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Evaluating lifting tasks using subjective and biomechanical estimates of stress at the lower back a

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A. WATKAR , K. LEE , F. AGHAZADEH & C. PARKS

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Industrial Engineering Department , Louisiana State University , Baton Rouge, LA, 70803, USA Published online: 30 May 2007.

To cite this article: A. WATKAR , K. LEE , F. AGHAZADEH & C. PARKS (1991) Evaluating lifting tasks using subjective and biomechanical estimates of stress at the lower back, Ergonomics, 34:1, 33-47, DOI: 10.1080/00140139108967286 To link to this article: http://dx.doi.org/10.1080/00140139108967286

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ERGONOMICS, 1991, VOL. 34, NO. 1, 33-47

Evaluating lifting tasks using subjective and biomechanical estimates of stress at the lower back A. WAIKAR,K.

LEE,F. AGHAZADEHand C. PARKS

Industrial Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA

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Keywords: Lifting; Biomechanical; Subjective; Psychophysical; Lower back.

The objective of this study was to evaluate five different lifting tasks based on subjective and biomechanical estimates of stress at the lower back. Subjective estimates were obtained immediately after the subjects performed the lifting tasks. Rankings for different tasks were obtained according to the perceived level of stress at the lower back. A biomechanical model was used to predict the compressive force at the L5/S1 disc for the weight lifted considering link angles for the particular posture. The tasks were also ranked according to the compressive f o r k loading at the L5/S1 disc. The weight lifted in these tasks for obtaining the subjective estimate of stress was the maximum acceptable weight of lift (MAWOL).This was determined separately for each subject using a psychophysical approach. Subjective estimates of stress were obtained for infrequent lifting, specifically for a single lift, as well as for lifting at a frequency of four lifts per min. The results showed that a lifting task acceptable from the biomechanical point of view may not be judged as a safe o r acceptable task by the worker based on his subjective perception. This may result in a risk of the worker not performing the recommended task or not following the recommended method.

1. Introductisn Lifting tasks, quite common in manual materials handling activities, have long been recognized as a source of lower back problems. The seriousness of the lower back injury problem is reflected in the large number of claims under the US Workman's Compensation Act of 1970. According to Snook (19781, statistics compiled by Liberty Mutual Insurance Company indicate that 79% of the manual material handling injuries were injuries to the lower back. The National Safety Council (1978) reported that in the USA;400,000 workers face disabling back injuries every year. More recent statistics (NIOSH 1981) also show that back injuries resulting from manual materials handling activities are a major source of lost time and compensation claims. Morris (1984) estimates that 28% of the US industrial population will experience disabling lower back pain some time in their lives with eight per cent of the total working population being disabled during each year. Lahey (1984) states that back injuries alone cost the industry an estimated US $14 billion a year. Although back pain ranks among the most widely experienced ailments in the western society, it is not well understood. Research is unclear on which factors are correlated with risk of back injury (National Safety Council 1987). Experts agree that further research is needed to reduce the incidence of back injury. Brown (1972) reported that it is generally supposed that more severe injuries are associated with lifting heavy objects. Since lifting is prevalent in many jobs, it is dificul t to eliminate lower back injuries. But by careful selection of workers, good training procedures in safe lifting, and designing the job to fit the worker, companies can reduce the number and severity of industrial over-exertion injuries (Snook 1987). 0014-0139/91 $ 3 0 0 0 1991 Taylor & Francis Ltd.

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A. Waikar et al.

Good job design reduces health-related hazards, and places less reliance on the worker's willingness to follow established training procedures, such as lifting properly. Ergonomists agree that jobs should be designed to match human capabilities, and people should be screened to match the job demands based on their individual capacities. For job design, data describing the material handling capabilities for various population groups have been reported by Snook (1978), Mital et a!. (1978), and Mital and Asfour (1983). To match 'the worker and the task, elaborate models have been developed to predict the lifting capability of an individual at a given population percentile (Mital and Ayoub 1980, Mital 1983). Considerable research in the area of manual materials handling (MMH) has been focused on the establishment of MMH limits, ergonomic principles for job design, employee placement, and employee training. In spite of the careful selection of workers, training, and good job design, there is no evidence that these recommendations have produced any great change in the incidence of lower back injuries. Reduction in the number, severity, and the resulting costs of these injuries has therefore been a major concern to many researchers and health-related agencies (Mital 1984). Mital and Kromodihardjo (1 986) conducted a study in which biomec hanical analysis of task variables in manual lifting was performed for maximum acceptable loads determined psychophysically. The experimental task involved symmetric and asymmetric lifting from floor to shelf height. They found that the maximum acceptable weight of lift determined psychophysically and the compressive forces estimated biomechanically had a high degree of correlation. A biomechanical model was developed by Chaffin (1969) for analysing lifting tasks. This was a seven-link, two dimensional static model which calculates joint forces and moments during manual material handling activities. The model has been revised and implemented on a microcomputer (Chaffin 1983). There are other biomechanical models (Kroemer et al. 1988) but the above-mentioned model is commonly used in industry. Balogun et al. (1986) stated that it is important to consider the subjective feelings of the individuals carrying the load since an ergonomic device must be subjectively acceptable to the worker. Subjective evaluations have also been used by researchers (Lee et al. 1988) in evaluating musculoskeletal stress in microscope and VDT work. The authors suspect that, in industry, some of the recommended lifting tasks or methods may not be subjectively acceptable to the workers, and therefore tasks or methods may not be practised as recommended. Therefore, it is of interest to investigate whether a task, recommended as a safe task from the biomechanical point of view, is also acceptable to the worker, based on subjective considerations. The objective of this study was, therefore, to evaluate five different lifting tasks based on biomechanical and subjective estimations of stresses at the lower back. The specific question addressed by this research was 'Are there discrepancies between the rankings for the lifting tasks based on the subjective estimates of injury potential to the lower back and the rankings based on compressive force at L5/S1 as predicted by the biomechanical model?.

2. Method 2.1 . Subjects The subjects for this experiment were ten randomly selected, male volunteers in the age range 20-30 years, from the student population at Louisiana State University. They were asked whether they had any history of back problems: only subjects in sound health participated in the experiment.

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Anthropometric measurements were recorded for the subjects in accordance with the procedures outlined by Roebuck et al. (1975). Table 1 shows the mean and standard deviation of all anthropometric measurements for the ten subjects. Several strength measurements were recorded for the subjects following the methodology specified by Ayoub et al. (1978), and ChafFin (1975). These are shown in table 2. The methodology outlines details of body position, posture, joint angles, points of force application, etc. It includes description of the measurement of composite strength as follows: 'Composite strength involves simultaneous exertion by legs, arms and torso muscles in a semi-squat position with the short handle adjusted to the height of 15 in above the platform. The subject is to take position such that the handle is between legs but not supported by them. The subject is to exert an upward, vertical force by extending the knees and simultaneously extending the torso'. Each strength measurement was replicated three times for each subject and the average value was taken as a measufe of strength. A 10 min rest break was provided between different strength measurements with a 2 min break between replications.

Table 1. Summary of anthropometric measurements of the subjects, n = 10. Mean

SD

Age lye=) Height (an) M Yweight (kg) Acromial heigb t (cms) Standing iliac crest bt (cm) Knuckle height (cm) Knee height (em) Forearm grip distance (an) Chest width (cm) Chest depth (cm) Abdominal depth (cm) Chest circumference (cm) Abdominal circumference (cm) Foreann circumference (cm) Biceps circumference (an) Thigh circumference (an) Calf circumference (cm) Note: Anthropometric measurements were recorded in aaordance with the procec outlined by Roebuck et al. (1975).

Table 2. Summary of static strength measurements of the subjects (in Newtons), n = 10.

Arm strength Stooped back strength

Composite strength Leg strength Shoulder strength Grip strength

Mean

SD

288.50 44456 777-88 901.16 391-05 384.37

51.95 77.54 144.29 251.93 59.08 76-26

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2.2. Experiment An experiment was conducted to evaluate five lifting tasks with different heights'of lift. The evaluations were based on the subjective and biomechanical estimates of stress at the lower back. The independent variable of interest in the study was the lifting task. Lifting tasks with five different heights of lift were studied. Essentially, the difference between the tasks was the height through which the load was lifted, i-e., the origin and terminal points of lift. The heights through which the load was lifted did not overlap. Specifically, the tasks involved floor to knuckle, knuckle to elbow, elbow to shoulder, shoulder to reach, and reach to overreach lifting. Reach height was measured to the top of the cutout handles while the subjects stood with their feet flat on the floor as described by Mital and Aghazadeh (1987). Similarly, overreach height in this study was defined as the maximum reach height of the individuals measured to the top of the cut-out box handles while the subjects stood with their heels raised, also as described by Mital and Aghazadeh (1 987). The absolute distance through which a-subject lifted the box varied with the anthropometric measurements of that subject. The subjective estimate of stress developed at the back while performing the lifting tasks in the experiment was the response variable. A randomized complete block design was employed as an experimental design in this study. Different tasks were used as treatments and subjects were used as blocks. The load was located 38.1 crn (15 in) from the ankles in front of the subject. This distance was kept constant throughout the experiment. This distance has also been used by other researchers in their lifting studies (Chaffin et al. 1978). Lifting was performed with a free-style technique in the sagittal plane. Some subjects used a posture similar to the semi-squat and semi-stoop back posture suggested by Brown (1 972). Since biomechanical models are more appropriate for infrequent lifting, the subjects were asked to lift the given load in each task once for subjective estimation of the stress at the lower back. To ensure that the accuracy in ranking the tasks based on a subjective estimate of stress was not affected because the subjects lifted only once, the subjects were also required to lift the same load with a frequency of four lifts per min in a separate experimental session. This was done to check if the physiological fatigue in frequent lifting would change the ratings supposedly based on subjective perceptions of the stress at the lower back. The frequency of four lifts per min has been used by other researchers in their lifting studies (Mital and Aghazadeh 1987, Snook and Irvine 1968). The weight lifted by the subjects in the two experimental sessions (frequent and infrequent lifting) was their maximal acceptable weight of lift (MAWOL) determined prior to these sessions. The determination of MAWOL has been explained in the experimental procedure. The experiment was conducted in a constant environment. The temperature in the room was maintained at 2 1-22"C, and the relative humidity was 45-55%. All other activities were minimized.

2.3. Equipment The strength measurement equipment consisted of the Isometric Strength Test Unit (Model No. ST-1)built by Prototype Design and Fabrication Co. of Ann Arbor, Michigan. The equipment used to perform the lifting experiment consisted of a sturdy wooden frame with two platforms. The platforms could be adjusted according to the

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Figure 1. Illustralion of the experimental set-up.

height of the subject and the lifting task for the specific trial. The experimental set-up is illustrated in figure I . Weights wcre lifted in a wooden box 0-52m wide (frontal plane), 0-37m deep (sagittal plane), and 0.24m high. The handles were cut out on the sides of the box (0.05 m x 0.15 m opening, 0-04m below the top edge). The walls of the box were 0-01m thick, 2.4. Experimental procedure The experiment was conducted in three sessions. In session I, the MAWOL was determined for each subject using a psychophysical metl?od. Subjects were given written instructions to be followed during the determination of the MAWOL. The subjects were asked to lift from thc floor to the overreach pos~tionfor 25 min at the rate of four lifts per min. The subjects follo~redrecorded bell sounds that rang at a frcquency of four times per min. The subjects were asked to lift at each bell sound, Lowering was done by two other persons before thc next lift. During this 25 min period, the subjects were allowed to increase or decrease the we~ghtuntil they determined the weight that was acceptable to them for lifting in an 8 h shift. Thw, the subjects were asked to adjust and arrive at the weight they felt comfortable with, for an 8 h shift. This weight was designated as the MAWOL for that subject. The weights used in the experiment were metal pieces (chunks). The smallcst adjustment possible was 1/41b. The subjects were not encouraged or motivated to llft more weight in any way. The weight determined as the MAWOL was kept confidential. Breaks were not provided dur~ngthe psychophyslca! estimation of MAWOL, but subjects wcrc asked to assume that they were simulating an 8 h work shift rncluding breaks while performing the experiment. The MAWOL was then used for that part~cuiarsubject in the lifting tasks in sesslons I1 and TI1 (~nfrequentand frequent lifting). Use ofthe MAWOLensured that

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A. Waikar et al.

the subjects only lifted the loads they felt they could handle. Also, the biomechanical model (discussed later) was used to ensure that the compressive force while lifting the MAWOL for each subject for each lifting task was within the acceptable limits suggested by NIOSH (1981). This reduced the risk of overexertion injury during the experiment. In session.11, the subjects ranked the lifting tasks for infrequent lifting, specifically for a single lift. Thus, the subjects performed each lifting task only once. This was done to minimize the effects of muscle fatigue on the subjective estimate of stress at the lower back. The subjects lifted a load equal to their MAWOL determined in session I. They used a freestyle technique, and lifted in the sagittal plane. A 2 min rest interval was provided between consecutive lifting trials. Tasks were performed in a randomized order. After each lifting task they were asked to estimate how stressful the task felt at the lower back. A scale of 0-lOO was used for the subjective estimate of stress. This was a modified Borg scale (Borg and Dahlstrom 1960)with zero indicating 'no discomfort' and I00indicating a 'very painful' feeling in the lower back. Borg and Dahlstrom (1960) have reported close accord between measures of effort and statements of perceived difficulty. The subjects placed the tasks in order of increasing difficulty based on the subjective estimate of stress. They were allowed to record the current task at the top, bottom or in between two tasks in the list depending upon the value of the subjective estimate of stress. The subjects were instructed to carefully rank a task based on the subjective estimate of stress developed at the lower back alone. The most stressful task was to be given a rank of 5 and the least stressful task a rank of 1. In session 111, the subjects estimated the stress and ranked the lifting tasks for frequent lifting based on a subjective estimate of stress at the lower back. As with session 11, the subjects lifted their corresponding MAWOL with a freestyle technique in the sagittal plane. Frequency of lift was four lifts per min. The subjects performed each lifting task for lomin, and then estimated the sttess on the lower back on a scale of 0-1 00. Again, the tasks were performed in a randomized order. A 1 h rest break was provided between trials for different tasks to minimize the e k t s of fatigue. As with session 11, the most stressful task was ranked 5 and the least stressful task 1.

2.5. Biomechanical estimation For a biomechanical evaluation of the lifting tasks, the compressive force developed at the L5/S1 disc was estimated using a computer program for Chaffin et al.'s (1983) computerized sagittal plane static lifting model. Even though the tasks were dynamic, a static model was used because unless the lifting acceleration is very high, the static model gives reasonable estimates of the compressive forces on L5/S 1. Furthermore, dynamic models are seldom used in industry even for job design involving dynamic tasks because there is no practical and highly reliable dynamic biomechanical model available. Freivalds et al. (1984) reported an average correlation between the load and the corresponding peak compressive force estimated using the dynamic biomechanical model of r =0-7.Also, an analysis using a dynamic model involves complex calculations (e.g., Fourier transforms) and needs expensive sophisticated equipment such as a high speed motion data analyser and collection system (Freivalds et a!. 1984). Input to the model used in this study included link lengths, link angles in each lifting task and the load lifted by the subject (MAWOL). Compressive forces at the U / S 1 disc were estimated, at the beginning of the lift, at the end of the lift, and in two discrete intermediate equidistant positions. The maximum value among the estimated

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compressive force values was used for ranking the tasks based on this criterion. In order to determine the angles between different links of the subject's body, photographs were taken for each subject at the positions described above. A camera was maintained at a fixed distance of 3 m from the subject. The background for the picture was a grid of 10x lOcm squares. This helped estimation of the angles between body links and the horizontal for the biomechanical analysis.

3. Results Data was analysed using the Statistical Analysis System (SAS 1979). The mean and standard deviation of the subjective estimates of stress at the lower back for the lifting tasks for a singe lift and for lifting at frequency of four lifts per min are given in table 3. The values were very close in the two cases. In addition, the relative ranking of the tasks was identical in the cases of ftequent and infrequent lifting. Table 4 shows the values of the average rankings for the lifting tasks. Table 5 gives the mean and standard deviation for the compressive forces at the L5/S 1 disc estimated by the model for all subjects for each lifting task. Relative ranks of the tasks based on the estimated resulting compressive force on L5/S 1 are also shown in table 5. The floor to k'nuckle lifting task was ranked as the most stressful task (biomechanical rank 5 ) based on biornechanical criteria. However, this was subjectively judged to be relatively low in stress (average subjective rank 2.3). Even though the elbow to shoulder and shoulder to reach tasks were ranked 2 and 3 respectively based on biornechanical criterion, the maximum compressive forces predicted were very close for these two tasks as seen in table 5. The biomechanical ranking of the tasks for individual subjects had the same order as the average biomechanical ranking seen in table 5. Table 3. Subjective estimate of stress at the lower back using 0-100 scale (100 most stressful), n=10.

Infrequent lifting (single lift) Task Floor to knuckle Knuckle to elbow Elbow to shoulder Shoulder to reach Reach to overreach

Frequent lifting (4 liftslmin)

Mean

SD

Mean

51.0

21 -83 13-37 15-49 13.33 22-01

56.0 3 7.0 59.0 72.0 94-0

37.0 58.0 800 92-0

SD 29.5 1 20.57 17.9 1

12.29 10-75

Table 4. Average rankings of the lifting tasks based on subjective estimate of stress at the lower back.

Task Floor to knuckle Knuckle to elbow Elbow to shoulder Shoulder to reach Reach to overreach

Infrequent lifting (single lift)

Frequent lifting (4 lifts/min)

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A. Woikar et al.

Table 5. Estimated maximum compressive force at L5/S1, and ranks based on biomechanical criterion. Task Floor to knuckle Knuckle to elbow Elbow to shoulder Shoulder to reach Reach to overreach

Rank

Mean*

SD

3899.53

64302

5

227070

532.16 336.34 34412

4

1150.79 1159-65 958.47

265.61

2 3 1

* Compressive force is in Newtons, n = 10. Sublectlve Estcrnata of Stress

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Bslornechon!cal Estimate of S t r a s s LEGEND :

F - K : F l w r to Knuckle E - S . E l bow to Shoulder R - OR

I .K

:

ii-E

K - E - Knuckle t o E l b o w S - R : Shoulder to Reach R e a c h ro Overreach

L-S

5,-R

"-OR

L I F T I N G TASK

Figure 2. Lifting tasks vs. maximum compressive forces on L5/S1, and subjective estimate of stress at thc lower back for infrequent lifting.

Figurc 2 shows the mean subjective estimate of stress at the lower back and the estimated maximum compressive force at L5/Sl disc for the lifting tasks for infrequent lifting (single lift). The tasks involving overreach and reach heights were rated more dificult than thc tasks involving knuckle and shoulder heights. Thc least difficult task, knuckle to elhow, was subjectively estimated to be approximately 60% less difficult than the reach to overreach task, The reach to overreach task was perceived as the most dificult task according to the experimental data. The subjects may have rated the reach to ovcrreach task as the most difficult because of the fear of losing balance while lifting a load at that height with the hcels raised. In spite of instructions, they may not have judged the difficulty oltask from thesubjective estimate ofstressat the lower back. The knuckle to elbow task may have becn ratcd as the least stressful because in this task, the upper body does not bend as much and provides a better sense of stability. From the maximum compressive forces at the L5/S I disc, i t can be seen that the task floor to knuckle is associated with highest compressive force. The task reach to overreach has the lowest compressive force associated with it. The compressive forces developed were approximately 75% higher for the floor to knuckle lifting task than the

Stress at the lower back Subjective E s t i m a t e of Stress

5

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LEGEND

:

Biomechonicol Estimote of Stress K - E : Knuckle ? o E l b o w F - K : Floor to Knuckle E - S : E l bow to Shoulder S - A . Shoulder to Reoch A - OR Reach t o Overreach

K-E

f-K

L-S

5

-

H-OR

LIFTING TASK

Figure 3. Average rankings of lifting tasks based on subjective estimates of stress at the lower back, and on biomechanical estimates of compressive force at LS/S1 for infrequent lifting.

Subjective Estimate o f Stress Based on 4 Lifn/rnin. Subjective Est~moteof Stress Based on Slngle L i f t LEGEND: F- K : Floor to Knuckle K - E : Knuckle to Elbow E- S : Elbow to Shoulder S - A : Shoulder l o Reach R-OR : Reoch t o Overreach

F-K

K-E

E-S

S-R

A-OR

L I F T I N G TASK

Figure 4. Subjective estimates of stress at the lower back for lifting tasks in frequent and infrequent lifting.

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reach to overreach task. Maximum compressive force is a function of the moment arm for the load handled by the subject. The moment arm distance is maximum for the floor to knuckle task in comparison with the other tasks considered in the study. The two values of the maximum compressive force were very close for the elbow to shoulder and shoulder to reach tasks. This is because the values of the link angles and the variation in link angles as the tasks are being performed, are very similar in these two tasks. The average ranking based on the subjective estimates of stress in infrequent lifting and the ranking based on biomechanical estimates of stress for the lifting tasks are plotted in figure 3. The tasks with the higher rank were judged to be more difficult. Figure 3 thus shows that the tasks that were in fact stressful from a biomechanical point of view were not subjectively perceived as stressful to the same relative extent. The differences are strikingly apparent in the cases of the floor t o knuckle and reach to overreach lifting tasks. Figure 4 shows the subjective estimates of stress at the lower back for frequent and infrequent lifting. It shows that the values of the subjective estimate of stress in the two cases were very close. It appears that for the duration of the task and for the frequency used, muscle fatigue did not affect the subjective evaluation of the lifting tasks for difficulty. A pairwise - t-test (a =0.05) revealed no significant difference between the subjective estimates of stresses in frequent and infrequent lifting. This was also true for data for each individual subject. The results of the analysis of variance (fixed effect model) performed on the data for the subjective estimate of stress showed that there was a significant difference between the lifting tasks (F(4,45)= 15.83, p

Evaluating lifting tasks using subjective and biomechanical estimates of stress at the lower back.

The objective of this study was to evaluate five different lifting tasks based on subjective and biomechanical estimates of stress at the lower back. ...
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