Effects of Moment Arms on the Internal Spinal Loads during Manual Material Handling

Transcription

1 Effects of Moment Arms on the Internal Spinal Loads during Manual Material Handling Murali Subramaniyam 1&2, Se Jin Park 1, Sangho Park 2 1 Korea Research Institute of Standards and Science, Daejeon, , South Korea 2 Chungnam National University, Daejeon, , South Korea ABSTRACT Objective: Effective estimates of internal spinal loading using biomechanical models are dependent on the accuracy of anatomical inputs, such as a muscle moment arm. In this study, the effects of the moment arms on the internal spinal loads were estimated during manual material handling tasks with a variety of postures. Background: Biomechanical models of the humans have been developed to predict the magnitude and pattern of spine loading during task performance. Accurate anatomical inputs for biomechanical models are necessary for valid estimates of internal spinal loading. Usually the extensor muscular and ligament forces of the lumbar spine are assumed to act 5 cm posterior (i.e., moment arm) to a disc centre of rotation. However, different moment arms (6, 7.5, and 8.5 cm) have been considered in the biomechanical studies. Method: In this study, the lumbar erector spinae moments (L4/L5 moment) during manual material handling with a variety of postures were estimated using a commercially available biomechanical model. Symmetrical hand loads considered for the tasks were 0 25 kg with 5 kg increments. Postures considered for the tasks were upright (0 degree) and stooped standing posture (10, 20, and 30 degrees). The muscle forces and disc compression forces about L4/L5 point were calculated from the estimated L4/L5 moment for the different moment arms. Results: When the load increases, flexor moment increases for upright posture and extensor moment decreases for the stooped standing postures. When the stooping angle increases, extensor moment increases to load. When the moment arm was higher (8.5 cm), the muscle forces were lower for the postures comparing with smaller moment arms (5, 6, and 7.5 cm). Similarly, when the moment arm was higher, the disc compression forces were also lower for the postures. Statistically significant difference (p < 0.05) was found in the muscle forces and disc compression forces between the moment arms. Conclusion: With the author s knowledge to calculate internal spinal loads, there is no standard moment arm available and may need further studies or standard for moment arm. Application: Effective estimates of internal spinal loads would be useful for calculating recommended weights for manual lifting tasks. Keywords: Moment arm, Muscle force, L4/L5 disc compression force, L4/L5 moment, Posture 1. Introduction For the last several decades, despite the growing advancement in mechanization and automation, manual material handling (MMH) has attracted great interest from researchers in many disciplines, since huge amount of work and financial losses, and human suffering caused by low back pain (LBP) and injuries. Consequently, it is a major concern to researchers and organizations to predict, control, and prevent such injuries [Elfeituri, 2001; Gundogdu et al., 2005]. One approach to LBP prevention involves the study of the forces acting on the spine during MMH. The main assumption underlying this approach is that the risk of injury to the low back associated with MMH is causally related to spinal loading [McGill, 1997]. When a person lifts a load, the mass of his/her upper body and the load to be lifted induce torques at various joints of the body. When we stand, the lower back is functioning to support the weight of the upper body.

2 Movements in the lumbar spine, including flexion and extension, are governed by a complex neuromuscular system involving both active and passive components, where muscles are active components and vertebral bones, intervertebral disks, ligaments, tendons, and fascia are passive components [Solomonow et al., 2003; Colloca and Hinrichs, 2005]. Stooped, restricted, kneeling, and other awkward postures adopted during manual materials handling have frequently been associated with LBP onset [Splittstoesser et al., 2007]. There is a lot of research that shows that sustained stooped postures and poor movement patterns cause stresses and strains that may have something to do with getting LBP. The discs at levels lower lumbar discs L4/L5 and lumbosacral joint (L5/S1) are the sites that appear to be most associated with clinical problems and the development of spinal diseases. Furthermore, elucidating how the posture or loading mode influences the biomechanical behavior at such level is also interest to researchers [Kuo et al., 2010]. Biomechanical modeling plays an important role in estimating individuals lifting capacities, comparing different lifting modes, and designing workspace conditions. Using such models, the potential for injuries can be estimated in advance and greatly reduce the need for often difficult, expensive and potentially invasive laboratory measurements [Hsiang and Ayoub, 1994; Pandy et al., 1992; Pandy et al., 1995]. Biomechanical models of the human trunk have been developed to predict the magnitude and pattern of spine loading during task performance. These models are necessary as direct quantification of muscle forces and spinal loading are currently infeasible [Jorgensen et al., 2001]. Effective estimates of internal spinal loading using biomechanical models are dependent on the accuracy of anatomical inputs, such as a muscle moment arm. Usually the extensor muscular and ligament forces of the lumbar spine are assumed to act 5 cm posterior to a disc centre of rotation [McGill and Norman, 1987]. However, different moment arms (6, 7.5, and 8.5 cm) have been considered in the biomechanical studies [Potvin, J. R., 1997; McGill and Norman, 1987; The biomechanical criterion is usually based on intervetebral joint compression forces that do not exceed some threshold limit. However, the determination of one limit is problematic due to the large variability in the vitro literatures. NIOSH have recommended limits of 3400 N for most workers and 6400 N for some workers [NIOSH, 1981; Waters et al., 1993]. However, Leamon [1994] has suggested that 5000 N may be a more appropriate conservative limit than 3400 N based on the data of Anderson [1983]. Also, Jager and Luttmann [1991] have determined that compressive strength is dependent on age, gender, and lumbar level and have calculated the average compressive strength to be 5700 N and 3900 N for males and females, respectively. From the biomechanical modeling point of view, an accurate estimate of spinal loading is based on moment arm length. Therefore, in this study, the lumbar erector spinae moments (L4/L5 moment) during manual material handling with a variety of postures were estimated using a commercially available biomechanical model. The muscle forces and disc compression forces about L4/L5 point were calculated from the estimated L4/L5 moment for the different moment arms 2. Method In this study, we have used a DHM tool CATIA V5 R18 from Dassault System s [ to biomechanical model and analyze the human postures to explore the lumbar spinal loads. The DHM technology is the human modeling solution for 3D CAD designers to perform ergonomics and human factor analyses. Several studies have been carried out to validate different DHM tools for ergonomics evaluation in the industries and showed that outcomes of the different tools are fairly accurate. Symmetrical hand loads considered for the tasks were 0 25 kg with 5 kg increments. Postures considered for the tasks were standing upright (0 degree) and stooped standing posture (10, 20, and 30 degrees). The muscle forces and disc compression forces about L4/L5 point were calculated from the estimated L4/L5 moment for the different moment arms. The moment arms considered were 5, 6.5, 7.5, and 8.5 cm. The anthropometric data utilized for the digital Korean model were based on size Korea 1997 database. Fiftieth percentile Korean male and female were considered. The internal spinal loads calculated were L4/L5 moment and L4/L5 compression force.

3 3. Results and Discussion During the MMH or lifting, to maintain the postural stability or static equilibrium, the support moment is needed by the trunk muscles, the necessary trunk muscle support moment is L4/L5 moment, and where negative values represent flexor and positive value represents extensor moment. The L4 and L5 vertebrae are compressed together by the mass of the upper body, external loads applied on their hands and also by the trunk muscles that are used to generate the support moment. The L4/L5 compression value represents the force acting upon the L4/L5 intervertebral joint. As per link segment static model, the L4/L5 moment (N.m) was calculated in the DHM tool using Eqn. 1. The magnitude of the muscle forces (N) can be estimated by dividing the L4/L5 moment with erector spine moment arm length (m) (Eqn. 2). L4/L5 Moment = M.U.B. * Lw + H.L. * Lp (Eqn. 1) Where, M.U.B Mass of the upper body (body mass above L4/L5) including mass of the head, neck, arms, hands, spine; H. L. Hand load; Lw Horizontal distance between L4/L5 disc center to center of mass of the upper body; Lp Horizontal distance between L4/L5 disc center to hand load. Muscle Force = (L4/L5 Moment) / M. A. (Eqn. 2) Where, M. A. Erector spine moment arm length in meter. Three main forces act on the lumbar spine at the L4/L5 level: the force produced by the weight of the upper body (body mass above L4/L5); the force produced by the weight of the object; and the force produced by the erector spinae muscles (which acts approximately at right angles to the disc inclination). These three forces used to calculate the L4/L5 compression (Eqn. 3). L4/L5 compression = Muscle force + (M.U.B. * sinυ) + (H. L. * sinυ) (Eqn. 3) Where, Υ - angle of upper body inclination or the angle of the trunk or the line between L4/L5 and C7/T L4/L5 Moment The L4/L5 moment estimated from the DHM tool for the stooped standing posture presented in Table 1 & 2 for male and female, respectively. The negative values represent flexor and positive values represent extensor moment. When the load increases, flexor moment increases for upright posture and extensor moment decreases for the stooped standing postures. When the stooping angle increases, extensor moment increases to load for genders. Table 1. L4/L5 Moment (N.m) for Male 0 deg 10 deg 20 deg 30 deg kg kg kg kg kg Table 2. L4/L5 Moment (N.m) for Female 0 deg 10 deg 20 deg 30 deg kg kg kg kg kg Muscle Force The muscle forces were calculated with different moment arms using Eqn. 2, and presented in Table 3 & 4 for male and female, respectively. Standing upright postures muscle force results are only given in the tables. When the moment arm was higher (8.5 cm), the muscle forces were lower for the postures comparing with other moment arms (5, 6, and 7.5 cm). Table 3. Muscle force (N) with different moment arm for Male (standing upright posture) kg kg kg kg kg

4 Table 4. Muscle force (N) with different moment arm for Female (standing upright posture) kg kg kg kg kg L4/L5 Compression The L4/L5 compression forces were estimated from the DHM tool for the postures. The L4/L5 compression force estimated from the DHM tool includes erector spinae muscle force with moment arm as 8.5 cm. L4/L5 compression without muscle force can be estimated using Eqn. 4., and L4/L5 compression force with 5 cm moment arm can be calculated using Eqn. 5. Similarly, for other moment arms L4/L5 compression force can be calculated. (L4/L5 compression) 8.5 cm (Muscle force) 8.5 cm = L4/L5 compression without muscle force Eqn. (4) L4/L5 compression without muscle force + (Muscle force) α cm = (L4/L5 compression) α cm Eqn. (5) Where α moment arms (5, 6.5, 7.5, 8.5 cm) Using DHM tool the L4/L5 compression force estimated for 8.5 cm moment arm and for other moment arms calculated using Eqn. 5. The results are presented in Table 5 & 6 for male and female, respectively. When the moment arm was higher, the disc compression forces were lower for the postures. Statistically significant difference (p < 0.05) was found in the muscle forces and disc compression forces between the moment arms. Table 5. L4/L5 compression force (N) with different moment arm for Male (standing upright posture) kg kg kg kg kg Table 6. L4/L5 compression force (N) with different moment arm for female (standing upright posture) kg kg kg kg kg When the moment arm was higher (8.5 cm), the muscle forces were lower for the postures comparing with other moment arms (5, 6, and 7.5 cm). When the moment arm was higher (8.5 cm), the disc compression forces were lower for the postures comparing with other moment arms (5, 6, and 7.5 cm). The biomechanical criterion is usually based on intervetebral joint compression forces that do not exceed some threshold limit. NIOSH have recommended limits of 3400 N for most workers and 6400 N for some workers [NIOSH, 1981; Waters et al., 1993]. The L4/L5 compression forces estimated in this study were within the NIOSH recommended limits. Even though the compression forces were within the threshold limits, the compression values were significantly different based on the moment arms. Effective estimates of internal spinal loads would be useful for calculating recommended weights for manual lifting tasks. With the author s knowledge to calculate internal spinal loads, there is no standard moment arm available and may need further studies or standard for moment arm. 4. Conclusions Biomechanical models of the humans have been developed to predict the magnitude and pattern of spine loading during task performance. In this study, the effects of the moment arms on the internal spinal loads were estimated during manual material handling tasks with a variety of postures. Effective estimates of internal spinal loads would be useful for calculating

5 recommended weights for manual lifting tasks. With the author s knowledge to calculate internal spinal loads, there is no standard moment arm available and may need further studies or standard for moment arm. References Elfeituri, F. E., A biomechanical analysis of manual lifting tasks performed in restricted workspaces, Int J Occup Saf Ergon., 7 (pp ), Gundogdu, O., Anderson, K. S. and Parnianpour, M., Simulation of manual material handling: biomechanical assessment under different lifting conditions, Technol Health Care., 13 (pp ), McGill, S. M., Biomechanics of low back injury, implications on current practice in industry & the clinic, J Biomech., 30 (pp ), Hsiang, S. H., and Ayoub, M. M., Development of methodology in biomechanical simulation of manual lifting, Intl J Indus Ergo., 13 (pp ), Pandy, M. G., Anderson, F. C., and Hull, D. G., A parameter optimization approach for optimal control of large-scale musculoskeletal systems, J Biomech Eng., 114 (pp ), 1992 Pandy, M. G., Garner, B. A., and Anderson, F. C., Optimal control of non-ballistic muscular movements: a constraint-based performance criterion for rising from a chair, J Biomech Eng., 117 (pp ), Chaffin, D. B., and Anderson, G. B. J., Occupational biomechanics, 2nd Edn, John Wiley & Sons, Inc, Kuo C. S., Hu, H. T., Lin, R. M., Huang, K. Y., Lin, P. C., Zhong, Z. C., and Hseish, M. L., Biomechanical analysis of the lumbar spine on facet joint force and intradiscal pressure? A finite element study, BMC Musculoskeletal Disord., 11 (p. 151), Solomonow, M., Baratta, R.V., Banks, A., Freudenberger, C., and Zhou, B. H., Flexion-relaxation response to static lumbar flexion in males and females, Clin Biomech., 18 (pp ), Colloca, C. J., and Hinrichs, R. N., The biomechanical and clinical significance of the lumbar erector spinae flexion-relaxation phenomenon: A review of literature, J Manipulative Physiol Ther., 28 (pp ), Splittstoesser, R. E., Yang, G., Knapik, G. G., Trippany, D. R., Hoyle, J. A., Lahoti, P., Korkmaz, S V., Sommerich, C. M., Lavender, S. A., and Marras, W. S., Spinal loading during manual materials handling in a kneeling posture, J Electromyogr Kinesiol, 17 (pp.25-34), Jorgensen, M. J., Marras, W. S., Granata, K. P. and Wiand, J. W., MRI-derived moment-arms of the female and male spine loading muscle, Clinical biomechanics, 16 (pp ), McGill, S. M. and Norman, R. W., Effects of an anatomically detailed erector spinae model on L4/L5 disc compression and shear, J. Biomechanics, 20 (pp ), Potvin, J. R., Use of NIOSH equation inputs to calculate lumbosacral compression forces, Ergonomics, 40 (pp ), Author listings Murali Subramaniyam: murali.subramaniyam@gmail.com Highest degree: M.Tech (CIM) Position title: Research Scholar, Division for Convergence Technology, Center for Medical Metrology, KRISS Areas of interest: Biomechanics, DHM, CAD/CAM, Ergonomics, Human Vibration, Human Factors Se Jin Park: sjpark@kriss.re.kr Highest degree: Ph.D. Position title: Director, Division of Convergence Technology, KRISS Areas of interest: Human Factors, Human Vibration, Human Sensibility, Biomechanics, Human Computer Interaction (HCI) Sangho Park: spark@cnu.ac.kr Highest degree: Ph.D. Position title: Professor, Department of Mechanical Design Engineering, Chungnam National University Areas of interest: CAD/CAM, Virtual Reality, Computer Graphics, DHM