Psychophysically determined infrequent lifting capacity of Chinese participants.

The aim of this study was to investigate the psychophysical infrequent lifting capacity (maximum acceptable weight of lift, MAWL) for a Chinese population. A nested-factorial experimental design with a participant factor nested within gender was employed. Forty-one Chinese participants (29 males, 12 females) participated in the study. Two frequencies (one lift every 8 h and one lift every 5 min) and six lifting heights (floor to knuckle, floor to shoulder, floor to reach, knuckle to shoulder, knuckle to reach, shoulder to reach) were evaluated. The results are compared with prior studies and they lead to the following conclusions. (1) The MAWLs were significantly affected by both the lifting frequency and lifting height. For lifting frequency, the MAWLs decreased markedly by nearly 30% from one lift every 8 h to one lift every 5 min. For lifting height, the MAWLs of the F-K was the greatest of all six lifting heights, followed by F-S, K-S, F-R, K-R and S-R was the smallest. (2) The MAWLs for Chinese females are significantly lower, but proportionately similar, to the MAWLs for Chinese males. (3) The Chinese participants had smaller capacities compared with the Occidental participants, and the rate of decrease in MAWL for the Chinese participants was much sharper than that of Occidental participants. (4) The MAWL of Chinese females was about 54-58% of the Chinese males, it is somewhat lower than those of 60-70% of the Occidental participants.     

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.     

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.     

Psychophysically determined symmetric and asymmetric lifting capacity of Chinese males for one hour's work shifts

A laboratory experiment was conducted to determine the effects of asymmetric lifting on psychophysically determined maximum acceptable weight of lift (MAWL) and the resulting heart rate, oxygen uptake and rating of perceived exertion. Thirteen male college students were recruited as participants. Each participant performed 12 different lifting tasks involving three lifting frequencies (one-time maximum, 1 and 4 lifts/min) and four twisting angles (including the sagittal plane and three different angles of asymmetry, i.e., 0, 30, 60, and 90°) from the floor to a 76 cm high pallet for one hour's work shift using a free-style lifting technique. The results showed that: (1) The MAWLs were significantly lower for asymmetric lifting than for symmetric lifting in the sagittal plane. The MAWL decreased with an increase in the angle of asymmetry, however, the heart rate, oxygen uptake and RPE remained unchanged; (2) Lifting frequency had no significant effect on the percentage decrease in MAWL from the sagittal plane values. Correction factors of 4, 9, and 13% for MAWL at 0, 30, 60, and 90°of asymmetric lifting, respectively, are recommended; (3) Both the physiological costs (heart rate and oxygen uptake) and rating of perceived exertion increased with an increase in lifting frequency though maximum acceptable weight of lift decreased. The most stressed body parts were the lower back and the arm; and (4) The percentage decrease in MAWL with twisting angle for the Chinese participants was somewhat lower than those of the Occidental participants. In addition, even though there was a decrease in MAWL, heart rate and RPE increased with an increase in the angle of a symmetric lifting for the Occidental participants, it was different from that of the Chinese participants.

Relevance to industry

It is generally believed that asymmetric lifting involving torso twisting is more harmful to back spine than symmetric lifting. However, the previous studies were conducted in Europe and North America, and the data were obtained from the Caucasian populations. This work, therefore, aims to investigate the influence of asymmetric lifting on the lifting capacity of the Chinese participants, and to compare the differences with the Occidental populations.     

A psychophysical study of manual lifting in hot environments

A study was conducted to assess the lifting capabilities of acclimatized individuals in hot environments. Six male subjects were selected for the study. The subjects were heat acclimatized for ten days prior to participation in the experiment. A psychophysical experiment was then carried out using three temperature levels (22, 27, and 32°C Wet Bulb Globe Temperature (WBGT), three frequencies of lift (0.1, 3 and 6 lifts/min), and one height of lift (floor to knuckle height). The results indicated that the maximum acceptable weights of lift selected by the subjects at 27°C WBGT were not significantly different from the weights selected at 22°C WBGT. On the other hand, the maximum acceptable weights selected at 32°C WBGT were reduced by approximately 13 percent.     

The effects of speed,frequency, and load on measured hand forces for a floor to knuckle lifting task

The purpose of this study was to describe and quantify measured hand forces during floor to knuckle lifting of various loads. Hand forces of five subjects were measured with a strain gauge apparatus for normal and fast speeds of lifting at 1,4, and 8 l/min. The pattern of hand force over time exhibited peaks in force in the shape of a spike for all fast lifts, indicating that subjects did not lift smoothly. For normal speed of lift, only one of the five subjects executed some lifts smoothly, indicating that it may be possible to lift smoothly, but most lifters probably do not. Peaks of horizontal and vertical components of hand force were tabulated by speed of lift, frequency, and load.     

The effects of speed, frequency, and load on measured hand forces for a floor to knuckle lifting task.

The purpose of this study was to describe and quantify measured hand forces during floor to knuckle lifting of various loads. Hand forces of five subjects were measured with a strain gauge apparatus for normal and fast speeds of lifting at 1, 4, and 8 l/min. The pattern of hand force over time exhibited peaks in force in the shape of a spike for all fast lifts, indicating that subjects did not lift smoothly. For normal speed of lift, only one of the five subjects executed some lifts smoothly, indicating that it may be possible to lift smoothly, but most lifters probably do not. Peaks of horizontal and vertical components of hand force were tabulated by speed of lift, frequency, and load.     

This month Surprise and delight: statistical methods in ergonomics

Matching the job demands to a person's physical characteristics is an effective method of reducing the risk involved in a manual materials-handling task. Seventy-three male and 73 female industrial workers served as subjects. Maximum Acceptable Load of Lifting (MAL) was determined psychophysical under each combination of the following task conditions: six ranges of lift (floor to knuckle, floor to shoulder, floor to reach, knuckle to shoulder, knuckle to reach and shoulder to reach); four lifting frequencies (2,4,6 and 81/min); and three box sizes (30·48,45·72 and 60·96cm). A factor-score-based model (R² = 0-924) is developed in this paper based on 100 subjects for predicting an individual's MAL and for describing his/her physical characteristics in terms of strength and anthropometric scores. The model was validated on the remaining 46 subjects and was shown to be superior to the previously developed models from the same data set.     

Review paper A comparative study of methods for establishing load handling capabilities

There has been much effort in recent years to quantify manual handling capabilities. Four main techniques have been used to this end; biomechanical modelling; the measurement of intra-abdominal pressure; psychophysics; and metabolic/physiological criteria. The aim of this study was to compare quantitatively the data produced from the first three techniques. The comparisons were limited to bimanual, sagittal plane lifting, which of all manual handling activities has been studied the most comprehensively, except that pushing and pulling data were compared from the psychophysics and intra-abdominal pressure (‘force limits’) databases. It was found that the data from ‘force limits’ proposed weights for bimanual lifting in the sagittal plane are lower than those reported to be psychophysically acceptable except for lifting close to and around the shoulder. The closest agreement between the databases was for lifting from an origin above knuckle height. The ‘force limits’ data were found to propose weights of lift which are at a minimum when lifting with a freestyle posture from the floor whereas the psychophysical technique proposes weights which are at a maximum when lifting from the floor. The psychophysical data were found to generate compressive forces at L5/S1 according to a static sagittal plane biomechanical model about 10% in excess of the NIOSH action limit (NIOSH 1981) when lifting from the floor, although over other lifting ranges the compressive forces were less than the NIOSH action limit. Lifting the (force limits) weights generated compressive forces which were on average 55% less than the AL (range 45 to 60%) when lifting in an erect posture. The data for pushing according to the psychophysical and ‘force limits’ database showed good agreement, but for pulling the ‘force limits’ weights were considerably greater than those selected psych ophysically. The implications of these findings are discussed.     

A comparative study of methods for establishing load handling capabilities

There has been much effort in recent years to quantify manual handling capabilities. Four main techniques have been used to this end; biomechanical modelling; the measurement of intra-abdominal pressure; psychophysics; and metabolic/physiological criteria. The aim of this study was to compare quantitatively the data produced from the first three techniques. The comparisons were limited to bimanual, sagittal plane lifting, which of all manual handling activities has been studied the most comprehensively, except that pushing and pulling data were compared from the psychophysics and intra-abdominal pressure ('force limits') databases. It was found that the data from 'force limits' proposed weights for bimanual lifting in the sagittal plane which [corrected] are lower than those reported to be psychophysically acceptable except for lifting close to and around the shoulder. The closest agreement between the databases was for lifting from an origin above knuckle height. The 'force limits' data were found to propose weights of lift which are at a minimum when lifting with a freestyle posture from the floor whereas the psychophysical technique proposes weights which are at a maximum when lifting from the floor. The psychophysical data were found to generate compressive forces at L5/S1 according to a static sagittal plane biomechanical model about 10% in excess of the NIOSH action limit (NIOSH 1981) when lifting from the floor, although over other lifting ranges the compressive forces were less than the NIOSH action limit. Lifting the 'force limits' weights generated compressive forces which were on average 55% less than AL (range 45 to 60%) when lifting in an erect posture. The data for pushing according to the psychophysical and 'force limits' database showed good agreement, but for pulling the 'force limits' weights were considerably greater than those selected psychophysically. The implications of these findings are discussed.     

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.     

Comparing the revised NIOSH lifting equation to the psychophysical,biomechanical and physiological criteria used in its development

The purpose of this study was to perform a systematic and comprehensive comparison between the recommended weights of lift (RWL) from the revised NIOSH lifting equation, and the specific biomechanical, psychophysical and physiological criteria that were stated to be used in the equation's development. A composite acceptable load (CAL) table was developed for the 216 conditions presented in the female lifting table of Snook and Ciriello (1991). Each CAL value was calculated to correspond to the most conservative load of the three design criteria. The RWLs caused lumbar compression forces close to the biomechanical limit of 3400 N for lifts below knuckle height, but resulted in compression forces that are much lower at higher lifting heights. At moderate lifting frequencies, the average RWL would be acceptable to more than 95% of female workers according to the psychophysical criterion. At the highest frequencies, the RWL was found to be an average of 63% of the CAL values based on the integration of the psychophysical and physiological criteria. It is recommended that ergonomists acknowledge the very conservative nature of the revised NIOSH lifting equation when using it to evaluate occupational lifting tasks.     

A psychophysical study of high-frequency arm lifting

Ergonomics research on worker lifting in industry, and the many tools and methods that have resulted from it, have most often concentrated on the maximum amount of weight that a worker is capable and willing to lift in a given situation. In most psychophysical research on lifting, the frequency is one of a number of controlled variables along with container size, lift range, etc. Most of the relatively few studies that have investigated frequency as the response variable have used relatively heavy loads. In the study reported here, the focus was on the lifting of light weights and the subject acceptance of maximum frequency of lift for a two-handed lifting task. The lift range was set at approximately knuckle to shoulder height and was intended to simulate industrial jobs where the worker is tasked with either loading or unloading relatively light weight items to or from a processing line operation. Twelve college-age male subjects were used. Two conditions of weight, 0.7 kg (1.5 lb.) and 4.45 kg (10 lb.) were used and the subject adjusted his frequency of lift by communicating with the researcher, who adjusted a metronome to pace the task. The subjects were instructed to work at as fast a rate as they could for an hour period without becoming overheated, overly tired, out of breath or in pain. Measurements of oxygen consumption and heart rate were taken to supplement the psychophysical measure of lift frequency. Two replications of each weight condition were performed. At the conclusion of the metronome-paced sessions, an additional session for each weight condition was performed where the subject was instructed to lift as fast and consistently as they could with no external cuing device. The mean frequencies of lift identified in the experiment were 31.21 lifts per minute and 23.50 lifts per minute for the 0.7 kg and 4.5 kg lift weights respectively. The two weight conditions were significantly different from each other in their effects on subject metabolic energy expenditure with the subjects tending to work significantly harder physiologically at the heavier weight.     

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.     

Spinal loads during individual and team lifting

The aim of this experiment was to compare lumbar spinal loads during individual and team lifting tasks. Ten healthy male subjects performed individual lifts with a box mass of 15, 20 and 25 kg and two-person team lifts with a box mass of 30, 40 and 50 kg from the floor to standing knuckle height. Boxes instrumented with force transducers were used to measure vertical and horizontal hand forces, whilst sagittal plane segmental kinematics were determined using a video based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. A significant reduction in L4/L5 torque and compression force of approximately 20% was found during team lifts compared to individual lifts. Two main reasons for the reduced spinal loads in team lifting compared to individual lifting were identified: (1) the horizontal hand force (i.e. pulling force) was greater in team lifting, (2) the horizontal position of the hands was closer to the lumbar spine during team lifts. The horizontal hand force and position of the hands had approximately equal contributions in reducing the spinal load during team lifting compared to individual lifting.     

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.     

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.     

The effects of caregiver experience on low back loads during floor and overhead lift maneuvering activities

This study investigated the effects of caregiver experience on peak external forces and moments generated at the L5/S1 joint of the low back when maneuvering loaded floor-based and overhead-mounted patient lifting devices. Twenty caregivers were divided into more-experienced and less-experienced groups based on the product of two factors: their years of lifting experience and the frequency of lifting the caregivers had done in the past. Ground reaction forces and moments as well as motion capture data were recorded while caregivers performed five different maneuvering tasks with both lifts in each of three conditions (caregiver subjects worked alone, as the primary caregiver in a pair, and as the secondary caregiver in a pair). Six outcome measures (net external forces and moments at the L5/S1 joint) were recorded. Multivariate analyses of variance of all net external forces and moments were done separately for the floor and overhead lifts. A significant effect of experience level was found for the floor lift (p = 0.006) but not for the overhead lift (p = 0.163). A follow-up univariate analysis of floor lift activities found significant differences between more-experienced and less-experienced caregivers for Turn, Push and Legs Up activities.

Relevance to industry

Previous work has shown that overhead lifts reduce the loads on caregivers compared to floor lifts. The findings of this study further underscore the need to purchase overhead lifts to protect less-experienced caregivers (including informal family caregivers) who are at increased risk of back injury when maneuvering floor lifts.