Review of:“Sex Differences in Human PerformanceEdited by M. A. BAKER (Chichester,England: John Wiley, 1987.) [Pp. xvi + 202.] £22·00. ISBN 0-471-91119-4.

An experiment was conducted to examine the role that maximal lifting power has in predicting maximum acceptable weight of lift (MAWL) for a frequency of one lift per 8 h. The secondary aim of the study was to compare the ability of power to predict MAWL to previously used measures of capacity including two measures of isometric strength, five measures of isokinetic strength, and isoinertial capacity on an incremental lifting test. Twenty-five male subjects volunteered to participate in the experiment. The isometric tests involved maximum voluntary contractions for composite lifting strength at vertical heights of 15 and 75 cm. Peak isokinetic strength was measured at velocities of 0.1, 0.2, 0.4, 0.6 and 0.8 m s^?1 using a modified CYBEX® II isokinetic dynamometer. Isoinertial lifting capactity was measured on the X-factor incremental lifting machine and peak power was measured on the incremental lifting machine by having subjects lift a 25 kg load as quickly as possible. The results indicate that peak isoinertial power is significantly correlated with MAWL, and this correlation was higher than any of the correlations between the other predictor variables and MAWL. The relationships between the isokinetic strength measures and MAWL were stronger than the relationships between the isometric measures and MAWL. Overall, the results suggest that tests used to predict MAWL should be dynamic rather than static.     

Significance of biomechanical and physiological variables during the determination of maximum acceptable weight of lift.

The aim was to identify which biomechanical and physiological variables were associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL) in a psychophysical study. Fifteen male college students lifted a box of unknown weight at 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as heart rate, trunk positions, velocities and accelerations were measured during the lifting, as well as estimated spinal loading in terms of moments and spinal forces in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques identified variables associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that heart rate, predicted sagittal lift moment and low back disorder (LBD) risk index were associated with decreases in the weight prior to the next lift. Thus, historical measures of LBD risk (e.g. compression, shear force) were not associated with decreases in weight prior to the next lift. Additionally, the magnitudes of the predicted spinal forces and LBD risk were all very high at the MAWL when compared with literature sources of tolerance as well as observational studies on LBD risk. Our findings indicate that the psychophysical methodology may be useful for the decision to lower the weight of loads that may present extreme levels of risk of LBD; however, the psychophysical methodology does not seem to help in the decision to stop changing the weight at a safe load weight.     

Significance of biomechanical and physiological variables during the determination of maximum acceptable weight of lift

The aim was to identify which biomechanical and physiological variables were associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL) in a psychophysical study. Fifteen male college students lifted a box of unknown weight at 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as heart rate, trunk positions, velocities and accelerations were measured during the lifting, as well as estimated spinal loading in terms of moments and spinal forces in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques identified variables associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that heart rate, predicted sagittal lift moment and low back disorder (LBD) risk index were associated with decreases in the weight prior to the next lift. Thus, historical measures of LBD risk (e.g. compression, shear force) were not associated with decreases in weight prior to the next lift. Additionally, the magnitudes of the predicted spinal forces and LBD risk were all very high at the MAWL when compared with literature sources of tolerance as well as observational studies on LBD risk. Our findings indicate that the psychophysical methodology may be useful for the decision to lower the weight of loads that may present extreme levels of risk of LBD; however, the psychophysical methodology does not seem to help in the decision to stop changing the weight at a safe load weight.     

Prediction of the maximum acceptable weight of lift from the frequency of lift

It is known that maximum acceptable weight of lift (MAWL) decreases as the frequency of lifting increases. The purpose of this study was to quantify the relationship between lifting frequency and the MAWL, and to generate models for predicting the mean MAWLs for males and females from frequency of lifting. Published experimental studies that have reported the MAWL at different lifting frequencies were identified and regression methods were used to evaluate the relationship between the frequency of lifting and the MAWL. The best fitting models were logarithmic but they accounted for less than 50% of the variance. This reflects the heterogeneity of the experiments included. Normalising the MAWL to the MAWL at one lift per minute improved the predictive power of the models, accounting for more than 80% of the variance. Linear and power models for predicting work rate in kg/min showed even higher levels of accuracy.     

Acceptable workloads for three common mining materials

A series of psychophysical lifting studies was conducted to establish maximum acceptable weights of lift (MAWL) for three supply items commonly handled in underground coal mines (rock dust bags, ventilation stopping blocks, and crib blocks). Each study utilized 12 subjects, all of whom had considerable experience working in underground coal mines. Effects of lifting in four postures (standing, stooping under a 1·5m ceiling, stooping under a l·2m ceiling, and kneeling) were investigated together with four lifting conditions (combinations of lifting symmetry and lifting height). The frequency of lifting was set at four per min, and the task duration was 15?min. Posture significantly affected the MAWL for the rock dust bag (standing MAWL was 7% greater than restricted postures and kneeling MAWL was 6·4% less than stooped); however, posture interacted with lifting conditions for both of the other materials. Physiological costs were found to be significantly greater in the stooped postures compared with kneeling for all materials. Other contrasts (standing versus restricted postures, stooping under 1·5?m ceiling versus stooping under l·2?m ceiling) did not exhibit significantly different levels of energy expenditure. Energy expenditure was significantly affected by vertical lifting height; however, the plane of lifting had little influence on metabolic cost. Recommended acceptable workloads for the three materials are 20·0?kg for the rock dust bag, 16·5?kg for the ventilation stopping block, and 14·7?kg for the crib block. These results suggest that miners are often required to lift supplies that are substantially heavier than psychophysically acceptable lifting limits.     

Acceptable workloads for three common mining materials.

A series of psychophysical lifting studies was conducted to establish maximum acceptable weights of lift (MAWL) for three supply items commonly handled in underground coal mines (rock dust bags, ventilation stopping blocks, and crib blocks). Each study utilized 12 subjects, all of whom had considerable experience working in underground coal mines. Effects of lifting in four postures (standing, stooping under a 1.5 m ceiling, stooping under a 1.2 m ceiling, and kneeling) were investigated together with four lifting conditions (combinations of lifting symmetry and lifting height). The frequency of lifting was set at four per min, and the task duration was 15 min. Posture significantly affected the MAWL for the rock dust bag (standing MAWL was 7% greater than restricted postures and kneeling MAWL was 6.4% less than stopped); however, posture interacted with lifting conditions for both of the other materials. Physiological costs were found to be significantly greater in the stooped postures compared with kneeling for all materials. Other contrasts (standing versus restricted postures, stooping under 1.5 m ceiling versus stopping under 1.2 m ceiling) did not exhibit significantly different levels of energy expenditure. Energy expenditure was significantly affected by vertical lifting height; however, the plane of lifting had little influence on metabolic cost. Recommended acceptable workloads for the three materials are 20.0 kg for the rock dust bag, 16.5 kg for the ventilation stopping block, and 14.7 kg for the crib block. These results suggest that miners are often required to lift supplies that are substantially heavier than psychophysically acceptable lifting limits.     

Maximum acceptable weight of lift reflects peak lumbosacral extension moments in a functional capacity evaluation test using free style, stoop and squat lifting

It is unclear whether the maximum acceptable weight of lift (MAWL), a common psychophysical method, reflects joint kinetics when different lifting techniques are employed. In a within-participants study (n = 12), participants performed three lifting techniques--free style, stoop and squat lifting from knee to waist level--using the same dynamic functional capacity evaluation lifting test to assess MAWL and to calculate low back and knee kinetics. We assessed which knee and back kinetic parameters increased with the load mass lifted, and whether the magnitudes of the kinetic parameters were consistent across techniques when lifting MAWL. MAWL was significantly different between techniques (p = 0.03). The peak lumbosacral extension moment met both criteria: it had the highest association with the load masses lifted (r > 0.9) and was most consistent between the three techniques when lifting MAWL (ICC = 0.87). In conclusion, MAWL reflects the lumbosacral extension moment across free style, stoop and squat lifting in healthy young males, but the relation between the load mass lifted and lumbosacral extension moment is different between techniques. PRACTITIONER SUMMARY: Tests of maximum acceptable weight of lift (MAWL) from knee to waist height are used to assess work capacity of individuals with low-back disorders. This article shows that the MAWL reflects the lumbosacral extension moment across free style, stoop and squat lifting in healthy young males, but the relation between the load mass lifted and lumbosacral extension moment is different between techniques. This suggests that standardisation of lifting technique used in tests of the MAWL would be indicated if the aim is to assess the capacity of the low back.     

A physiological study of manual lifting of loads in Indians

The employment of workers solely for lifting of loads is common in the developing countries. This task can be described in terms of its three principal variables, viz. the weight of the load, the height of the lift and the rate of lifting, but Jew attempts to quantitate the contributions of these variables in determining its strenuousness have been made.

Based on the observed range of variation in an industrial lifting operation, a total of 525 lifting experiments comprising combinations of three different weights of compact loads, lifts to three separate heights from the ground level and three different rates of lifting were carried out on 21 subjects selected from amongst the load lifters.

Comparison of the observed energy expenditures of these tasks with the maximum working capacities of the subjects showed that many of the tasks were unduly heavy. Regression equations depicting the relation between the energy expenditure of lifts of different heights with the other two variables are given. A chart linking these variables has also been prepared; this may be helpful in adjusting those lifting tasks which are continued for prolonged periods so that they are of ‘acceptable’ heaviness.     

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= 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.     

An investigation of perceived exertion via whole body exertion and direct muscle force indicators during the determination of the maximum acceptable weight of lift

The objective of this study was to identify the perceived exertion mechanisms (direct muscle force and whole body exertion) associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL). Fifteen males lifted a box of unknown weight at a rate of 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as the predicted muscle forces and heart rate were measured during the lifting exertion, as well as the predicted spinal loading in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques were used to identify variables that were associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that the force in the left erector spinae, right internal oblique, and left latissimus dorsi muscles as well as heart rate were associated with decreases in the weight prior to the next lift. It appears that a combination of local factors (muscle force) and whole body exertion factors (heart rate) provide the feedback for the perceived exertion when decreasing the weight. The up-change model indicated that the forces of the right erector spinae, left internal oblique, and the right latissimus dorsi muscles were associated with the decision to increase the weight prior to the next lift. Thus, local factors provide feedback during the decision to increase the weight when starting from light weights. Collectively, these findings indicate that psychophysically determined weight limits may be more sensitive to muscular strain rather than spinal loading.     

An investigation of perceived exertion via whole body exertion and direct muscle force indicators during the determination of the maximum acceptable weight of lift

The objective of this study was to identify the perceived exertion mechanisms (direct muscle force and whole body exertion) associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL). Fifteen males lifted a box of unknown weight at a rate of 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as the predicted muscle forces and heart rate were measured during the lifting exertion, as well as the predicted spinal loading in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques were used to identify variables that were associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that the force in the left erector spinae, right internal oblique, and left latissimus dorsi muscles as well as heart rate were associated with decreases in the weight prior to the next lift. It appears that a combination of local factors (muscle force) and whole body exertion factors (heart rate) provide the feedback for the perceived exertion when decreasing the weight. The up-change model indicated that the forces of the right erector spinae, left internal oblique, and the right latissimus dorsi muscles were associated with the decision to increase the weight prior to the next lift. Thus, local factors provide feedback during the decision to increase the weight when starting from light weights. Collectively, these findings indicate that psychophysically determined weight limits may be more sensitive to muscular strain rather than spinal loading.     

Rating of acceptable loads: Lifting with and without handles

Rating of acceptable load (RAL) was used to determine the load-handling capacity of women (n = 54) and men (n = 49) and the effects of handles on the acceptable weight. The RAL test was administered using 30×30×30 cm boxes, one without handles and the other with handles 20 cm above the base. Subjects were asked to fill a box with the weight that they considered would be acceptable for lowering from table (72 cm) to floor and again lifting back to the table at 5-min intervals over an 8-hour working day. The tests were done in their actual place of work.

Subjects selected heavier acceptable loads for the box with handles. The overall means for women and men for the box with handles were 8.8 kg for women and 19.1 kg for men and for the box without handles 7.5 kg and 14.9 kg, respectively. The greater difference of the selected weight of two the boxes for men than for women suggests that the heavier the box more important it is to have it equipped with handles.     

Maximum acceptable weights for asymmetric lifting of Chinese females

This study used the psychophysical approach to evaluate the effects of asymmetric lifting on the maximum acceptable weight of lift (MAWL) and the resulting heart rate, oxygen uptake and rating of perceived exertion (RPE). A randomized complete block factorial design was employed. Twelve female college students lifted weights at three different lifting frequencies (one-time maximum, 1 and 4 lifts/min) in the sagittal plane and at three different asymmetric angles (30 degrees, 60 degrees, and 90 degrees ) from the floor to a 68-cm height pallet. This lifting experiment was conducted for a 1-h work period using a free-style lifting technique. The MAWLs for asymmetric lifting were significantly lower than those for symmetric lifting in the sagittal plane. The MAWL decreased with the increase in the angle of asymmetry. However, the heart rate, oxygen uptake and RPE remained unchanged. Though the MAWL decreased significantly with lifting frequency, both the physiological costs (heart rate and oxygen uptake) and rating of perceived exertion increased with the increase in lift frequency. The most stressed body part was the arm. Lifting frequency had no significant effect on the percentage decrease in MAWL from the sagittal plane values. On average, decreases of 5%, 9% and 14% for MAWL at 30 degrees, 60 degrees and 90 degrees asymmetric lifting, respectively, were revealed. This result was in agreement with the findings of Chinese males studied by Wu [Int. J. Ind. Ergonom. 25 (2000) 675]. The percentage decrease in MAWL with twisting angle for the Chinese participants was somewhat lower than those for Occidental participants. In addition, even though there was an increase in heart rate and RPE with the increase in the symmetrical lift angle for Occidental participants, it was different from the Chinese participants. Lastly, the 1991 NIOSH equation asymmetry multiplier is more conservative in comparison with the results of the present study.     

Lifting belts: a psychophysical analysis

The literature supporting the use of lifting belts has not demonstrated consistent trends. It was hypothesized that if lifting belts provide a biomechanical or motivational advantage then the participants in a psychophysical lift test should select a higher maximum acceptable weight of lift (MAWL). Eleven male and five female subjects participated in one session with a lifting belt and one session without a belt. The order of the belt sessions was counterbalanced. Each session was comprised of two psychophysical lifting tests varying only in the initial weight of the box. Repeated measures ANOVA showed no difference between the MAWL between belt sessions or between the two tests within a given session. Following the test, subjective evaluations of the belt were measured via a questionnaire. The psychophysical test results showed no change as a function of the belt condition. Thus, these results do not support the hypothesis that lifting belts increase trunk strength or motivate individuals to select a greater MAWL. Therefore, it must be concluded that belts do not offer a biomechanical or motivational advantage to the user.     

Psychophysical and physiological responses to lifting symmetrical and asymmetrical loads symmetrically and asymmetrically

Eighteen adult males (mean age 22·6 years, weight 78·6kg and height 176·6cm) participated in a study designed to investigate the effects of symmetrical and asymmetrical lifting on the maximum acceptable weight of lift and the resulting physiological cost. Each subject performed sixty different lifting tasks involving two lifting heights, three lifting frequencies and five containers. For each lifting task, the load was lifted either symmetrically (sagittal lifting) or asymmetrically (turning 90^° while continuing to lift). The heart rate and oxygen uptake of the individuals at the maximum acceptable weight of lift were measured. At the end of the experiment, subjects also verbally indicated their preference for symmetrical and asymmetrical lifting. When lifting asymmetrically, subjects accepted approximately 8·5% less weight. There was, however, no difference in the physiological costs when lifting symmetrically or asymmetrically. Lifting asymmetrical loads also resulted in lower maximum acceptable weights. No difference in either oxygen uptake or heart rate was observed when the centre of gravity of the load was offset by 10·16 or 20·32 cm from the mid-sagittal plane in the frontal plane towards the preferred hand. All subjects indicated, verbally, that asymmetrical lifting tasks were physically more difficult to perform.     

Psychophysical lifting capabilities for overreach heights

Overreach height, in this study, is defined as the maximum reach height of individuals measured to the top of the cut-out box-handles while subjects stand with their heels raised. Since such postures are inherently unstable, knowing how much weight individuals are willing to lift across overreach lifting heights is important. Ten young adult male students (mean age 25·9 years, mean weight 70·8 kg and mean height 175 cm) voluntarily participated in a study designed to investigate the effect of lifting heights above reach height on the maximum acceptable weights of lift. The weight was lifted using a ‘free-style’ technique in the mid-sagittal plane from the floor, knuckle and shoulder heights to overreach heights (individuals stand with their heels raised to deposit the load). The maximum acceptable weight of lift, on the average, declined by approximately 14%, compared with the maximum acceptable weight of lift for reach heights, when the box was lifted to overreach heights. The magnitude of decline in the maximum acceptable weight was highest for the floor to overreach height compared with the knuckle to overreach and shoulder to overreach lifting heights.     

The effect of floor slope on sub-maximal lifting capacity and technique

Inclined surfaces, where both the lifter and load are on the slope, may be encountered in a jobsite situation. The purpose of this study was to determine if facing up or down a sloped surface (10 degrees and 20 degrees ) would affect maximal acceptable weights of lift (MAWL) using a 10 min psychophysical approach with symmetric freestyle technique at 4 lifts/min. Seventeen healthy men and 18 women determined floor to knuckle height MAWL while facing uphill, downhill, and on a level surface. Motion capture was also performed to examine sagittal plane joint angles and foot placement relative to a milk crate. Slope did not alter MAWL (p>0.05) with the men lifting more than the women in every condition (p<0.001) (25 kg vs. 15 kg, respectively). Foot placement relative to the box was altered by slope such that both horizontal position behind and vertical position below the box increased as slope changed from the downhill to uphill conditions (both p<0.001). Forward torso lean as well as hip, knee, and ankle (plantar) flexion generally decreased as slope changed from the downhill to uphill conditions (all p<0.001). Torso and knee motion appeared to be protected compared to the other joints, changing the least. Though trends were the same in both sexes, interactions did exist in vertical foot position and hip angle (both p0.001). In conclusion, the body is highly adaptive to floor slope, maintaining MAWL at least in the short term. However, while slight technique differences exist between men and women, care should be taken by all when facing uphill due to the tendency to stand further from the load horizontally and when facing downhill due to increased torso lean.     

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.