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Effect of a Weight Transfer Device on Muscle Activities, Joint Flexions, and Low Back Loads in the Stooped Posture

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Brent Leonard Ulrey
Abstract:
Repetitive work in the stooped posture is a known risk factor for developing low back disorders (LBDs). Use of the stooped posture is widespread throughout the world in the agriculture, construction, and mining industries. An on-body weight transfer device was tested as a possible intervention for reducing the risk of developing LBDs. Eighteen healthy subjects (11 male and 7 female), with no history of LBDs, performed stooped posture tasks in a laboratory study designed to simulate harvesting of low-growing crops. Surface electromyograms of the erector spinae, rectus abdominis, biceps femoris, and tibialis anterior muscles were recorded. Total torso, lumbar, hip, knee, and ankle joint flexions were measured with a combination of inclinometers and electrogoniometers. Results show that when wearing the device in the static stooped posture, biceps femoris activity was reduced by 17% (p <0.0001), lumbar flexion was reduced by 12% (p <0.01), ankle plantar-flexion increased by 5% (p <0.05), hip and knee flexion were not significantly altered, and the lumbar erector spinae of those subjects who did not experience flexion-relaxation of the erector spinae was reduced by 26% (p <0.01). During torso flexion and extension between stooped and upright when wearing the device, peak values of back muscle activity and lumbar flexion were significantly reduced and hip flexion significantly increased, while peak tibialis anterior activity significantly increased during extension only. Abdominal muscles had a low level of activity throughout the procedure. Whole-body musculoskeletal models, which included individual passive torso stiffness and anthropometry, predicted that when wearing the device in the static stooped posture, compression and shear forces on the L5-S1 intervertebral disc were reduced by 13% and 12% respectively ( p <0.0001). Internal loads in the hip, knee, and ankle joints were also significantly reduced. Many low back pain sufferers have reduced, or no, relaxation of back muscles in the stooped posture. Therefore, the device may be beneficial for those with existing LBDs. Furthermore, by limiting lumbar flexion, the device could reduce the risk of developing LBDs for those who work while adopting the stooped posture. Follow up field studies are needed to confirm the long-term potential benefits of such an intervention approach.

Contents List of Figures.................................vi List of Tables.................................xiii Abstract....................................xv 1 Introduction 1 Hypothesis................................5 Specific Aims..............................5 2 Background 6 2.1 Musculoskeletal Disorders.......................6 2.2 Biomechanics of Spinal Postures....................9 2.3 Flexion-Relaxation Response......................13 2.4 Passive Resistance and Viscoelastic Response............16 3 Laboratory Experiment 22 3.1 Introduction...............................22 3.2 Methods.................................23 3.2.1 Subjects.............................23 3.2.2 Instrumentation........................24 3.2.3 Weight Transfer Device.....................29 3.2.4 Experimental Design......................30 3.2.5 Protocol.............................32 -ii-

3.2.6 Analysis.............................35 3.3 Results.................................36 3.3.1 Device Effect..........................36 3.3.2 Weight Effect..........................41 3.4 Discussion...............................43 3.5 Additional Results...........................48 3.5.1 Flexion-Relaxation and Gender Effects............48 3.5.2 Response During Flexion and Extension Phases.......57 3.5.3 Further Discussion.......................77 3.6 Conclusion................................78 4 Biomechanical Model 79 4.1 Introduction...............................79 4.2 Passive Torso Extensor Moment Calculation.............81 4.2.1 Methods.............................81 4.2.2 Results and Discussion.....................85 4.2.3 Forward Stepwise Regression Analysis............93 4.3 Model..................................95 4.3.1 Device and Weight Effects...................116 4.3.2 Flexion-Relaxation and Gender Effects............123 4.4 Discussion...............................126 4.5 Conclusion................................136 -iii-

5 Conclusion 137 5.1 Key Findings..............................138 5.2 Limitations and Future Work.....................140 References 146 Appendix A Tilt-Sensing Using a Dual-Axis MEMS Accelerometer 156 Appendix B Experimental Design 159 B.1 Split-Plot................................159 B.2 ANOVA.................................160 B.2.1 Assumptions...........................163 B.2.2 Properties of Observations...................164 B.2.3 Sum of Squares Equations...................165 B.2.4 F Tests..............................165 B.3 Multiple Comparison Test.......................166 B.4 Sample Size...............................167 Appendix C Subject Results 168 Subject 1................................169 Subject 2................................173 Subject 3................................177 Subject 4................................181 Subject 5................................185 -iv-

Subject 6................................189 Subject 7................................193 Subject 8................................197 Subject 9................................201 Subject 10................................205 Subject 11................................209 Subject 12................................213 Subject 13................................217 Subject 14................................221 Subject 15................................225 Subject 16................................229 Subject 17................................233 Subject 18................................237 -v-

List of Figures 3.1 Photograph showing the location of the electromyogram electrodes and the inclinometers on a subject’s back...............26 3.2 Photograph showing the electrogoniometers on a subject’s right leg 28 3.3 Photograph of a subject standing and stooping while wearing the BNDR device,and performing the task in the laboratory experiment 34 3.4 Normalized muscle activities averaged for all subjects in the static stooped posture,with and without the weight transfer device....38 3.5 Torso flexions averaged for all subjects in the static stooped posture, with and without the weight transfer device.............39 3.6 Hip,knee,and ankle flexions averaged for all subjects in the static stooped posture,with and without the weight transfer device....40 3.7 Histogram of the normalized lumbar erector spinae activity of all subjects in the static stooped posture,at all weight-level combinations 44 3.8 Residuals and the Q-Q plot from the ANOVA of the normalized lumbar erector spinae activity of all subjects in the static stooped posture,at all weight-level combinations...............45 3.9 Normalized lumbar erector spinae activity for three subjects who had extreme respones in the static stooped posture.........47 3.10 Normalized lumbar and thoracic erector spinae activities in the static stooped posture,with and without the device,categorized by the presence of flexion-relaxation..................52 3.11 Significant differences in lower lumbar flexion and hip flexion be- tween the male and female subjects in the static stooped posture..53 3.12 Significant differences in normalized lumbar erector spinae activity, thoracic erector spinae activity,and the T3 angle with respect to vertical when wearing the device in the static stooped posture...54 3.13 Significant differences in normalized biceps femoris activity and lower lumbar flexion between males and females in the static stooped posture,categorized by the presence of flexion-relaxation......55 -vi-

3.14 Normalized lumbar and thoracic erector spinae activities in the static stooped posture,categorized by the presence of flexion- relaxation,at the three weight levels.................56 3.15 Peak normalized muscle activities averaged for all subjects in the flexion/short lift phase,with and without the weight transfer device 60 3.16 Peak torso flexions averaged for all subjects in the flexion/short lift phase,with and without the weight transfer device.........61 3.17 Peak hip,knee,and ankle flexions averaged for all subjects in the flexion/short lift phase,with and without the weight transfer device 62 3.18 Peak normalized muscle activities averaged for all subjects in the extension phase,with and without the weight transfer device....63 3.19 Peak flexion angles for sections of the torso,and the T3 angle with respect to vertical,averaged for all subjects in the extension phase, with and without the weight transfer device.............64 3.20 Peak hip,knee,and ankle flexions averaged for all subjects in the extension phase,with and without the weight transfer device....65 3.21 Peak normalized lumbar and thoracic erector spinae activities in the flexion/short lift phase,with and without the device,categorized by the presence of flexion-relaxation..................67 3.22 Peak normalized lumbar and thoracic erector spinae activities,and peak knee flexion in the flexion/short lift phase,categorized by the presence of flexion-relaxation at the three weight levels.......68 3.23 Significant flexion-relaxation effects in peak normalized lumbar and thoracic erector spinae activities,and the T3 angle with respect to vertical.................................69 3.24 Peak normalized biceps femoris activity for males and females dur- ing the flexion/short lift phase,with and without the device....70 3.25 Peak lower lumbar flexion of the male and female subjects during the flexion/short lift phase,separated into flexion-relaxation cate- gories...................................71 3.26 Peak normalized lumbar and thoracic erector spinae activities in the extension phase,with and without the device,categorized by the presence of flexion-relaxation...................73 -vii-

3.27 Significant effects of the presence of flexion-relaxation on peak T3 angle with respect to vertical,and peak thoracic flexion,during the extension phase.............................74 3.28 Peak lower lumbar flexion of the male and female subjects during the extension phase,categorized by the presence of flexion-relaxation 75 3.29 Peak normalized lumbar and thoracic erector spinae activities in the extension phase,categorized by the presence of flexion-relaxation, at the three weight levels........................76 4.1 Photograph showing the hip constraint apparatus used in the ex- periment for calculation of the passive torso extensor moment...83 4.2 Diagram of the two-dimensional model used for the passive torso extensor moment calculation......................86 4.3 Values of passive torso extensor moment versus percent torso flexion for all subjects.............................88 4.4 Values of passive torso extensor moment versus percent torso flexion for all subjects,separated by gender..................88 4.5 Values of passive torso extensor moment versus percent torso flexion for all subjects,separated into flexion-relaxation categories.....89 4.6 Values of passive torso extensor moment versus percent torso flexion for females and males,separated into flexion-relaxation categories.90 4.7 Values of passive torso extensor moment versus percent lumbar flex- ion for all subjects,with different markers for gender and compared with values from the literature.....................92 4.8 Screen shots of the graphical representation of the model in the AnyBody Modeling System......................99 4.9 Muscle activities predicted by the model in the static stooped pos- ture,averaged for all subjects at the three weight levels,with and without the device...........................103 4.10 Reaction forces in the L5-S1 joint predicted by the model in the static stooped posture,averaged for all subjects at the three weight levels,with and without the device..................106 -viii-

4.11 The active torso extensor moment about the L5-S1 joint predicted by the model in the static stooped posture,averaged for all subjects at the three weight levels,with and without the device.......108 4.12 The passive torso extensor moment about the L5-S1 joint predicted by the two-dimensional model,averaged for all subjects at the three weight levels,with and without the device..............108 4.13 Hip joint reactions predicted by the model in the static stooped posture,averaged for all subjects at the three weight levels,with and without the device.........................109 4.14 Knee joint reactions predicted by the model in the static stooped posture,averaged for all subjects at the three weight levels,with and without the device.........................111 4.15 Ankle joint reactions predicted by the model in the static stooped posture,averaged for all subjects at the three weight levels,with and without the device.........................114 4.16 Muscle activities predicted by the model,averaged for all subjects, with and without the device......................118 4.17 Reactions in the L5-S1 joint,and the active and passive torso ex- tensor moments,predicted by the models,averaged for all subjects, with and without the device......................119 4.18 Hip joint reactions predicted by the model,with and without the device..................................120 4.19 Knee joint reactions predicted by the model,with and without the device..................................121 4.20 Ankle joint reactions predicted by the model,with and without the device..................................122 4.21 Significant differences in ankle antero-posterior force and L5-S1 ac- tive extensor moment between the male and female subjects pre- dicted by the model...........................124 4.22 Significant differences in hip antero-posterior and medio-lateral forces, and knee and ankle axial moments between the flexion-relaxation Yes and No groups predicted by the model..............125 -ix-

4.23 Normalized activity of the muscles in the laboratory experiment, and the corresponding muscle activities predicted by the model, averaged from all subjects in the static stooped posture,with and without the device...........................127 4.24 Normalized activity of the muscles in the laboratory experiment, and the corresponding muscle activities predicted by the model, averaged fromall subjects in the static stooped posture,categorized by the presence of flexion-relaxation in the laboratory experiment.128 4.25 Normalized activity of the muscles in the laboratory experiment, and the corresponding muscle activities predicted by the model,for subject 17,who experienced flexion-relaxation,in the static stooped posture..................................129 4.26 Active torso extensor moments predicted by the whole-body model, and the passive torso extensor moments calculated from the two- dimensional model,categorized by the presence of flexion-relaxation in the laboratory experiment......................130 4.27 Normalized activity of the muscles in the laboratory experiment, and the corresponding muscle activities predicted by the model,for subject 4,who did not experience flexion-relaxation,in the static stooped posture.............................131 4.28 Compression and shear forces in the L5-S1 joint predicted by the model in the static stooped posture,averaged for all subjects at the three weight levels,with and without the device.Recommended safe limits for compression and shear were exceeded by some subjects 134 5.1 Muscle activity of the lumbar erector spinae in the laboratory ex- periment,and the corresponding muscle activity predicted by the model,averaged from all subjects in the static stooped posture, and from subjects 4 and 17,categorized by the presence of flexion- relaxation in the laboratory experiment................144 A.1 Photograph of an Analog Devices ADXL203 accelerometer.....156 A.2 Voltage outputs of each channel of an Analog Devices ADXL203 dual-axis accelerometer in 360 ◦ of rotation..............157 C.1 Results for Subject 1 from the laboratory experiment,and the model predictions............................169 C.2 Results for Subject 2 from the laboratory experiment,and the model predictions............................173 -x-

C.3 Results for Subject 3 from the laboratory experiment,and the model predictions............................177 C.4 Results for Subject 4 from the laboratory experiment,and the model predictions............................181 C.5 Results for Subject 5 from the laboratory experiment,and the model predictions............................185 C.6 Results for Subject 6 from the laboratory experiment,and the model predictions............................189 C.7 Results for Subject 7 from the laboratory experiment,and the model predictions............................193 C.8 Results for Subject 8 from the laboratory experiment,and the model predictions............................197 C.9 Results for Subject 9 from the laboratory experiment,and the model predictions............................201 C.10 Results for Subject 10 from the laboratory experiment,and the model predictions............................205 C.11 Results for Subject 11 from the laboratory experiment,and the model predictions............................209 C.12 Results for Subject 12 from the laboratory experiment,and the model predictions............................213 C.13 Results for Subject 13 from the laboratory experiment,and the model predictions............................217 C.14 Results for Subject 14 from the laboratory experiment,and the model predictions............................221 C.15 Results for Subject 15 from the laboratory experiment,and the model predictions............................225 C.16 Results for Subject 16 from the laboratory experiment,and the model predictions............................229 C.17 Results for Subject 17 from the laboratory experiment,and the model predictions............................233 -xi-

C.18 Results for Subject 18 from the laboratory experiment,and the model predictions............................237 -xii-

List of Tables 3.1 Age,height,and weight statistics of the subjects...........24 3.2 List of independent variables in the laboratory experiment.....31 3.3 List of dependent variables in the laboratory experiment......31 3.4 Device main effect p-values and percent changes from the split-plot ANOVA for all subjects in the static stooped posture........37 3.5 Weight main effect p-values and percent changes from the split-plot ANOVA and the Tukey tests for all subjects in the static stooped posture..................................42 3.6 Age,height,and weight statistics of the subjects,categorized by gender and flexion-relaxation factors.................50 3.7 Age,height,and weight statistics of the female and male subjects, categorized by the flexion-relaxation factor..............51 3.8 Device main effect p-values and percent changes from the split-plot ANOVA for all subjects during the flexion/short lift phase.....58 3.9 Device main effect p-values and percent changes from the split-plot ANOVA for all subjects during the extension phase.........59 4.1 Results of forward stepwise regression on the passive torso extensor moment.................................94 4.2 Results of linear regression on the passive torso extensor moment data for each subject,and each subject’s gender,age,height,weight, and flexion-relaxation response.....................96 4.3 The maximumflexion range of motion of the lumbar motion segments 97 4.4 List of dependent variables from the model..............102 4.5 Device main effect p-values and percent changes from the split-plot ANOVA of the outputs from the models of all subjects in the static stooped posture.............................117 B.1 Example of randomization of device and weight levels for three sub- jects in a split-plot design.......................160 -xiii-

B.2 Analysis of variance table and expected values of mean squares for a split-plot model............................162 B.3 Standard errors for a split-plot design.................163 -xiv-

Abstract Effect of a Weight Transfer Device on Muscle Activities, Joint Flexions,and Low Back Loads in the Stooped Posture Repetitive work in the stooped posture is a known risk factor for developing low back disorders (LBDs).Use of the stooped posture (bent forward and down at the waist and/or mid-back while maintaining straight legs) is widespread throughout the world in the agriculture,construction,and mining industries.An on-body weight transfer device was tested as a possible intervention for reducing the risk of developing LBDs. Eighteen healthy subjects (11 male and 7 female),with no history of LBDs, performed stooped posture tasks in a laboratory study designed to simulate har- vesting of low-growing crops.Surface electromyograms of the erector spinae,rectus abdominis,biceps femoris,and tibialis anterior muscles were recorded.Total torso, lumbar,hip,knee,and ankle joint flexions were measured with a combination of inclinometers and electrogoniometers. Results show that when wearing the device in the static stooped posture,biceps femoris activity was reduced by 17% (p < 0.0001),lumbar flexion was reduced by 12% (p < 0.01),ankle plantar-flexion increased by 5% (p < 0.05),hip and knee flexion were not significantly altered,and the lumbar erector spinae of those subjects who did not experience flexion-relaxation of the erector spinae was reduced by 26% (p < 0.01).During torso flexion and extension between stooped and upright when wearing the device,peak values of back muscle activity and lumbar -xv-

flexion were significantly reduced and hip flexion significantly increased,while peak tibialis anterior activity significantly increased during extension only.Abdominal muscles had a low level of activity throughout the procedure. Whole-body musculoskeletal models,which included individual passive torso stiffness and anthropometry,predicted that when wearing the device in the static stooped posture,compression and shear forces on the L5-S1 intervertebral disc were reduced by 13% and 12% respectively (p < 0.0001).Internal loads in the hip, knee,and ankle joints were also significantly reduced. Many low back pain sufferers have reduced,or no,relaxation of back muscles in the stooped posture.Therefore,the device may be beneficial for those with existing LBDs.Furthermore,by limiting lumbar flexion,the device could reduce the risk of developing LBDs for those who work while adopting the stooped posture.Follow up field studies are needed to confirm the long-term potential benefits of such an intervention approach. -xvi-

1 Chapter 1 Introduction During a conference entitled “Stooped and Squatting Postures in the Workplace” held in Oakland,CA in 2004,the need for specific research on the relationship between development of low back disorders (LBDs) and working in the stooped position was emphasized (Fathallah et al.2004).Researchers have concluded that prolonged or repetitive work in stooped postures leads to development of LBDs (McCurdy et al.2003;Meyers et al.1997).While there have been some studies dedicated to work performed in awkward postures,the majority of ergonomics research has been focused on work performed in the sitting or erect standing posi- tions,and there have been relatively few studies specifically focused on the stooped posture (Fathallah et al.2004,2008;Gallagher 2005).Furthermore,there has been a lack of a clear definition of the stooped posture,which confuses the results that exist (Fathallah et al.2004).The following definition for a stooped posture was suggested at the “Stooped and Squatting Postures in the Workplace” conference,

2 “bent forward and down at the waist and/or mid-back while maintaining straight legs” (Fathallah et al.2004).One third to one-half of farm workers in Califor- nia (400,000 to 600,000 workers) perform work in stooped postures (Miller and Fathallah 2004).Work in awkward postures is prevalent in most developed and developing nations (Fathallah et al.2004,2008).Therefore,there are a large number of workers routinely performing tasks with high risk of developing LBDs. In congruence with two national research agendas (NIOSH 1997;NRC and IOM 2001),and the discussion at the aforementioned conference,this study furthers the knowledge of the development and prevention of musculoskeletal disorders (MSDs) associated with work in the stooped posture. People naturally choose the stooped posture when lifting light loads in uncon- fined areas (Burgess-Limerick and Abernethy 1997) or heavier loads in confined areas (Gallagher 2005;Gallagher et al.2002).Additionally,the full squat position, which has often been proposed as the best choice for lifting heavier objects (i.e., lifting with the legs,not the back),has no proven biomechanical advantage for preventing low back pain,and can increase the risk of developing musculoskeletal disorders (MSDs) in the knees (Burgess-Limerick 2003;Fathallah et al.2004;van Dieen et al.1999).Compared to kneeling positions,larger leg muscles are avail- able for recruitment in the stooped position,which increases the lifting capacity (Gallagher et al.1988).Stooping is also chosen over kneeling or squatting because the stooped posture allows for greater motility and reach (Fathallah et al.2004). Many jobs in agriculture,construction,and mining involve sustained or repetitive

3 tasks close to the ground,both with and without lifting tasks.Even without the additional loads due to lifting,sustained or repetitive low load bearing activities in the stooped position can cause LBDs (McGill 1997).Results of biomechani- cal studies on the spine show a reduction in the passive stiffness in intervertebral discs,ligaments,tendons,fascia,and connective tissues after sustained or repeti- tive flexed postures (Adams and Dolan 1996).This reduction in passive stiffness alters spinal muscle control and reduces spinal load bearing capability,leaving the spine vulnerable to injuries (Dickey et al.2003;Jackson et al.2001;Olson et al. 2004;Solomonow et al.1999,2003a,b). Several commercial products have recently become available which claim to re- duce lower back loading in the stooped posture by moving some of the spinal load to the legs.These products are referred to as weight transfer devices (WTDs),or load transfer devices.Three available devices are the HappyBack (ErgoAg,Aptos, CA),the Bending Non-Demand Return (BNDR) (Limbic Systems Inc,Ventura, CA),and the Bendezy (Bendezy LiteTop,Mount Barker,Western Australia).Bar- rett and Fathallah performed a preliminary evaluation of these devices in 2001 and found that WTDs reduce muscle activity in the back;however,leg muscle fa- tigue may increase,and the wearers complained of comfort problems (Barrett and Fathallah 2001).Other devices have since been studied by different investigators. Mirka et al.(2003) tested a chest harness device with a bucket counter-weight that generated a torso extensor moment,and found that during simulated sweet potato harvesting the moment about the L5-S1 joint was reduced by 60Nm when the

4 bucket was full;however,back muscle activity increased.Abdoli-E et al.(2006) analyzed a personal lift assist device (PLAD) and found that it reduced loads in the L4-L5 disc,and reduced back muscle activity during symmetrical lifting tasks. The aforementioned devices are all passive,and store elastic energy when a person bends over.A recent study has investigated an active device that adds energy to assist with extension of the torso.The device was designed and tested by Wehner (2009),and shows promise in reducing back loads and muscle activities. Possible drawbacks of a powered device for use by agricultural workers is the need to carry a battery pack,and a relatively high cost per unit compared to passive devices. The current study determined how the muscle activity,the lumbar curvature, and the loads in the spine and legs were changed by a passive weight transfer device — the BNDR.Data from both a laboratory study and biomechanical modeling were used for analyses.This study is the first investigation to determine if the use of a WTD changes the lumbar curvature in the stooped posture.The hypothesis and specific aims follow.

5 Hypothesis The BNDRweight transfer device significantly reduces the back mus- cle activity and the loads imposed on the passive tissues of the spine in the static stooped posture,without significantly increasing the loads on the hip, knee,and ankle joints. Specific Aim 1 Through a laboratory study of the stooped posture,determine how the BNDR weight transfer device affects the muscle activity in the trunk and lower extremity. Specific Aim 2 Through a laboratory study,determine if the use of the BNDR weight transfer device alters the lumbar curvature. Specific Aim 3 Through biomechanical modeling of the stooped posture,de- termine loads on the passive tissues of the lower back,and on the joints of the lumbar spine,hip,knee,and ankle,with and without the BNDR weight transfer device.

6 Chapter 2 Background 2.1 Musculoskeletal Disorders Musculoskeletal disorders (MSDs) are conditions that affect the nerves,tendons, muscles,and supporting structures of the body (NRC and IOM 2001).MSDs in- clude low back pain,shoulder disorders,wrist disorders,tendonitis,and peripheral neuropathies.In a 1999 survey by the Bureau of Labor Statistics,nearly 1 million people in the United States reported taking time away from work to treat or re- cover from musculoskeletal pain or loss of function (NRC and IOM2001).In 2001, the Panel on Musculoskeletal Disorders in the workplace established by the United States National Research Council (NRC) in conjunction with the United States Institute of Medicine (IOM) estimated that the annual cost of work-related MSDs was $45 to $54 billion (NRC and IOM2001).This estimate includes loss of wages, loss of productivity,loss of tax revenues,and workers’ compensation claims.How-

7 ever,this is a conservative estimate because it only includes the monetary impact of reported cases,and many work-related cases are not reported (NRC and IOM 2001).If non-work related MSDs are included,the estimated annual cost increases to approximately $215 billion (NRC and IOM 2001).Others have estimated that in 1992 work related lower back disorders (LBDs) alone had aggregate costs of $49 billion annually (Leigh et al.1997).On self-reporting surveys,between 19 and 61% of people reported having low back pain in the past year,and 9-17% reported low back pain lasting 2 weeks or more during their lifetime (NRC and IOM 2001). In an effort to organize the research efforts on MSDs,the federal government published two research agendas.The first agenda was published by the National In- stitute for Occupational Safety and Health (NIOSH) in 1996.The NIOSHNational Occupational Research Agenda (NORA) for MSDs is based on input from medical practitioners and industrial health and safety experts (Marras 2004).Several years later in 2001,the NRC/IOM National Panel on Musculoskeletal Disorders in the Workplace,published a literature review and another research agenda on MSDs (NRC and IOM 2001).The NRC/IOM agenda is based on input from researchers in the fields of medicine,science,and ergonomics.Both agendas emphasize the need for developing standards for MSD classification,for developing methods for quantifying physical and psychological factors,and for additional human studies for quantifying relationships between exposures and outcomes (Waters 2004).Recent studies of California nursery and migrant farm workers show that there are still many risks which should be addressed with employer and employee education,task

8 alterations,and engineering controls (McCurdy et al.2003;Meyers et al.1997). MSDs are not isolated in the United States but are prevalent in most societies, including Canada,Western Europe,Australia,India,Japan,and most other devel- oped or developing nations (Burgess-Limerick 2003;Fathallah et al.2004;Maeda et al.1980;Marshall and Burnett 2004;NRC and IOM 2001;van Dieen et al. 1999). General knowledge of MSDs has increased over the years in the areas of epi- demiology,biomechanics and physiology of tissue loading,biomechanics of tissue tolerance,pathways of pain perception in the musculoskeletal system,individual factors,genetic factors,psychosocial factors,and effectiveness of primary and sec- ondary interventions (Marras 2004).The NRC and IOM report,“there is a clear relationship between back disorders and physical load;that is,manual material handling,load moment,frequent bending and twisting,heavy physical work,and whole-body vibration” (NRC and IOM 2001,page 9).Even with this growth of knowledge,a complete understanding of the etiology of MSDs remains (Marras 2004;NRC and IOM2001;United States Senate Committee on Health,Education, Labor,and Pensions 2002). Surveys of outpatient health care facilities in the United States in 1989 esti- mated that there were “19.9 million visits for low back pain,8.1 million for neck pain,and 5.2 and 2.7 million for hand and wrist pain respectively,” and further- more,“projections suggest that these figures are rising” (NRC and IOM 2001, page 44).Since low back pain (a type of LBD) is a large portion of the total

9 number of MSDs,understanding the causal factors and developing preventative mechanisms for LBDs is a high priority for a large portion of the population: workers,employers,the medical field,the government,and insurance companies. Working in the stooped posture has been identified as a high risk factor for devel- oping LBDs (Fathallah et al.2004).An understanding of the effects of prolonged or repetitive work in the stooped posture and developing preventative methods or mechanisms may reduce the number of work-related LBDs.Weight transfer devices may reduce the loads on the spine in the stooped posture,and ultimately reduce the number of LBDs related to working in the stooped posture.The following sections summarize previous research of biomechanics,physiologic response,and modeling of the spine as it relates to the stooped posture. 2.2 Biomechanics of Spinal Postures The human body can assume many different positions to facilitate the completion of various tasks.The majority of lifting and non-lifting tasks that are close to the ground are accomplished by 6 postures:(1) kneeling with one knee on the ground, (2) kneeling with both knees on the ground,(3) sitting,(4) squatting,(5) semi- squatting,or (6) stooping.Prone and semi-prone postures are also used in some agricultural applications (Fathallah et al.2004).The mechanics and constraints of the task are a main factor in the choice of posture.Workers in confined spaces such as mines,airline cargo holds,and sewage pipes are not able to stand erect; therefore,these workers must choose a stooped or kneeling posture (Friedrich et al.

10 2000a,b;Gallagher et al.1988).Sheep shearers in Australia use the stooped posi- tion to sheer 150-200 sheep per day (Marshall and Burnett 2004).The majority of strawberry,blueberry,and eggplant harvesters use the stooped posture (Estill et al.1996;Maeda et al.1980;McCurdy et al.2003;Meyers et al.1997).Tree nursery workers have tasks that require substantial and prolonged trunk flexion, and sustained kneeling (Faucett et al.2007;Meyers et al.1997).Therefore,the physical constraints of the task have a major affect on the choice of posture,and the stooped posture is commonly used for tasks close to the ground or in confined areas. A person’s ability to lift heavy loads is reduced in postures other than stooping and standing.Compared to the standing and stooping posture,while kneeling the psychophysical lifting capacity is reduced 7-21%,trunk extensor strength is re- duced 16%,and bodily motility is greatly reduced (Gallagher 1997,2005;Gallagher et al.1988).The reduction in lifting capacity is most likely due to the inability to rotate the pelvis,and not due to changes in muscle activity (Gallagher 1997, 2005).Squatting lifting capacity is reduced when compared to stoop (Burgess- Limerick 2003;Kumar 1995).This is due to the fact that during full squat the quadriceps are at a length greater than their optimal length (Burgess-Limerick 2003).During stooped lifting,the hip extensor muscles are responsible for trunk extension through rotation of the hip,with the bulk of the power coming from the gluteals and the hamstrings (Gallagher and Hamrick 1991).The reduction of motility increases the amount and frequency of bending and twisting,which were

11 mentioned previously as main risk factors of developing LBDs.The loss of motil- ity in the kneeling position also decreases the stability and balance.Therefore, working in the kneeling position has several disadvantages and cannot be emphat- ically recommended as an alternative to stooping.An additional advantage of the stoop posture is that after 20 minutes of stoop lifting,lifting capacity is not much different than after 20 minutes of standing lifts (Gallagher and Hamrick 1992). Spinal loads vary with posture as well.Compared to the standing position,the stoop,squat,and kneeling postures all result in higher loads on the spine.A model by Arjmand and Shirazi-Adl (2006) predicts that,when compared to the standing position,spinal flexions of 40 ◦ and 65 ◦ increase the L4-L5 spinal compression by 251% and 337% respectively.Spinal compression is a reliable index of physical stresses placed on the spine,and is a risk factor for injury when loads are excessive (Kumar 1996).The lower portions of the spine bear the brunt of mechanical loads;therefore,the loads in the L4-L5 and L5-S1 joints are often calculated as a measure of risk (Kumar 1996).In upright postures with healthy and well hydrated intervertebral discs,the discs resist a greater portion of compression and a smaller portion of the shear force than the neural arch and apophyseal joints (Dolan and Adams 2001).In erect postures,the apophyseal joints resist about 16% of the compression and most of the shear,and in flexed postures,the apophyseal joints resist most of the shear force and none of the compressive force (Adams and Hutton 1985). The intervertebral discs are composed of a fluid-like center,the nucleus pulpo-

12 sus,which is surrounded by the annulus fibrosis.The annulus consists of concentric lamellae of fibrocartilage and fibrous tissue wrapped in alternating directions.The nucleus pulposus migrates posteriorly during spinal flexion and anteriorly during spinal extension (Fredericson et al.2001).As flexion increases,the amount of pos- terior bulging of the disc increases (Fredericson et al.2001),which can lead to disc prolapse or herniation.In full flexion,measurements from X-rays show that the posterior annulus is stretched by about 50%and the anterior annulus is compressed by about 30% compared to measurements from erect postures (Adams and Hut- ton 1982,1986).Peak compressive loads within the intervertebral discs move from the posterior annulus in erect postures to the anterior annulus in flexed postures (Dolan and Adams 2001).The anterior portion of the annulus is the thickest part, and is capable of withstanding higher compressive forces than the posterior annu- lus (Adams and Hutton 1985).Moderate flexion equalizes the compressive load across the spine;however,if excessive compression occurs during full flexion,“the anterior vertebral body may be crushed or there may be sudden posterior prolapse of the intervertebral disc” (Adams and Hutton 1982,1985).In addition,the line of action of the largest trunk extensor muscles (longissimus thoracis and iliocostalis lumborum) is changed during full flexion,reducing their ability to support anterior shear forces (McGill et al.2000). The intervertebral discs are avascular and receive nutrients by diffusion and fluid transfer from the surrounding vertebral bodies,and from the fluid contacting the annulus fibrosis (Adams and Hutton 1983,1985,1986).Therefore,sedentary

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Abstract: Repetitive work in the stooped posture is a known risk factor for developing low back disorders (LBDs). Use of the stooped posture is widespread throughout the world in the agriculture, construction, and mining industries. An on-body weight transfer device was tested as a possible intervention for reducing the risk of developing LBDs. Eighteen healthy subjects (11 male and 7 female), with no history of LBDs, performed stooped posture tasks in a laboratory study designed to simulate harvesting of low-growing crops. Surface electromyograms of the erector spinae, rectus abdominis, biceps femoris, and tibialis anterior muscles were recorded. Total torso, lumbar, hip, knee, and ankle joint flexions were measured with a combination of inclinometers and electrogoniometers. Results show that when wearing the device in the static stooped posture, biceps femoris activity was reduced by 17% (p <0.0001), lumbar flexion was reduced by 12% (p <0.01), ankle plantar-flexion increased by 5% (p <0.05), hip and knee flexion were not significantly altered, and the lumbar erector spinae of those subjects who did not experience flexion-relaxation of the erector spinae was reduced by 26% (p <0.01). During torso flexion and extension between stooped and upright when wearing the device, peak values of back muscle activity and lumbar flexion were significantly reduced and hip flexion significantly increased, while peak tibialis anterior activity significantly increased during extension only. Abdominal muscles had a low level of activity throughout the procedure. Whole-body musculoskeletal models, which included individual passive torso stiffness and anthropometry, predicted that when wearing the device in the static stooped posture, compression and shear forces on the L5-S1 intervertebral disc were reduced by 13% and 12% respectively ( p <0.0001). Internal loads in the hip, knee, and ankle joints were also significantly reduced. Many low back pain sufferers have reduced, or no, relaxation of back muscles in the stooped posture. Therefore, the device may be beneficial for those with existing LBDs. Furthermore, by limiting lumbar flexion, the device could reduce the risk of developing LBDs for those who work while adopting the stooped posture. Follow up field studies are needed to confirm the long-term potential benefits of such an intervention approach.