LIFTING IN STOOPED AND KNEELING POSTURES: 

EFFECTS ON LIFTING CAPACITY, METABOLIC COSTS, 

AND ELECTROMYOGRAPHY OF EIGHT TRUNK MUSCLES 

Sean Gallagher * 

U.S. Department of the Interior, Bureau of Mines, Pittsburgh Research Center, Pittsburgh, PA 15236 (U.S.A.) 

William S. Marras 

Department of Industrial and Systems Engineering, The Ohio State University, Columbus, OH 43210 (U.S.A.) 

and Thomas G. Bobick 

U.S. Department of the Interior, Bureau of Mines, Pittsburgh Research Center, Pittsburgh, PA 15236 (U.S.A.) 

ABSTRACT 

Twelve heulfhy, experienced underground coal 

miners perforn~ed lifiing capacify rests in stooped 

and kneeling postures using a modified psychophysical procedure. Subjecfs adjusfed weigh1 in a lifring 

box fo rhe maximum 117e.v could handle wifhout 

undue fafigue in an asymmetric lifring fask. Liffing 

periods were 20 min in durafion and fhe frequency 

was 10 lifrs/ntin. Tesfs were perfornzed under a 

48-in. roof fhaf r-esfricfed [he subjecf 's posfure. Psychophysical, physiological, and biomechanical dependen[ measures included ,he muximum accepfable 

weighf lifi (MA WL), hearf rare (HR), rate oj 

oxygen consumpfion (vo,), venfilafion uol~rme rafe 

(V,), respirafory exchange ratio (R), and in- 

* Please address reprint requests to: Sean Gallagher, U.S. 

Bureau of Mines, P.O. Box 18070. Cochsans Mill Rd., Pillsburgh, PA 15236, U.S.A. 

I egra I ed elec~ron~yography (EIMG) of eigh I ~runk 

muscles. Resulfs indicufed fhaf fke MA WL was 

significanf(y lower- when kneeling fhan when sfooped 

(p < 0.05). Furlhermore, mefabolic demands were 

greafer in ferms of HR (p < 0.005) and VO, (p < 

0.05) when kneeling. Left and righf erecfores spinae 

muscles exhibited increased EMG acfivify it7 rhe 

kneeling posture (p < 0.001). If was concluded 117~1 

ps~ychophysical lifiing capacity is decreased, while 

the mefabolic stress and inrernal load on [he spine 

are increased, in fhe kneeling posfure. Resulrs of [his 

Bureau of Mines sfu4 ir~dicafe fhaf ir may be 

advisable to reduce fhe weigh1 of n~aferials fhaf are 

handled repefifiveiy in [he kneeling posrure. 

INTRODUCTION 

Underground miners who work in low-seam 

coal mines (< 48-in. roof height) often lift heavy 
, j

relies on the assumption that people are capable 

of determining workloads that are safe for them. 

The physiological method is useful in determining 

the metabolic demands on the body due to the 

lifting task, and can provide an Indication of 

muscular fatigue (U.S. Department of Health and 

Human Services, 1981; Jorgensen, 1985). However, this technique may neglect the biomechanical 

stresses of lifting. Finally, the biomechanical 

method can estimate the forces imposed on the 

lumbar spine, but is not sensitive to factors such 

as muscular fatigue (Ayoub et al., 1983). There

TABLE 1 

Selected physical characteristics of the mining test subjects in 

this study compared to anthropometric data on male underground miners collected by Texas Tech University (Ayoub et 

al., 1981) 

Measurement U.S.B.M. Texas Tech 

low-coal miners low-coal miners 

Mean S.D. N Mean S.D. N 

Age (years) 36 8 12 35 11 135 

Weight (kg) 83.8 13.9 12 81.9 16.8 132 

Stature (cm) 173.8 6.7 12 174.4 6.5 136 
te aveage subjectvey deteed wegt cose

for two test conditions in each posture. Physiological dependent measures included heart rate 

(HR), oxygen utilization (vo,), ventilation volume 

(V,), and the respiratory exchange ratio (R). Integrated electromyographic (EMG) data were collected from eight trunk muscles in six of the 

subjects and served as the dependent variables for 

the biomechanical portion of the experiment. 

Apparatus 

A schematic of the equipment used in this 

experiment is shown in Fig. 2. The weight of the 

gp000 wp. SC 

2000 data acquisition system was used for on-line 

collection and the data were stored on a microcomputer for subsequent data analysis. In both 

postures it was assumed that the back was in a 

static position for the purposes of the EMG analysis. Integrated EMG data were normalized with 

respect to the maximum EMG obtained for each 

muscle during maximum trunk flexion and exten- 

* Reference to specific products does not irnply endorsement 

by the Bureau of Mines. 
dance with Snook and Irvine (1967), as modified 

by Snook (1985b). In this study, the subjects were 

told to adjust the weight in the box so tlie load 

could be handled for 20 min and to assunie that 

this 20 rnin of lifting would have to be performed 

four times during a workday. It should be noted 

that nlultiple psychophysical trials to establish a 

consistent and repeatable load were not possible 

in this experiment; thus, the subjects provided a 

one-time estimate of their lifting capacity in the 

two postures. 

The subject lifted the box at a frequency of 10 

per niin for two 20-min periods in each posture. 

One lilting period started with a heavy box, weigh- 

last 10 s of every minute. The final 15 heart rate 

values were averaged and taken as the mean HR 

for that test condition. Respiratory nieasurements 

were obtained approximately every 30 s during the 

last 5 min of each lifting period, and were averaged by the nuniber of values obtained during this 

period. Integrated EMG data were collected from 

six subjects at a rate of 100 Hz for a period of 3 s 

during a lift at niinute 2 and nlinute 18 of each 

lifting period. A 10-min rest break was provided 

between tests so that subjects could rest and attend to personal needs. Test conditions were 

randomized and presented in a counterbalanced 

fashion. 
Stooped Kneeling Significance 

MAWL (kg) 30.9 

(_+4.1) 

HR (bpm) 123 

(_+12) 

VO~ 14.9 

(mL/kg/min) ( + 2.9) 

V, (L/rnin) 32.7 

(f 6.1) 

R 0.83 

( ? 0.09) 

Note: bpm = beats per minute; mL/kg/nlin = milliliters per 

kilogram body weight per nlinute; n.s. = not significant; numbers in parentheses represenr the standard devio~ion. 

during this study, respectively. The prominent 

characteristic of these figures is that the right and 

left erectores spinae demonstrated increased activity i11 the kneeling posture compared to stooped. 

Tables 3 and 4 present the pooled within-groups 

correlation matrices for maximum and mean EMG 

activity, respectively. These tables show many significant intercorrelations between the eight trunk 

muscles studied for both maximum and mean 

EMG data. Generally stated, the activity of back 

muscles seem to be more highly correlated with 

the activity of other back muscles; activity of the 

abdominals also tended to be intercorrelated. 
TABLE 4 

Pooled within-groups correlation matrix for mean integrated EMG data 

LLD RLD LES RES LEO RE0 LRA RRA 

LLD 1 .OO 

RLD 0.44 1 .OO 

LES 0.16 0.27 1 .OO 

R ES 0.21 0.48 " 0.79 1 .OO 

LEO 0.34 a 0.15 0.41 0.26 1 .OO 

RE0 0.17 0.14 0.07 0.03 0.75 ' 1 .OO 

LRA 0.34 a 0.07 0.13 0.07 0.41 0.24 1 .OO 

RRA -0.13 - 0.29 0.12 - 0.06 0.28 0.37 " - 0.04 1 .OO 

Note: M = 40. Refer to Fig. 2 for muscle abbreviations. 

" p < 0.05, bp < 0.01, "p < 0.001. 
g

found to adequately handle the follow-up analyses 

to MANOVA (Borgen and Seling. 1978). Therefore, the results of univariate ANOVA F ratios 

abdominis correlated highly with the function separating left and right integrated EMG activity. 

These muscles apparently help to control asymTABLE 7 

Univariate ANOVA F-ratios for differences in maximum and mean EMG act~vity between stooped and kneeling postures 

Trunk Muscle -- 

LLD RLD LES RES LEO RE0 LRA RRA 

F (Maximum EMG) 0.7 2.2 42.7 " 28.4 a 3.5 1.2 0.0 0.9 

F (Mean EMG) 0.1 0.0 35.4 a 30.1 " 0.9 1.3 0.3 0.4 

Note: d.f. = 1.33 for maxinlum EMG: d.f. = 1.32 for mean EMG. See Fig. 2 for nluscle abbreviations. 

a I; significant at 0.001 level. 
are not able to contribute a great deal of useful 

force when lifting in the kneeling posture. This 

means that the work of lifting must be performed 

primarily by the muscles of the upper body, particularly shoulder and arm muscles and muscles 

associated with extension of the vertebral column. 

The back muscles would be expected to provide a 

substantial increase in force output to execute the 

lift. 

In comparison with other asymmetric lifting 

studies, the maximum acceptable weights of lift 

reported in this study, while in the general range 

reported by others (Asfour et al., 1984; Garg and 

Badger, 1986; Mital and Fard, 1986), tend toward 

if a smaller muscle mass is used to acco~nplish an 

equivalent workload. the heart rate and oxygen 

uptake adjustment by the body will be increased 

(Stegemann, 1981). The increased metabolic cost 

of lifting when kneeling indicates that the onset of 

fatigue may be more rapid when working in this 

posture. The oxygen consumption required of the 

subjects to perform lifting tasks when stooped 

appears to be in line with that reported by others 

relative to the weight and frequency of lift (Asfour 

et al., 1984; Mital and Fard, 1986); the oxygen 

consumption is somewhat higher in the kneeling 

posture than would be expected for the weight 

lifted at this frequency. 
ing to note that these same muscles have been 

found to be significantly affected by the velocity 

of lifting (Kim and Marras, 1987). Therefore, the 

role of the latissimus dorsi and rectus abdominis 

during lifting may be specialized in two ways: (I) 

to provide the force necessary for acceleration of 

the trunk, and (2) to provide disproportionate left 

and right side forces when necessary to perform 

asymmetric tasks. 

The dynamic nature of the lifting tasks performed in this study requires that caution be 

exercised when interpreting the EMG results 

(Marras, 1987). As mentioned previously, the as

g py 

have considerable biomechanical disadvantages. 

For instance. the load on the intervertebral disks 

of the lumbar spine is known to be substantially 

increased in the stooped posture as opposed to 

upright standing (Nachemson, 1976). The shear 

forces acting on the intervertebral disks would 

also be expected to be considerable in this position. Another disadvantage of the stooped posture 

is that the back muscles are at their greatest 

length. Therefore, according to the relationship 

between length and strength of a muscle, less force 

can be exerted by these muscles. On the other 

hand, the stooped lifting posture would appear to 
that must compensate for these forces are smaller. 

In fact, it rnay be that the back is one primary 

area where these forces must be absorbed when 

kneeling, whereas in the stooped posture the ankles 

and knees may help to reduce the burden placed 

on the back. In addition, the greater mobility 

afforded by working in the stooped position may 

allow increased opportunity to "fine tune" postural adjustments to reduce the stress on overworked muscles. Therefore, despite the fact that 

the trunk is more upright in the kneeling posture. 

many biomechanical factors may offset ths advantage, malung the kneeling pose equally (if not 

those screened to exclude prior serious back 

trauma or other health-related problen~s) were used 

in this investigation. Thus, these recommended 

weights may exceed the capabilities of less healthy 

workers, or those ~~naccuston~ed to lifting in the 

postures described in this paper. 

The implications of the recommended weights 

described above are noteworthy due to the fact 

that the most commonly handled supply item in 

~~nderground coal mines is the 50-lb rock dust bag. 

Based on the acceptable weight-lifting recommendations described above, 50 Ib is an acceptable weight of lift for the stooped posture; how- 
increased metabolic and biomechanical stresses of 

lifting on two knees, serious consideration should 

be given to reducing the weight of materials that 

must be handled in the kneeling posture. 

ACKNOWLEDGMENTS 

We wish to thank Mr. R.L. Unger, Mr. S. 

Lavender, Mr. S. Rangarajulu, and Mr. H.J. Kellner for their assistance in the performance of this 

project. 

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