Occupational spine biomechanics: A journey to the spinal frontier

doi:10.1016/j.jelekin.2008.07.004

Journal of Electromyography and Kinesiology

Volume 18, Issue 6, December 2008, Pages 891-899

https://doi.org/10.1016/j.jelekin.2008.07.004 Get rights and content

Abstract

This paper provides a brief introduction to the variety of research areas focusing on spine biomechanics as it pertains to understanding and preventing low back injuries in the workplace. While certainly not a comprehensive review of the literature, some of the earliest, pioneering studies are presented from the following areas: (1) spine tissue testing, (2) estimating spine tissue loading, (3) manual materials handling studies, (4) prolonged or repetitive spine loading, (5) ergonomic assessment tools, (6) sudden/unexpected loading and (7) spine stability. Where possible, some of our own research contributions are integrated into the relevant sections. This paper concludes with a suggestion of some future research directions to continue and enhance the important impact of occupational spine biomechanics.

Introduction

It is well known that spine injuries, most often in the lower back, are prevalent in the workplace. Many researchers in the discipline of occupational biomechanics have dedicated themselves to furthering our knowledge of the mechanical characteristics of the spine and its neural control, so that we might further understand its normal function during manual materials handling (MMH) tasks and potential mechanisms of injury. This paper will provide a very brief overview of the various fields of emphasis in occupational spine biomechanics. While an attempt will be made to provide some historical context to the development of this field, this paper is certainly not a comprehensive history of the vast and impressive variety of research published on this topic. Rather, an effort will be made to introduce the main areas of emphasis, while citing some of the earliest pioneering work. Where relevant, some of our own research will be integrated into this paper. However, this certainly does not presume that they have had the same impact as the landmark studies also cited.

Section snippets

Acute loading

For decades, lumbar intervertebral disc compression force was the main variable of interest when assessing low back injury (LBI) risk in the workplace. NIOSH (1981) integrated biomechanical and epidemiological data and recommended a compression force Action Limit of 3400 N. Jager and Luttmann (1991) and Genaidy et al. (1993) have presented predictive models for further delineating compression force tolerance based on variables like gender, age, percentile, lumbar level etc. While much of the

Direct measures

Nachemson and Morris (1964) inserted a pressure–sensitive needle into L3/4 intervertebral discs to determine the effects of various static loading trials. While later efforts were made to convert these pressures to compression force (Nachemson, 1981), they did not actually represent direct measures. In fact, it is not currently possible to make such direct in vivo measurements of spine tissue loading.

Simple biomechanical models

Given the infeasibility of direct spine load measures, biomechanical models were developed to

Early studies

Some of the earliest trunk loading studies were conducted, first with no hand loads (Floyd and Silver, 1951, Floyd and Silver, 1955) and then while subjects supported external loads (Whitney, 1958, Grieve, 1958, Davis, 1959). Davis (1959) presented one of the first concepts of an internal single equivalent muscle model and Floyd and Silver (1955) published EMG records indicating the flexion–relaxation phenomenon for the first time.

Lifting and lowering

Since the early-1950’s, lifting and lowering has been the

Prolonged or repetitive spine loading

Almost all of the early occupational biomechanics research on MMH was focused on acute loading and tissue damage. Early research related to repetitive load handling was concerned more with physiological cost and fatigue (e.g. Jorgensen and Poulsen, 1974, Legg and Myles, 1981), and not tissue injury implications. One of the first studies of the mechanical consequences of repetitive spine loading was performed by Parnianpour et al. (1988). They had subjects perform repetitive, isodynamic trunk

Ergonomic assessment tools

Some of the earliest work to establish safe limits for manual materials handling was performed at Liberty Mutual Insurance (e.g. Snook and Irvine, 1966, Snook and Irvine, 1967). This large series of studies culminated in the “Liberty Mutual” or “Snook” tables (Snook and Ciriello, 1991). Ayoub and colleagues presented a large number of equations for predicting MMH capacity based on a set of measured task variables (e.g. Ayoub et al., 1978). Garg et al. (1978) presented equations to predict the

Sudden/unexpected loading

Magora (1973) was among the first to demonstrate a relationship between sudden trunk loading and the prevalence of low back pain (LBP). Since that time, a number of researchers have made substantial contributions to our understanding of the response of the spine to sudden loading and unloading, and the potential mechanical contributions this might make to spine tissue injury (see Carlson et al., 1981, Marras et al., 1987, Lavender et al., 1989, Hodges and Richardson, 1996, Cholewicki et al.,

Spine stability

Until the late-1980’s, most attempts to understand the causes of work related low back injuries focused on loads exceeding some tissue tolerance level. However, this did not explain the large number of injuries that were occurring with loads that would not be considered hazardous. This paradox necessitated alternative explanations for occupational spine injuries. In fact, in the absence of muscles, buckling has been shown to occur with loads as low as 19 N on the osteoligamentous thoracolumbar

Future directions

This paper has presented a very brief overview of the progression of research in occupational spine biomechanics. While the many contributions of a vast number of researchers have had a positive impact on the health of workers, much work remains to be done. The following are some suggested areas needing further emphasis: (1) while most of the in vitro tissue testing studies, to date, have concentrated on compression loading, further research is needed to increase our understanding of tissue

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2021, Applied Ergonomics
Citation Excerpt :
For example, studies by Garg et al. (1982), McGill and Norman (1985) and de Looze et al. (1994) all showed that either the compressive forces or joint moments in the lumbar spine were significantly higher when employing 2-D dynamic models, even without the representation of trunk muscles. In 1986, McGill and Norman (1986) made a major advancement with the publication of a 3-D EMG-driven dynamic model with representations of 7 ligaments and 48 muscles (Potvin, 2008). Since then, several other detailed 3-D dynamic models have been published, e.g. by Granata and Marras (1995) and Kingma et al. (1996).
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.
Relationship between leg and back strength with inter-joint coordination of females during lifting
2016, International Journal of Industrial Ergonomics
The role of strength and fatigue in the lifting technique is not very clear, especially with regards to inter-joint coordination. We examined the relationships between muscle strength and endurance with inter-joint coordination of the knee-hip (KH) and hip-back (HB) during a lifting task performed until exhaustion. Thirteen healthy females were recruited to participate in the study. Significant negative correlations were found between HB maximum relative phase angle and leg lifting strength (r = −0.805), knee extensor strength (r = −0.705), knee flexor strength (r = −0.633), back extensor strength (r = −0.593) and back flexor strength (r = −0.596). The greater the strength of these muscles, the more synchronized the hip-back inter-joint coordination. However, no significant relationships were found with endurance test performance. Moreover, although the lifting task induced muscle fatigue, there were no significant fatigue-induced changes in lifting coordination.
Image driven subject-specific finite element models of spinal biomechanics
2016, Journal of Biomechanics
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Establishing the loads present in the human spine and its associated tissues is of significant importance for understanding normal spinal function and problems such as degeneration (Adams, 2012), functional instability (Mulholland, 2008), manual handling injury (Potvin, 2008), and back pain (Yang et al., 2011) and for designing successful regenerative therapies and implants for degenerative disc disease (Weber et al., 2015).
Finite element (FE) modelling is an established technique for investigating spinal biomechanics. Using image data to produce FE models with subject-specific geometry and displacement boundary conditions may help extend their use to the assessment spinal loading in individuals. Lumbar spine magnetic resonance images from nine participants in the supine, standing and sitting postures were obtained and 2D poroelastic FE models of the lumbar spine were created from the supine data. The rigid body translation and rotation of the vertebral bodies as the participant moved to standing or sitting were applied to the model. The resulting pore pressure in the centre of the L4/L5 disc was determined and the sensitivity to the material properties and vertebral body displacements was assessed. Although the limitations of using a 2D model mean the predicted pore pressures are unlikely to be accurate, the results showed that subject-specific variation in geometry and motion during postural change leads to variation in pore pressure. The model was sensitive to the Young׳s modulus of the annulus matrix, the permeability of the nucleus, and the vertical translation of the vertebrae. This study demonstrates the feasibility of using image data to drive subject-specific lumbar spine FE models and indicates where further development is required to provide a method for assessing spinal biomechanics in a wide range of individuals.
Development of an equation for calculating vertebral shear failure tolerance without destructive mechanical testing using iterative linear regression
2013, Medical Engineering and Physics
Equations used to determine vertebral failure tolerances without the need for destructive testing are useful for scaling applied sub-maximal forces during in vitro repetitive loading studies. However, existing equations that use vertebral bone density and morphology for calculating compressive failure tolerance are unsuitable for calculating vertebral shear failure tolerance since the primary site of failure is the pars interarticularis and not the vertebral body. Therefore, this investigation developed new equations for non-destructively determining vertebral shear failure tolerance from morphological and/or bone density measures. Shear failure was induced in 40 porcine cervical vertebral joints (20 C3-C4 and 20 C5-C6) by applying a constant posterior displacement to the caudal vertebra at 0.15 mm/s. Prior to destructive testing, morphology and bone density of the posterior elements were made with digital calipers, X-rays, and peripheral quantitative computed tomography. Iterative linear regression identified mathematical relationships between shear failure tolerance, and morphological and bone density measurements. Along with vertebral level, pars interarticularis length and lamina height from the cranial vertebra, and inferior facet height from the caudal vertebra collectively explained 61.8% of shear failure tolerance variance. Accuracy for this relationship, estimated using the same group of specimens, was 211.9 N or 9.8% of the measured shear failure tolerance.
Towards establishing an occupational threshold for cumulative shear force in the vertebral joint - An in vitro evaluation of a risk factor for spondylolytic fractures using porcine specimens
2013, Clinical Biomechanics
Citation Excerpt :
To enhance spondylolytic injury models and lay the groundwork for additional threshold limit values that consider cumulative shear load exposure, it is appropriate to evaluate the influence that the magnitude of repetitively applied sub-maximal shear force has on fatigue of the vertebral joint, and in particular the PI. In response to a call put forward by Potvin (2008), quantifying the relationship between the magnitude of repetitively applied forces and the vertebral joint's shear fatigue life will provide a basis to establish appropriate cumulative threshold values. Controlled in vitro studies have the capability to isolate specific injury mechanisms and quantify tissue damage thresholds that can be used for assessments of injury risk.
Injury models for spondylolytic fracture of the pars interarticularis have long considered repetitive shear loading as a risk factor without quantifying the relationship between shear force magnitude and fatigue life. This investigation sought to quantify the relationship using a basic in vitro approach.
Thirty-two (16 C3–C4, 16 C5–C6) porcine cervical specimens were exposed to repetitive shear loading to 20%, 40%, 60%, or 80% of their calculated ultimate anterior shear failure tolerance. Shear force was cyclically applied at 1 Hz for 21,600 cycles or until bone failure was detected. Cumulative shear force and the number of cycles sustained until failure were calculated. Failure patterns were also documented.
Cumulative shear and the number of cycles sustained prior to failure demonstrated a strong non-linearly decreasing relationship with increased force magnitude. In particular, sustained cumulative shear by the 40% group was 2.52 and 2.63 MN ∗ s higher than for the 60% and 80% groups (P < 0.0001). Despite undergoing an average of 230 more loading cycles, cumulative shear force sustained by the 60% group was not statistically different from the 80% group. Bilateral fractures of the cranial vertebra's pars interarticularis were most common, but less consistent at higher force magnitudes.
Our investigation suggested that pars interarticularis damage may begin non-linearly accumulating with shear forces between 20% and 40% of failure tolerance (approximately 430 to 860 N). Models of pars interarticularis injury and estimates of cumulative shear exposure may be enhanced from a tissue-based weighting method for low-back shear.
Musculoskeletal disorder risk during automotive assembly: Current vs. seated
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Skeletal muscle can generate large internal forces on the joints, tendons and nerve during movement that may lead to MSDs (Cutlip et al., 2009). Surface electromyography (EMG) and EMG-assisted spine loading models have been used to assess internal forces acting on the spine and the risk of low back disorders (Kim and Marras, 1987; Marras and Sommerich, 1991a; McGill and Norman, 1986; Garnder-Morse et al., 1995; Potvin, 2008). Furthermore, EMG of the shoulder muscles has also been used to assess exposure to physical demands in the workplace and subsequent risk of shoulder injury (Lee et al., 1997; Southard et al., 2007; Bao et al., 2009; Porter et al., 2010).
Musculoskeletal disorder risk was assessed during automotive assembly processes. The risk associated with current assembly processes was compared to using a cantilever chair intervention. Spine loads and normalized shoulder muscle activity were evaluated during assembly in eight regions of the vehicle. Eight interior cabin regions of the vehicle were classified by reach distance, height from vehicle floor and front to back. The cantilever chair intervention tool was most effective in the far reach regions regardless of the height. In the front far reach regions both spine loads and normalized shoulder muscle activity levels were reduced. In the middle and close reach regions spine loads were reduced, however, shoulder muscle activity was not, thus an additional intervention would be necessary to reduce shoulder risk. In the back far reach region, spine loads were not significantly different between the current and cantilever chair conditions. Thus, the effectiveness of the cantilever chair was dependent on the region of the vehicle.

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Jim Potvin received a B.H.K. in Kinesiology from the University of Windsor (1986) and an M.Sc. (1988) and Ph.D. (1992) in Biomechanics from the University of Waterloo. He is currently an Associate Professor in the Department of Kinesiology at McMaster University. He researches and teaches primarily in the areas of biomechanics and the ergonomics of musculoskeletal injuries. His basic research focuses on the study of spine mechanics during load handling and the quantification of the effects of muscle fatigue during repetitive or prolonged tasks. He also conducts applied research with a focus on developing valid ergonomic methods to quantify injury risk in the workplace; including the assessment of manual materials handling tasks and the evaluation of upper limb disorders.

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2008 ISEK Congress Keynote LectureOccupational spine biomechanics: A journey to the spinal frontier

Abstract

Introduction

Section snippets

Acute loading

Direct measures

Simple biomechanical models

Early studies

Lifting and lowering

Prolonged or repetitive spine loading

Ergonomic assessment tools

Sudden/unexpected loading

Spine stability

Future directions

Clin Biomech

J Biomech

J Biomech

J Biomech

Hum Movement Sci

Clin Biomech

J Biomech

Clin Biomech

Clin Biomech

Clin Biomech

J Biomech

Lancet

J Biomech

Int J Ind Ergonom

Clin Biomech

Orthop Clin North Am

Int J Ind Ergonom

Clin Biomech

Clin Biomech

J Biomech

J Biomech

Clin Biomech

Clin Biomech

Clin Biomech

J Biomech

The relevance of torsion to the mechanical derangement of the lumbar spine

Spine

Effects of operator stance on pushing and pulling tasks

IEEE Trans

Stability of the lumbar spine: a study in mechanical engineering

Acta Orthop Scand Suppl

Fatigue fractures of human lumbar vertebrae

Clin Biomech

An evaluation of predictive methods for estimating cumulative spinal loading

Ergonomics

Motor responses in the human trunk due to load perturbations

Acta Physiol Scand

Prolonged activity of lumbar erectores spinae: an electromyographic and dynamometric study of the effect of training

Ann Phys Med

The in vivo dynamic response of the human spine to rapid lateral bend perturbations: Effects of pre-load and step input magnitude

Spine

Lumbar spine stability can be augmented with an abdominal belt and/or increased intra-abdominal pressure

Eur Spine J

Spondylolytic fractures

J Bone Joint Surg Br

Posture of the trunk during the lifting of weights

Brit Med J

Pressures in the trunk cavities when pulling, pushing and lifting

Ergonomics

The function of the erectores spinae muscles in certain movements and postures in man

J Physiol

Role of muscles in lumbar spine stability in maximum extension efforts

J Orthop Res

Prediction of metabolic rates for manual materials handling jobs

Am Ind Hyg Assoc J

Stoop or squat: a biomechanical and metabolic evaluation

IIE Trans

Spinal compression tolerance limits for the design of manual material handling operations in the workplace

Ergonomics

A combined finite element and optimization investigation of lumbar spine mechanics with and without muscles

Spine

Stability of dynamic trunk movement

Spine

Manual lifting and handling

Physiotherapy

2008 ISEK Congress Keynote Lecture
Occupational spine biomechanics: A journey to the spinal frontier