Reducing the biomechanical stress of lifting by training

The effectiveness of two methods of training in reducing biomechanical stress during lifting was examined in a sample of 12 men aged 18–40 years. Subjects carried out three 40 min sessions, one session per day, of a simple symmetrical lifting task. No training was given before the first session, which acted as the control condition. before each of the next two sessions training was given in the form of either written guidelines or interactive personal tuition in a crossover design. Interaction of order with method was expected and was of interest. The effectiveness of each training method in relieving the stress of lifting was measured in four ways: by ratings of perceived exertion on the Borg scale; by video analysis of adherence to guideline kinematics; by chronic spinal compression measured by precision stadiometry; and by the relative compression force at L5/S1 calculated using Chaffin's model. Although the differences in the results of the four assessment methods make reservations necessary it is suggested that while brief personal tuition may demonstrably reduce lifting stress even in a simple lifting task, written guidelines for the untrained may be counterproductive and may interfere with habitual skills. It is recommended that the efficiency of lifting training methods be estimated objectively, such as by the methods employed here, before being adopted.     

The muscular load on the lower back and shoulders due to lifting at different lifting heights and frequencies

The aim of the study was to investigate the muscular load on the lower back and shoulders and the circulatory load on employees at a post center during repetitive lifting of mail transport boxes. A mock-up was designed in the laboratory, a total of nine combinations of lifting height and frequency were studied. Surface EMG was recorded bisymmetrically from m. erector spinae (L3-level) and m. trapezius. The circulatory load was evaluated by measuring the heart rate. The results show a trade off between the low back and shoulders. The maximum load on the low back occurred at the low lifting height (36.3 and 54.4 cm) whereas the maximum load on the shoulders occurred at the high lifting height (144.9 and 163.0 cm).     

The effects of load knowledge on stresses at the lower back during lifting

This study investigated the effects of load uncertainty on the lifting characteristics of 40 male volunteers during the initial portion of a lift. Twenty subjects were experienced weightlifters while another 20 were subjects who had never lifted weights nor held a job that required them to on a regular basis. The subjects each lifted a container 20 × 45 × 40 cm, with handles, from floor to waist height 12 times with loads of 68, 10·2 or 13·6 kg. The loads were lifted under conditions of either havingor not having verbal and visual knowledge of the load magnitude prior to the lift. The subjects were allowed to perform the lift in a manner of their choosing. A 2 (groups) × 3 (loads) × 2 (load knowledge) ANOVA was performed on the data. Maximim force (F_max) value analysis revealed group and technique differences. The experienced lifters had lower stress levels at L4/L5 and utilized two technique strategies that were dependent upon the load knowledge condition, whereas the non-lifters used the same strategy for all lifts. Maximum moment values (M_max were significantly higher for the inexperienced lifters under all conditions, indicating a greater dependence on the low back musculature for initiating the lifting of a load.     

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.     

Tolerability to prolonged lifting tasks assessed by subjective perception and physiological responses

Prolonged physical exertion is regulated subjectively by the perception of effort. This preliminary study was conducted to validate the use of subjective perceptions of effort in assessing objectively tolerable workloads for prolonged lifting tasks. Eight healthy male subjects underwent incremental and 30-minute endurance lifting tests. Cardiorespiratory parameters were monitored with an oxygen uptake analyser and mechanical parameters were calculated using a lift dynamometer. Ratings of perceived exertion were given on Borg's 10-point scale. Physiological responses to repetitive lifting were matched with subjective perceptions. The relationship between the perception of exertion and the duration of the endurance tests was described by power functions; Y=aXⁿ in which 0 > n >1. A single-variable statistical regression for power functions was performed to obtain the individual ‘iso-perception’ curves as functions of the mechanical work exerted. It was found that the ‘iso-perception’ curve corresponding to a ‘moderate’ perception of effort may represent the individual ‘tolerance threshold’ for prolonged lifting tasks, since physiological responses at this intensity of effort did not change significantly and the respiratory exchange ratio was less than one. The individually tolerable power over lime for lifting tasks has been estimated.     

A biomechanical evaluation of lifting speed using work- and moment-related measures

A biomechanical evaluation of lifting speed was conducted in the laboratory. The study investigated the effects of lifting speed on several predetermined biomechanical cost functions. The lifting tasks consisted of five lifting speeds labelled as the slowest, slow, normal, fast and fastest, and three weights, 50, 65 and 80% of their maximum acceptable weight of lift. The speed at each level was determined individually by each subject according to their capability. The study found that work-related measures, including the total net muscle work, total absolute net muscle work and work done to the load, decreased significantly as the lifting speed increased (p<0.05, <0.001 and <0.001, respectively). The time integral of sum of squared ratio of joint moment and strength also decreased significantly (p<0.001). This indicates that lifting at a faster speed tends to reduce the work the body has to do. The peak speed of load occurred at 70% of total lifting time for the slowest lifts, but at 30% of total lifting time for other lifting speeds. Performing lifts at the minimum speeds changed the usual speed coordination technique the subjects used.     

A biomechanical evaluation of lifting speed using work- and moment-related measures.

A biomechanical evaluation of lifting speed was conducted in the laboratory. The study investigated the effects of lifting speed on several predetermined biomechanical cost functions. The lifting tasks consisted of five lifting speeds labelled as the slowest, slow, normal, fast and fastest, and three weights, 50, 65 and 80% of their maximum acceptable weight of lift. The speed at each level was determined individually by each subject according to their capability. The study found that work-related measures, including the total net muscle work, total absolute net muscle work and work done to the load, decreased significantly as the lifting speed increased (p < 0.05, < 0.001 and < 0.001, respectively). The time integral of sum of squared ratio of joint moment and strength also decreased significantly (p < 0.001). This indicates that lifting at a faster speed tends to reduce the work the body has to do. The peak speed of load occurred at 70% of total lifting time for the slowest lifts, but at 30% of total lifting time for other lifting speeds. Performing lifts at the minimum speeds changed the usual speed coordination technique the subjects used.     

Assessment of an EMG-based method for continuous estimates of low back compression during asymmetrical occupational tasks

Variables, such as peak and accumulated moments and spine compression forces, have been shown to be risk factors for occupational low back pain. Estimates of these forces during prolonged, dynamic, asymmetric tasks using biomechanical models is complex and time-consuming. A simple technique for continuous measurement of these variables over a prolonged period is needed to measure the distribution of spinal loading during both sagittal plane lifts and complex asymmetrical jobs. The aim of this study was to determine whether a linear normalization of erector spinae EMG to spine compression force, called compression normalized EMG (CNEMG), could be used to estimate spinal loading for simulations of asymmetrical occupational tasks. The estimates of spine compression force obtained using the normalized EMG are presented in the form of an amplitude probability distribution function and are compared with estimates of a three-dimensional biomechanical model. The per cent time a worker spends above particular levels of spinal loading of interest, such as the NIOSH action limit for compression, are displayed. Five males performed simulated occupational tasks. The exposure time at a specific level of spine compression force for a combination of three tasks, estimated by CNEMG, was, on average, within 6.5% of the time calculated by the biomechanical model. However, if the task combination was dominated by an axial twisting moment, then the difference was, on average, 13.4%. The difference in magnitude of spine compression at a specific probability was, on average, 14.9% and when axial trunk twist dominated, 30.7%. It is concluded that CNEMG can estimate probability at a specific level of spine compression force when the task combination is characterized by a predominant extensor moment in the sagittal plane. Estimates of spine compression at a specific probability, and estimates obtained during task combinations dominated by an axial twisting moment, are poor.     

An electromyographic analysis of seated and standing lifting tasks

The objective of this project was to compare the muscular effort exerted during manual lifting tasks performed in standing versus seated posture. Six male undergraduate and graduate students performed 12 different static and dynamic lifts in both sitting and standing positions. During each effort electromyographic (EMG) data were collected on four muscles groups (low back, upper back, shoulder, and abdominals). Four contractions were designed to elicit maximum muscular effort in the four groups being monitored. The remaining data were then expressed as a percentage of maximum EMG. Each subject performed the following: maximum static lift when sitting; maximum static lift when standing; sitting, static lift with 15·9 kg; standing, static lift with 15·9 kg; dynamic sit-forward lift with 15·9 kg, dynamic stand-forward lift with 15·9 kg, dynamic sit-twist with 15·9 kg, dynamic stand-vertical lift with 15·9 kg. Each of the lifts was performed with a wooden tray with slotted handles. Root mean square (RMS) values of the EMG data were calculated for three second periods. EMG activity in the low back, upper back, and shoulder was greater during sitting lifting than during standing lifting. The sit-twist lift resulted in the highest EMG in the abdominal muscles. Dynamic lifts resulted in more muscle activity than did static lifts. From these data it was concluded that sitting-lifting results in greater stress in the low back, upper back, and shoulders than does lifting while standing.     

Assessment of an EMG-based method for continuous estimates of low back compression during asymmetrical occupational tasks.

Variables, such as peak and accumulated moments and spine compression forces, have been shown to be risk factors for occupational low back pain. Estimates of these forces during prolonged, dynamic, asymmetric tasks using biomechanical models is complex and time-consuming. A simple technique for continuous measurement of these variables over a prolonged period is needed to measure the distribution of spinal loading during both sagittal plane lifts and complex asymmetrical jobs. The aim of this study was to determine whether a linear normalization of erector spinae EMG to spine compression force, called compression normalized EMG (CNEMG), could be used to estimate spinal loading for simulations of asymmetrical occupational tasks. The estimates of spine compression force obtained using the normalized EMG are presented in the form of an amplitude probability distribution function and are compared with estimates of a three-dimensional biomechanical model. The per cent time a worker spends above particular levels of spinal loading of interest, such as the NIOSH action limit for compression, are displayed. Five males performed simulated occupational tasks. The exposure time at a specific level of spine compression force for a combination of three tasks, estimated by CNEMG, was, on average, within 6.5% of the time calculated by the biomechanical model. However, if the task combination was dominated by an axial twisting moment, then the difference was, on average, 13.4%. The difference in magnitude of spine compression at a specific probability was, on average, 14.9% and when axial trunk twist dominated, 30.7%. It is concluded that CNEMG can estimate probability at a specific level of spine compression force when the task combination is characterized by a predominant extensor moment in the sagittal plane. Estimates of spine compression at a specific probability, and estimates obtained during task combinations dominated by an axial twisting moment, are poor.     

The effects of military body armour on the lower back and knee mechanics during toe-touch and two-legged squat tasks

While effective in the prevention of otherwise lethal injuries, military body armour (BA) has been suggested to reduce warfighter's performance and increase injury-related musculoskeletal conditions. Providing the significant role of joint biomechanics in both performance and risk of injury, the immediate and prolonged effects of wearing BA on biomechanics of the lower back and knee during toe-touch (TT) and two-legged squat (TLS) tasks were investigated. The immediate effects of BA were an increase of >40 ms (p ≤ 0.02) in flexion duration of the dominant joint and an ～1 s (p ≤ 0.02) increase in overall task duration as well as an ～18% (p = 0.03) decrease in the lumbopelvic rhythm ratio near the mid-range of trunk flexion. In general the prolonged duration of wearing BA (i.e. 45 min of walking) was not found to cause more changes in our measures than walking without BA.     

An electromyographic analysis of seated and standing lifting tasks.

The objective of this project was to compare the muscular effort exerted during manual lifting tasks performed in standing versus seated posture. Six male undergraduate and graduate students performed 12 different static and dynamic lifts in both sitting and standing positions. During each effort electromyographic (EMG) data were collected on four muscles groups (low back, upper back, shoulder, and abdominals). Four contractions were designed to elicit maximum muscular effort in the four groups being monitored. The remaining data were then expressed as a percentage of maximum EMG. Each subject performed the following: maximum static lift when sitting; maximum static lift when standing; sitting, static lift with 15.9 kg; standing, static lift with 15.9 kg; dynamic sit-forward lift with 15.9 kg, dynamic stand-forward lift with 15.9 kg, dynamic sit-twist with 15.9 kg, dynamic stand-vertical lift with 15.9 kg. Each of the lifts was performed with a wooden tray with slotted handles. Root mean square (RMS) values of the EMG data were calculated for three second periods. EMG activity in the low back, upper back, and shoulder was greater during sitting lifting than during standing lifting. The sit-twist lift resulted in the highest EMG in the abdominal muscles. Dynamic lifts resulted in more muscle activity than did static lifts. From these data it was concluded that sitting-lifting results in greater stress in the low back, upper back, and shoulders than does lifting while standing.     

Perceived safety and biomechanical stress to the lower limbs when stepping down from fire fighting vehicles

Injuries related to emergency vehicles represent 19% of compensated work accidents for fire fighters, 37% of which occur while stepping down from their vehicles. This study compared the impact forces, the use of upper limbs and the perception of danger of fire fighters as they step down from five different locations on fire trucks. The results show that stepping down from the crew cab facing the street produces impact forces averaging 3.2 times the subject's body weight, but is also perceived as the safest way to descend in one of the two groups of fire fighters that participated in the study. Stepping down from the same location, but facing the truck, produced significantly less impact force and a better distribution of the energy over time. This may be achieved through better control of the descending leg, ankle flexion, and the use of grab bars. A re-design of the access to emergency vehicles should take into account both the safety needs and reduction in biomechanical stress of fire fighters.     

Maximum acceptable repetitive lifting workloads for an 8-hour work-day using psychophysical and subjective rating methods

The psychophysical method used by Snook (1978) to determine maximum acceptable workloads for repetitive lifting during an 8-hour work-day in industrial populations was evaluated for application in military ergonomics. Under the conditions ofthe present experiment, the mean load selected by 10 soldiers (17·5 kg) was lower than reported by Snook (1978) for industrial workers and by Garg and Saxena (1979) for college students. When the soldiers lifted and lowered their selected load for an 8-hour work-day, the average heart rate was 92 beats min^?1 and the mean oxygen cost was 21% of their maximum oxygen uptake (determined for uphill treadmill running). There was no evidence of cardiovascular, metabolic or subjective fatigue. A subjective rating method tended to identify slightly lower loads than the psychophysical method. The results indicate that with good subject cooperation and firm experimental control in a laboratory, the psychophysical method can identify loads that soldiers can lift repetitively for an 8-hour work-day without metabolic, cardiovascular or subjective evidence of fatigue     

Evaluating worker vibration exposures using self-reported and direct observation estimates of exposure duration

The objective of this study was to compare objective and subjective methods of collecting exposure time data for hand arm vibration (HAV) and whole-body vibration (WBV), and to evaluate the impact of inaccurate exposure times’ on the calculation of the average vibration exposure over an 8 h working day A(8).The study was carried out in the engineering services and maintenance departments of a construction and property management company. Worker exposure time data was collected using three methods, questionnaire surveys, daily worker interviews and 8 h direct workplace observations. Vibration magnitudes (m/s²) were measured for a range of hand tools and vehicles, and daily vibration exposure estimates A(8) were calculated using exposure times observed, reported in interview and self reported in the questionnaire.Results from the study showed that self-reported exposure time estimates from the questionnaire survey were a factor of 9.0 (median value) times greater for HAV and a factor of 6.0 (median value) times greater for WBV when compared with direct observation estimates. Exposure times reported in interview were higher, than those observed, but more reliable than those self reported in the questionnaire; a factor of 2.1 (median value) times greater for HAV and a factor of 1.4 (median value) times greater for WBV. A(8) values calculated using questionnaire exposure times were up to 66% and 75% greater for sources of HAV and WBV respectively when compared to A(8) values calculated using observed exposure times.For the purposes of carrying out a reliable risk assessment, results from this study indicate that direct measurements of worker exposure time are not recommended over questionnaires especially where work is highly variable for example in construction and property management. Worker interviews or direct workplace observation methods were found to be reliable alternative methods for collecting exposure time.     

A biomechanical and subjective comparison of two powered ambulance cots

This study investigated biomechanical effects of different leg folding/unfolding mechanisms used for loading/unloading two powered cots (Cots A and B) into and from a simulated ambulance. Sixteen experienced emergency medical service (EMS) workers loaded and unloaded cots with weights of 45, 68 and 91 kg placed on the cots to simulate patients. Peak back and shoulder/arm muscle activity was reduced 52–87% when using Cot A in comparison to Cot B. Peak ground reaction force (PGRF) was reduced by 74% with Cot A. Adding weight resulted in increased muscle activity and PGRF when using Cot B, but had little effect when using Cot A. Task time was longer with Cot A, though was not perceived unfavourably by participants. This study confirmed that it is possible to substantially reduce physical stress imposed on EMS workers when loading and unloading a cot to and from an ambulance through improvements in cot design.     

Evaluation of the effect of backpack load and position during standing and walking using biomechanical, physiological and subjective measures

Recommendations on backpack loading advice restricting the load to 10% of body weight and carrying the load high on the spine. The effects of increasing load (0%-5%-10%-15% of body weight) and changing the placement of the load on the spine, thoracic vs. lumbar placement, during standing and gait were analysed in 20 college-aged students by studying physiological, biomechanical and subjective data. Significant changes were: (1) increased thorax flexion; (2) reduced activity of M. erector spinae vs. increased activation of abdominals; (3) increased heart rate and Borg scores for the heaviest loads. A trend towards increased spinal flexion, reduced pelvic anteversion and rectus abdominis muscle activity was observed for the lumbar placement. The subjective scores indicate a preference for the lumbar placement. These findings suggest that carrying loads of 10% of body weight and above should be avoided, since these loads induce significant changes in electromyography, kinematics and subjective scores. Conclusions on the benefits of the thoracic placement for backpack loads could not be drawn based on the parameter set studied.     

Evaluation of the effect of backpack load and position during standing and walking using biomechanical,physiological and subjective measures

Recommendations on backpack loading advice restricting the load to 10% of body weight and carrying the load high on the spine. The effects of increasing load (0%–5%–10%–15% of body weight) and changing the placement of the load on the spine, thoracic vs. lumbar placement, during standing and gait were analysed in 20 college-aged students by studying physiological, biomechanical and subjective data. Significant changes were: (1) increased thorax flexion; (2) reduced activity of M. erector spinae vs. increased activation of abdominals; (3) increased heart rate and Borg scores for the heaviest loads. A trend towards increased spinal flexion, reduced pelvic anteversion and rectus abdominis muscle activity was observed for the lumbar placement. The subjective scores indicate a preference for the lumbar placement. These findings suggest that carrying loads of 10% of body weight and above should be avoided, since these loads induce significant changes in electromyography, kinematics and subjective scores. Conclusions on the benefits of the thoracic placement for backpack loads could not be drawn based on the parameter set studied.     

Effect of gloves on prehensile forces during lifting and holding tasks

The effect of gloves on the spatio-temporal characteristics of prehensile forces during lifting and holding tasks was investigated. Participants (n = 10) lifted a force transducer equipped object (weight = 0.29 N) with various types of gloves and barehanded using a two-fingered precision grip. Rubber surgical gloves of varied thicknesses (0.24, 0.61 and 1.02 mm) were worn to examine the effect of glove thickness on a rayon surface. It was found that grip force increased with thickness because the participants employed a higher safety margin above the minimum force required to hold the object. The safety margin for the barehanded condition was the smallest. The performance time for lifting the object was not influenced by the variation of glove thickness. The findings suggest that glove thickness, which presumably modifies the cutaneous sensation, influences grip force regulation. The effect of glove material (rubber and cotton) was also examined in relation to slippery (rayon) and non-slippery (sandpaper) surfaces. It was found that the participants used a larger grip force with the cotton glove than the rubber glove for the slippery surface, but not with the non-slippery surface. With use of the rubber glove, a relatively low grip force level was maintained for both slippery and non-slippery surfaces. The performance time for the cotton glove was longer than that for the rubber glove. The findings suggest that the rubber glove provides better efficiency of force and temporal control than the cotton glove in precision handling of small objects.