Effect of lifting belts on trunk muscle activation during a suddenly applied load

The National Institute for Occupational Safety and Health suggests there is insufficient biomechanical or epidemiological evidence to recommend the use of back belts in industry. From a biomechanical perspective, previous work suggests that lifting belts stiffen the torso, particularly in the frontal and transverse planes. To determine whether lifting belts stiffen the torso and alter the trunk muscle response during a sudden loading event, we tested the hypotheses that (a) lifting belts alter peak muscle activity recorded with electromyography (EMG) during sudden loading and (b) lifting belts have a larger impact on trunk muscle response when sudden loads are applied asymmetric to the torso's midsagittal plane. A sudden load was delivered to 10 men and 10 women without history of low back disorder via a cable attached to a thoracic harness; motion was restricted to the lumbar spine. Results indicate that gender was not a significant factor in this study. The lifting belt reduced the peak normalized EMG of the erector spinae muscles on average by 3% during asymmetric loading, though peak normalized EMG was increased by 2% during symmetric loading. Lifting belts have been shown to slightly reduce peak erector spinae activity during asymmetric sudden loading events in a constrained paradigm; however, the effects of lifting belts are too small to provide effective protection of workers. Actual or potential applications include the assessment of lifting belts as protective devices in workers based on the effects of lifting belts on the trunk muscle activity.     

A novel optimization model for predicting trunk muscle forces during asymmetric lifting tasks

Low back pain (LBP) is a social and economic problem throughout industry. Repetitive asymmetric postures frequently occur in manual materials handling tasks and such asymmetric lifting has been epidemiologically linked to LBP. Therefore, biomechanical lifting models must be developed to predict muscle forces during asymmetric lifting tasks. This paper proposes an optimization model that was revised to take into account the activities of trunk muscles during asymmetric lifting tasks. Also, three optimization models (minimize maximum muscle intensity: MIN_I_MAX, minimize sum of magnitudes of the muscle forces raised to power 3: MIN_F³, and minimize sum of the muscle intensities raised to power 3: MIN_I³) are compared under various asymmetric lifting conditions. The revised model not only reflects the twisting effect of muscle force vectors for eight primary trunk muscles when trunk rotation is involved, but also accurately predicts the forces of left erector spinae, left latissimus dorsi and left external oblique muscles when compared with EMG signals obtained from experiments. Furthermore, MIN_I_MAX exhibits the best prediction capability among the three optimization models.Relevance to industryA novel optimization model proposed herein considers the twisting effect of muscle force vectors for eight trunk muscles when trunk rotation is involved. An accurate biomechanical model which reflects the asymmetric lifting conditions would significantly facilitate the evaluation of job and workplace design as well as provide a practical clinical evaluation technique.     

The effects of a sloped ground surface on trunk kinematics and L5/S1 moment during lifting

There are many work environments that require workers to perform manual materials handling tasks on ground surfaces that are not perfectly flat (e.g. in agriculture, construction, and maritime workplaces). These sloped ground surfaces may have an impact on the lifting strategy/technique employed by the lifter, which may, in turn, alter the biomechanical loading of the spine. Describing the changes in kinematics and kinetics of the torso is the first step in assessing the impact of these changes and is the focus of the current research. Subjects' whole-body motions were recorded as they lifted a 10 kg box while standing on two inclined surfaces (facing an upward slope: 10° and 20°), two declined surfaces (facing a downward slope: ? 10° and ? 20°), and a flat surface (0°) using three lifting techniques (leg lift, back lift and freestyle lift). These data were then used in a two-dimensional, five-segment dynamic biomechanical model (top-down) to evaluate the effect of these slopes on the net moment about the L5/S1 joint. The results of this study showed an interesting interaction effect wherein the net L5/S1 moment was relatively insensitive to changes in slope angle under the back lift condition, but showed a significant effect during the leg lift and freestyle lifting conditions. The results show that under the freestyle lifting condition the peak L5/S1 moment was significantly higher for the inclined surfaces as compared to the flat surfaces (6.8% greater) or declined surfaces (10.0% greater). Subsequent component analysis revealed that both trunk flexion angle and angular trunk acceleration were driving this response. Collectively, the results of this study indicate that ground slope angle does influence the lifting kinematics and kinetics and therefore needs to be considered when evaluating risk of low back injury in these working conditions.     

Lower back muscle forces in pushing and pulling

Abstract

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).     

Age-related biomechanical differences during asymmetric lifting

This study investigated age-related biomechanical differences during asymmetric lifting. Eleven younger and twelve older participants were instructed to lift loads of different weights to an asymmetric destination. The trunk kinematics and low back moments were examined. The results showed that older adults adopted safer lifting strategies compared with younger adults. Specifically, the peak trunk sagittal flexion angle was 32% lower and the peak trunk transverse twisting angle was 22% lower in older adults compared with those in younger adults. It was also found that the average low back moment in the deposit phase was 32% higher in older adults than that in younger adults, most probably due to the age-related increased body weight. Based on these findings and the fact of age-related decreased muscle strengths, physical exercise programs were suggested to be more effective than instructions of safe lifting strategies in LBP risk reduction during asymmetric lifting for older adults. For younger adults, safe lifting strategy instructions might be effective to reduce risks of LBP.     

Effect of foot movement and an elastic lumbar back support on spinal loading during free-dynamic symmetric and asymmetric lifting exertions

The aim of this study was to assess the effect of an elastic lumbar back support on spinal loading and trunk, hip and knee kinematics while allowing subjects to move their feet during lifting exertions. Predicted spinal forces and moments about the L5/S1 intervertebral disc from a three-dimensional EMG-assisted biomechanical model, trunk position, velocities and accelerations, and hip and knee angles were evaluated as a function of wearing an elastic lumbar back support, while lifting two different box weights (13.6 and 22.7 kg) from two different heights (knee and 10 cm above knee height), and from two different asymmetries at the start of the lift (sagittally symmetric and 60°asymmetry). Subjects were allowed to lift using any lifting style they preferred, and were allowed to move their feet during the lifting exertion. Wearing a lumbar back support resulted in no significant differences for any measure of spinal loading as compared with the no-back support condition. However, wearing a lumbar back support resulted in a modest but significant decrease in the maximum sagittal flexion angle (36.5 to 32.7°), as well as reduction in the sagittal trunk extension velocity (47.2 to 40.2°s^-1). Thus, the use of the elastic lumbar back support provided no protective effect regarding spinal loading when individuals were allowed to move their feet during a lifting exertion.     

The effects of horizontal load speed and lifting frequency on lifting technique and biomechanics

Lifting loads that have a horizontal velocity (e.g. lifting from a conveyor) is often seen in industry and it was hypothesised that the inertial characteristics of these loads may influence lifting technique and low back stress. Seventeen male participants were asked to perform lifting tasks under conditions of four horizontal load speeds (0 m/s, 0.7 m/s, 1.3 m/s and 2.4 m/s) and two lifting frequencies (10 and 20 lifts/min) while trunk motions and trunk muscle activation levels were monitored. Results revealed that increasing horizontal load speed from 0 m/s to 2.4 m/s resulted in an increase in peak sagittal angle (73° vs. 81°) but lower levels of peak sagittal plane angular acceleration (480°/s² vs. 4°/s²) and peak transverse plane angular acceleration (200°/s per s vs. 140°/s per s) and a consistent increase in trunk muscle co-activation. Participants used the inertia of the load to reduce the peak dynamics of the lifting motion at a cost of increased trunk flexion and higher muscle activity.

Statement of Relevance: Conveyors are ubiquitous in industry and understanding the effects of horizontal load speed on the lifting motions performed by workers lifting items from these conveyors may provide some insight into low back injury risk posed by these tasks.     

Assessment of engineering controls designed for handling unstable loads: An electromyography assessment

Low back injury due to manual lifting is historically prevalent in labor intensive industries. Improving risk management options is necessary to reduce the risk of low back injury. Workers lifting unstable loads are at greater risk of back injury compared to workers lifting stable loads. This study focused on the effect of engineering controls on trunk muscle activity. Engineering controls were designed to control the instability of a liquid load. Thirty-nine healthy subjects manually lifted asymmetrically in the transverse direction stable loads, unstable loads, and unstable loads with engineering controls. Trunk and load kinematic and trunk muscle electromyography data were collected during lifting. Unstable loads with engineering controls significantly (p < 0.001) reduced trunk muscle activity compared to unstable loads. Engineering controls should be implemented to reduce the risk of injury to workers handling unstable liquid loads.Relevance to industryManually handling containers filled with liquids is necessary in many industrial workplaces. Risk management solutions for low back injury due to manual lifting of such loads should focus on reducing muscular demand. This study demonstrates that engineering controls designed to increase the stability of a liquid load reduced muscular demand.     

Prediction models for the cross-sectional areas of lower lumbar intervertebral discs and vertebral endplates

Current approaches to obtain lumbar morphometry data usually require expensive medical imaging technology, long processing time, and are often limited by small sample size. This study develops regression models for the cross-sectional areas (CSAs) of the lower lumbar (i.e., from L3/L4 to L5/S1 level) intervertebral discs (IVDs) and vertebral endplates (EPs) using both simple and complex anthropometric variables. CSAs were measured using OsiriX^© software, based on 3T magnetic resonance imaging (MRI) scans from a sample of 13 females and 22 males, aged between 20 and 40, and asymptomatic of low back disorders. Comprehensive body anthropometry data were collected and included in the regression analyses. Several multiple regression models were developed with varying levels of complexity. Subject stature, elbow dimensions, and ankle dimensions were statistically significant predictors for the CSAs of IVDs and EPs. Gender exhibited a more predictive relationship with the CSAs when compared to body weight and age. In general, regression models using newly proposed best subset procedure resulted in smaller prediction errors, compared to the models using easy-to-measure variables (i.e., gender, age, height, and weight). However, simple regression models are still worthy of investigation given the low cost, ease of data collection, and satisfactory model performance.     

Evaluating lifting tasks using subjective and biomechanical estimates of stress at the lower back

The objective of this study was to evaluate five different lifting tasks based on subjective and biomechanical estimates of stress at the lower back. Subjective estimates were obtained immediately after the subjects performed the lifting tasks. Rankings for different tasks were obtained according to the perceived level of stress at the lower back. A biomechanical model was used to predict the compressive force at the L5/S1 disc for the weight lifted considering link angles for the particular posture. The tasks were also ranked according to the compressive force loading at the L5/S1 disc. The weight lifted in these tasks for obtaining the subjective estimate of stress was the maximum acceptable weight of lift (MAWOL). This was determined separately for each subject using a psychophysical approach. Subjective estimates of stress were obtained for infrequent lifting, specifically for a single lift, as well as for lifting at a frequency of four lifts per min. The results showed that a lifting task acceptable from the biomechanical point of view may not be judged as a safe or acceptable task by the worker based on his subjective perception. This may result in a risk of the worker not performing the recommended task or not following the recommended method.     

The effects of a sloped ground surface on trunk kinematics and L5/S1 moment during lifting

There are many work environments that require workers to perform manual materials handling tasks on ground surfaces that are not perfectly flat (e.g. in agriculture, construction, and maritime workplaces). These sloped ground surfaces may have an impact on the lifting strategy/technique employed by the lifter, which may, in turn, alter the biomechanical loading of the spine. Describing the changes in kinematics and kinetics of the torso is the first step in assessing the impact of these changes and is the focus of the current research. Subjects' whole-body motions were recorded as they lifted a 10 kg box while standing on two inclined surfaces (facing an upward slope: 10 degrees and 20 degrees), two declined surfaces (facing a downward slope: -10 degrees and -20 degrees), and a flat surface (0 degrees) using three lifting techniques (leg lift, back lift and freestyle lift). These data were then used in a two-dimensional, five-segment dynamic biomechanical model (top-down) to evaluate the effect of these slopes on the net moment about the L5/S1 joint. The results of this study showed an interesting interaction effect wherein the net L5/S1 moment was relatively insensitive to changes in slope angle under the back lift condition, but showed a significant effect during the leg lift and freestyle lifting conditions. The results show that under the freestyle lifting condition the peak L5/S1 moment was significantly higher for the inclined surfaces as compared to the flat surfaces (6.8% greater) or declined surfaces (10.0% greater). Subsequent component analysis revealed that both trunk flexion angle and angular trunk acceleration were driving this response. Collectively, the results of this study indicate that ground slope angle does influence the lifting kinematics and kinetics and therefore needs to be considered when evaluating risk of low back injury in these working conditions.     

Foot positioning instruction, initial vertical load position and lifting technique: effects on low back loading

This study investigated the effects of initial load height and foot placement instruction in four lifting techniques: free, stoop (bending the back), squat (bending the knees) and a modified squat technique (bending the knees and rotating them outward). A 2D dynamic linked segment model was combined with an EMG assisted trunk muscle model to quantify kinematics and low back loading in 10 subjects performing 19 different lifting movements, using 10.5 kg boxes without handles. When lifting from a 0.05 m height with the feet behind the box, squat lifting resulted in 19.9% (SD 8.7%) higher net moments (p < 0.001) and 17.0% (SD 13.2%) higher compression forces (p < 0.01) than stoop lifting. This effect was reduced to 12.8% (SD 10.7%) for moments and a non-significant 7.4% (SD 16.0%) for compression forces when lifting with the feet beside the box and it disappeared when lifting from 0.5 m height. Differences between squat and stoop lifts, as well as the interaction with lifting height, could to a large extent be explained by changes in the horizontal L5/S1 intervertebral joint position relative to the load, the upper body acceleration, and lumbar flexion. Rotating the knees outward during squat lifts resulted in moments and compression forces that were smaller than in squat lifting but larger than in stoop lifting. Shear forces were small ( < 300 N) at the L4/L5 joint and substantial (1100 - 1400 N) but unaffected by lifting technique at the L5/S1 joint. The present results show that the effects of lifting technique on low back loading depend on the task context.     

Foot positioning instruction,initial vertical load position and lifting technique: effects on low back loading

This study investigated the effects of initial load height and foot placement instruction in four lifting techniques: free, stoop (bending the back), squat (bending the knees) and a modified squat technique (bending the knees and rotating them outward). A 2D dynamic linked segment model was combined with an EMG assisted trunk muscle model to quantify kinematics and low back loading in 10 subjects performing 19 different lifting movements, using 10.5 kg boxes without handles. When lifting from a 0.05 m height with the feet behind the box, squat lifting resulted in 19.9% (SD 8.7%) higher net moments (p < 0.001) and 17.0% (SD 13.2%) higher compression forces (p < 0.01) than stoop lifting. This effect was reduced to 12.8% (SD 10.7%) for moments and a non-significant 7.4% (SD 16.0%) for compression forces when lifting with the feet beside the box and it disappeared when lifting from 0.5 m height. Differences between squat and stoop lifts, as well as the interaction with lifting height, could to a large extent be explained by changes in the horizontal L5/S1 intervertebral joint position relative to the load, the upper body acceleration, and lumbar flexion. Rotating the knees outward during squat lifts resulted in moments and compression forces that were smaller than in squat lifting but larger than in stoop lifting. Shear forces were small ( < 300 N) at the L4/L5 joint and substantial (1100 – 1400 N) but unaffected by lifting technique at the L5/S1 joint. The present results show that the effects of lifting technique on low back loading depend on the task context.     

Modification of an EMG-assisted biomechanical model for pushing and pulling

Pushing and pulling tasks using carts and material handling devices have become more prevalent in occupational environments in an attempt to reduce the musculoskeletal risks associated with lifting. However, little change in low back disorder rates have been noted as tasks change from lifting to pushing and pulling indicating that we do not understand the mechanics of pushing and pulling well. Biomechanical assessments of pushing and pulling tasks using person-specific biologically assisted models offer a means to help understand how the spine is loaded under pushing and pulling conditions. However, critical components of these models must be adjusted so that they are sensitive to the different physiologic responses in the torso muscles expected during pushing and pulling compared to lifting tasks.The objective of this study was to modify an electromyography (EMG)-assisted biomechanical model designed to evaluate lifting tasks so that it can better represent the biomechanical forces expected during pushing and pulling tasks. Several key modifications were made. Based upon a literature review, changes in muscle cross-sectional area and muscle origins and insertions were made to better represent the geometry of the torso muscles. It was also necessary to adjust the length–force and velocity–force muscle relationships. Empirically derived length–force and velocity–force relationships were developed to independently represent the flexor and extensor musculature. These modifications were then systematically incorporated into the model.The model was exercised over several pushing and pulling conditions to assess the effect of these modifications on its ability to predict externally measured spinal moments. Results indicated that the alterations made to the preexisting EMG-assisted model resulted in acceptable model performance for pushing, pulling, and lifting activities.

Relevance to industry

The use of carts and material handling devices has become increasingly prevalent in industry, though little research has been done to examine the body's response. The modifications made to the biomechanical model would enable its use in the evaluation and design of material handling devices and pushing and pulling tasks.     

Workplace design guidelines for asymptomatic vs. low-back-injured workers

While numerous efforts have attempted to provide quantitative guidelines for the prevention of initial low back disorders during material handling tasks, none have appeared in the literature that address the issue of recurrent low back disorders due to materials handling when returning to the workplace. A study comparing the spine loads of low back pain patients and asymptomatic controls was conducted. Subjects lifted weights varying from 4.5-11.4 kg at four vertical heights, two horizontal distances and five task asymmetries collectively representing common industrial lifting situations. Spine loading was calculated using a validated EMG-assisted biomechanical model. Spine loads observed during lifting tasks were compared to spine tolerance values believed to initiate low back injuries. In addition, the percentage of patients successfully performing the lift was noted and used as an indication of the willingness of the subject to perform the task. These evaluations are summarized in a series of three lifting guidelines indicating safe, medium risk and high risk lifting tasks for low back patients as well as asymptomatic workers. It is believed that adherence to these guidelines can minimize the risk of recurrent low back disorders due to occupational lifting.     

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.     

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.     

Method for determining musculotendon parameters in subject-specific musculoskeletal models of children developed from MRI data

Accurate knowledge of muscle-tendon parameters in biomechanical models is critical for accurate simulation and analyses of human movement. An excellent example of this is the creation of subject-specific models from magnetic resonance imaging (MRI). When Hill-type muscle models are used to calculate muscle forces, the determination of muscle attachment points, optimal fiber length, tendon slack length and maximum isometric force all have a significant influence on the joint moment-angle behavior of the model. In the present study a method was developed for customizing the values of muscle-tendon parameters in a generic model to create subject-specific biomechanical models from MRI. The method was applied by generating musculoskeletal models for the biomechanical simulation platform OpenSim, but the workflow is equally well applicable to other simulation platforms. New computational algorithms are described for identifying joint centers and for reconstructing the centroids of the muscle bellies from MRI. A?process is also described for the extraction of the muscle paths and for identifying the positions of ??via-points?? used to model muscles wrapping over bones. Finally, a new algorithm is described for adjusting the values of optimal fiber length, tendon slack length and maximum isometric force based on a comparison of the model results with experiment. We tested our computational algorithms by developing subject-specific biomechanical models of five typically developed children (age 9.5±1.7?years) from MRI. The joint moment-angle relationships calculated for the subject-specific models were similar to those determined for corresponding scaled generic models. The results indicate that the proposed methodology is suitable for developing subject-specific models of healthy children. Future studies should investigate how abnormalities of the musculoskeletal system, such as tibial torsion and muscle spasticity, can be integrated into the modeling process.     

Development and validation of a three dimensional dynamic biomechanical lifting model for lower back evaluation for careful box placement

One of the major causes to low back injury is the box lifting activity, thus for many years biomechanics has been utilized by designers for ergonomic evaluations of the box lifting activity which includes the placement of the box. More recently these ergonomic investigations have focused on the careful placement of the box. The AnyBody (AB) biomechanical models and optimization within the AB software system in conjunction with motion capture has been shown to obtain adequate estimates of joint reaction forces of the body. To date there has not been a dynamic 3D box lifting model developed and validated for carefully placing a box using the AB modeling system and motion capture. Thus the focus of this paper is on the development, verification and validation of a box lifting full body model for lower back evaluations for a dynamic lifting activity for carefully placing a box on a shelf.     

Trunk muscle activation and low back loading in lifting in the absence of load knowledge

People who know the actual mass of an object to be lifted normally prepare themselves before attempting a lift to control the movement and to minimize low back loading. In this study, the trunk muscular reactions and low back torque were investigated in the situation in which the individual did not know the actual mass but only had some idea of the range within which the mass lay. Nine males lifted boxes weighing 6.5 or 16.5 kg under the condition in which they knew the actual mass before attempting a lift (the 'known' condition) and the condition in which they only had the information that the mass would be within the range of 6.5-16.5 kg (the 'unknown' condition). The ground reaction forces and body movements were measured in the trials and, from these, the L5/S1 torques were calculated. The activation of back and abdominal muscles was also measured. For the 6.5 kg weight, a higher (16%) back muscle activation in grasping the box and a higher (10%) peak L5/S1 torque in actual lifting were observed in the 'unknown' compared with the 'known' weight condition. For the 16.5 kg weight, the back muscle activation was lower (10%) during grasping, and higher (10%) during lifting in the 'unknown' compared with the 'known' weight condition. Knowledge of the load had no effect on the activation of the abdominal muscles. It was concluded that in the so-called 'unknown' conditions, the risks of low back injury were increased in comparison with the conditions where the actual weight was known in advance.