Error Sum of Squares) statistics and higher [Formula: see text] values than the static model. Isoinertial 1.83 m maximum strength (T1) and the isoinertial elbow-height strength, tested on an incremental weightlifting machine, were found to be the best screening test. This was different from a previous study that recommended T1 as the most promising single screening test for Occidentals.     

Study on the variation of peak isometric strength and EMG activity in static field-simulated lifting postures

Peak isometric strength was measured from 10 adult Indian construction workers in eight different field-simulated (FS) postures. This peak-strength data in these FS postures were compared with symmetric postures. In symmetric postures, the vertical load positions were kept the same as FS postures and the points of force exertion were fixed at 40 cm distance in front of the subject. From both symmetric and FS static strength data, it was shown that the maximum peak strength occurred at medium vertical height level and decreased with both increase and decrease of the vertical height level. The maximum and minimum peak strengths were obtained in different FS postures as 222.85±61.15 N and 85.65±19.89 N, respectively. It was observed that the lifted weight in the field was 12.0 kg, which corresponds to 54.54% and 137.3% (i.e. >100%) of these maximum and minimum peak-strength values. This result indicated the prevalence of high risk factors in the field. During this study, surface EMGs from four different muscles (i.e. trapezius, external oblique, rectus abdominis and erector spinae) were collected while exerting the peak isometric strength. From ANOVA analysis, it was shown that the erector spinae and trapezius activities were significantly (p⩽0.05) related to the peak-strength value, whereas external oblique and rectus abdominis activities were not. It was also observed that RMS of erector spinae activity increases with ipsi-lateral increase of asymmetry angle along with the decrease in maximum static peak-strength level in FS postures.Relevance to industryThe peak static strength and EMG activities were measured in FS postures to highlight the potential risk factors in building construction industry. This study was conducted on adult female building construction workers, where the female construction workers were not well represented in the literature. Moreover, from this study, it is very clear that the actual field postures are of more complex in nature than the simulation studies, as mentioned in earlier studies.     

Development of a safety index for manual lifting tasks

The concept of a safety index (SI) for assigning a worker to a particular manual lifting task is developed, and a simple formula for its calculation is presented. The proposed index is based upon the combined measure of acceptability of the biomechanical and physiological stress responses of the worker to a lifting task. Individual capacity norms, as opposed to the norms usually given based on population percentiles, are also defined. Numerical examples are given to illustrate the SI approach.     

Maximum acceptable weights and maximum voluntary isometric strengths for asymmetric lifting

A laboratory study was conducted to determine the effects of asymmetric lifting on psychophysically determined maximum acceptable weights and maximum voluntary isometric strengths. Thirteen male college students lifted three different boxes in the sagittal plane and at three different angles of asymmetry (30,60 and 90°) from floor to an 81-cm high table using a free-style lifting technique. For each lifting task, the maximum voluntary isometric strength was measured at the origin of lift.

The maximum acceptable weights and the static strengths for asymmetric lifting were significantly lower than those for symmetric lifting in the sagittal plane for three box sizes (P<0·01). The decrease in maximum acceptable weight and static strength from the sagittal plane values increased with an increase in the angle of asymmetry (P < 0·01). Box size had no significant effect (P≥ 0·05) on the percentage decrease in maximum acceptable weight or voluntary isometric strength from the sagittal plane values. Correction factors of 7,15 and 22% for maximum acceptable weights and 12, 21 and 31% for static strength at 30, 60 and 90% of asymmetric lifting are recommended. Lastly, in the absence of epidemiological data, a comparison of maximum acceptable weight and static strength in the sagittal plane with the NIOSH guidelines for action and maximum permissible limits indicates that the guidelines may be conservative.     

A relation between dynamic strength and manual materials-handling strategy affected by knowledge of strength

OBJECTIVE: We studied the relation between dynamic (isokinetic) strength and the batch-assorting strategy to initiate a manual materials-handling task and the effect of knowledge of strength on that relation. BACKGROUND: The debated, complex relationship between muscular strength and the risk of injury can be better understood from a behavioral perspective by examining performance strategies in physical acts such as lifting. METHODS: Thirty-two participants (16 men and 16 women) were first tested for their isokinetic strengths of trunk extension, knee extension, shoulder extension, and shoulder abduction. The participants were then divided into two groups, one provided with knowledge feedback of their strength testing results and the other not provided with such feedback. Participants subsequently performed the same load-handling task in which they carried batches of various weight plates while allowed to assort batches of more than one plate into any combination. RESULTS: Dynamic strength, as represented by a total isokinetic strength score, and knowledge feedback both had significant effects on measures quantifying the batch-assorting strategy. CONCLUSION: Individuals with greater strength tended to adopt a strategy corresponding to a heavier load per carry and fewer carries per batch. Receiving knowledge feedback evoked a tendency toward handling a heavier load, and this tendency was more salient in the weaker individuals. APPLICATION: Potential applications include the use of strength testing in worker selection and training as well as in job design to promote better strategies of balancing productivity and injury prevention.     

Multidirectional manual arm strength and its relationship with resultant shoulder moment and arm posture

Previous work has quantified manual force capabilities for ergonomics design, but the number of studies and range of conditions tested are limited in scope. Therefore, the aims of this study were to collect seated manual arm strength (MAS) data from 24 females in several unique exertion directions (n = 26) and hand locations relative to the shoulder (n = 8), and to investigate the associations between MAS and shoulder/elbow moments. MAS was generally highest when the direction of force application was oriented parallel to the vector from the shoulder to knuckle, and weakest when oriented orthogonal to that vector. Moderate correlations were found between MAS and: (1) resultant shoulder moment (r = 0.34), (2) resultant moment arms (r = ?0.545) and (3) elbow flexion/extension moment (r = 0.481). Our strength data will be used in the development of a comprehensive MAS predictive method, so that strength capabilities can be predicted to help design acceptable tasks in the workplace.

Practitioner Summary: This study sought to enhance our understanding of one-handed manual arm strength capabilities for ergonomics task evaluations. Our findings provide researchers and practitioners with manual strength data for off-axis force directions, as well as hand locations not previously measured. These data will contribute to future methods for predicting strength capabilities.