Anthropometric, muscle strength, and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting.

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Anthropometric, muscle strength, and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting a

H. R. STÅLHAMMAR & V. LOUHEVAARA

a

a

Department of Physiology , Institute of Occupational Health , Topeliuksenkatu 41 a A, Helsinki, SF-00250, Finland Published online: 31 May 2007.

To cite this article: H. R. STÅLHAMMAR & V. LOUHEVAARA (1992) Anthropometric, muscle strength, and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting, Ergonomics, 35:9, 1033-1044, DOI: 10.1080/00140139208967380 To link to this article: http://dx.doi.org/10.1080/00140139208967380

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ERGONOMICS,

I 992, VOL. 3 5 , NO. 9, 1033- 1 044

Anthropometric, muscle strength, and spinal mobility characteristics as predictors in the rating of acceptable loads in parcel sorting H. R. STALHAMMAR and V. L ~ U H E V M ~ U

Downloaded by [The University of Manchester Library] at 09:39 20 October 2014

Department of Physiology, Institute of Occupational Health, Topeliuksenkatu 4 1 a SF-00250, Helsinki, Finland Keywords:

A,

Musculoskeletal system; Psychophysics; Muscle contraction; Range of motion; Weight-lifting; Manual materials handling.

The rating of acceptable load (RAL) attained with a standard test ( R A b , ) and a work-simulating test (RAL,) for postal parcel sorting was related to

anthropometric, muscle strength, and spinal mobility characteristics of 1 8 male sorters. The subjects comprised a subsample of 103 experienced male soners who carried out the FUL tests at postal sorting centres. The dynamic hand-grip endurance correlated significantly (p=0.036) to the R A k , results. Correspondingly, there was a significant correlation (p=O-044) between the ' ratio of maximal isometric strength of trunk extension to body weight and the RAL,,,. The dynamic hand-grip endurance predicted 26% of the variation in the R A L , ; in the RALw the maximal isometric strength of trunk flexion to body weight ratio predicted 24%. The subjects who rated heavier weights for R A b , tended to have a better trunk mobility. The dynamic endurance of hand-grip muscles, trunk strength, and spinal flexibility seemed to be the most powerful predictors for the psychophysically assessed 'acceptable loads' in experienced workers performing manual materials handling tasks. 1. Introduction The assessments of the load which can be manually handled safely are based on estimations of biomechanical (Schultz 1982, Leskinen et al. 1983, Chaffin 1988) and physiological (Jdrgensen 1 985, Genaidy and Asfour 1 987) stress factors. These approaches are not suitable for evaluating different manual materials handling (MMH)tasks, for screening individuals' musculoskeletal capacity, nor for assessing 'acceptable' loads simply and quickly at the worksites in order to prevent musculoskeletal disorders. Griffin et a/. (1 984) demonstrated the repeatability of a quick and simple psychophysical assessment of acceptable weights for dynamic lifting: the rating of acceptable load (RAL). Foreman et al. (1984) showed that the RAL test was both repeatable and consistent. This conclusion was drawn from the experiments where the RAL test was repeated four times at intervals of five days, and daily over a period of two weeks. Later Baxter er al. (1986) emphasized that the procedure and the instructions to the subjects must be carefully worded and standardized. Griffin er al. ( 1984) found that subjects with previous back pain chose loads that weighed less on average (4.1 kg for men and 2.2 kg for women) than subjects without previous back pain. Troup et al. ( 1 987) demonstrated with 289 1 individuals that the average weights chosen by men with frequent symptoms of back disorders were 88Oh of the weights of those who reported no symptoms. The corresponding ratio for women was 79%. The psychophysical tests of acceptable load have proven to be equally sensitive both to the prevalence of back pain and maximal isometric strength 00 1 4-0 1 39/92 53.00 8 1 992 Taylor & Francis Ltd.


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H . R. Sthlhammar and V. Louhevaara

(Troup el al. 1987). In ergonomic evaluations the RAL tests were well suited for testing the effects of changes in task variables (StAlhammar el a!. 1989). A minimum requirement for safe lifting is the ability to exert the required force. Strength assessments have been generally used in determining individual risk of injury to a person involved in lifting activities. Also anthropometric attributes are important, when one is considering the mechanical stress of the body in terms of dimensions of the levers and mobility of the joints. The purpose of the present study was to examine thc relation of the anthropometric, muscle strength and spinal mobility characteristics to two RAL tests: the standard RAL test (RAk,) and the work-simulated RAL test ( R A L ) .

2.1 . Subjects

2. Methods

The subjects were 18 healthy male sorters of postal parcels (table 1). This wellmotivated group volunteered for the study and was a subsample from 103 experienced sorters working at five postal sorting centres (St3lhammar et a/. 199 1 ) . 2.2. Testsfor rhe rating ofacceptable loads (RAL) The standard RAL test (RAL,) (Griffin et al. 1984), employed a box (30 x 30 x 30 cm) with handles, located 20 cm above the base. The task was to assess the load for the box which a subject individually felt to be acceptable for lifting between a table (height 72 cm) and floor at 5 min intervals for an 8 h working day. The work-simulated RAL test (RAL) was administered for the simulation of postal parcel sorting. The box for the RAL, was also 30 x 30 x 30cm in size, but without handles. In this test the researcher changed the weights in the box at the subject's bidding, until the subject felt that the load was acceptable for lifting and transfening from the table to a parcel container ( 1 25 x 105 x 167 cm) and back to the table at a rate of 4-6 boxes min-I for an 8-hour working day. In the R A L , test the contents of the box was invisible to the subjects, contrary to the RAL,. A random selection of unlabelled bags of lead shot weighing between 0-5-2-5 kg were used in both tests. The RAL, and RAL, tests were conducted at five postal sorting centres during habitual day and evening work shifts. The detailed instructions for the tests have been reported by StAIhammar et a/.(1991).

2.3. Laboratory measurements 2.3.1. Anlhropomelric measurements: The height and weight of the subjects, clad in shorts and without shoes, were measured in the laboratory and used for the determination of the body mass index (BMI -weight/height2) (Keys et al. 1972, Heliovaara and Aromaa 1980). At the worksite, the weight of the subjects, in working clothes, was also measured before the RAL tests. The girths of the right body segments (upper arm and forearm, thigh and calf) were measured with a flexible tape. The girth of the upper arm was measured at the midpoint between the acromion and olecranon with relaxed muscle bellies. The girths of the forearm and thigh were the butt measures, e.g., about 5 cm to the distal direction from the elbow joint axis and from the hip joint axis. The girth of the calf was measured precisely on the calf muscle belly. The length of the spine was the distance measured from L5IS1 disc level to the C7

Rating acceptable loads

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in upright position, and the length of the legs was the distance from the right hip to the floor. The arms measure was the distance between the right and left finger tips when the subject was leaning against the wall. The four skinfolds (triceps, biceps brachii, subscapular, suprailiac) were measured from the right side with a skinfold caliper (John Bull). The sum of skinfolds was calculated from these four measures. The percent of body fat was calculated applying the equations of Durnin and Rahaman (1967).

,

2.3.2. Muscle strength and endurance measurements: The isometric maximal voluntary contraction (MVC)was measured for both hand-grip muscles with a waterfilled rubber tube with a built-in amplifier and pressure transducer (Honeywell Microswitch-division 142PC30G) connected to an indicator and a power supply (Smolander el al. 1 984). The hand-grip contractions were done in a sitting position, and the contracting arm was extended to the side at a 10" angle to the vertical (Lou hevaara et al. 1 990a). The static muscle endurance was assessed by recording the time for holding the muscle contraction at 50°h of the subject's isometric M V C of the right hand. The subjects could visually control their force level by observing the pressure indicator. The time was measured until breaking point, when the subjects could no longer maintain the 50% force level after being verbally reminded once. The dynamic muscle endurance was determined by measuring the time for the repetitive muscle contractions (50 timeslmin) at the 50% level of the subject's isometric MVC of the left hand. The force control was arranged as in static endurance measurements, and the frequency was controlled with a metronome. The isometric MVC of the trunk flexors and extensors was measured by a calibrated dynamometer (Asmussen et a!. 1959) which consists of a frame with a strain gauge transducer (Philips Strain Gauge). Trunk flexor strength was measured while the subject stood upright with his back against the dynamometer. For trunk extension, the test trials were repeated in the same way with the subject standing facing the dynamometer. The subjects were allowed a minimum of two trials of each strength and endurance test. The time interval between the maximal strength tests was at least 30 s. The isometric and dynamic endurance times were assessed twice on separate days. The guidelines of Chafin (1 975) were applied to both the strength and endurance measurements. The trunk strength measurements have been described in detail by Nygdrd et al. (1987). 2.3.3. Measurements of forward and lateral trunk bending: Forward bending and lateral bending (right and left) in a standing position were measured with an anthropometer. The subjects stood on a 19cm high platform so that they could bend over the edge of the standing level. 0-level was the distance from the finger tips to the floor, when the subject was standing upright. The range of motion (mobility) is given in centimeters from the @level to the reach point, when the subject was bending. The lateral bending was performed in one plane (frontally) without any forward bending. The muscle strength, endurance, and trunk bending measurements were carried out in the laboratory before the subjects participated in the study on simulated sorting of postal parcels and in the assessments of cardiorespiratory work capacity (Louhevaara et al. 1988). 2.3.4. Statistics: The results were analysed using conventional stat istical methods.

H. R. Stcilhammar and V. Louhevaara


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The relationships between the variables of the study were analysed with the correlation analysis. A multivariate analysis was used for describing the relations between the dependent variables-RAbt and R A k a n d the independent variables characterizing the subjects of the study. For the correlation and regression analyses, the number of subjects was limited to 17 because the results of one subject's endurance tests were missing. The differences between the two groups were also compared-Low RAL:under median and High RAL: above median-according t o the results of the R A L , and R A L , test carried out with 103 sorters (Stillhammar et a!. 1991). Their median value was 15.8 kg for R A k , and 9.0 kg for R A L . The statistical differences between the two groups were tested with the Wilcoxon Rank Sum test, because the data on the dependent variables were skewed to the right, and for some variables the sample distribution did not seem to be normal.

Table 1. Means, standard deviations, range of the anthropometric, muscle strength, spinal mobility characteristics, and RAL tests of the 18 subjects. Mean A nrhropometry Age (a)

Height (cm) Weight (kg) Weight with clothes on (kg) BMI (kg/m2) Sum of skinfolds (mm) Body fat (%) Back length (cm) Leg length (cm) Arms (cm)

Circumfprences Upper arm (crn) Forearm (cm) Thigh (cm) Calf (cm) Isometric slrengrh Hand grip/right (kPa) Hand grip/left (kPa) Trunk extension (N) Trunk extension ratio (Nlkg) Trunk flexion (N) Trunk flexion ratio (N/kg) Static hand-grip endurance (s) Dynamic hand-grip endurance (s)

Trunk bending Forward bending (cm) Right lateral bending (cm) Left lateral bending (cm)

SD

Range


Rating acceptable loads 3. Results The mean values for the anthropometric, muscle strength, spinal mobility characteristics and RAL tests of the subjects are given in table 1. Tables 2 and 3 summarize the comparable results of the measurements in the high and low groups for R A k , and R A L , respectively. A statistically significant difference was found in the circumference of the forearm, which was bigger for the high R A L , than for the low R A L , (p-0-041). There was a tendency for those who had better trunk mobility (forward bending, p=O- 124, lateral bending right, p=0.077, and left, p=0-077) to rate heavier weights for RAL,. No statistically significant differences were found for the R A L . The relations between the R A k , and R A L , and the measured individual characteristics are given in table 4. The dynamic hand-grip endurance correlated significantly (p=0-036) to the R A L , results, and the isometric trunk extension strength to body weight ratio (p=0-044) to the R A L , correspondingly. The RAL, was significantly correlated to the R A L , (r= 0.87, p= 0-0001). Table 5 gives the best predicting equations for the R A L , and the R A L . The most powerful single predictor for the variation in the R A L , was the dynamic endurance of the hand-grip muscles (R2=0-26),and in the RAL, the isometric trunk flexion strength to body weight ratio (R2=O.24), respectively.

4. Discussion The subjects were healthy and well motivated in this -study. According to the cardiorespiratory tests, they were in moderate physical condition in their respective age groups (Louhevaara et al. 1990b). The fact that the subjects were quite homogeneous probably decreased the variance of the present functional capacity measures. As the subject sample was small, the results must be regarded as tentative. The anthropometric characteristics appeared to have a minor effect on the rating of acceptable loads. There were practically no statistically significant differences between the two subgroups (low and high) both in the R A k , and RAL, tests in ant hropometry and circumference measures. Only the circumference of the forearm was bigger for the high R A k , than for the low R A L , (p=O.O4 1). A similar weak association of the ant hropometric characteristics with low back pain was earlier found by Pope et al. ( 1 985) and Troup et al. (1 987). When spinal mobility was considered a trend was apparent, in that those who had a more flexible upper trunk chose heavier weights in the R A L , . In the RAL, test there was no association between spinal mobility and the 'acceptable weight of the parcels'. However, the average acceptable weight of the parcel (9.7 kg) was significantly lighter than that in the RAL,. In a prospective survey of 289 1 individuals, Troup et al. (1 987) noted that the three aggravating factors which most significantly associated with poor psychophysical test results were walking, bending forward, and dressing-all requiring mobility of the body. The general assumption that greater spinal mobility is associated with improved back health and less back complaints is still poorly understood, because the results of the determinants for low back pain are contradictory (Biering-Sdrensen i 984, Troup et al. 1987, Battie er al. 1 990). The results of the present study are in line with these contradictory findings, because in the R A b , test an association was observed between a mobile back and heavier weights, but not in the R A L , test. Maximal isometric trunk flexion and extension strength of the subjects were 1 I %

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H . R. Sthihammar and V. Louhevaara

Table 2. Means and standard deviations for the anthropometric, muscle strength, spinal mobility characteristics of 18 sorters assigned into two groups according to the median (RAb-, 1 5.8 kg) of 103 ,sorters of postal parcels (StAlhammar et al. 1991): under median Law RAL (n- 1 1) and above median High RAL (n = 7). RAL,

Low


Mean

SD

High Mean SD

P(z)

A nrhropornerry Age (a) Height (cm) Weight (kg) Weight with clothes on (kg) BMI (kg/m2) Sum of skinfolds (mm) Body fat (%) Back length (cm) Leg length (cm) Arms (cm)

Circumferences Upper arm ( c m ) Forearm (cm) Thigh (cm) Calf (cm) Isomerric slrength Hand griplright (kPa) Hand griplleft (kPa) Trunk extension (N) Trunk extension ratio (N/kg) Trunk flexion (N) Trunk flexion ratio (Nlkg) Static hand-grip endurance (s) Dynamic hand-grip endurance (s) Trunk bending Forward bending (cm) Right lateral bending (cm) Left lateral bending (cm) R A b , (kg) RALw (kg)

and 38% lower, respectively, than those found in a cohort of men of the same age (Viitasalo et a/. 1985) and 2-3% lower than for ageing men (mean 52 years) engaged in physical work (NygArd et al. 1987). Moreover, many of the subjicts involved in the latter study had acute back symptoms, but they were at work during the tests. As compared to the earlier findings of Viitasalo ef al. ( 1 985) and NygArd et a/.(1 987), the isometric capacity of the trunk muscles of the subjects can be classified as moderate. Asymmetric loading of the back muscles during twisting and turning movements decreases the maximum isometric strength during exertions (Warwick et al. 1980, Garg and Badger 1986) as well as the maximum acceptable weights assessed by the psychophysical methods (Ljungberg et al. 1982, Garg and Badger 1986). Warwick


1039

Table 3. Means and standard deviations for the anthropometric, muscle strength, spinal mobility characteristics of 18 sorters assigned into two groups according to the median ( R A L = 9 . 0 kg) of 103 sorters of postal parcels (Stahammar et al. 1991): under median Low RAL (n= 11) and above median High RAL (n=7). -

RAL High

Low


Mean

SD

Mean

SD

P(z)

Anthropomerry Age (a) Height (cm)

Weight (kg) Weight with clothes on (kg) BMI (kg/m2) Sum of skinfolds (mm) Body fat (%) Back length (cm) Leg length (cm) Arms (cm)

Circurnferences Upper a m (cm) Forearm (cm) Thigh (cm)

Calf (cm) Isome~ric strength Hand griplright (kPa) Hand gripfleft (kPa) Trunk extension (N)

Trunk extension ratio (Nlkg) Trunk flexion (N) Trunk flexion ratio (N/kg) Static hand-grip endurance (s) Dynamic hand-grip endurance (s) Trunk bending Forward bending (cm) Right lateral bending (cm) Left lateral bending (cm)

(1 980) reported a decrease of 38Oh in isometric lifting strength when the subject was rotated 90" or 135" from the sagittal plane at shoulder-height placement of the workpiece, and a decrease of 54% in 90" rotation at knee height, respectively. In their psychophysical experiments of horizontal lifting, Ljungberg et al. ( 1 982) reported that the subjects chose acceptable loads only half as heavy as those reported by Snook et al. (1970) for vertical lifting. Considering the reduction of maximum isometric strength during asymmetric exertions, and the results of Louhevaara et bl. ( 1 990a) who stated that about one-third of the parcels in an actual work situation were lifted up or down above shoulder height or below knuckle height with a twisted back, the sorters must have a good musculoskeletal capacity to avoid overloading the spine.

H. R. Stdlhammar and V. Louhevaara

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Table 4. The strength of the relation between the RALs (RAL, R A L ) and the anthropometric, muscle strength and spinal mobility variables. Correlation coefficients ( r ) and statistical significances (p) are given for the relation (n = 17).

A n~hropometty


'

Height Weight Weight with clothes on

BMI Sum of skinfolds Body fat Back length Leg length Arms Circumferences Upper arm

Forearm Thigh Calf Isometric strength Hand griplright Hand grip/left Trunk extension Trunk extension ratio Trunk flexion Trunk flexion ratio Static hand-grip endurance Dynamic hand-grip endurance Trunk bending Forward bending Right lateral bending Left lateral bending RA5,

Table 5. The best predicting equations for the acceptable loads (RAb, and R A h ) . Standard errors are given in parenthesis. -

Prediction equation RAk, -- 23-0 (2 1.3)+ 0.94 (0-39)(LLATBEN)+ 0.08 (0.02) (DYNEND) -4.45 (1 -89) (BACKST) RAL,= 30.89 (9-44)+0-02(0-01 ) (DYNEND)1-98(0.90) (BACKST) -0-71 (0.51) (ABDST)

LLATBEN = left lateral bending DYNEND = dynamic hand-grip endurance BACKST =trunk extensor strengtldweight ratio ABDST =trunk flexor strength/weight ratio

R2

F

P

0.63

7.21

0.004

0.44

3-43

0.049


Rating acceprable loads

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The R A k , test results correlated highly to the RAL, (r=0.87), but the results differed significantly, reflecting sensitivity to the differences of the tasks. There is no discrepancy between the average weight ofthe parcel (5 kg) habitually handled in sorting centres and the sorters' rating ofacceptable load (9-5kg) as to the handled weight as a risk factor(StAlhammar et al. 199 I ). This isdue to the fact that, besides theacceptable load in parcel sorting, there are many uncontrolled risk factors which can be more hazardous than pure weight, such as body configuration, muscle contraction type, and direction and level of force exertion during asymmetric lifting tasks. Kumar (1988) reported that the increased reach of the task required 11-41% more energy. Louhevaara er .al. ( 1 990a) stated that during sorting work the back of the worker was bent or bent and twisted 26Oh of the time during actual sorting (72% of the average shift length 39 1 min) and arms elevated below or above shoulder level 47% of the time. The fact that the arms were elevated for half of the sorting time indicates that parcel sorting stresses partly the back, bur especially the shoulder region which is exposed to prolonged, static, postural load during sorting periods, even though the handled weights are 'acceptable'. Regression analyses, using R A k , and R A L , test results as the dependent variables, and anthropometric, muscle strength, and mobility data as independent variables, revealed the insu ficiency of a single multivariate predictor. Since the dynamic hand-grip endurance was predicted 26% of the variation in the R A k , and in the R A L , the isometric trunk flexion strength to body weight ratio predicted 24%. The predictors in the equations were limited to three, due to the small subject sample. Since the strength variables were not intercorrelated, the 'strength score' could not be used to represent all strength data as suggested by Jiang and Ayoub (1987). In this study the strength variables were both static and dynamic, and the predictors in the equations also included ratios for strength and body weight. The strengthlbody weight ratios are relevant for describing lifting ability, but the use of these ratio variables in the correlations and predictive equations must be considered with care. Lateral bending, dynamic hand-grip endurance, and isometric trunk flexion strength to body weight ratio explained 63% of the variance in RAL,. The equations for the RAL, included dynamic hand-grip endurance, isometric trunk flexion and extension strength to body weight ratios which explained 44% of the variance in RAL,. In spite of the differences between the R A k , and RAL, tests, the hand-grip endurance and trunk strength appear to be good predictors of the acceptable weight of the load in both RAL tests. Previously, Yates et al. (1980) have suggested that the arm, shoulder, and back muscle strength have a significant role as a limiting factor in lifting. The results of this study confirm the earlier findings of Mital et a!. (1 986a) who pointed out that repetitive, endurance type of dynamic strength of individuals is a better predictor of psychophysically acceptable weight than either the maximal dynamic strength or maximal isometric strength. This means that physical responses which assess an employee's dynamic endurance factors at work can provide more reliable assessments of lifting capacity-especially with lifting frequencies higher than 1.5 lifts per minute (Mital et a!. 1987). Also the observations of this study are i n agreement with earlier data on the variable combination of general predictive models of lifting capacity (Pytel and Kamon 198 1, Kamon el al. 1982, Aghazadeh and Ayoub 1985, Mital et al. 1986b) preferring the use of dynamic strength and endurance variables as independent factors. Future studies should also consider the

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H. R. Stdlhammar and V. Louhevaara


possibility of specific psychological variables to improve the predictive power of equations. 5. Conclusions The study showed that the dynamic endurance of hand muscles and the maximal strength of trunk muscles are relevant factors in the assessment of musculoskeletal load and lifting ability. The results suggest that the dynamic, submaximal workrelated procedures are more relevant and feasible than isometric testing for the prediction of the capability of individuals in manual materials handing. The dynamic tests are superior to isometric procedures with regard to safety, simplicity, and the time spent on testing.

Acknowledgements This study was supported by the Central Post and Telecommunications Office, the Labour Union for Postal Workers, and the Ministry of Finance (Finland). The authors especially wish to acknowledge the indispensable help of Mr Pentti Kokko and Mr Ossi Pesonen in the practical arrangements of the study. The generous help and co-operation of the personnel at the postal sorting centres is also gratefully acknowledged.

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Received 19 June 1991. Revise accepted 2 1 August 199 1 .

Physiological, anthropometric, strength, and muscle power characteristics correlates with running performance in young runners.

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