Biomechanical model to predict loads on lumbar vertebra of a tractor operator

https://doi.org/10.1016/j.ergon.2015.02.006Get rights and content

Highlights

  • Biomechanical model to predict loads at L4/L5 vertebra of tractor drivers.

  • Inputs were stature and weight of operators, operating conditions and reaction forces.

  • Maximum compressive and shear forces ranged 943–1367 N and 422–991 N, respectively.

Abstract

A biomechanical model is important for prediction of loads likely to arise in specific body parts under various conditions. The biomechanical model was developed to predict compressive and shear loads at L4/L5 (lumbar vertebra) of a tractor operator seating on seats with selected seat pan and backrest cushion materials. A computer program was written to solve the model for various inputs viz. stature and weight of the tractor operators, choice of operating conditions, and reaction forces from seat pan and backrest cushions. It was observed that maximum compressive and shear forces ranged 943–1367 N and 422–991 N, respectively at L4/L5 of tractor operators steering the tractor with leg and hand control actions and occasionally viewing the implement at back. The compressive forces were maximum (1202–1367 N) with coir based composite seat backrest cushion materials (thickness of 80 mm, density of 47.19 kg/m3) and were minimum (943–1108 N) with high density polyurethane foam (thickness of 44 mm, density of 19.09 kg/m3) for the seats.

Relevance to industry

The biomechanical model of a tractor operator is important for theoretical understanding the problem of sitting and is also valuable in prediction of compressive and shear loads at L4/L5 of operator under various operating conditions. It will help in design of tractor seat for operator's comfort.

Introduction

The nature of tasks on a tractor necessitates a number of actions to be performed by the operator, which puts varying physiological demands on the body. Examples of these tasks are steering of tractor, looking backward to observe and control the machine/implement, and force required to operate clutch, brake, and hydraulic control lever. The task and workplace determine the postures and create a pattern of loading on the structures of the body of the individual. The seat is one component affecting these loads. Tractor seat design can be used as a means to modify loads on the body structures to reduce operator's discomfort. The load on the spine and its distribution in time, as well as muscle activity resulting from postural and external loading, are important determinants of discomfort and pain in the back (Eklund et al., 1983). Knowledge of these loads is important for the design of tractor seat and for the evaluation of work tasks as well as for estimation of postural loads.

Pheasant and Harris (1982) studied the biomechanical factors, which influenced human strength in the operation of a tractor pedal. The variables investigated were horizontal distance in front of the seat reference point (SRP), vertical distance above and below SRP, lateral distance from the midline, direction of thrust and use of steering wheel for ‘bracing’. They represented seat–man–pedal assembly by a kinetic chain of four linkages.

Eklund et al. (1983) described a method for calculating biomechanical load on the spine (at L3) and back muscles for a seated task. This method utilised a small computer online to a force platform and the results were obtained within a few seconds. It dealt with vertical forces arising from support of the hands and arms, weight lifting and vertical acceleration as well as trunk movements but did not deal with horizontally imposed forces.

Corlett and Eklund (1984) discussed loading on the spine and active muscles of the back during upright sitting. It was shown that the backrest was located at the lumbar spine area so that the centre of gravity (CG) of the super-incumbent body parts could be positioned above the vertebrae, permitting the gravity load to be transmitted to the seat without counteracting torque which muscles would have to provide.

Viano and Andrzejak (1992) reviewed issues relating to seats including design for comfort, mechanics of discomfort, and low back pain. They focussed on the interface between seating technology and occupant comfort and found that seating features and riding comfort required more specific information on the biomechanics of discomfort by pressure distribution, body support, ride vibration, material breathability, and other factors. These inputs stimulated mechanisms of discomfort that need to be quantified in terms of mechanical requirements for seat design and function. They concluded that the new seat requirements need to be based on biomedical science and should include the needs of customers.

Hubbard et al. (1993) described a group of computer models and a drawing template that represented the geometry and movements of people of different sizes for design of automobile seating. These models included a two-dimensional articulated drafting template and a three-dimensional computer model of the skeletal system with soft tissue thickness. They represented the external body contours on the back of the torso, model of forces and moments between body segments based on seated posture, body segment masses, and seat surface forces. The developed models and template provided biomechanical information that assisted designers in creating seats that fitted and moved with people. It was observed that the geometric characteristics of people and their geometric interactions with seats were directly related to the physiological features of the seating comfort.

Oberg (1993) suggested a biomechanical model of the lumbar spine load due to twisted trunk postures during tractor ploughing. Video recording of tractor driver's work in a transversal plane was done to quantify head and trunk twisting angles and its duration's. A laboratory mock up of the interior of the tractor cabin was set up to reproduce and to measure these working postures using a three dimensional Mac Reflex position and motion analysis system. It was observed that the lumbar thoracic spine was kept twisted 37° during a whole furrow and head was twisted in relation to the shoulder further 90° to the right during 90% of the total duration and 20° to the left during 10% of the duration.

Boden and Oberg, 1995, Boden and Oberg, 1997 observed that the exerted torque from passive tissues at different twisting angles in sitting position for tractor drivers changed as the passive tissues were resisting or assisting the twisting action. They concluded that up to 20° rotation, the trunk twisting stiffness was approximately 0.1 N m/degree and after 40° rotation, the corresponding value was 0.4 N m/degree.

Hansson and Öberg (1996) described a method for analysis of static biomechanical load of an agricultural tractor driver when manoeuvring the controls inside the cab. The measurement procedure was standardized and the transfer of data from the opto-electronic measurements to the biomechanical model was computerized, which increased the accuracy and made it possible to analyse measurements of time series within a reasonable time. However, since the method was based on calculation of static load, it was mainly suited for analysis of static working tasks or tasks including only slow movements.

Srdjevic and Cveticanin (2004) proposed a method for identifying principal parameters of the n-DOF (n degrees-of-freedom) biomechanical model of a seated human driver. The model was considered as a structure represented by masses of human body segments, mechanical springs and dampers, and also as a biomechanical system with dynamic performances described in terms of response functions in the frequency domain, such as the driving-point mechanical impedance, and seat-to-head transmissibility function. The problem addressed was to identify the structural components that provided the best dynamic model performance for the driver-seat system. The proposed method was comprehensive in approach and efficient in application.

In order to assess the tolerability of a given posture, measurement of lumbar stresses is important. The stresses are generally expressed in terms of forces and moments acting on a given articulation of the lumbar spine and in particular on its intervertebral disc (Occhipinti et al., 1985). Few methods of estimating the spinal and muscular load exist. Measurement of intradiscal pressure, back muscle electromyogram (EMG) and intra-abdominal pressure are limited by the costs of the equipment, the expertise required and difficulty in field use. Biomechanical analysis based on mathematical models, however, have advantages in this respect, and have also been shown to agree well with the other methods (Andersson et al., 1980, Schultz and Andersson, 1981).

Most of the biomechanical models were developed to quantify the spinal stress in manual material handling activities in order to compare different handling techniques. However, only a few biomechanical models were developed for prediction of lumbar stress or load on the back of a sitting person (Eklund et al., 1983, Occhipinti et al., 1985). These models are for sitting on a chair under static condition. Nevertheless, no application procedure for analysis of the compressive loads on the lumbar disc was reported for tractor operator. Moreover, working condition of a tractor operator is different from working on a chair in an office. In fact, the situations usually illustrated in the literature, i.e. sitting upright using a backrest, head straight and arms hanging, are rather different from operation of tractor during ploughing, harrowing etc. The development of a biomechanical model for a tractor operator is important for theoretical understanding the problem of sitting. It is also valuable in prediction of the loads likely to arise in specific body parts, under various conditions.

Section snippets

Theoretical considerations

The development of a biomechanical model for a tractor operator will help in calculating compressive and shear loads in a horizontal plane at any selected spinal level, for example at L4 (lumbar vertebra 4). It will also help in calculation of momental load in the sagittal plane around any chosen spinal disc.

The tractor operator has to adopt different spinal postures under dynamic conditions, which creates an uneven pressure on the lumbar discs, especially the L4–L5 discs. It may force the disc

Material and methods

Seven commercially available seat cushion materials of different densities, thickness and compositions were selected for the study. These cushion materials were generally used in different combinations in Indian automobile and tractor seats. Table 2 shows that the selected cushion materials of different thicknesses were medium density PU foam (M), high density PU foams (H and 2H), synthetic rubber foams (D and 1.9D), and locally available coir type (C and 1.5C). The prefix number to each

Results and discussion

The biomechanical model of tractor operator was developed to predict the loads likely to arise in specific body parts, under various operating conditions and was solved for heights and weights of the three selected tractor operators. The 5th, 50th and 95th percentile heights of the operators were 1591, 1643 and 1755 mm with corresponding weights of 53.0, 74.5 and 84.0 kg. The model was solved for four main activities of a tractor operator under dynamic condition. The mean values of compressive

Conclusions

The developed biomechanical model for a tractor operator was useful to predict compressive and shear loads at L4/L5 (lumbar vertebra) of driver seating on the tractor seat with selected seat pan and backrest cushion materials under different operating conditions. The maximum compressive forces at L4/L5 ranged from 943 to 1367 N and shear forces ranged from 422 to 991 N when tractor operator was steering the tractor with leg and hand control actions and occasionally viewing the implement at the

Acknowledgements

The authors thank the Director, Central Institute of Agricultural Engineering, Bhopal and the Head, Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur, India for providing facilities to conduct this study.

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