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.     

Initial force and postural adaptations when pushing and pulling on floor surfaces with good and reduced resistance to slipping

The objective of the present study was to determine whether differences in the frictional properties of a floor surface may affect the kinematics and kinetics of pushing and pulling. Eight male participants were required to push and pull a four-wheeled trolley over two level surfaces, on which were mounted floor coverings with good (safety floor) and reduced (standard floor) frictional properties. A psychophysical approach was used to determine the initial maximum acceptable horizontal force required to move the trolley over a short distance (3 m). Three-dimensional (3D) hand and ground reaction forces and 3D postures were measured during initial force exertions. The results showed that psychophysically derived measures of initial horizontal force and horizontal components of hand forces did not differ significantly between floor surfaces. Despite the ability to exert similar forces, the measured maximum coefficient of friction varied according to floor surface. These changes reflected significant alterations in vertical and horizontal components of ground reaction and vertical hand forces, suggesting that participants had maximized the frictional properties available to them. Postures also changed as a consequence of floor surface, with significant changes occurring in knee flexion and trunk extension. This study has shown that handlers involved in the pushing and pulling of trolleys are capable of adjusting posture and the direction of hand and foot forces in order to compensate for reduced levels of floor friction. This has particular relevance when assessing the musculoskeletal loads imposed on the handler and the likely mechanisms of injury resulting from variations in floor conditions when workers undertake pushing and pulling tasks in the workplace.     

Initial force and postural adaptations when pushing and pulling on floor surfaces with good and reduced resistance to slipping

The objective of the present study was to determine whether differences in the frictional properties of a floor surface may affect the kinematics and kinetics of pushing and pulling. Eight male participants were required to push and pull a four-wheeled trolley over two level surfaces, on which were mounted floor coverings with good (safety floor) and reduced (standard floor) frictional properties. A psychophysical approach was used to determine the initial maximum acceptable horizontal force required to move the trolley over a short distance (3 m). Three-dimensional (3D) hand and ground reaction forces and 3D postures were measured during initial force exertions. The results showed that psychophysically derived measures of initial horizontal force and horizontal components of hand forces did not differ significantly between floor surfaces. Despite the ability to exert similar forces, the measured maximum coefficient of friction varied according to floor surface. These changes reflected significant alterations in vertical and horizontal components of ground reaction and vertical hand forces, suggesting that participants had maximized the frictional properties available to them. Postures also changed as a consequence of floor surface, with significant changes occurring in knee flexion and trunk extension. This study has shown that handlers involved in the pushing and pulling of trolleys are capable of adjusting posture and the direction of hand and foot forces in order to compensate for reduced levels of floor friction. This has particular relevance when assessing the musculoskeletal loads imposed on the handler and the likely mechanisms of injury resulting from variations in floor conditions when workers undertake pushing and pulling tasks in the workplace.     

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 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     

Biomechanically determined hand force limits protecting the low back during occupational pushing and pulling tasks

Though biomechanically determined guidelines exist for lifting, existing recommendations for pushing and pulling were developed using a psychophysical approach. The current study aimed to establish objective hand force limits based on the results of a biomechanical assessment of the forces on the lumbar spine during occupational pushing and pulling activities. Sixty-two subjects performed pushing and pulling tasks in a laboratory setting. An electromyography-assisted biomechanical model estimated spinal loads, while hand force and turning torque were measured via hand transducers. Mixed modelling techniques correlated spinal load with hand force or torque throughout a wide range of exposures in order to develop biomechanically determined hand force and torque limits. Exertion type, exertion direction, handle height and their interactions significantly influenced dependent measures of spinal load, hand force and turning torque. The biomechanically determined guidelines presented herein are up to 30% lower than comparable psychophysically derived limits and particularly more protective for straight pushing.

Practitioner Summary: This study utilises a biomechanical model to develop objective biomechanically determined push/pull risk limits assessed via hand forces and turning torque. These limits can be up to 30% lower than existing psychophysically determined pushing and pulling recommendations. Practitioners should consider implementing these guidelines in both risk assessment and workplace design moving forward.     

Secular changes in psychophysically determined maximum acceptable weights and forces over 20 years for male industrial workers

The most frequent and expensive cause of compensable workplace injuries loss is manual material handling (MMH). In an attempt to minimise these losses, refinement of existing MMH guidelines is a component of redesigning high risk MMH jobs. In the development of the present MMH 1991 guidelines (Snook and Ciriello 1991), maximum acceptable weights (MAWs) and forces (MAFs) were derived from studies conducted in a 21 year time span before the above publication date. The question arises whether the present generation of workers have the same psychophysically determined weights and forces as those reflected in the guidelines. Therefore, the present study investigated whether secular changes had occurred in key MMH tasks in trials performed by present day local industrial workers. A total of 23 male industrial workers performed 20 variations of lifting, lowering, pushing, pulling and carrying tasks. A psychophysical methodology, identical to that of the authors' previous experiments, was used whereby the subjects were asked to select a workload they could sustain for 8 h 'without straining themselves or without becoming unusually tired, weakened, overheated or out of breath'. The results revealed that MAWs of lifting, lowering and carrying averaged 69% of the guideline values. MAFs of pushing and pulling showed less of a drop, averaging 82% and 94% respectively for initial and sustained forces. The results also indicated that the effects of the variables frequency, height, lifting vs. lowering, pushing vs. pulling were similar to earlier reported results, even though the absolute weights or forces were lower. It was concluded that consideration to change existing guidelines, reflecting this new psychophysical set point, may be appropriate if these significant performance decreases are confirmed in other locations, with greater subject numbers, and by other investigators.     

The effect of handle height and cart load on the initial hand forces in cart pushing and pulling

The objective of this study was to measure the three-dimensional hand forces people exert to initiate a cart push or pull for two cart loads: 73 and 181 kg, and three handle heights: knuckle, elbow, and shoulder heights. The cart used was equipped with 15.24 cm (6 in) diameter wheels. The floor was covered with carpet tiles. The laboratory-measured hand force exertions were compared to the minimum forces needed to push/pull the cart under the same conditions and to the psychophysical initial push/pull force limits. For pushing and pulling, the measured anterior-posterior hand forces were 2–2.4 times the minimum required forces. For the heavier cart load, lower forces were applied as handle height increased. Pull forces were 7% higher than push forces. The smallest vertical forces were measured at elbow height. Strength capability and gender did not have an effect on the applied forces. The mean strength percentile for the male sample was 64%, while the mean strength percentile for the female sample was 13% as determined from the Adjusted Torso Lift Strength Test and population strength data for this test. The comparison with the psychophysical limits indicated that the tasks were well within the maximum acceptable initial forces for males, but not for females.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a force-transducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45 degrees (SD 3.3 degrees) downward to near horizontal, while the pulling force direction changed from pulling upward by 14 degrees (SD 15.3 degrees) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a forcetransducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45° (SD 3.3°) downward to near horizontal, while the pulling force direction changed from pulling upward by 14° (SD 15.3°) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

Lower back muscle forces in pushing and pulling

In the investigation of lower back stress, the muscle forces of the erector spinae and the rectus abdominis are often calculated using the two-dimensional biomechanical model. These muscle forces are used to estimate the compressive forces at L5/S1 disc. This paper presents a study of the muscle forces predicted by a two-dimensional biomechanical model during pushing and pulling and myoelectric activity from the corresponding muscles. The goal was to investigate whether a simple two muscle torso model would reasonably estimate the muscle actions in pushing and pulling tasks. Six subjects participated in the experiment. EMG (rms) value was used as an indicator of muscle forces. The results show high correlation between the predicted muscle forces and the measured root-mean-square EMG values in trunk pushing and pulling (r2 = 0.93) and hand pushing and pulling (r2 = 0.96) in an erect posture with hips braced but low in hand pushing and pulling using a free posture (r2 = 0.37).     

Oxygen consumption prediction models for individual and combination materials handling tasks

An experiment was conducted to develop models to predict oxygen consumption of males and females engaged in common materials handling tasks including lifting, lowering, pushing, pulling, (de)palletising and combination tasks involving lifting or lowering a box and carrying it a set distance and lifting or lowering it to the destination. Nineteen male and 19 female subjects participated in the study. A psychophysical approach was used to set load limits for individual subjects for the oxygen consumption protocol. The 8398 oxygen consumption values collected were entered into the initial regression analyses and 168 potential outliers were removed before the final models were run. In addition to relevant task variables, body weight was a significant predictor variable in all models. The r² values for the final models ranged from 0.54 to 0.82 and the root mean square errors ranged from 90.2 ml to 294.8 ml.     

The load on the lumbar spine during asymmetrical bi-manual materials handling

Previous biomechanical analyses of typical load manipulation tasks were mainly limited to sagittal-plane activities or to static cases. This paper includes the biomechanical determination and assessment of lumbar load during asymmetrical bi-manual materials handling tasks which involve lateral turning of the body, trunk inclination, and sagittal flexion and lateral bending of the spine. Diagonal lifting tasks were analysed for different values for load weight (0-40 kg) and task duration (0·75-1·5 s). Whereas a constant grasp height of 15 cm was assumed, the height for releasing the load differed (50, 100, 150 cm). A dynamic spatial human model (‘The Dortmunder’) was used for calculating the torque in the sagittal, frontal, and transversal planes through the lumbosacral joint and for determining the compressive and the sagittal and lateral shear force at the L5-S1 disc. The trajectories of body segments and load are computer-simulated on the basis of postures adopted during the movement. During diagonal lifting of loads, lumbosacral torque in the sagittal plane is considerably larger than the lateral bending and torsional torque components. Dynamic analyses result in higher maximum values in the lumbar-load time curves than static analyses. The shorter the time for task execution, the higher the resultant dynamic effects and, in consequence, the higher the lumbar load. Lumbosacral compression and shear increase with increasing load-release heights due to higher acceleration and retardation of body and load when the same grasp position and task duration are assumed. The maximum load-bearing capacity of the lumbar spine was determined on the basis of strength data for isolated lumbar segments provided in the literature. The compressive strength falls within the same range as the compressive forces calculated for asymmetrical lifting of loads up to 40 kg. On account of the wide scattering of the compressive strength values, the main influences were determined (age and gender). At an age of 40 years, strength is approx. 6·7 kN for males and 4·7 kN for females (decrease with age per decade: 1·0 kN males, 0·6 kN females). In order to avoid overestimating an individual's lumbar compressive strength, predicted values should be reduced, e.g., by the standard deviation in the male or female samples (2·6 kN or 1·5 kN). Although only a few maximum shear force values are available in the literature, comparison with the calculated values for diagonal lifting leads to the conclusion that sagittal and lateral shear should not be ignored in the assessment of lumbar load during asymmetrical handling tasks.     

Dynamic biomechanical modelling of symmetric and asymmetric lifting tasks in restricted postures

This article describes investigations of dynamic biomechanical stresses associated with lifting in stooping and kneeling postures. Twelve subjects volunteered to participate in two lifting experiments each having two levels of posture (stooped or kneeling), two levels of lifting height (350 or 700 mm), and three levels of weight (15,20, or 25 kg). One study examined sagitally symmetric lifting, the other examined an asymmetric task. In each study, subjects lifted and lowered a box every 10 s for a period of 2 min in each treatment combination. Electromyography (EMG) of eight trunk muscles was collected during a specified lift. The EMG data, normalized to maximum extension and flexion exertions in each posture, was used to predict compression and shear forces at the L3 level of the lumbar spine. A comparison of symmetric and asymmetric lifting indicated that the average lumbar compression was greater in sagittal plane tasks; however, both anterior-posterior and lateral shear forces acting on the lumbar spine were increased with asymmetric lifts. Analysis of muscle recruitment indicated that the demands of lifting asymmetrically are shifted to ancillary muscles possessing smaller cross-sectional areas, which may be at greater risk of injury during manual materials handling (MMH) tasks. Model estimates indicated increased compression when kneeling, but increased shear forces when stooping. Increasing box weight and lifting height both significantly increased compressive and shear loading on the lumbar spine. A multivariate analysis of variance (MANOVA) indicated complex muscle recruitment schemes—each treatment combination elicited a unique pattern of muscle recruitment. The results of this investigation will help to evaluate safe loads for lifting in these restricted postures.     

Mechanical loading on the low back in three methods of refuse collecting

The mechanical loading on the low back was studied in three different current methods of refuse collecting: in polythene bags, two-wheeled mini-containers and large four-wheeled containers. To this end the most prominent activities of each collecting method were performed in a laboratory. On the basis of movement analysis, force measurements and biomechanical modelling, spinal compressive and shear forces were estimated. From these forces and from the frequency of activities during the working day (assessed in a preliminary field study) the low-back stress in each collecting method was evaluated. In the bag-method, peak forces when throwing the bags ranged from 3341 to 5179?N (average compression) and from 284 to 673?N (shear) among the different conditions studied. The act of picking up bags also showed rather high forces (exceeding the NIOSH limit for disc compression in most cases). The frequency of exposure to these forces in the field is rather high (workers pick up and throw on average 807 times each day). The mini-container method compares favourably to the bags method. Peak compressive and shear force in tilting/pushing and pulling mini-containers ranged from 1657 to 2654?N and from 123 to 248?N respectively. Also, the frequency of stressful events in the Held is lower in this method. In the large container method extremely high peak forces (e.g. compression ranged from 4991 to 5810N) were observed in the task of putting the empty container back from street level to sidewalk level (surmounting the kerb). The frequency of activities like pushing, pulling and lifting the large container in the field is much lower compared with activities in the other methods. On the basis of the frequency and magnitude of spinal forces it was concluded that the mini-containers should be preferred to the bags. If kerbs are removed at container places and tasks are performed by two instead of a single person, the large container method would form another good alternative to the stressful task of collecting refuse in bags.     

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.     

Comparison of prediction models for the compression force on the lumbosacral disc

The main objective of this research was to compare three representative methods of predicting the compressive forces on the lumbosacral disc: LP-based model, double LP-based model, and EMG-assisted model. Two subjects simulated lifting tasks that are frequently performed in the refractories industry of Korea, in which vertical and lateral distances, and weight of load were varied. To calculate the L5/ SI compressive forces, EMG signals from six trunk muscles were measured, and postural data and locations of load were recorded using the Motion Analysis System. The EMG-assisted model was shown to reflect well all three factors considered here, whereas the compressive forces from the two LP-based models were only significantly affected by weight of load. In addition, low lifting index (LI) values were observed for relatively high L5/S1 compressive forces from the EMG-assisted model, suggesting that the 1991 NIOSH lifting equations may not fully evaluate the risk of dynamic asymmetric lifting tasks.     

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, and (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.     

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.     

Psychophysical modelling for combined manual materials-handling activities

Most psychophysical studies in manual materials handling (MMH)are involved only with single MMH activities, i.e. lifting, lowering, carrying, holding, pushing or pulling. Very little research has been reported on the determination of operator capacities for combinations of MMH activities (e.g. lifting a box, then carrying the box, or carrying a box, then lowering the box). These kinds of combined activities are prevalent in industry and in our daily lives. The objective of this study was to utilize the psychophysical approach to examine the effects of combinations of lifting, carrying and lowering activities. Twelve male students served as subjects for the study. The capacities that were determined as the maximum acceptable workloads for a 1-h work period for four individual MMH activities—lifting from floor to knuckle height (LFK), lifting from knuckle to shoulder height (LKS), lowering from knuckle to floor height (LOW) and carrying for 3·4 m (C) —and three combined MMH activities—LFK + C, LFK + C + LKS and LFK + C + LOW—were determined psychophysically under three frequency conditions: one time maximum, one handling per minute and six handlings per minute. Combined MMH capacities models were developed using the following three methods: a limiting individual MMH capacity, isoinertial 1·83-m maximum strength and fuzzy-set theory. The advantages and disadvantages of different models were discussed.