upper body push-pull strength of normal young adults in sagittal plane at three heights

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E L S E V I E R International Journal of Industrial Ergonomics 15 (1995) 427-436

Industrial Ergonomics

Upper body push-pull strength of normal young adults in sagittal plane at three heights

Shrawan Kumar Department of Physical Therapy, University of Alberta, Edmonton, Canada T6G 2G4

Received March 23, 1994; accepted in revised form June 24, 1994

Abstract

Twenty young adults (ten males - mean age = 21.1 years; ten females - mean age = 21.1 years) were tested for their two-handed push-pull strength in sagittal plane at heights of 35 cm (low), 100 cm (medium) and 150 cm (high) in isometric and isokinetic modes. The lower extremities of the subjects were stabilized in a custom-designed device at hip, knees and ankle. The twelve experimental conditions (2 activities - push and pull x 3 heights x 2 modes) were randomized. The push-pull strengths were measured using a modified Static Dynamic Strength Tester with a SM 500 load cell. The analogue data were sampled and collected at 50 Hz through a Metrabyte DAS 20 in an IBM XT. Males as well as females were strongest in pulling at medium height in isometric mode. The isometric pushing strengths ranged between 41% to 68%, and 27% to 44% for males and females respectively when normalized against mean pulling strength of males at medium height. The isokinetic strengths were invariably significantly lower than isometric strength (p < 0.01).

Relevance to industry

The results indicate that appropriately job-simulated and gender-adjusted isokinetic strength will be a better design criterion compared to maximal strength measured in an optimal exertion posture. The results obtained here may also be of use in design of wheelchairs.

Keywords: Push-pull; Strength; Design criteria

I. Introduct ion

Manual materials handl ing is a c o m m o n industrial activity. Nearly half of all manual materials handl ing activities consist of pushing and pulling (Bari l-Gingras and Lortie, 1990). The subjects tend to minimize lifting and lowering in favour of pushing and pulling. Such activities are routinely pe r fo rmed in shipping and receiving, moving, warehousing, farming and agriculture, policing,

fire fighting, depar tmen t stores, supermarkets and others. As much as 20% of all overexert ion injuries were ascribed to push-pull activities by N I O S H (1981). This could be extrapolated to represent 5% of all compensable occupat ional injuries. In recent years the magni tude of overex- er t ion injuries has continually increased being 48% of all compensa ted work injuries in Canada (Statistics Canada , 1991). Though an exact magnitude contr ibut ion of push-pull activities in the

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428 S. Kumar / International Journal of lndustrial Ergonomics 15 (1995) 427-436

rise of overexertion injuries is not clear, it can be logically interpreted to have risen from before. Despite such common practice of push-pull activities the data are still scanty in this area.

Most of the studies reported in this field have been done in isometric mode (Kroemer and Robinson, 1971; Laubach et al., 1972; Martin and Chaffin, 1972; Ayoub and McDaniel, 1974; Kroe- met, 1974; Davis and Stubbs, 1978; Chaffin et al., 1983). In these isometric studies the effect of variables such as body weight, height of force application, distance between body and point of force application, coefficient of friction between floor and footware, support for body members, frequency of exertion and volitional postures were studied. Kroemer and Robinson (1971) and Kroe- mer (1974) studied horizontal push-pull force exertion when standing in working positions on various surfaces. Their studies were performed in standing posture when the subjects had either braced themselves against a vertical wall, or had anchored their feet at a rigid footrest on the floor, or stood on various surfaces with varying degrees of coefficient of friction. They reported a 50% increase in push-pull force due to an increase in coefficient of friction from 0.3 to 0.6. Martin and Chaffin (1972), Ayoub and McDaniel (1974), and Chaffin et al. (1983) found that the height at which push-pull forces were applied was the most important variable in affecting the force output. Chaffin et al. (1983) studied volitional postures during maximal push and pull exertions in sagittal plane at 67 cm, 109 cm and 152 cm heights. They concluded that the foot placement,

handle height and body postures all affected the push-pull strength. Davis and Stubbs (1978), using magnitude of the intra-abdominal pressure as the criterion, recommended safe levels of one- and two-handed push-pull exertions in symmetrical and asymmetrical planes.

Snook (1978) reported maximal acceptance load for push and pull activities determined by psychophysical methodology. He had his subjects push or pull against an instrumented handle while they walked on a treadmill. This study simulated pushing-pulling a cart with variable resistance at a variable height.

The dynamic studies on push-pull strengths have generally been restricted to one-handed effort (Garg et al., 1988; Garg and Belter, 1990; Mital and Genaidy, 1989; Mital and Faard, 1990; Imrhan and Ayoub, 1990; Imrhan and Rama- krishnan, 1992). The study by Garg et al. (1988) was performed with a special reference to lawn- mowers. They reported that peak and average dynamic pulling strengths were 55% and 34% of the static strengths. They also found that the dynamic strength was strongly correlated with the peak velocity. Garg and Beller (1990) reported a progressive decrease in strength capability with increase in the height of effort application. They also confirmed the finding of Kumar et al. (1988) that there was an inverse relationship between the velocity of exertion and the strength generated. The effect of the height of the pull, the direction of the pull, the body posture and reach distance was reported by Mital and Faard (1990).

Two-handed dynamic push-pull activities were

Table 1 The anthropometric details of the experimental sample

Gender Parameter Age Height Weigh t Knuckle Knee Hip Shou lde r Horizontal yrs cm kg height height h e i g h t height reach

cm cm cm cm cm

Mean 21.1 173.0 71.2 77.0 53.1 92.3 143.0 64.2 Males Std. Dev. 2.5 6.4 9.4 4.2 2.8 3.2 6.1 2.3

Max 25.0 179.0 82.6 80.2 58.0 98.5 149.0 68.3 Min 18.0 157.0 48.7 66.4 48.4 87.0 127.8 57.0 Mean 21.1 163.8 61.3 74.0 49.3 85.1 135.3 58.8

Females Std. Dev. 2.5 5.1 6.0 3.8 2.1 2.2 5.0 1.8 Max 26.0 173.5 68.5 81.0 52.5 88.3 145.0 62.0 Min 18.0 158.0 48.5 68.5 46.0 81.1 129.0 57.0

S. Kumar / International Journal of Industrial Ergonomics 15 (1995) 427-436 429

investigated by Andres and Chaffin (1991) and Gagnon et al. (1992) for determining volitional postures and validating biomechanical models yielding low-back compressive forces. Fothergill et al. (1991) found that two-handed strength com- monly exceeded one-handed strengths at lower handle height. Fothergill et al. (1992) reported a significant effect of handle type and handle position on push-pull strength. Daams (1993) reported that free body posture is most advanta- geous in maximizing force output. Kumar et al. (1991) had also reported two-handed push-pull strength at three heights in sagittal plane both in isometric and isokinetic modes. They reported the isokinetic strengths to be significantly lower than the isometric values. The current study is an expansion of that study.

2. Materials and methods

2.1. Subjects

Ten normal young male and ten normal young female subjects (each group with a mean age of 21.1 years) were paid volunteers for the study. The relevant anthropometric details of the experimental sample are given in Table 1. All these subjects were screened for upper extremity or back injury and any other musculoskeletal disor- ders within the past 12 months. Following recruit- ment, subjects were provided with information on the purpose and protocol of the project and signed the informed consent form. The subjects were familiarized with the activities at a submaximal intensity for the required task the day before the experiment. Prior to the familiarization session, the subjects were given full instruction and demonstration of the tasks they had to do and the postures they had to assume. Prior to the experiment, the subjects were placed on the push-pull platform and their lower extremities were stabilized in the device.

ZZ Tasks

The subjects were required to perform a two- handed push or pull at the command "go". They

were instructed to exert to their maximal capacity in sagittal plane without jerking on the handle. These push-pull activities were performed in isometric as well as isokinetic modes. The isometric activity lasted five seconds. For isokinetic push- pull, the equipment was set at a velocity of 50 cm per second. The isokinetic pulling was initiated with arms fully extended while isokinetic pushing was started close to the body. The subjects were informed that their maximal effort was required but they will not be encouraged during the trial. However, before every experimental condition they were reminded to give their maximal effort. These sagitally symmetrical push-pull activities were performed at the heights of 35 cm, 100 cm, and 150 cm. Thus a total of 12 conditions were tested (2 speeds: isometric and isokinetic x 2 directions: push and pull x 3 heights: 35 cm, 100 cm and 150 cm). For isometric activities, subjects were required to bring their effort to a maximum within the first two seconds and maintain the level of the effort for another three seconds. For isokinetic exertions, the subjects were asked to rapidly increase their effort to a maximum without jerking within the first five centimeters of displacement and continue the same way through the rest of the range.

2.3. Equipment and set-up

The push-pull strengths were measured using a Static Dynamic Strength Tester (SDST) (Kumar, 1991) with modifications for adjusting heights for push/pull activities (Fig. 1). The modifications consisted of a height adjustable horizontal bar fitted with two central rollers to allow the passage of the steel strap of the SDST. This horizontal bar could be raised or lowered by a steel cable operated by a cranking lever mounted at the base of the strength tester. The rollers allowed smooth and friction-reduced travel of the steel strap with a provision of reversing the direction of force application by 180 ° . Such an arrangement allowed to change directions between push and pull instantly. To the end of the metal strap, a 53 cm wide handle was attached to simulate an industrial box or carton. Intervening between the handle and the end of the metal strap, a load cell

430 S. Kumar / International Journal of Industrial Ergonomics 15 (1995) 427-436

(SM 500) was placed to measure the magnitude of the force during push-pull activities. In order to maintain a fixed distance of travel for the handle due to change in direction from pulling to pushing, an extension bar measuring 53 cm was inserted during the pushing activities.

In order to stabilize the lower extremities of the subject, a special device was constructed (Fig. 2) with a stable metal base and two uprights made of smooth cylindrical oil pipes. Three slid- ing padded upholstered supports for the front of the legs and thigh travelled up and down on these uprights and could be fixed at any position. Each of these padded supports were, in turn, fitted with 3-inch wide nylon straps with adjustable buckles. The support platform was welded on horizontal metal tubes which were bolted to the ground by metal brackets. The position of the platform was fixed on the ground to allow a 53

Fig. 2. A lower extremity stabilized subject during testing.

Fig. 1. Modified Static and Dynamic Strength Tester for testing push-pull strengths.

cm distance between the subject's ankle and the handle for all trials. This subject platform was attached rigidly to the Static and Dynamic Strength Tester at one end and the wall on the other to achieve a rigid fixation.

A 386 microcomputer equipped with Metra- byte DAS 20 was used for data collection and analogue to digital conversion. The push/pul l force as well as the handle displacement were sampled at 50 Hz. A special modular software package was developed for the data acquisition. The first module accepted the subject data and created a random sequence for the experimental conditions. The second module started the data acquisition, created files for data, and acquired and saved them to the files created before. The third module instantly plotted the acquired data on the screen for the purposes of quality control. The fourth and final phase of the software per-

S. Kurnar / International Journal of Industrial Ergonomics 15 (1995) 427-436

Table 2 Isometric push/pull strength in Newtons

431

Gender Parameter Push Pull

Low Med High Low Med High

Male

Female

Peak strength Mean 362 520 S.D. 112 174

Average strength Mean 314 488 S.D. 102 132

Peak strength Mean 246 339 S.D. 51 76

Average strength Mean 210 277 S.D. 41 64

482 356 763 744 165 61 202 243 378 295 646 615 118 53 189 200 272 309 521 427

68 82 95 76 220 247 439 352

53 64 97 58

mitted the preliminary analysis and calculation of descriptive statistics.

2.4. Experimental design and data analysis

The combination of two directions (push and pull), two modes (isometric and isokinetic), and three heights (low, medium, and high) yielded twelve experimental conditions. Each condition was treated as a cell. The sequence of these twelve conditions was randomized. Each of the conditions was tested three times sequentially. Between trials the subjects were allowed at least two minutes of rest. The mean of the three trials was taken to represent the mean of the cell. The tests were carried out in two sessions on two different days at the same time of the day with one intervening day. From the collected data, the peak and average strengths were extracted and compared. In this comparison, all values were normalized against the mean peak isometric

pulling strength of males at medium height. The descriptive statistics were calculated and sub- jected to analysis of variance to deduce the main effects of and interaction between gender, activity, mode and height.

3. Results

The means and standard deviations for the peak and average isometric and isokinetic pushing and pulling strengths are presented in Tables 2 and 3 respectively. The pulling strengths, mean as well as average, were always greater than that of the pushing strengths except among males for the lowest height. The highest strength was recorded during the medium height isometric pull in both genders. However, among the two genders, males were strongest. The pulling strength of males at medium height was chosen as the normalizing factor to allow an intuitive compara-

Table 3 Isokinetic push-pull strength in Newtons



Male

Female

Peak strength Mean 291 S.D. 80

Average strength Mean 207 S.D. 65

Peak strength Mean 186 S.D. 34

Average strength Mean 112 S.D. 25

438 386 342 553 526 119 117 65 105 176 240 189 254 379 373 107 46 44 79 119 267 219 232 363 288

44 47 48 58 60 139 112 165 254 196 38 31 23 52 43

432 s. Kumar / International Journal of lndustrial Ergonomics 15 (1995) 427-436

tive feel for d i f fe ren t capabi l i t i es in re la t ion to maximal effor t in these activities. Al l o the r s t reng ths and the i r d i f fe rences have been re- p o r t e d as p e r c e n t a g e o f this factor . The i somet r ic push ing s t reng th r anged be tween 41% to 61% for males and 27% to 44% for females . The i somet r ic pul l ing s t reng th for females r anged be tween 32% to 68%, but for males it r anged b e t w e e n 38% to 100%. The d i f fe rences be tween push-pu l l s t rength values were p r o n o u n c e d at m e d i u m and high levels, and small at the low level. Pul l ing activity always scored a h igher va lue c o m p a r e d to pushing except among low level i somet r ic activit ies of males . The pu l l ing s t r eng th among o the r condi - t ions at the lowest level for bo th gende r s was 5% to 80% g rea t e r t han the push ing s t rength. This

d i f fe rence for m e d i u m height a m o n g males r anged be tween 15% to 32% and among females between 12% to 24%. A t a 150 cm height , this d i f fe rence r anged be tween 19% to 34% and 9% to 21% among males and females respect ively.

The isokinet ic s t rength values were significant ly lower than the co r r e spond ing i sometr ic s t reng th (Tab le 4). F o r males , the isokinet ic pushing s t reng th va lues were 11% to 25% less than the co r r e spond ing i sometr ic values. This reduc- t ion in pul l ing for m e d i u m and high heights among males was be tw e e n 28% and 35%, though at low levels it was u n d e r 5%. A m o n g females , this r educ t ion for push ing r anged be tween 8% to 18% and for pul l ing be tw e e n 10% and 24%. C o m p a r - ing the co r r e spond ing condi t ions of males and

Table 4 The difference between the isometric and isokinetic strengths of the corresponding activities expressed as percent of the normalizing factor. (Isometric - Isokinetic)



Male Peak 9 10 12 2 27 28 Average 14 32 24 5 35 31

Female Peak 8 9 7 10 21 19 Average 13 18 15 11 24 21

(Normalizing Factor - male pulling strength at 100 cm)

Table 5 The difference between the strengths of ma!e and female strengths of corresponding activities expressed as percentage of normalizing factor. (Male strength - Female strength)

Mode Parameter Push Pull


Isometric Peak 15 24 28 6 32 41 Average 14 28 20 6 27 34

Isokinetic Peak 14 22 22 15 25 32 Average 13 24 28 12 16 41


Table 6 Female push-pull strength expressed as percentage of corresponding male strength

Mode Parameter Push Pull


Isometric Peak 67 65 56 86 68 57 Average 66 56 58 83 68 57

Isokinetic Peak 64 61 56 67 65 54 Average 54 58 59 65 67 52


Table 7 The drop in push-pull strength at low and high heights compared to the medium height expressed as percentage of normalizing factor


Low High Low High

Male Isometric Peak 21 5 54 3 Average 23 15 46 4

Isokinetic Peak 19 7 27 3 Average 4 7 16 0

Female Isometric Peak 12 9 28 12 Average 0 7 25 11

Isokinetic Peak 11 7 17 10 Average 4 4 12 8


females on the common normalized scale, females were generally 6% to 41% weaker than males (Table 5). However, when females were expressed as a percent of the corresponding values of the males, they were 14 to 48% weaker than males (Table 6).

The medium height (100 cm) invariably allowed subjects to generate maximal strength. The differences between the medium height and the high height for pulling activities were between 0% to 4%, and 8% to 12% among males and females respectively. But these differences between medium and low heights range between 16% to 54%, and 12% to 28% among males and females respectively. A trend of greater difference between medium level and low level as

compared to medium and high levels was obvious for pushing activities as well (Table 7). The difference between the medium and high levels was generally under 10%, whereas that between the medium and the low levels was up to 23%. The females were significantly weaker than the males in their pushing and pulling strengths. Their push/pull strengths ranged between 32% to 60% of males in isometric pulling strength and between 21% to 47% in isokinetic pulling strengths.

Whereas the duration for isometric activity for pushing and pulling were identical, the times at which the peak strength occurred were different at different heights (Table 7). As the height increased from low to high, the time at which the peak strength could be exerted for pushing and pulling increased for males going from 0.3 second to 3.7 seconds for pushing and 2.6 seconds to 3.4 seconds for pulling. A similar pattern is seen for females as well. The time of peak for isokinetic activity though differs at different levels of exertion, it does not show a consistent pattern of change. However, the displacement at which the peak strengths could be exerted for pushing occurred after 2/3 of displacement of the total displacement allowed. On the contrary, the peak strength during pulling activity occurred around 1/3 of displacement from the initial position (Table 8).

The analysis of variance revealed significant main effects due to gender, activity, mode, and height (p < 0.01). Such significant main effects

Table 8 The duration of the activities, time of peak strength, total displacement and displacement at the peak strength

Gender Mode Parameter Low Medium High

Push Pull Push Pull Push Pull

Male Isometric Duration (s) 4.8 4.8 4.8 4.8 4.8 4.8 Time of peak (s) 2.3 2.6 3.1 3.0 3.7 3.4

Isokinetic Duration (s) 1.8 4.0 2.2 4.5 2.4 4.4 Time of peak (s) 0.7 1.3 1.0 1.2 1.0 1.5 Total displ. (cm) 15.7 42.8 16.3 47.2 17.1 46.3 Displ. at peak (cm) 9.7 15.3 13.5 14.4 12.8 17.9

Female Isometric Duration (s) 4.7 4.7 4.7 4.8 4.8 4.7 Time of peak (s) 2.9 3.0 3.4 3.4 3.9 3

Isokinetic Duration (s) 2.4 3.8 2.6 4.5 3.0 4.5 Time of peak (s) 1.0 1.0 1.2 1.4 1.3 1.3 Total displ. (cm) 16.0 40.5 17.5 45.6 17.5 45 Displ. at peak (cm) 11.1 12.5 14.8 17.3 14.5 15.4

434 S. Kumar / lnternational Journal of lndustrial Ergonomics 15 (1995) 427-436

were found for peak as well as average strengths. The two-way interactions between activity and mode, activity and height were also significant for peak as well as average strengths (p < 0.01).

4. Discussion

Ergonomic literature has not emphasized pushing-pulling to the same extent as lifting and lowering, though they all constitute manual materials handling. Perhaps reports such as NIOSH (1981) and Statistics Canada (1991), which point out that only 20% of all manual materials handling injuries are due to pushing-pulling, convey the message of reduced significance in comparison to lifting and lowering. However, the work of Baril-Gingras and l_~rtie (1990) has demon- strated that workers indulge in these activities approximately half the time. Its contribution to human back morbidity may not be reliably deci- phered simply due to the higher frequency of injury precipitated during lifting and lowering. On the basis of a rigorous study, Kumar (1990) has advocated that cumulative load on the back is an important risk factor. The activities of pushing and pulling, therefore, may have a significant contribution in the accumulation of the pool of physical stress which eventually may play a significant role in precipitation of low back injuries.

Since in pushing-pulling activities most body members are involved (upper extremities, torso and lower extremities), each one of them is likely to have a role in determination of the magnitude capability. Since the stresses generated in these activities affect the human back more than any other body part, it will be valuable to determine the push-pull capability using only arm-back ag- gregate. Such a position is advocated specially because the contribution of lower extremities in an unrestricted posture in push-pull activities will increase the strength exertion capacity of individ- uals, thereby increasing the biomechanical stress on the spine. In the current project, therefore, the lower extremities of the subjects were stabilized to eliminate the postural difference and consequent variable contribution. It is for this reason, it is suggested, that the data obtained in

this project may have greater relevance to the linking of the push-pull capability of people and the safety of their backs. This will be the case in many industrial sectars where pushing-pulling are common activities, such as: farming and agriculture, moving, warehousing, policing, firefighting, manufacturing and others. If data on push-pull strength capability, solely due to the contribution of back, were available, ergonomists might be more effective in increasing the safety of workers. It is, however, acknowledged that the workplaces present a variety of situations calling for various postures. The data presented here is not suggested to provide information about either job- simulated strength capability or capability of workers regardless of workplace factors.

Given a high frequency of occurrence of push- pull activities in work environments where manual materials handling is common (Baril-Gingras and Lortie, 1990), it will be of value to determine a standardized relationship between different push-pull exertion efforts. Due to the horizontal direction of force application in push-pull activities, the workers may switch back and forth between these. If a common standardization scheme is developed, all such activities can be readily compared to each other in an effort to determine the relative exertion involved in different activities. Furthermore, a quantitative knowledge of the gender-based difference will assist the ergonomists in designing the jobs and also determining the placement. It is for these reasons that a normalization procedure was used. The mean pulling strength of the male sample at the medium height was chosen as the normalizing factor. The latter was based on the logic that in industrial environment generally the highest capability is used as the design criterion. Since the pulling strength at the medium height registered highest force, normalizing against this factor may enable an understanding of the global relationship of the push-pull activities at different heights in both genders.

The findings of this study show a considerable difference in the pushing-pulling capabilities of men and women (p < 0.01). Most industries re- quire push-pull activities to be performed at different height and velocities. The results reported


here clearly indicate that these task variables significantly affect the human capability. Other authors have also reported a significant effect of height in push-pull strength (Martin and Chaffin, 1972; Ayoub and McDaniel, 1974; Chaffin et al., 1983; Garg and Belier, 1990; Mital and Faard, 1990; Kumar et al., 1991; Gagnon et al., 1992). Like other studies, this study also found that the medium height enables maximal force production which declines with increasing or decreasing heights (p < 0.01). Such an outcome is suggested to be due to the mechanical disadvantage for the body in force application. Daams (1993) proposes a free posture for such strength measurement. However, this strategy has a potential of ad- versely affecting the safety. During most mea- surements, an optimum posture may be allowed which may not be the case on the shop floor. Such a postural disadvantage may increase the load by an undetermined and variable amount even if the job requirement remains constant but the immediate surroundings vary. The support for this argument may be derived from the work of Kroemer et al. (1971) and Kroemer (1974), who built bracing structures in their rig considerably increasing the capability.

The effect of the velocity on the force generated was significant in this study (p < 0.01), ac- counting for a difference ranging from 10% to over 30%. Such a role of velocity on strength production has been reported by other authors as well (Garg and Belier, 1990; Mital and Faard, 1990; Kumar et al., 1991). Kumar et al. (1988) in another study reported a progressive decrease in strength production with increasing exertion velocity. Since in the current study only one dynamic velocity was used, the effect of increasing velocity on push-pull strength cannot be speci- fied. It is, however, expected that the strength may have an inverse relationship with the velocity.

On the basis of observations made in the current study, Kumar (1990), and that of Baril- Gingras and Lortie (1990), it is suggested that job-simulated (posture and velocity) isokinetic strength be used as a design criterion for industrial work. Further, where push and pull both activities are involved, the push strength should

be used. These standards must also be gender-adjusted.

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upper body push-pull strength of normal young adults in sagittal plane at three heights

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