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Shear Happens! Suggested guidelines for ergonomists to reduce the risk of low back injury from shear loading
Abstract
This paper describes the cause of excessive shear forces and the link between shear forces on the low back and injury. Guidelines are provided for the ergonomist to minimize the danger.
... This equates to the leg of a 91 kg patient. There are additional parameters to this guideline that include avoiding bending or twisting (McGill et al. 1998;Waters et al. 1993). This 15.9 kg limit is frequently exceeded during regular patient handling tasks, which can put healthcare workers at risk of musculoskeletal injury (Bartnik and Rice 2013;Larson, Murtagh, and Rice 2018;Larson and Rice 2015). ...
... Many studies focus on methods for bringing these forces under the recommended limits during patient handling tasks (Bartnik and Rice 2013;Larson, Murtagh, and Rice 2018;Larson and Rice 2015;McGill et al. 1998;Waters et al. 1993). These studies have shown risks associated with the manual handling of patients, including high forces at the low back even when using safe patient handling materials. ...
Healthcare workers have a high rate of low back injury due to patient handling tasks. These workers receive training in patient handling methods such as adjusting bed height, but often ignore them. In this study, 35 healthcare workers completed patient boosts at a self-chosen bed height and again with the bed in a higher standardised position. Motion capture and force data were collected for analysis. Given the choice, less than half of participants adjusted the bed at all and none of them moved the bed to the highest position (99.1 cm). The self-chosen bed position yielded significantly higher low back force than the higher position at L4-L5 and L5-S1 (p = 0.02, p = 0.01 respectively). Low back forces can be reduced by raising the bed prior to engaging in patient handling tasks, which is a simple step that can reduce forces placed on healthcare workers’ low backs.
Practitioner summary: Healthcare workers experience high rates of low back pain secondary to patient handling tasks. In this cross-sectional crossover study, healthcare workers consistently chose a low bed height when boosting a patient, which resulted in higher low back loads compared to the highest bed height.
... McGill et al. recommended that the shear force of the lumbar vertebral should be less than 500 N to prevent LBP [31]. Data distribution indicated that all shear forces exceeded the injury threshold (500 N) [12] at a 50 kgf external load (Fig. 7). ...
Patient transfer is the primary cause of lower back pain among caregivers because it requires awkward postures and movements such as twisting, lifting, and lowering with heavy external loads such as body weight. To prevent lower back pain, the relationship between lumbar loads and external loads from patient weight should be investigated to explore the hazardous limits of external loads during patient transfer. However, this investigation requires frequent trials and heavier loads than the hazardous limit. Therefore, we have used a computational musculoskeletal simulation for patient handling without the actual measured load data of human subjects. A previous study used a musculoskeletal simulation of sit-to-stand assistance motion; however, this simulation did not consider twisting and lowering patient transfer. Hence, this study aims to investigate the relationship between lumbar loads and external loads during patient transfer, including twisting and lowering. The musculoskeletal simulation for this investigation was implemented using the 3D Static Strength Prediction Program. First, the implemented musculoskeletal simulation was validated by comparison with related research using actual measured motion data and an optical motion capture system. Furthermore, the relationship between lumbar loads (compressive and shear forces of L5/S1) and external loads during patient transfer was investigated using a validated musculoskeletal simulation. According to the results, the compressive and shear forces of L5/S1 during patient transfer exceeded the limits of safety when the external load was more than 40 kgf. These findings will contribute to the prevention of lower back pain due to patient transfer.
... Previous investigations that have specifically examined lumbar spine kinetics and kinematics in anthropometric testing devices (ATDs) 2,3 and cadavers 4 demonstrate that the peak exposures in the lumbar spine during a simulated rear impact collision are below existing injury assessment reference values and are within the range of loads and postures experienced during activities of daily living and manual materials handling. [5][6][7][8] Gates et al 2 compared estimates of lumbar kinetics in a Hybrid III and a Biofidelic Rear Impact Dummy (BioRID) ATDs at rear impact collision severities ranging from 8 to 24 km/h. The measured peak compressive force, across all collision severities, was well below the National Institute of Occupational Safety and Health (NIOSH) action limit (3400 N) for occupational exposures. ...
Background:
Recent work has demonstrated that low back pain is a common complaint following low-speed collisions. Despite frequent pain reporting, no studies involving human volunteers have been completed to examine the exposures in the lumbar spine during low-speed rear impact collisions.
Methods:
Twenty-four participants were recruited and a custom-built crash sled simulated rear impact collisions, with a change in velocity of 8 km/h. Randomized collisions were completed with and without lumbar support. Inverse dynamics analyses were conducted, and outputs were used to generate estimates of peak L4/L5 joint compression and shear.
Results:
Average (SD) peak L4/L5 compression and shear reaction forces were not significantly different without lumbar support (compression = 498.22 N [178.0 N]; shear = 302.2 N [98.5 N]) compared to with lumbar support (compression = 484.5 N [151.1 N]; shear = 291.3 N [176.8 N]). Lumbar flexion angle at the time of peak shear was 36° (12°) without and 33° (11°) with lumbar support.
Conclusion:
Overall, the estimated reaction forces were 14% and 30% of existing National Institute of Occupational Safety and Health occupational exposure limits for compression and shear during repeated lifting, respectively. Findings also demonstrate that, during a laboratory collision simulation, lumbar support does not significantly influence the total estimated L4/L5 joint reaction force.
... The NIOSH recommended action limit is 3400 N (350 kg) and the maximum permissible limit is 6400 N (650 kg) for compressive force on L5-S1 [59]. The recommended shear force limit for occasional lifting task (≤100 loading/day) is 1000 N and for repetitive lifting task (100-1000 loadings/day) is 700 N [60], [61]. Considering the subject's height, weight, box size and distance between the box and body, the predicted L5-S1 joint compression force is within the expected limit and below the NIOSH action limit. ...
Objective:
In this study, a novel hybrid predictive musculoskeletal model is proposed which has both motion prediction and muscular dynamics assessment capabilities.
Methods:
First, a two-dimensional (2D) skeletal model with 10 degrees of freedom is used to predict a symmetric lifting motion, outputting joint angle profiles, ground reaction forces (GRFs), and center of pressure (COP). These intermediate outputs are input to the scaled musculoskeletal model in OpenSim for muscle activation and joint reaction load analysis. Finally, the experimental validation is carried out.
Results:
Static Optimization tool is used to estimate the muscle activation data in OpenSim for the predicted lifting motion. Joint reaction forces of the lumbosacral joint (L5-S1) are generated using the OpenSim Joint Reaction analysis tool. The predicted joint angles, muscle activations, and peak joint reaction forces are compared with experimental data and data from literature to validate the hybrid model.
Conclusion:
The proposed hybrid model combines the skeletal models rapid motion prediction with OpenSims complex muscular dynamics assessment, and it can serve as a new generic tool for motion prediction and injury analysis in ergonomics and biomechanics.
Occupational applicationsAcross a series of standing single-handed exertions performed at different lateral angles, distances, heights, and loads, lumbar axial twist exceeded an angular threshold of 9° in select exertions. Specifically, 9° of rightward axial twist was exceeded for all exertions performed laterally (90° from the body midline). Additionally, for those at the body midline, 9° of leftward axial twist was exceeded for upward exertions and exertions performed at far distances (tertiary reach envelope). Further, the data supports that for many exertions, lumbar flexion-extension and shoulder elevation would be unlikely to increase the potential for injury as angles remained within the in vivo lumbar neutral zone and were not considered overhead. Given the relationship between lateral hand exertions and lumbar axial twist, it is generally recommended that standing single-handed exertions not be performed beyond 60° from the midline. In addition to the current recommendations related to reach distance, future ergonomic reach envelope guidelines could benefit from incorporating recommendations on reach angle from the body midline.
Objective:
Adequacy of the Revised NIOSH Lifting Equation (RNLE) in maintaining lumbosacral (L5-S1) loads below their recommended action limits in stoop, full-squat, and semi-squat load-handling activities was investigated using a full-body musculoskeletal model.
Background:
The NIOSH committee did not consider the lifting technique adapted by workers when estimating the recommended weight limit (RWL). It is currently unknown whether the lifting technique adapted by workers would affect the competence of the RNLE in keeping spine loads below their recommended limits.
Method:
A full-body subject-specific musculoskeletal model (Anybody Modeling System, AMS) driven by a 10-camera Vicon motion capture system (Vicon Motion Systems Inc., Oxford, UK) was used to simulate different static stoop, semi-squat, and full-squat load-handling activities of ten normal-weight volunteers (mean of ∼70 kg corresponding to the 15th percentile of adult American males) with the task-specific NIOSH RWL held in hands.
Results:
Two-way repeated measures ANOVA revealed a significant effect of lifting technique on both the L5-S1 compression (p = 0.003) and shear (p = 0.004) loads with semi-squat technique resulting in significantly larger loads than both stoop and full-squat techniques (p < 0.05). While mean of L5-S1 loads remained smaller than their recommended limits, it is much expected that they pass these limits for heavier individuals, that is, for the 50th percentile of adult American males.
Conclusion:
Spinal loads are expected to pass their recommended limits for heavier individuals especially during semi-squat lifting as the most frequently adapted technique by workers.
Application:
Caution is required for the assessment of semi-squat lifting activities by the RNLE.
Kas ve iskelet sistemi rahatsızlıkları, meslek hastalıklarının büyük bölümünü oluşturur. Bu hastalıklar çoğunlukla tekrarlı manuel iş yapma esnasında çalışanın yanlış beden duruşundan ve çalışma ortamının ergonomik olmamasından kaynaklanmaktadır. Kas ve iskelet sistemi rahatsızlıkları çalışanların yaşam kalitesinin düşmesine, işletmelerde nitelikli iş gücünün kaybına ve işlerin aksamasına neden olmaktadır. Bu proje kapsamında otomotiv yan sanayisi için kauçuk hortum üreten bir fabrikada vulkanizasyon ve maça imalat bölümleri REBA (Rapid Entire Body Assesment) Analizi ile incelenmiştir. Potansiyel yanlış beden duruşları tespit edilerek REBA Analizi puanları hesaplanmıştır. Mevcut çalışma koşulları, çalışanın beden duruşu CATIA V5 programı ile modellenmiştir. Çalışanların hazırlanan dijital modeli üzerinden biyomekanik analizi yapılmıştır. Olumsuz koşulları ortadan kaldırmak ve çalışanlarda ilerleyen yıllarda kas ve iskelet sistemi hastalıkları meydana getirebilecek yanlış beden duruşlarını giderilmek için yeni çalışma ortamı dizayn edilmiştir. Çalışanların iyileştirilen modeller üzerinde REBA Analizi ve CATIA V5 Programı üzerinden biyomekanik analizi yapılmış, iyileştirme sonuçları değerlendirilmiştir.
Background
Historically, there has been a lack of focus on the lumbar spine during rear impacts because of the perception that the automotive seat back should protect the lumbar spine from injury. As a result, there have been no studies involving human volunteers to address the risk of low back injury in low velocity rear impact collisions.
Methods
A custom-built crash sled was used to simulate rear impact collisions. Randomized collisions were completed with and without lumbar support. Measures of passive stiffness were obtained prior to impact (Pre), immediately post impact (Post) and 24 h post impact (Post-24). Low back pain reporting was monitored for 24 h following impact exposure.
Findings
None of the participants developed clinically significant levels of low back pain after impact. Changes in the passive responses persisted after impact for the length of the low stiffness flexion and extension zone. The length of the low stiffness zone was longer in the Post and Post-24 trial for low stiffness flexion and longer in the Post-24 for low stiffness extension.
Interpretation
Findings from this investigation demonstrate that during a laboratory-simulation of an 8 km/h rear-impact collision, young healthy adults did not develop low back pain. Changes in the low stiffness zone of the passive flexion/extension curves were observed following impact and persisted for 24 h. Changes in passive stiffness may lead to changes in the loads and load distributions during movement within the passive structures such as the ligaments and intervertebral discs following impacts.
Musculoskeletal models may enhance our understanding of the dynamic loading of the joints during manual material handling. This study used state-of-the-art musculoskeletal models to determine the effects of load mass, asymmetry angle, horizontal location and deposit height on the dynamic loading of the knees, shoulders and lumbar spine during lifting. Recommended weight limits and lifting indices were also calculated using the NIOSH lifting equation. Based on 1832 lifts from 22 subjects, we found that load mass had the most substantial effect on L5-S1 compression. Increments in asymmetry led to large increases in mediolateral shear, while load mass and asymmetry had significant effects on anteroposterior shear. Increased deposit height led to higher shoulder forces, while the horizontal location mostly affected the forces in the knees and shoulders. These results generally support the findings of previous research, but notable differences in the trends and magnitudes of the estimated forces were observed.
Fatigue fracture of human lumbar vertebrae under cyclic axial compressive load has been investigated in vitro for load magnitudes between 20% and 70% of the ultimate compressive strength and cycle numbers between 1 and 5000. In addition, the dependence of the ultimate compressive strength of lumbar vertebrae on trabecular bone density and geometric dimensions was investigated. Seventy specimens of human lumbar motion segments were subjected to a fatigue test; 35 specimens were subjected to an ultimate strength test. The results state the probability of a lumbar vertebra encountering a fatigue fracture in relation to the magnitude of the cyclic load and the number of load cycles. In addition, it is shown that the ultimate compressive strength of a vertebra can be predicted with an error of less than 1 kN on the basis of bone density and endplate area.
Lumbar load, as indicated by the moment of force and the force at the lumbosacral disc, was determined for one-handed bricklaying tasks using a dynamic 3-D model, ‘The Dortmunder’. The grasp height differed (90, 50, 10 cm). By contrast, the final postures were assumed almost upright in all cases. This resulted in considerable variance in the postures during the computer-simulated movements. The task duration varied (2.0, 1.5, 1.0 s). The lower the grasp height and the shorter the time, the higher the lumbar load (moment of force at L5-S1 up to 140 Nm, compressive force up to 6 kN), and the larger the differences between dynamic and static calculations. Increasing brick mass (0, 5, 10 kg) leads to an upward shift in the moment and compression curves (20 Nm or 1 kN per 5 kg). For the assessment of lumbar load during the analysed bricklaying tasks, the lumbosacral moment of force was first classified accoding to Tichauer (1978). Bricklaying involving a 50 cm grasp height requires ‘selection of labor, careful training and rest pauses’. Lower grasp heights of bricks of 10 kg should not occur throughout ‘the entire working day’. Lumbosacral force was then compared with lumbar strength values provided in the literature. These vary within a wide range (0.8–13 kN). Strength mean ± s.d. amounts to 5.0 ± 2.2 kN for the total sample (n = 507), to 5.8 ± 2.6 kN for males (n = 174) and to 4.0 ± 1.5 kN for females (n = 132). Strength dependes primarily on age. Assuming linear regression models, strength (in kN) is 10.53-0.97/decade for males (r2 = 0.39) and 7.03-0.59/decade for females (r2 = 0.35). A strength prediction model considering 3 additional factors was developed (r2 = 0.62) in order to explain most of the remaining variance. Since average values may overestimate an individual's strength, the mean or regression model value should be reduced by the s.d. of the respective sample. This would result in a lumbar load limit of 5.5 kN for 25-year-old men and 2.6 kN for 55-year-old men. Corresponding values for women are 4.1 and 2.3 kN. If the brick-supply stack is 90 cm high, the lumbar load limits will not be exceeded for any person in these age groups. By contrast, all limits would be exceeded for a 10-kg 1-s brick transfer from a grasp height of 10 cm. In conclusion, to ensure that the predicted lumbar load during bricklaying remains below the limits, the brick-supply stack should be above 50 cm.
The purpose of this paper is to introduce some concepts of low back injury for use towards developing better injury risk reduction strategies and advancing rehabilitation of the injured spine. Selected issues in low back injury are briefly reviewed and discussed, specifically, the types of tissue loads that cause low back injury, methods to investigate tissue loading, and issues which are important considerations when formulating injury avoidance strategies such as spine posture, and prolonged loading of tissues over time. Finally, some thoughts on current practice are expressed to stimulate discussion on directions for injury reduction efforts in the future, particularly, the way in which injuries are reported, the use of simple indices of risk such as load magnitude, assessment of the injury and development of injury avoidance strategies. This paper was written for a general biomechanics audience and not specifically for those who are spine specialists.
OBJECTIVE: To determine the relative importance of modelled peak spine loads, hand loads, trunk kinematics and cumulative spine loads as predictors of reported low back pain (LBP). BACKGROUND: The authors have recently shown that both biomechemical and psychosocial variables are important in the reporting of LBP. In previous studies, peak spinal load risk factors have been identified and while there is in vitro evidence for adverse effects of excessive cumulative load on tissue, there is little epidemiological evidence. METHODS: Physical exposures to peak and cumulative lumbar spine moment, compression and shear forces, trunk kinematics, and forces on hands were analyzed on 130 randomly selected controls and 104 cases. Univariable and multivariable odds ratios of the risk of reporting were calculated from a backwards logistic regression analysis. Interrelationships among variables were examined by factor analysis. RESULTS: Cases showed significantly higher loading on all biomechanical variables. Four independent risk factors were identified: integrated lumbar moment (over a shift), 'usual' hand force, peak shear force at the level of L(4)/L(5) and peak trunk velocity. Substituting lumbar compression or moment for shear did not appreciably alter odds ratios because of high correlations among these variables. CONCLUSIONS: Cumulative biomechanical variables are important risk factors in the reporting of LBP. Spinal tissue loading estimates from a biomechanical model provide information not included in the trunk kinematics and hand force inputs to the model alone. Workers in the top 25% of loading exposure on all risk factors are at about six times the risk of reporting LBP when compared with those in the bottom 25%. RELEVANCE: Primary prevention, treatment, and return to work efforts for individuals reporting LBP all require understanding of risk factors. The results suggest that cumulative loading of the low back is important etiologically and highlight the need for better information on the response of spinal tissues to cumulative loading.
OBJECTIVE: The purpose of this study was to investigate the effect of load rate on the mechanical characteristics of spinal motion segments under compressive loading. DESIGN: An in vitro experiment using a porcine model which ensured a homogeneous population for age, weight, genetic background and physical activity. BACKGROUND: Spinal motion segments comprise of viscoelastic materials, and as a result the rate of loading will modulate mechanical characteristics and fracture patterns of the segments. METHODS: Twenty-six cervical porcine spines were excised immediately post-mortem with all soft tissue intact. Spines were then separated into two specimens each consisting of three vertebral bodies and the two intervening intervertebral discs (C2-C4 and C5-C7) and loaded to failure under five loading rates (100, 1000, 3000, 10 000 and 16 000 N s(-1)). After the specimens failed, they were dissected to determine the mode of failure. RESULTS: Dynamic loading increases the ultimate load compared with quasi-static loading (100 N s(-1)), whereas the magnitude of dynamic loading (1000-16 000 N s(-1)) appears not to have a significant affect. Stiffness behaved in a similar manner. The displacement to failure of specimens decreased as load rate increased, although there was a diminishing effect at high load rates. Furthermore, failure at low load rates occurred exclusively in the endplate, whereas failure of the vertebral body appeared with greater frequency at higher load rates. CONCLUSIONS: The mechanical characteristics and resulting injuries of porcine spinal motion segments were affected as the loading rates changed from quasi-static to dynamic. The modulating factors of the mechanical characteristics of the spine need to be understood if valid models are to be designed which will increase the understanding of spinal function, and are important for choosing better injury prevention and rehabilitation programmes.
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