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Dynamic comparison of the two-hand stoop and assisted one-hand lift methods
Jourd ofS.+ry Research Vol. 21, pp. 53-59.1990 8 1990 National Safety Council and Pergamon Press plc
00224375/?XI $3.00 + .OO printed in the USA
Dynamic Comparison of the Two-hand Stoop and Assisted One-hand Lift Methods T. M. Cook, S. Mann, and G. E. Lovested
Low-back pain arising from repetitive lifting of objects out of deep containers is a significant problem in many industries and results in a great deal of time lost from work. This study measured electromyographic (EMG) activity of the lumbar paraspinal muscles, vertical acceleration of three different loads, and the reaction force of the supporting upper extremity to compare the assisted one-hand lift and the two-hand stoop lift techniques. The results of the study indicate that the assisted one-hand lift requires significantly less EMG activity than the two-hand stoop method when lifting loads out of deep containers. The acceleration of the load was not significantly different for three different loads, but the force on the supporting upper extremity was significantly related to the weight of the load lifted. Results of this study seem to indicate that, under certain circumstances, the one-hand lift is a less stressful method of lifting than the two-hand stoop method.
Low-back pain is a significant problem in the manufacturing and warehousing industry. It results in a great deal of time lost by workers as well as billions of dollars in workers’ compensation costs in the United States annually (Akeson & Murphy, 1977). Lifting parts or materials out of large containers is a common industrial practice. One large farm implement manufacturer has reported that between 45% and 55% of all parts were processed through various manufacturing departments using deep steel containers (Lovested, 1981). These steel containers (tubs and skids) and wooden pallet boxes present special lifting problems to workers. As the level of parts
ThomasCook, PhD, is assistant professorin the physical therapygraduateprogram,Collegeof Medicine,University of Iowa. Susan Mann, MS, is director, Department of Physical Therapy, Mercy Hospital, Davenport, IA, and Gary E. Lovested. CSP, is an occupational safety specialist with Deere & Company, Moline, IL. Summer 1990lVolutne 21lNumber 2
or material stored in the container decreases, the worker must bend over farther to reach and lift the item. In these situations most workers use a two-hand scoop lift, which, because of the large horizontal distance involved, is likely to result in high lifting moments and high compressive forces on the low back (U. S. Department of Health and Human Services, 198 1). Repetitive lifting using this method further compounds the risk of injury. Traditional measures to overcome this problem include ergonomic redesign of the lifting task or teaching workers how to lift. Although job redesign is the more preferable and longer-term of the two, worker lifting programs are often given instead. The worker training usually consists of teaching the worker to use the two-hand squat lift or “bended knees” lift to keep the load close to the body and thus minimize the horizontal moment arm. While the two-hand squat technique 53
FIGURE 1 TWO-HAND STOOl’ (A) AND ASSISTED ONEHAND (B) METHODS FOR LIFllNG ITEMS OUT OF DEEP CONTAINERS
, *
I___..
_~___ B
-
may be a viable alternative under some circumstances, it is not possible to use this method when lifting objects from large containers, since the container sides restrict the worker from bending the knees. An alternative method has been proposed for lifting objects from large deep containers, which, it is hypothesized, will reduce physical stress to the low back (Lovestead, 1985). This method is referred to as the assisted one-hand lift and involves bending over a container and lifting an object with one hand while pushing down on the top edge of the container with the other hand (Figure 1). This study compared the assisted one-hand method with the two-hand stoop method of lifting various loads from large containers in terms of low-back muscle activity, load acceleration, and upper-extremity supporting force. The purposes of the study were: (1) to determine whether there was a significant difference in the mean myoelectrical activity at the third lumber (L3) level of the erector spinae muscles between the two lifting methods, (2) to determine whether there was a significant difference in the peak acceleration of the object being lifted between the two methods, and (3) to determine what, if any, relationship exists between the mean reaction force created by the supporting upper extremity during the assisted one-hand lift and the weight of the object being lifted. METHOD Twenty-four male subjects volunteered to participate in the study. They had a mean age 54
of 26.9 years with a range of 22-35 years; a mean height of 1.77 m, with a range of 1.571.88 m; a mean weight of 75.4 kg, with a range of 61.1-88.2 kg. None reported symptoms or history of low-back pain. The object lifted was a wooden box 28 cm x 27 cm x 34 cm with three dowel handles 5 cm from the top, allowing the subject to make a two-hand lift by grasping the two outside dowels or a one-hand lift by grasping the middle dowel. Bags of lead pellets were removed and replaced within the box to adjust the total weight being lifted. The box was placed within a recess in the bottom of a wooden simulated parts container 73.6 cm x 73.6 cm x 91.4 cm (Figure 1). Surface EMG electrode assemblies with silver-silver chloride electrodes were used to record action potentials for both the left and right lumbar paraspinal muscles as described elsewhere (Cook & Neumann, 1987). The signal conditioner component of each amplifier produced the instantaneous root-meansquare (RMS) value of the signal, which was smoothed with a time constant of 55 msec. The electrode assemblies were applied vertically at the L3 level over the convexity of the paraspinal muscle bellies as determined by isometric contraction and palpation. The distance from the midline to the electrodes was approximately 4-5 cm on most subjects. Both RMS and raw signals were monitored for artifacts on an oscilloscope. A piezo resistive, critically damped (+2.5 g) accelerometer was attached to the box and used to measure the vertical acceleration of the load as it was lifted. A cantilever with four strain gauges was attached to the upper edge of the container at the location of the placement of the left hand and was used to measure the left upper-extremity reaction force on the edge of the container during the assisted one-hand lift. A multichannel FM tape recorded was used to record EMG, left upper extremity reaction force, and vertical acceleration signals, as well as a trigger signal that cued the subject to begin each lift. All EMG data were normalized to a percentage of the individual’s maximum voluntary isometric contraction (MVIC) (Soderberg & Cook, 1984). After an appropriate warmup period, EMG activity from maximal voluntary erector spinae muscle contractions was Journal of Safety Research
recorded while the subject assumed a position of standing trunk flexion and with two hands isometrically pulled up on a cable tensitometer 31.75 cm from the floor for 4 s. Subjects performed a total of three MVICs with a 60-s rest period between each contraction. The erector spinae activity from the three MVICs was averaged and became the base against which all lifting data for that subject was normalized as a percentage (%MVIC). During the experiment, the investigator closed a trigger switch, which produced a clicking sound from an audio speaker and served as a cue to the subject to begin lifting. This trigger signal was also recorded on the FM tape recorder for use in digitizing the data during tape playback. The initial distance of the handles from the floor was 31 cm. The final distance of the handles from the floor was 114.4 cm when the box was rested on the top edge of the container for a total vertical excursion of 83.4 cm. The horizontal distance from the midpoint of the box in its initial location to the top edge of the container was 36.6 cm for all lifts. Three loads, 3.75 kg, 6.81 kg, and 13.64 kg, were used in randomly ordered blocks of six lifts each. Within each block, subjects randomly varied the type of lift, using one-hand assisted lift or two-hand stoop lift. Consequently, all subjects performed a total of 18 lifts. Each lift began when the sub ject heard the clicking sound. The subject lifted the load to the top edge of the container and paused for several seconds before lowering it back to its original position. Subjects were instructed to lift at whatever rate was most natural to them. No measurements were taken during lowering. There was one lift per minute, so that fatigue was not a factor. At the end of the session subjects performed post-test MVICs to
enable a comparison of pre- and post-test EMG values for reliability. A digital laboratory computer was used to process the data recorded on the FM tape and to calculate the mean values of EMG activity and left upper extremity reaction force during each lift. The trigger signal recorded during the experiment was used to begin computer digitizing. Sampling rate was 250 samples/s and continued 5 s following the trigger. The computer determined the mean of the right and left EMG signals and the upper extremity reaction force signal from the entire 5-s sampling period. These mean values, which included both the trunk flexion motion involved in bending over to grasp the load and the extension motion of lifting it, were used as the data for further analysis. Peak vertical acceleration values were obtained by displaying the vertical acceleration data recorded on an FM tape on a storage oscilloscope and then determining the peak acceleration level for each lift. EMGs, upper extremity reaction force, and acceleration data were submitted to an analysis of variance (ANOVA) with subject, lift method, and load as independent variables. An additional four-way ANOVA was performed on the EMG data using the right and left sides as additional independent variables. The means of the various experimental conditions were statistically compared against each other by using Tukey’s test for mean comparisons and multiple A-posteriori t test comparisons with Bonferroni adjustments. In all tests, a significance level of 0.05 was used. RESULTS Paired t-tests for both right- and left-side EMG data indicated no statistically significant
TABLE 1 EMG DATA FOR TWO TYPES OF LIm (ALL LOADS)
Two handed t.i
tC
Ki
):I1I
stoop Meat)
107.1
65.9
Assisted One-handed 75.3 48.2
P .OOOl
EM{:
s.d.
(ZMVIC)
n
204
204
(F=94.30)
Lel1
Mea11
113.7
103.0
-001
EMC
s.d.
68.6
64.1
(%MVIC)
n
207
Summer 1990/Vohme 21lNumber 2
208
(F=41.23)
55
TABLE 2 EMG DATA FOR THREE LOADS (BOTH TYPES OF LIFI’) Load
(kg)
%-&i
3.75
6.81
13.64
P .OOOl
EMG (%MVlC)
Mean* 6.d. n
76.6 47.1 140
88.5 53.4 131
108.7 71.9 137
Lett Et-K (XMVLC)
Mean” s-d. n
96.5 61.0 140
103.5 63.7 138
125.2 71.6 138
*
All
values
are
significantly
different:
differences between pre- and post-test maximum voluntary contractions. For both the right- and left-side EMG data, the ANOVA showed no signi~cant two-way interaction for method of lift and weight lifted. There were significant main effects for method of lift and for load on both sides. F tests revealed that both the right and left EMG activities for the assisted one-hand lift method were significantly less than for the two-hand stoop lift (Table 1). Tukey’s test for mean comparisons revealed that both the right and left EMG values were signi~cantly different for all three weights lifted (Table 2). Across both methods of lifting, as the weight of the load increased there was an associated increase in EMG activity. The results of the four-way ANOVA indicated a significant interaction between side (left and right EMG) and method (assisted one-hand and two-hand lifts) with F=6.87 and P<.OO93. There were significant main effects for side, method, and load lifted. Post hoc t-tests revealed signi~c~~y greater leftside EMG activity than right-side EMG activity during the assisted one-hand lift for all three loads. There was also significantly less right-side EMG activity during the assisted one-hand lift compared to the two-hand lift {Figure 2). For peak load acceleration ANOVA demonstrated significant interactions between weight lifted and method of lift (F=3.54, p=.O324). Post hoc t-tests were performed to examine any differences in load acceleration between the assisted one-hand lift and the two-hand stoop lift, and to also determine differences among weights lifted. The ttests revealed no significant differences (w.05) between load accelerations for the two methods of lift or for the load lifted. The 56
pc.05
(F=30.54) .0001 (1”=77.23)
(Tukey’s)
average vertical accelerations of the two lighter loads were slightly higher for the stoop lift than for the assisted one-hand lift, although the differences were not statistically significant (Figure 3). For the dependent variable, left upperextremity mean reaction force, there was a signi~c~t effect for the load lifted during the assisted one-hand lift (F=28.74, and P<.OOOl) (Figure 4). Tukey’s test for mean comparisons revealed significant differences (~~05) in left upperextremity mean reaction forces between the 13.6-kg load and each of the other two loads. No differences were found between the 3.75 kg and the 6.81-kg loads.
FIGURE2 EMG (%hWIC) OF RIGHT AND LFZT EmCTOR SPINAE USING TWO-HAND S’IOOP AND ASSISTED ONE-IfAND METHODS ‘IO LIFT THREE
DICED LOADS. EACH SYMBOL REPRE!fi~ THE MEAN OF THREE TRIALS FOR TWENTYFOUR SUBJEXTS. VERTICAL BARS INDICA-I?‘i ONE STANDARD DEVIATION
FIGURE 3 PEAK VERTICAL ACCELERATION OF THREE DIFFERENT LOADS USING THE TWO-HAND STOOP AND ASSISTED ONE-HAND LIFT METHODS. EACH SYMBOL REPRESENTS THE MEAN OF THREE TRIALS FOR TWENTY-FOUR SUBJECTS. VERTICAL BARS INDICATE ONE STANDARD DEVIATION
DISCUSSION Within the limitations of this study, the results indicate that there is significantly more erector spinae EMG activity during the twohand stoop lift than during the assisted onehand lift for both the left and right sides. During the assisted one-hand lift there is significantly less EMG activity on the lifting side (right erector spinae in this study) than on the supporting side (left erector spinae). During the two-hand lift, back muscle activity is, for practical purposes, symmetrical with no significant difference between sides. There appears to be a slight asymmetry (4%6% greater activity on the right side), which may be due to handedness, given that 90% of the population is right-handed (Donish & Basmajian, 1972). During the assisted onehand lift, asymmetry was pronounced, with approximately 30% less right EMG activity than left. This asymmetry is most likely due to spinal torsion during the assisted one-hand lift, with the trunk rotating to the left during flexion, and to the right during extension, thus requiring first eccentric, then concentric muscle activity by the left-side erector spinae. Associated reductions in EMG activity occur on the right side. As the load increased, there was a significant increase in both left and right EMG in both the two-hand stoop and assisted onehand lifts. Differences in EMG activity Summer 1990/Volutne 2lINumber 2
FIGURE 4 LEFT UPPER EXTREMITY REACTION FORCE FOR THREE DIFFERENT LOADS LIFTED USING THE ASSISTED ONE-HAND METHOD. SOLID LINE IS THE BEST FIT REGRESSION LINE. EACH SYMBOL REPRESENTS THE MEAN OF THREE TRIALS FOR TWENTY-FOUR SUBJECTS. VERTICAL BARS INDICATE ONE STANDARD DEVIATION
between the two-hand stoop lift and the assisted one-hand left were generally maintained as the load increased within the range of loads investigated here. More muscle activity is required to lift heavier loads, and more compressive force may be generated on the lumbar spine due to the increased force necessary to overcome the inertia of a heavier load. Two important factors must be kept in mind when interpreting the EMG findings in this study. First, as with most EMG studies of functional activities, all EMG values were normalized to a maximal voluntary isometric contraction and expressed as a percentage of an MVIC. Although this procedure is widely accepted and reported in the literature, MVIC testing is somewhat dependent on each subject’s motivation to expend a maximal effort. EMG measurements taken during dynamic activities are influenced by a number of factors such as changing muscle length and movement of the skin with respect to the underlying muscle tissues. During the isometric tests, the erector spinae were lengthened substantially, thus pulling the muscle fibers apart and decreasing the available muscle tension. Ortegren et al., in 1981 found that amplitude decreased with decreased floor-tohandle distance. This may explain EMG activity >lOO% during dynamic lifting. In spite of these limitations, EMG activity does provide a useful indication of the relative tension in the paraspinal muscles during the two 57
lifting methods used in this study. Studies by Ortegren and co-workers indicated a very strong linear relationship between disc pressure and integrated erector spinae EMG for both symmetrical and asymmetrical lifts justifying EMG activity as a good predictor of disc pressure. Second, EMG and upper extremity reaction force measurements derived for each &ft in this study were determined by averaging values over a 5-s time period. This time period encompassed forward bending to grasp the load as well as the act of lifting the load to the front edge of the container. The instantaneous maximal amplitudes of these measures were not examined as part of this study but are likely to be much higher than the averages reported here. Such averages were used as the primary data in this study based on the concept that they provide good estimates of the total muscle effort and force-time product required to accomplish the entire functional task. Schultz & Andersson (1981) indicated that the difference between dynamic and static calculations for predicting external and internal loading during lifting is insignificant when small accelerations and inertial forces are involved. Leskinnen et al. (1983) found in a study investigating four different types of lifting that there were no significant differences in load acceleration among different methods of lift. Post hoc t-tests revealed no significant differences in load acceleration between the assisted one-hand lift and the two-hand stoop lift or among loads lifted. The lack of differences in acceleration may be due to the limited range of weights used in this study. Load acceleration may be different for extremely heavy loads. The assisted one-hand lift is limited by the amount of weight one can safely grasp with one hand. If the load had been heavy enough, it might have accelerated at a slower rate due to difficulty in grasping and holding it with one hand. Numerical results showed a slight trend to accelerate the load more while lifting the lighter loads using the two-hand stoop lift method. During the assisted one-hand lift there was a significant increase in the left upper-extremity reaction force as the load increased (Figure 4). Ekholm (1982) determined that 70% of the maximum loading moments during lifting are produced by body segments, and 30% are produced by the load, indicating that the load 58
contributes less to the stress of lifting than the body segments themselves. During the assisted one-hand lift, the left upper extremity reaction force may assist in reducing the load of the upper trunk more than reducing the load of the object lifted, although the two effects are difficult to separate. The mean reaction force was double the weight lifted at 3.75 kg, virtually equal to the weight lifted at 6.81 kg, and was l/3 less than the 13.64 kg weight. The regression line superimposed on Figure 4 would predict that during the assisted onehand lift without any load, subjects would be willing to produce a force of 5 kg to assist in trunk extension. Further research is needed to investigate this extrapolation. At loads of approximately 8 kg, the mean upper extremity reaction force is equal to the weight lifted. As the load increases above approximately 8 kg, the upper-extremity reaction force appears to contribute progressively less force to assist back muscle activity. At these higher load levels, increased torsional moments on the spine are likely to require significant contributions by other muscle groups, along with marked increases in the erector spinae. Before such high load levels are reached, however, grip strength is likely to become a major limiting factor in using the assisted onehand method. SUMMARY As measured by EMG, the assisted onehand lift appears to produce less stress on the lumbar spine than does the two-hand stoop lift for loads up to 14 kg, at least under the conditions simulated in this study. Peak vertical accelerations of three different magnitude loads were not found to be significantly different between the two lifting methods. The upper extremity reaction force showed a linear, but not equal, relationship to the load lifted using the onehand lift, an asymmetrical lift with a use limited to loads that can be grasped and lifted using one hand. Results of this study imply that adoption of this technique may reduce the potential of low-back injury when lifting items of approximately 14 kg or less out of deep containers, especially when the other lift methods are not possible. Journal of Safety Research
REFERENCES
Engineering in Medicine. 12,87- 89. Lovested, G. E. (1981). Parts lifting study/ John Deere
Akeson, W., & Murphy, R. (1977). Editorial comment: Low back pain. Clinical Orthotxxiics. 129.2-3. Cook, T: M.. & Neumann.b. A. (1987). The effects of load placement on the EMG activity of the low back muscles during load carrying by men and women. Ergonomics, 30.1413-1423. Davis, P. R., Troop, J. D. G., & Bumard, J. H. (1965). Movements of the thoracic spine when lifting: A chronocyclogmphic study. Journal of Anatomy, 99,13-26. Donish, E. W. & Basmajian, J. V. (1972). Electromyography of the deep muscles of the back.
parts distribution warehouse, Moline, IL: Unpublished study by Deere & Co. Lovested,.G. E. (1985). Materials handling safety in industry, in: Materials Handlinn Handbook, New York John Wiley and Sons. Ortegren, R., Andersson, G. B., & Nachemson, A. L. (1981). Studies of relationships between lumbar disc pressure, myoelectric back muscle activity and intraabdominal (intragastric) pressure. Spine, 6,98-103. Schultz, A., & Andersson, G. (1981). Analysis of loads on the lumbar spine. Spine 6,76-82. M. (1984). Soderberg, G. L., & Cook T. Electromyogtaphy in biomechanics. Physical Therapy,
American Journal of Anatomy, 133.2536. Ekholm, J., Arborelius, U. P., & Nemeth, G. (1982). The
load on the lumbosacral joint and trunk muscle activity during lifting. Ergonomics, 25(2), 145-61. Leskinnen, T, Stalhammar, H., Dourinka, Il. et al. (1983). The effect of inertial factors on spinal stress when lifting.
Summer 1990/VoItune
21lNumber
2
64,1813-1820.
U. S. Department of Health and Human Services. (1981). Work practices guide for manual lifting. (NIOSH Publication No. 81-122). Cincinnati, OH: Physiology and Ergonomics Branch.
59
00224375/?XI $3.00 + .OO printed in the USA
Dynamic Comparison of the Two-hand Stoop and Assisted One-hand Lift Methods T. M. Cook, S. Mann, and G. E. Lovested
Low-back pain arising from repetitive lifting of objects out of deep containers is a significant problem in many industries and results in a great deal of time lost from work. This study measured electromyographic (EMG) activity of the lumbar paraspinal muscles, vertical acceleration of three different loads, and the reaction force of the supporting upper extremity to compare the assisted one-hand lift and the two-hand stoop lift techniques. The results of the study indicate that the assisted one-hand lift requires significantly less EMG activity than the two-hand stoop method when lifting loads out of deep containers. The acceleration of the load was not significantly different for three different loads, but the force on the supporting upper extremity was significantly related to the weight of the load lifted. Results of this study seem to indicate that, under certain circumstances, the one-hand lift is a less stressful method of lifting than the two-hand stoop method.
Low-back pain is a significant problem in the manufacturing and warehousing industry. It results in a great deal of time lost by workers as well as billions of dollars in workers’ compensation costs in the United States annually (Akeson & Murphy, 1977). Lifting parts or materials out of large containers is a common industrial practice. One large farm implement manufacturer has reported that between 45% and 55% of all parts were processed through various manufacturing departments using deep steel containers (Lovested, 1981). These steel containers (tubs and skids) and wooden pallet boxes present special lifting problems to workers. As the level of parts
ThomasCook, PhD, is assistant professorin the physical therapygraduateprogram,Collegeof Medicine,University of Iowa. Susan Mann, MS, is director, Department of Physical Therapy, Mercy Hospital, Davenport, IA, and Gary E. Lovested. CSP, is an occupational safety specialist with Deere & Company, Moline, IL. Summer 1990lVolutne 21lNumber 2
or material stored in the container decreases, the worker must bend over farther to reach and lift the item. In these situations most workers use a two-hand scoop lift, which, because of the large horizontal distance involved, is likely to result in high lifting moments and high compressive forces on the low back (U. S. Department of Health and Human Services, 198 1). Repetitive lifting using this method further compounds the risk of injury. Traditional measures to overcome this problem include ergonomic redesign of the lifting task or teaching workers how to lift. Although job redesign is the more preferable and longer-term of the two, worker lifting programs are often given instead. The worker training usually consists of teaching the worker to use the two-hand squat lift or “bended knees” lift to keep the load close to the body and thus minimize the horizontal moment arm. While the two-hand squat technique 53
FIGURE 1 TWO-HAND STOOl’ (A) AND ASSISTED ONEHAND (B) METHODS FOR LIFllNG ITEMS OUT OF DEEP CONTAINERS
, *
I___..
_~___ B
-
may be a viable alternative under some circumstances, it is not possible to use this method when lifting objects from large containers, since the container sides restrict the worker from bending the knees. An alternative method has been proposed for lifting objects from large deep containers, which, it is hypothesized, will reduce physical stress to the low back (Lovestead, 1985). This method is referred to as the assisted one-hand lift and involves bending over a container and lifting an object with one hand while pushing down on the top edge of the container with the other hand (Figure 1). This study compared the assisted one-hand method with the two-hand stoop method of lifting various loads from large containers in terms of low-back muscle activity, load acceleration, and upper-extremity supporting force. The purposes of the study were: (1) to determine whether there was a significant difference in the mean myoelectrical activity at the third lumber (L3) level of the erector spinae muscles between the two lifting methods, (2) to determine whether there was a significant difference in the peak acceleration of the object being lifted between the two methods, and (3) to determine what, if any, relationship exists between the mean reaction force created by the supporting upper extremity during the assisted one-hand lift and the weight of the object being lifted. METHOD Twenty-four male subjects volunteered to participate in the study. They had a mean age 54
of 26.9 years with a range of 22-35 years; a mean height of 1.77 m, with a range of 1.571.88 m; a mean weight of 75.4 kg, with a range of 61.1-88.2 kg. None reported symptoms or history of low-back pain. The object lifted was a wooden box 28 cm x 27 cm x 34 cm with three dowel handles 5 cm from the top, allowing the subject to make a two-hand lift by grasping the two outside dowels or a one-hand lift by grasping the middle dowel. Bags of lead pellets were removed and replaced within the box to adjust the total weight being lifted. The box was placed within a recess in the bottom of a wooden simulated parts container 73.6 cm x 73.6 cm x 91.4 cm (Figure 1). Surface EMG electrode assemblies with silver-silver chloride electrodes were used to record action potentials for both the left and right lumbar paraspinal muscles as described elsewhere (Cook & Neumann, 1987). The signal conditioner component of each amplifier produced the instantaneous root-meansquare (RMS) value of the signal, which was smoothed with a time constant of 55 msec. The electrode assemblies were applied vertically at the L3 level over the convexity of the paraspinal muscle bellies as determined by isometric contraction and palpation. The distance from the midline to the electrodes was approximately 4-5 cm on most subjects. Both RMS and raw signals were monitored for artifacts on an oscilloscope. A piezo resistive, critically damped (+2.5 g) accelerometer was attached to the box and used to measure the vertical acceleration of the load as it was lifted. A cantilever with four strain gauges was attached to the upper edge of the container at the location of the placement of the left hand and was used to measure the left upper-extremity reaction force on the edge of the container during the assisted one-hand lift. A multichannel FM tape recorded was used to record EMG, left upper extremity reaction force, and vertical acceleration signals, as well as a trigger signal that cued the subject to begin each lift. All EMG data were normalized to a percentage of the individual’s maximum voluntary isometric contraction (MVIC) (Soderberg & Cook, 1984). After an appropriate warmup period, EMG activity from maximal voluntary erector spinae muscle contractions was Journal of Safety Research
recorded while the subject assumed a position of standing trunk flexion and with two hands isometrically pulled up on a cable tensitometer 31.75 cm from the floor for 4 s. Subjects performed a total of three MVICs with a 60-s rest period between each contraction. The erector spinae activity from the three MVICs was averaged and became the base against which all lifting data for that subject was normalized as a percentage (%MVIC). During the experiment, the investigator closed a trigger switch, which produced a clicking sound from an audio speaker and served as a cue to the subject to begin lifting. This trigger signal was also recorded on the FM tape recorder for use in digitizing the data during tape playback. The initial distance of the handles from the floor was 31 cm. The final distance of the handles from the floor was 114.4 cm when the box was rested on the top edge of the container for a total vertical excursion of 83.4 cm. The horizontal distance from the midpoint of the box in its initial location to the top edge of the container was 36.6 cm for all lifts. Three loads, 3.75 kg, 6.81 kg, and 13.64 kg, were used in randomly ordered blocks of six lifts each. Within each block, subjects randomly varied the type of lift, using one-hand assisted lift or two-hand stoop lift. Consequently, all subjects performed a total of 18 lifts. Each lift began when the sub ject heard the clicking sound. The subject lifted the load to the top edge of the container and paused for several seconds before lowering it back to its original position. Subjects were instructed to lift at whatever rate was most natural to them. No measurements were taken during lowering. There was one lift per minute, so that fatigue was not a factor. At the end of the session subjects performed post-test MVICs to
enable a comparison of pre- and post-test EMG values for reliability. A digital laboratory computer was used to process the data recorded on the FM tape and to calculate the mean values of EMG activity and left upper extremity reaction force during each lift. The trigger signal recorded during the experiment was used to begin computer digitizing. Sampling rate was 250 samples/s and continued 5 s following the trigger. The computer determined the mean of the right and left EMG signals and the upper extremity reaction force signal from the entire 5-s sampling period. These mean values, which included both the trunk flexion motion involved in bending over to grasp the load and the extension motion of lifting it, were used as the data for further analysis. Peak vertical acceleration values were obtained by displaying the vertical acceleration data recorded on an FM tape on a storage oscilloscope and then determining the peak acceleration level for each lift. EMGs, upper extremity reaction force, and acceleration data were submitted to an analysis of variance (ANOVA) with subject, lift method, and load as independent variables. An additional four-way ANOVA was performed on the EMG data using the right and left sides as additional independent variables. The means of the various experimental conditions were statistically compared against each other by using Tukey’s test for mean comparisons and multiple A-posteriori t test comparisons with Bonferroni adjustments. In all tests, a significance level of 0.05 was used. RESULTS Paired t-tests for both right- and left-side EMG data indicated no statistically significant
TABLE 1 EMG DATA FOR TWO TYPES OF LIm (ALL LOADS)
Two handed t.i
tC
Ki
):I1I
stoop Meat)
107.1
65.9
Assisted One-handed 75.3 48.2
P .OOOl
EM{:
s.d.
(ZMVIC)
n
204
204
(F=94.30)
Lel1
Mea11
113.7
103.0
-001
EMC
s.d.
68.6
64.1
(%MVIC)
n
207
Summer 1990/Vohme 21lNumber 2
208
(F=41.23)
55
TABLE 2 EMG DATA FOR THREE LOADS (BOTH TYPES OF LIFI’) Load
(kg)
%-&i
3.75
6.81
13.64
P .OOOl
EMG (%MVlC)
Mean* 6.d. n
76.6 47.1 140
88.5 53.4 131
108.7 71.9 137
Lett Et-K (XMVLC)
Mean” s-d. n
96.5 61.0 140
103.5 63.7 138
125.2 71.6 138
*
All
values
are
significantly
different:
differences between pre- and post-test maximum voluntary contractions. For both the right- and left-side EMG data, the ANOVA showed no signi~cant two-way interaction for method of lift and weight lifted. There were significant main effects for method of lift and for load on both sides. F tests revealed that both the right and left EMG activities for the assisted one-hand lift method were significantly less than for the two-hand stoop lift (Table 1). Tukey’s test for mean comparisons revealed that both the right and left EMG values were signi~cantly different for all three weights lifted (Table 2). Across both methods of lifting, as the weight of the load increased there was an associated increase in EMG activity. The results of the four-way ANOVA indicated a significant interaction between side (left and right EMG) and method (assisted one-hand and two-hand lifts) with F=6.87 and P<.OO93. There were significant main effects for side, method, and load lifted. Post hoc t-tests revealed signi~c~~y greater leftside EMG activity than right-side EMG activity during the assisted one-hand lift for all three loads. There was also significantly less right-side EMG activity during the assisted one-hand lift compared to the two-hand lift {Figure 2). For peak load acceleration ANOVA demonstrated significant interactions between weight lifted and method of lift (F=3.54, p=.O324). Post hoc t-tests were performed to examine any differences in load acceleration between the assisted one-hand lift and the two-hand stoop lift, and to also determine differences among weights lifted. The ttests revealed no significant differences (w.05) between load accelerations for the two methods of lift or for the load lifted. The 56
pc.05
(F=30.54) .0001 (1”=77.23)
(Tukey’s)
average vertical accelerations of the two lighter loads were slightly higher for the stoop lift than for the assisted one-hand lift, although the differences were not statistically significant (Figure 3). For the dependent variable, left upperextremity mean reaction force, there was a signi~c~t effect for the load lifted during the assisted one-hand lift (F=28.74, and P<.OOOl) (Figure 4). Tukey’s test for mean comparisons revealed significant differences (~~05) in left upperextremity mean reaction forces between the 13.6-kg load and each of the other two loads. No differences were found between the 3.75 kg and the 6.81-kg loads.
FIGURE2 EMG (%hWIC) OF RIGHT AND LFZT EmCTOR SPINAE USING TWO-HAND S’IOOP AND ASSISTED ONE-IfAND METHODS ‘IO LIFT THREE
DICED LOADS. EACH SYMBOL REPRE!fi~ THE MEAN OF THREE TRIALS FOR TWENTYFOUR SUBJEXTS. VERTICAL BARS INDICA-I?‘i ONE STANDARD DEVIATION
FIGURE 3 PEAK VERTICAL ACCELERATION OF THREE DIFFERENT LOADS USING THE TWO-HAND STOOP AND ASSISTED ONE-HAND LIFT METHODS. EACH SYMBOL REPRESENTS THE MEAN OF THREE TRIALS FOR TWENTY-FOUR SUBJECTS. VERTICAL BARS INDICATE ONE STANDARD DEVIATION
DISCUSSION Within the limitations of this study, the results indicate that there is significantly more erector spinae EMG activity during the twohand stoop lift than during the assisted onehand lift for both the left and right sides. During the assisted one-hand lift there is significantly less EMG activity on the lifting side (right erector spinae in this study) than on the supporting side (left erector spinae). During the two-hand lift, back muscle activity is, for practical purposes, symmetrical with no significant difference between sides. There appears to be a slight asymmetry (4%6% greater activity on the right side), which may be due to handedness, given that 90% of the population is right-handed (Donish & Basmajian, 1972). During the assisted onehand lift, asymmetry was pronounced, with approximately 30% less right EMG activity than left. This asymmetry is most likely due to spinal torsion during the assisted one-hand lift, with the trunk rotating to the left during flexion, and to the right during extension, thus requiring first eccentric, then concentric muscle activity by the left-side erector spinae. Associated reductions in EMG activity occur on the right side. As the load increased, there was a significant increase in both left and right EMG in both the two-hand stoop and assisted onehand lifts. Differences in EMG activity Summer 1990/Volutne 2lINumber 2
FIGURE 4 LEFT UPPER EXTREMITY REACTION FORCE FOR THREE DIFFERENT LOADS LIFTED USING THE ASSISTED ONE-HAND METHOD. SOLID LINE IS THE BEST FIT REGRESSION LINE. EACH SYMBOL REPRESENTS THE MEAN OF THREE TRIALS FOR TWENTY-FOUR SUBJECTS. VERTICAL BARS INDICATE ONE STANDARD DEVIATION
between the two-hand stoop lift and the assisted one-hand left were generally maintained as the load increased within the range of loads investigated here. More muscle activity is required to lift heavier loads, and more compressive force may be generated on the lumbar spine due to the increased force necessary to overcome the inertia of a heavier load. Two important factors must be kept in mind when interpreting the EMG findings in this study. First, as with most EMG studies of functional activities, all EMG values were normalized to a maximal voluntary isometric contraction and expressed as a percentage of an MVIC. Although this procedure is widely accepted and reported in the literature, MVIC testing is somewhat dependent on each subject’s motivation to expend a maximal effort. EMG measurements taken during dynamic activities are influenced by a number of factors such as changing muscle length and movement of the skin with respect to the underlying muscle tissues. During the isometric tests, the erector spinae were lengthened substantially, thus pulling the muscle fibers apart and decreasing the available muscle tension. Ortegren et al., in 1981 found that amplitude decreased with decreased floor-tohandle distance. This may explain EMG activity >lOO% during dynamic lifting. In spite of these limitations, EMG activity does provide a useful indication of the relative tension in the paraspinal muscles during the two 57
lifting methods used in this study. Studies by Ortegren and co-workers indicated a very strong linear relationship between disc pressure and integrated erector spinae EMG for both symmetrical and asymmetrical lifts justifying EMG activity as a good predictor of disc pressure. Second, EMG and upper extremity reaction force measurements derived for each &ft in this study were determined by averaging values over a 5-s time period. This time period encompassed forward bending to grasp the load as well as the act of lifting the load to the front edge of the container. The instantaneous maximal amplitudes of these measures were not examined as part of this study but are likely to be much higher than the averages reported here. Such averages were used as the primary data in this study based on the concept that they provide good estimates of the total muscle effort and force-time product required to accomplish the entire functional task. Schultz & Andersson (1981) indicated that the difference between dynamic and static calculations for predicting external and internal loading during lifting is insignificant when small accelerations and inertial forces are involved. Leskinnen et al. (1983) found in a study investigating four different types of lifting that there were no significant differences in load acceleration among different methods of lift. Post hoc t-tests revealed no significant differences in load acceleration between the assisted one-hand lift and the two-hand stoop lift or among loads lifted. The lack of differences in acceleration may be due to the limited range of weights used in this study. Load acceleration may be different for extremely heavy loads. The assisted one-hand lift is limited by the amount of weight one can safely grasp with one hand. If the load had been heavy enough, it might have accelerated at a slower rate due to difficulty in grasping and holding it with one hand. Numerical results showed a slight trend to accelerate the load more while lifting the lighter loads using the two-hand stoop lift method. During the assisted one-hand lift there was a significant increase in the left upper-extremity reaction force as the load increased (Figure 4). Ekholm (1982) determined that 70% of the maximum loading moments during lifting are produced by body segments, and 30% are produced by the load, indicating that the load 58
contributes less to the stress of lifting than the body segments themselves. During the assisted one-hand lift, the left upper extremity reaction force may assist in reducing the load of the upper trunk more than reducing the load of the object lifted, although the two effects are difficult to separate. The mean reaction force was double the weight lifted at 3.75 kg, virtually equal to the weight lifted at 6.81 kg, and was l/3 less than the 13.64 kg weight. The regression line superimposed on Figure 4 would predict that during the assisted onehand lift without any load, subjects would be willing to produce a force of 5 kg to assist in trunk extension. Further research is needed to investigate this extrapolation. At loads of approximately 8 kg, the mean upper extremity reaction force is equal to the weight lifted. As the load increases above approximately 8 kg, the upper-extremity reaction force appears to contribute progressively less force to assist back muscle activity. At these higher load levels, increased torsional moments on the spine are likely to require significant contributions by other muscle groups, along with marked increases in the erector spinae. Before such high load levels are reached, however, grip strength is likely to become a major limiting factor in using the assisted onehand method. SUMMARY As measured by EMG, the assisted onehand lift appears to produce less stress on the lumbar spine than does the two-hand stoop lift for loads up to 14 kg, at least under the conditions simulated in this study. Peak vertical accelerations of three different magnitude loads were not found to be significantly different between the two lifting methods. The upper extremity reaction force showed a linear, but not equal, relationship to the load lifted using the onehand lift, an asymmetrical lift with a use limited to loads that can be grasped and lifted using one hand. Results of this study imply that adoption of this technique may reduce the potential of low-back injury when lifting items of approximately 14 kg or less out of deep containers, especially when the other lift methods are not possible. Journal of Safety Research
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Engineering in Medicine. 12,87- 89. Lovested, G. E. (1981). Parts lifting study/ John Deere
Akeson, W., & Murphy, R. (1977). Editorial comment: Low back pain. Clinical Orthotxxiics. 129.2-3. Cook, T: M.. & Neumann.b. A. (1987). The effects of load placement on the EMG activity of the low back muscles during load carrying by men and women. Ergonomics, 30.1413-1423. Davis, P. R., Troop, J. D. G., & Bumard, J. H. (1965). Movements of the thoracic spine when lifting: A chronocyclogmphic study. Journal of Anatomy, 99,13-26. Donish, E. W. & Basmajian, J. V. (1972). Electromyography of the deep muscles of the back.
parts distribution warehouse, Moline, IL: Unpublished study by Deere & Co. Lovested,.G. E. (1985). Materials handling safety in industry, in: Materials Handlinn Handbook, New York John Wiley and Sons. Ortegren, R., Andersson, G. B., & Nachemson, A. L. (1981). Studies of relationships between lumbar disc pressure, myoelectric back muscle activity and intraabdominal (intragastric) pressure. Spine, 6,98-103. Schultz, A., & Andersson, G. (1981). Analysis of loads on the lumbar spine. Spine 6,76-82. M. (1984). Soderberg, G. L., & Cook T. Electromyogtaphy in biomechanics. Physical Therapy,
American Journal of Anatomy, 133.2536. Ekholm, J., Arborelius, U. P., & Nemeth, G. (1982). The
load on the lumbosacral joint and trunk muscle activity during lifting. Ergonomics, 25(2), 145-61. Leskinnen, T, Stalhammar, H., Dourinka, Il. et al. (1983). The effect of inertial factors on spinal stress when lifting.
Summer 1990/VoItune
21lNumber
2
64,1813-1820.
U. S. Department of Health and Human Services. (1981). Work practices guide for manual lifting. (NIOSH Publication No. 81-122). Cincinnati, OH: Physiology and Ergonomics Branch.
59