Biomechanical evaluation of lifting tasks: A microcomputer-based model

Computers ind. Engng Vol. 14, No. 2, pp. 153--160, 1988

0360-8352/88 $3.00+0.00 Pergamon Press pie

Printed in Great Britain

B I O M E C H A N I C A L E V A L U A T I O N OF LIFTING TASKS: A MICROCOMPUTER-BASED MODEL TAREK M. KHALIL and MOHAMEDZ. RAMADAN Department of Industrial Engineering, Universityof Miami, Coral Gables, FL 33124, U.S.A.

(Received for publication 24 April 1987) Al~trlet--This paper discusses a static and dynamic biomechanicalevaluation of sagittal lifting activities via a microcomputer model. The input to the developed model includes operator's anthropometric data, and sex. The model provides the reactive forces and torques at the various joints of the body expressed in both British and metric systems. Also, the model shows the calculated compressive force on the spine at the fifth lumbar/first sacral joint (L5/$1), and both kinematic and kinetic informatious are displayed. The model has a menu of five options: (1) to analyze stress imposed on the L5/$1 during a dynamic activity;(2) to determine maximum weight to be allowedduring a dynamic motion; (3) to check stress on the spine (L5/$1) for specifiedstatic postures; (4) to determine maximum weight to be allowed for a static posture; or (5) to stimulate the lifting action and determine critical postures while performing lifting tasks based on static biochemical analysis.

1. INTRODUCTION Manual handling of materials has been recognized as a major source of low back injuries by researchers and organizations concerned with human health and safety [1--4]. Many variables have a bearing on how much weight can be lifted safely. These include human variables such as sex, age, body dimensions, muscle strength, physical fitness, and technique of lifting [5]; task variables such as load, location of load, vertical height range, frequency of lifting, speed of lifting, size of container, and coupling [6, 7]; and finally environmental variables such as heat, noise, vibration, humidity, or mental stress [8]. Generally, it is believed that a majority of the manual material handling injuries occur because the worker exceeds his/her physical capabilities. Therefore, a designer needs a tool to provide guidelines for designing manual material handling operations which would not create excessive strain on the worker. With the rapid computation capabilities of the m o d e r n microcomputer available, it becomes scientifically desirable and economically feasible to perform detailed mechanical analysis of people's interaction with their tasks and working environment. A generalized model that is capable of predicting safe levels of loads to be lifted by workers, and of calculating stresses imposed while lifting these loads, could be very useful. This study presents a microcomputer-based, interactive model that can assist job designers in determining safe load handling values in the design of manual material handling tasks. 2. MODEL DESCRIPTION The biomechanical model attempts to establish the stresses imposed on critical points of the musculoskeletal system. The model uses current available information concerning body segment parameters assuming that the body is made up of rigid links joined at known articulations. Utilizing these segment parameters and Newtonian mechanics, the reactive forces and torques equations could be developed at the various articulations of the body in different configurations. Figure I shows that the human body is modeled as a two-dimensional, eight-link system representing movement around seven joints. These are the ankle, knee, hip, L5/$1, shoulder, elbow, and wrist. Because the majority of back injuries occur at the lower back at the level of the fifth lumbar and first sacral vertebrae (L5/$1) and their adjoining disc, stresses imposed on that region during lifting are of particular interest. It was decided to model the trunk as two links to allow the calculation CA%| 1 4 : 2 - r

153

TAREK M. KHALIL a n d MOHAMED Z. RAMADAN

154

of spinal compressive forces and moments at the L5/S1 disc. The two trunk links are: (1) Hip joint to center of L5/S1 disc; (2) Center of L5/S1 disc to the shoulder joint. The ankle joint is assumed to be a fixed position which provides a reference for the model.

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The posture is defined by the angles between body links and the horizontal, measured in an anticlockwise direction. Angles 03 and 04 are computed in the model from the angle of inclination of the trunk and the knee with respect to the horizontal (08 and 02) [9, 10]. The model is implemented on an IBM-PC using advanced BASIC language. The program makes use of stored data as well as of inputed data. The stored data include: (1) the weight of the body segment links which is based on the ratios of segment weight to body weight based on data provided by Miller and Morrison [11], and Morris et al. [12] (Table 1); (2) location of the center of gravity for each link based on data provided by Dempster [13], and Garg and Chaffin [14] (Table 2); and (3) radius of gyration for each link as given by Chaffin and Andersson [15] and based on data provided by Plagenhoef [16]. The inputed data to the model are discussed later in the input--output section. To check stress on the L5/S1 disc during both static and dynamic analysis, the model assumes that a safe stress is one where maximum compression force calculated on the spine must be less than the compressive force limits cited by Chaffin [17]. These are 5670 N (578 kg) for males and 3394 N (346 kg) for females. Maximum weight allowed for a Table 1. Body segment weight as a presented regression equation SW = a + b t TW (based on MiUer and M o r m o n [11], and Morris et al. [12]). Body segment Hands Forearms Upper arms Head, neck and trunk Upper legs Lower legs Feet TW = total body weight in lbs. SW = segment weight in Ibs.

a

b

+ 0.7 - 0.5 - 2.9 + 12.0 + 3.2 - 1.9 + 1.5

+0.01 +0.04 +0.08 +0.47 +0.18 +0.11 +0.02

Biomechanical evaluation of lifting tasks

155

Table 2. Link centers of mass as a percentage of segment length (based on Demster [13], and Garg and Chaffm [14]) Link centers of mass from from proximal end distal end

Segment Hand Forearm Upper arm Head, neck and trunk above L5/S1 Trunk below L5/S1 disc Upper leg Lower leg Foot

50.6 % 43.0 43.6 43.21 50.0 56.7 56.7 57.1

49.4 % 57.0 56.4 56.79 50.0 43.3 43.3 42.9

defined posture is based on the consideration that the same allowable stress limit on the L5/$1 will not be exceeded. Utilizing the model, a biomechanical analysis can be made either in a dynamic mode or static mode. 2.1.

Dynamic mode

For dynamic analysis, the behavior of the body inertia creates forces which are an integral part of the total kinematic system. This model relies on a relationship which was developed many years ago by Slote and Stone to describe the displacement-time relationship for arm movement [18]. By studying segment displacements with respect to time, the angular velocity and angular acceleration are obtained by 1st and 2nd derivatives of the displacement equation. Starting from the ankle joint, the tangential and normal accelerations and their components in horizontal and vertical directions are computed at the center of gravity of each segment based on the following equations: Tangential acceleration = c02 , ri Normal acceleration = cti * ri where: ri = the distance of the center of gravity of segment i to the articulation; t~i = angular velocity of segment i; ct~ = angular acceleration of segment i. The inertial force components in X and Y directions are calculated at the center of mass of each link by multiplying the link mass by the corresponding linear acceleration components. This methodology was previously used by Curtis [19]. Forces and torques at each joint are computed based on the equilibrium equations, as shown in Fig. 2, taking into account the masses of the body segments, mass of the handled weight, and additive effects of acceleration of both weight handled and body segments as used by Ayoub and EI-Bassoussi [20]. A complete description of the motion of each link during the entire lifting action must be provided for this analysis. This can be obtained through recorded photographic data either by using goniometric method or by means of video spot locator system [15]. The movement action should be photographed from beginning to end of the lifting action. The user of the computer program inputs the data either by specifying postures at constant increments of time or by specifying the posture corresponding to actual elapsed time. The dynamic model again recognizes that the maximum load that can be handled safely should create compression force at the L5/$1 less than the compressive force limit of the intervertebral disc [17]. To compute this maximum load, it is advisable to use a conservative approach by calculating it at the maximum velocity of movement. This is attained when motion is performed with no load carried in the hands. It is, therefore,

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recommended that segment displacement data inputed to the model be obtained experimentally during motion with no load at the hands. 2.2.

Static mode For the static analysis mode the same body-segment data are used, but all accelerations are set to zero. The static model is initiated by specifying the body posture. The body posture could be defined by six articulation angles. These are: ankle, knee, trunk,

Biomechanicalevaluationof liftingtasks

157

shoulder, elbow, and wrist angles. These could be measured from a lateral photograph of a worker in the position under study by means of photographic techniques as explained in Chaffin and Andersson [15]. All angles are measured in a counter-clockwise direction from the horizontal plane. Once a body position has been defined, the model calculates the inclination angles of both hip- L5/$1, and L5/S1 shoulder links by knowing the trunk and knee angles with respect to the horizontal plane [9, 10]. This feature made it feasible to calculate compressive force on the L5/$1 during the lifting action. The additional force due to the erectorspinae muscle tension required to balance the torque at the lumbosacral disc is calculated by assuming a line of action of muscle force parallel to the vertebral column and at distance of 5 cm posterior to the center of the discs [21]. The forces and torques acting on the joints are produced from the masses of the body segments, weight lifted and the effects of gravity. A feature in the model is designed to recognize critical postures while performing a lifting task to a given height. This is the posture at which the stresses at the L5/$1 reach the compressive force limit as given earlier. The six heights specified are: floor to knuckle, floor to shoulder, floor to reach, knuckle to shoulder, knuckle to reach, and shoulder to reach. The body positions are simulated within the specified range of the lifting trajectory. Body articulation angles are incremented within the range of motion using maximum, minimum and increment values stored into the program memory. Each articulation angle is then decreased or increased by its incremental value to present a logical balanced posture. This procedure is continued through all articulation angles until every position has been considered.

2.3.

Model input~output

The input data required include subject's weight, sex, and link lengths (i.e. the straight line distances between the articulation points). The latter is based on actual body dimension or may be taken from displaced table of percentile values. At this point in the execution of the program, there are five options to calculate the mechanical stresses in the body. Fig. 3 shows the menu which is provided to the user of these options. A user may select any option desired from the menu. If the static option is selected, the user must provide body posture parameters corresponding to the angle of the links from the horizontal. If the dynamic option is selected, the user must provide body posture angles at different time intervals throughout the trajectory of motion.

Standing

I Determine

max.

allowable

weight to be lifted

l Static posture

l Dynamic activity

Check stress on the LS/SI disc

[ Static posture

l

Determine critical postures during lifting

Dynamic activity

Fig. 3. Main menuoptions. The microcomputer model prints the following output data: (1) A complete configuration for a given motion as shown in Fig. 4A, or for a defined posture as shown in Fig. 4B. (2) Position of each joint in the space (Cartesian Coordinates) considering the ankle joint as a reference. (3) Angular velocity and acceleration for each link.

158

TAREK M . KHALIL a n d MOHAMED Z . RAMADAN

(4) Forces in the X and Y directions at each joint. (5) Torque at each joint. (6) Compression force at LS/S1 joint•

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I

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B. s t a t i c

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Fig. 4. Print out of the model.

In addition to the above information, special messages may appear depending on the option selected. 3. APPLICATIONS

Case study 1 In a study to determine the stresses acting on the musculoskeletal system during back lifting (stooped posture) statically and dynamically, the model was fed with anthropometric data for the fiftieth percentile values of males. The predicted compressive force at L5/S1 based on both the static and dynamic models were computed and plotted as shown in Fig. 5. Only the trunk angle was changed from horizontal position to 80 degrees from the horizontal moving in an counter-clockwise direction. The range of trunk extension KN 5.0

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B i o m e c h a n i c a l e v a l u a t i o n o f lifting t a s k s

159

was divided into five points corresponding to five discrete angles with respect to the horizontal. The subject held 300 N in his hands for both lifting evaluations. The dynamic analysis was conducted for an angular velocity of 25 and 40 degrees per see. It is clear from the figure that the most stressful posture occurs quite early after the lifting action has started. Maximum compressive forces generated from the dynamic model are higher than those obtained from the static model except at the beginning, middle, and end of the lifting action. At those postures the compression force values are almost equal because the accelerations are zeroes. The figure also shows that as the trunk angular velocity increases the load on the spine increases. Case study 2 Anthropometric data for the fiftieth percentile values of males and females were used in this illustration. Table 3 shows body positions and maximum allowable weights to be handled based on the static model. Table 3. Body positions and maximum allowable weight to be lifted for males and females Joint angles (degrees) Posture configuration

01

02

085

80

145

75

Maximum weight to be lifted, N

05

06

07

Male

Female

55

-100

-60

-60

453.1

261.9

135

15

- 80

-80

-80

330.9

177.3

60

200

50

-110

-20

-20

331.0

190.8

90

90

90

- 45

60

60

498.1

310.4

90

100

100

- 90

-20

-20

434.3

267.8

$ 08 is the measured trunk angle used to calculate 03 and 04.

4.

DISCUSSION

AND CONCLUSION

The prediction of musculoskeletal stresses using biomechanical models is one of the safest and most practical approaches used by task designers and medical professionals. The effectiveness of a biomechanical model depends on its ability to realistically capture the intricate features of the human body. Existing biomechanical models used in the analysis of lifting tasks have varied in complexity from simple two link models to more complex several link models. Most of these models analyze the stresses on the human body in a static posture. Only few use a dynamic analysis of the body throughout the trajectory of motion. Dynamic analysis is much more complex and time consuming than static analysis, and as the number of links in the model increases the level of difficulty in computation grows rapidly. It should be noted that the use of biomechanical models for evaluation of stresses on the musculoskeletal systems has its advantages and its limitations. The advantages include the ability to obtain quantitative data on the forces and torques applied on the various joints, bones and muscles without having to resort to oftenly hazardous and psychologically unacceptable invasive techniques. The major drawback is that results obtained from biomechanical models are dependent on the assumptions made in developing the model and in how realistic do these assumptions duplicate the complex structure and function of the human system. Therefore in the interpretation of results one should be quite familiar with the assumptions and limitations of the model used. Another problem that frequently exists is the lack of accurate in vivo information about stress limits of soft and hard human tissues. Most available data are based on cadaver studies or laboratory specimens. Data derived from such specimens may not duplicate exact stress behaviour of these tissues in a living

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TAREK M. KHALIL and MOHAMED Z. RAMADAN

human. Again here one should exercise a great amount of judgement in the interpretation of model output data. The model outputs however still can provide very useful approximation of stress values that otherwise would not be available. These values can be used in conjunction with the guidelines of job analysis and design. The recent widespread use of microcomputers permits an analyst to more freely use complex biomechanical models to study the relationship between posture, type of task, and stresses imposed at various joints and muscles of the human body. The model developed here considers the body as an eight-link system. It differs from more simplified biomechanical models in that it recognizes the stresses at the hip as being different from stresses at the L5/S1 by identifying the two articulation points separately. It recognizes both static and dynamic types of tasks in its analysis. It provides several features in its "menu" that make it a useful practical tool for task analysts, workplace designers and safety professionals. The computer program is capable of calculating maximum allowable weight to be lifted, checks the level of stress based on a specific load carried in the hands or based on a specific posture assumed. It can also determine critical postures in the trajectory of lifting from floor to a certain level of height. The proposed computerized model shows the differences in results calculated with static and dynamic modes. Using a single case study it has been shown that dynamic aspects impose added stresses that should be taken into account when lifting tasks are considered. With the wide availability of the microcomputer the authors suggest that the use of this and similar biomechanical models can have an impact on operator's performance and on the prevention of costly workplace injury. REFERENCES 1. J. D. Troup. The relation of lumbar spine disorders to heavy manual work and lifting. Lancet 1,857-861 (1965). 2. A. Magora and I. Tauste. A n investigationof the problem of sick leave in the patientsufferingfrom low back pain. Ind. Med. Surgery 38, 11 0969). 3. NIOSH. Work PracticesGuide for Manual Lifting.Technical Report 0981). 4. D. H. Liles,S. Deivanayagam, M. M. Ayoub and P. Makajan. A job severityindex for the evaluation and control of liftinginjury. H u m a n Factors26(6),683-693 (1984). 5. S. H. Snook and C. H. Irvin.M a x i m u m acceptable weight of lift.J.AIHA 28(7),323-329 0967). 6. R. E. Knipfer. Predictive Models for the Maximum Acceptable Weight of Lift. Unpublished doctoral dissertation,Texas Tech University, Lubbock, Texas (1974). 7. J. R. Brown. Liftingas an industrialhazard. J.AIHA 34, 292 (1973). 8. E. R. Tichauer. A pilot study of the biomechanics of liftingin simulated industrialwork situations.J. Safety Res. 3, 98-115 (1971). 9. B. Fisher. A Biomechanical Model for the Analysis of Dynamic Activities. M.S. Thesis, University of Michigan, Ann Arbor, Michigan (1967). 10. A. Freivalds, D. B. Chaffin, A. Garg and K. S. Lee. A dynamic biomechanical evaluation of lifting maximum acceptable loads. J. Biornechanics 17(4), 251-262 (1984). 11. D. I. Miller and W. E. Morrison. Prediction of segmental parameters using the Hanavan human body model. Medicine Sci. Sports 7(3), 207-212 (1975). 12. J. M. Morris, D. B. Lucas and B. Bressler. Role of the trunk in stability of the spine. J. Bone Jt Surg. 43A, 327-339 (1961). 13. W. T. Dempster. Space requirements of the seated operator. WADC-TR-55-159, Aerospace Medical Research Laboratories, Ohio (1955). 14. A. Garg and D. B. Cbaffln. A computerized biomechanical stimulation of human strength. AIIE Trans. 7(1), 1-15 (1975). 15. D. B. Chaffin and G. Andersson. Occupational Biomechanics. J. Wiley, New York (1984). 16. S. C. Plagenhoef. Methods for obtaining kinetic data to analyze human motions. Res. Q. 37(1), 103-112 (1963). 17. D. B. Chaffin. A computerized biomechanical model development of and use in standing gross body actions.J. Biomech. 2, 429 ~A.I(1969). 18. L. Slote and G. Stone. Biomechanical power generated by forearm flexion.H u m a n Factors5, 5 443--452 0963). 19. M. Curtis. Computer program for obtaining kinetic data on human movement. J. Biomech. I, 221-234 0968). 20. M. M. Ayoub and M. S. EI-Bassoussi. Dynamic biomechanical model for sagittal plane lifting activities. Safety in Manual Materials Handling (Edited by Drury, C. G.), pp. 78-185. NIOSH, Cincinnati, Ohio. 21. D. B. Chaffin. A biomechanical model for analysis of symmetric sagittal plane rifting. AIIE Trans. 2(1), 16--27 (1970).