An electromyographic analysis of two techniques for squat lifting and lowering.

Epidemiologic studies attribute a considerable proportion of the incidence of low back pain (LBP) to lifting related injuries.[1-4] In an attempt to the occurrence of low back injury and decrease compensation costs, instruction in techniques for safe load handling has been advocated.[5-8] It is generally accepted that the squat technique for handling loads, as opposed to the stoop technique, best minimizes the risk of low back injury.[9,10] There are, however, varied positions that the low back and pelvis can assume while performing squat techniques, and controversy exists as to the optimal alignment. Theories have been proposed as to the safest squat technique, but to date, no one method has been shown to be more effective than any other in preventing LBP.[11,12]

One type of squat technique is that in which the lumbar spine is maintained in its normal lordosis[13] and the pelvis is aligned in an anterior tilt (AT). Advocate of this technique contend hat it minimizes stretch on the posterior elements of the lumbar spine and thereby decreases he stress on these structures. In addition, maintaining the normal lordotic curve throughout lifting and lowering results in a load being sustained on a relatively rigid lumbar spine. Consequently, the lumbar spine may incur less stress and better stabilization if an AT is maintained. A second type of squat technique is that in which the lumbar spine s aligned in kyphosis and the pelvis is in a posterior tilt (PT). Support of this technique originally arose from the observation that increased lordosis appeared to be related to LBP.[14] Recent literature,[15,16] however, suggests otherwise. Theoretically, in the more kyphotic style of lifting, the flexion moment is minimized by tension in the posterior ligaments and lumbodorsal fascia,[17] whereas the erector spinae musculature remains relatively quiescent. The decreased erector spinae muscle activity should result in less compressive force to the posterior elements of the lumbar spine. Furthermore, in addition to the hip extensors acting to maintain the posterior pelvic tilt, the oblique abdominal muscles could be recruited for this purpose, thereby providing dynamic abdominal bracing.[18]

In a previous study in our laboratory,[19] the AT lift referred to as a "back-bowed-in lift") was compared with a squat lift in which the lumbar spine was aligned in relatively less lordosis with no attempt to maintain an anterior pelvic tilt (referred to as a "back-bowed-out lift"). Results demonstrated greater erector spinae muscle activity during the back-bowed-in lift as opposed to the back-bowed-out lift, particularly during the crucial initial period of the lift when the stress on the low back is maximal.[20,21] In contrast to the pattern of erector spinae muscle activity, oblique abdominal muscle activity was greater in the initial portion of the lift than in the later portion, regardless of the lifting style. Furthermore, there was no significant difference in the amount of oblique abdominal muscle activity between the two styles. Because we believe that the recruitment of trunk musculature during lifting is crucial in minimizing the stress on the inert structures of the lumbar spine, we concluded from this study that the back-bowed-in (or AT) position provided the optimal muscular support for the lumbar spine when handling loads. We based our conclusion on the following, Erector spinae muscle recruitment is important in counteracting the bending moment in the lumbar spine during the initial portion of the lift. Without activation of the erector spinae muscles, and the resultant loss of the lordotic posture, the bending moment would then be counteracted by the noncontractile elements of the lumbar spine,[22] with potentially damaging consequences to these structures. In addition, the erector spinae musculature may play a role in counteracting the deleterious anterior shear forces at the lumbar spine produced by the weight of the upper body and the load.[23] Furthermore, it has been suggested that oblique abdominal muscle activation may provide support to the lumbar spine by (1) enhancing the retinacular function of the thoracolumbar fascia[17,24] and (2) increasing intra-abdominal pressure.[25]

This study was an extension of our previous study[19] and compared the AT position with a PT position, in which an active effort was made to maintain the pelvis in a posterior tilt. Furthermore, although the lumbar spine may potentially be injured during lowering of loads, there is a lack of information on the events associated with lowering loads. An added element of this study, therefore was an examination of lowering of loads. The purpose of this study was to examine the effects of two different alignments of the lumbar spine and pelvis (AT versus PT) and three different loads on the electromyographic (EMG) activity of the erector spinae and oblique abdominal masculature during both squat lifting and lowering.

Method

Subjects

Fifteen healthy subjects (3 men, 12 women), ranging in age from 20 to 33 years (X=24.6, SD=4.05) participated in this study None of the subjects had a history of either significant episodes of LBP or pathological conditions of the knee. Participants were recruited from the student and staff populations of Washington University School of Medicine (St Louis, Mo). Before testing, the experimental procedures were explained to the subjects and all questions were answered. Subjects then read and signed a consent form approved by the university's Human Studies Committee.

Instrumentation

The EMG signal was acquired and processed as previously described by Mayhew et al.[26] Standard silver-silver chloride electrodes(*) were used as transducers for the EMG signals. The signals were first passed through preamplifiers (built in our laboratory) with a bandwidth of 17.5 Hz to 26 kHz, an input impedance of 100,000 M [OMEGA], and a common mode rejection ratio (CMRR) of 105 dB. The signal was conducted over a 1.5-m-long shielded cable to an amplifier (built in our laboratory) with a bandwidth of 0 to 20 kHz, a CMRR of 140 dB, and an input impedance of 100,000 M [OMEGA]. After amplification, the signal was full-wave rectified with a precision rectifier and low-pass filtered with a second-order, low-pass Butterworth filter (cutoff frequency of 5.86 Hz). After rectification and averaging with an upper cutoff of 20 kHz and a time constant of 12 milliseconds, the averaged rectified output was displayed on a multichannel pen recorder(dagger) with a maximum cutoff frequency of 125 Hz.

Subjects lifted a 0.9-kg crate containing variable disk weights. A pressure-sensitive switch was secured to the bottom of the crate to indicate the beginning of the lift and the end of the lower, and a single-axis electrogo-niometer built in our laboratory was secured to the subject's hip. Information from the goniometer was used to indicate the end of the lift and the beginning of the lower (Fig. 1). These data were recorded on the pen recorder.

Procedure

Before application of the EMG electrodes, the skin at the placement site was rubbed with alcohol and slightly abraded to ensure good surface contact and to reduce skin resistance. Pairs of electrodes were applied bilaterally on the skin surface overlying the bellies of the lumbar erector spinae muscles at the L3-4 spinal level and the oblique abdominal muscles midway between the 12th rib and the iliac crest. The interelectrode separation for each pair of active electrodes was 2 cm. A ground electrode for each pair of active electrodes was applied to the nearest bony prominence (on the lower thoracic spinous processes for erector spinae muscle electrodes and laterally on the 10th ribs for oblique abdominal muscle electrodes).

The axis of die electrogoniometer was placed at the level of the greater trochanter on the subject's left side. One arm of the electrogoniometer was aligned with the longitudinal axis of the trunk and secured with an elastic bandage, and the other arm of the electrogoniometer was aligned with the longitudinal axis of the thigh and secured with an elastic bandage.

The squat lifting and lowering technique in which the lumbar spine is aligned in normal lordosis and the pelvis is aligned in an anterior tilt will be referred to as the "anterior tilt (AT) squat" (Fig. 2). The squat lifting and lowering technique in which the lumbar spine is aligned in kyphosis and the pelvis is aligned in a posterior tilt will be referred to as the "posterior tilt (PT) squat" Fig. 3).

To determine a percentage of maximal EMG activity during the activity the subjects performed maximal voluntary isometric contractions (MVICs) of both the erector spinae and oblique abdominal musculature. The MVIC of the lumbar erector spinae muscle group was obtained by having the subject execute an upper torso lift from the prone position with arms out to the side while resistance was applied to the shoulders bilaterally. The MVIC of the oblique abdominal muscle group was obtained by having the subject perform a partial sit-up from the supine position with knees straight and arms straight in front of the body while resistance was applied to the shoulders bilaterally. The positioning used to determine the MVICs was chosen based on our experience during pilot research. Subjects were randomly assigned a sequence for performing the squats, starting with either the AT or the PT style. Both styles of squat were performed with each of three loads. A light load was the 0.9-kg crate only, a moderate load was the crate containing 6.8 kg of disk weights for female subjects and 11.4 kg of weights for male subjects, and a heavy load was the crate containing 13.6 kg of weights for female subjects and 22.7 kg of weights for male subjects. The weights selected, which were deemed submaximal loads for all subjects, were in accordance with safe and acceptable limits set by the Industrial Labor Organization.[27] Subjects performed the squat lifts and lowers at their preferred rate, completing two repetitions for each load for each type of squat condition.

Data Reduction and

Normalization 

Based on the total duration (as determined by the pressure-sensitive switch and the electrogoniometer), both the lift and the lower were divided into two equal phases. The EMG activity of the erector spinae and the oblique abdominal musculature was then quantified by manually digitizing area under the curve for each phase(double dagger) The areas for the lift and the lower were normalized for the duration of the phase, and the lift and the lower were then expressed as a percentage of the time-normalized value of die peak EMG activity obtained during an MVIC of the respective muscle. The values for EMG activity of two trials for the same condition were average, and values for the right and left muscles were then averaged.

Data Analysis

We used a three-way analysis of variance for repeated measures (2x2x3) to analyze the effect of the following factors on the amount of EMG activity: (1) style of lifting and lowering (AT versus PT), (2) phase of lifting and lowering (first half versus second half), and (3) load (light, moderate, and heavy). Electromyographic activity was analyzed separately for the erector spinae and oblique abdominal muscles, as were the lifting and lowering activities. Scheffe's posttest analyses were used to analyze significant main effects and interactions. Results were considered to be significant at the alpha level of .05.

Results

Time Course of

Lowering Activities

Because the time course of an activity can alter the EMG amplitude profile and be construed as a confounding factor of the dependent variable, we examined the duration of the activities. The means, standard deviations, and ranges for the duration of each activity are reported in Tables 1 and 2. We believe that the difference in timing between the compared activities was nominal (an average of about 0.1 second) and does not warrant concern for how this factor might confound the interpretation of our data.

Erector Spinae Muscles

During lifting, erector spinae muscle EMG activity was greater during the AT style than during the PT style (P<.05) (Tabs. 3, 4). There was also a difference in EMG activity of the erector spinae muscles between the first and second halves of the lift (P<.05) (Tabs. 3, 4). Both the effect of style of lifting and especially the effect of phase of lifting, however, were qualified by the presence of a style of lifting x phase of lifting interaction, in which the comparison of erector spinae muscle EMG activity between the first and second halves of the lift differed or the two styles f lifting (P<.05) (Tabs. 3, 4; Fig. 4). Scheffe's post hoc tests revealed significant differences (P<.05) for these factors, with the first half of the AT lift showing greater activity than the second half of the AT lift and either half of the PT the and with the second half of the AT lift showing greater activity than either half of the PT lift. In addition, EMG activity of the erector spinae muscles increased with increasing loads (P<.05) (Tabs. 3, 4; Fig. 5). Scheffe's post hoc tests revealed significant differences (P<.05) for this factor, with the heavy load showing greater activity than both the moderate and the lot loads and with the moderate load showing greater activity than the light load. During lowering, erector spinae muscle EMG activity was greater during the AT style than during the PT style (P<.05) (Tabs. 5, 6). This effect was qualified by the presence of a style of lowering x phase of lowering interaction, in which the comparison of erector spinae muscle EMG activity between the first and second halves of the lower differed the two styles of lowering (P<.05) (Tabs. 5, 6; Fig. 6). Scheffe's post hoc tests revealed significant differences (P<.05) for these factors, with the first half of the AT lower showing greater activity than either half of the PT lower and the second half of the AT lower showing greater activity than either half of the PT lower. As with lifting, erector spinae muscle EMG activity during lowering was again found to increase with increasing loads (P<.05) (Tabs. 5, 6; Fig. 7). Scheffe's post hoc tests revealed significant differences (P<.05) for this factor, with both the heavy and the moderate loads showing greater activity than the light load.

[TABULAR DATA OMITTED]

Oblique Abdominal Muscles

During lifting, oblique abdominal muscle EMG activity was greater during the first half of the lift as opposed to the second half for both lifting styles (P<.05) (Tabs. 3, 7). As with erector spinae muscle EMG activity, oblique abdominal muscle EMG activity also increased with increasing loads during lifting (P<.05) (Tabs. 3, 7; Fig 8). Scheffe's post hoc tests revealed significant differences (P<.05) for this factor, with both the heavy and the moderate loads showing greater activity than the light load. During lowering of the load, the pattern of oblique abdominal muscle EMG activity was reversed from that of lifting, whereby oblique abdominal muscle activity was greater during the second half of the lower as opposed to the first half (P<.05) (Tabs. 5, 8). Oblique abdominal muscle activity was determined not to be significantly different between the styles for either the lifting or lowering activities (Tabs. 7, 8).

[TABULAR DATA OMITTED]

Because of technical difficulties related to the EMG signal from the oblique abdominal musculature, data from all subjects could not be analyzed. We were able to analyze the oblique abdominal muscle EMG data from 12 of the subjects for the lifting activity and from 13 of the subjects for the lowering activity.

Discussion

During lifting, the greatest loading moment at the lumbar spine occurs during the very early portion of the activity,[20,21] when the body must overcome inertial forces in order to complete the lift successfully. For this reason, the lift was analyzed relative to an early phase and a later phase, with the muscle strategy considered to be the most critical for protecting the lumbar spine during the early phase of lifting. Although of the load does not involve overcoming great inertial forces, the lowering activity was also analyzed relative to two phases.

Lifting and Erector 

Spinae Muscle Activation

For reasons discussed earlier, both erector spinae and oblique abdominal muscle activation are important during lifting. Contraction of the erector spinae musculature is necessary to counteract the bending moment on the spine resulting from the load. Although the activity of the erector spinae muscles will generate undesirable compressive forces to the lumbar spine, the spine is able to withstand a considerable amount of these forces. What the spine cannot tolerate, however, are the damaging anterior shear forces produced by the weight of the upper body and the load.[28] The erector spinae muscle activity provides a mechanism for counteracting these damaging anterior shear forces.[23] The recruitment pattern of the erector spinae muscles is therefore apparently beneficial during load handling.

Results of this study demonstrate greater erector spinae muscle activity during the AT (X = 48% [+ or -] 18% of MVIC) as compared with the PT lift (X = 28% [+ or -] 13% of MVIC), particularly during the initial portion of the AT lift (X = 55% of MVIC). Possibly, the elongated state of the back extensor muscles, which occurs with the PT position, inhibits activation of these muscles,[29] leaving the ligaments and fascia to counteract the bending moment. Theoretically, then, if when lifting with a PT position a sudden change in load occurs, or an individual slips or falls, the erector spinae muscles would be at a disadvantageous length to generate the tension needed to resist this change.[30] The compensatory adjustment (counteraction of the bending moment) is then left to the already elongated posterior ligametous systern, which is not well adapted to resist sudden changes in tension. As a result, there is ligamentous strain.[31] Ligaments and fascia do not have the healing capacity that muscles have.[32,33] Consequently, because of the limited healing capacity of them inert tissues, we believe this ligamentous strain may result in chronic LBP.

We have assumed that muscle tension increases with EMG activity. Although the relationship between EMG activity and tension in a muscle of unvarying length is essentially linear.[34,35] the slope of this linear relationship changes at different points in the muscle's length-tension curve.[36] Because we examined EMG activity in the erector spinae musculature with the pelvis in two different positions, there were probably two different lengths of these muscles. We must therefore consider the changing relationship of EMG activity and tension at varying muscle lengths. Moritani and deVries[36] have shown that systematic shortening of a muscle, which may occur during an isometric contraction, results in elevated EMG activity. Conceivably, then, when a muscle is working in a shortened position, the curve of the EMG activity-tension relationship may shift to the left, that is, for a given amount of tension, there is more EMG activity. We believe, however, that the magnitude of differences in erector spinae muscle EMG activity between the two styles of lifting investigated in our study was great enough to warrant the conclusion that the erector spinae musculature produces greater tension during the AT lift than during the PT lift. Future investigations, however, need to examine the EMG activity-tension-length length relationships in the trunk musculature.

Lowering and Erector

Spinae Muscle Activation

As with lifting, erector spinae muscle activity was greater during the AT lower (X = 45%[+ or -] 23% of MVIC) than during the PT lower (X = 20% [+ or -] ll% of MVIC). The AT position then would result in more support from the erector spinae musculature during lowering. In contrast to the phasic differences in muscle activity observed with lifting phasic differences were not demonstrated in lowering. Erector spinae muscle activity while lowering a load was fairly uniform throughout the entire activity with no real distinction between the first and second halves of the lowering activity for either style. Perhaps critical timing of erector spinae muscle support during lowering is not important and just a uniform degree of muscular tension is needed throughout the entire activity. We believe this makes sense for the following reasons: (1) the lowering activity is all negative work and (2) there are no substantial inertial forces to overcome while lowering.

Lifting and Oblique

Abdonimal Muscle Activation

Cantilever systems have been used to model the lumbar spine and forces imposed on it during load handling.[25,37] According to this type of model, however, a large amount of compression is placed on the lumbar spine as a result of the forces developed by die erector spinae muscles to counteract the bending moment on the spine.[38] In an effort to account for the spine's ability to handle these loads without injury, mechanisms for reducing the potentially deleterious compressive forces at the lumbar spine have been described. These mechanisms have included increasing intra-abdominal pressure[25.39,40] and tensing the thoracolumbar fascia,[17] both thought to be a function of oblique abdominal muscle activity.[17,25,39]

Results of this study demonstrate greater oblique abdonimal muscle activity in the first half of the lift than in the second half, regardless of the lifting style. Furthermore, there was no significant difference in the amount of oblique abdominal muscle activity between the two lifting styles. Interestingly, abdominal muscle activity was minimal during both lifting styles (X = 13% [+ or -] 10% of MVIC for AT lift, X = 15% [+ or -] 12% of MVIC for PT lift). We believe, therefore, that the increase in intra-abdominal pressure that occurs with lifting is not solely a result of oblique abdominal muscle activity This concurs with findings by others,[41,42] which left us to question the importance of oblique abdominal muscle activity during lifting for the purpose of increasing intra-abdominal pressure.

The importance of the oblique abdominal muscles' contribution to support of the lumbar spine through its role in tensing the thoracolumbar fascia is as yet to be determined. The adequacy of the forces developed through this mechanism in lending any substantial support to the lumbar spine during load handling, however, is questionable.[43]

Aspden[44] has suggested that die cantilever model of the spine may not be correct. He proposes an alternative model in which the spine is described as an arch and the trunk muscles have a role in enabling the spine to respond to

external loads by adjusting the curvature of die arch and die compressive stresses (or trust) within it. According to this model, if the line of thrust does not he within the cross-section of the arch, tensile stresses will be developed in the spine. Stability of the spine is then dependent on the spine's ability to sustain these tensile forces. Aspden has shown that during lifting with a flexed spine, the line of thrust does fall outside of the cross-section of the arch, theoretically increasing the tensile sile forces. When lifting with a lordosis, however, the line of thrust falls within the arch, and stability of the spine is ensured. Predicted loads on the lumbar spine during load handling using this arch model results in dramatically reduced forces as compared with those predicted using the cantilever model.

In light of Aspden's argument[44] the importance of abdominal muscle activity for effectively increasing intra-abdominal pressure and tensing the thoracolumbar fascia during load handling may not be as great as once thought. If abdominal muscle activity does contribute - even minimally - to support of the lumbar spine, however, the results of our study have demonstrated no differences in either die pattern or the amount of oblique abdominal muscle activity between the two squat styles. There appears, therefore, to be no advantage to the PT lift relative to abdominal muscle activity.

Lowering mg Oblique

Abdominal Muscle Activation

As with lifting, there was no difference in the amount of oblique abdonimal muscle activity between the two styles of lowering. AM, oblique abdominal muscle activity was minimal for both styles of lowering X = 12% [+ or -] 10% of MVIC for AT lower, X = 14% [+ or -] 8% of MVIC for PT lower). Although a significant difference was found between the first half and the second half of the lowering activity, we are unable to attribute any clinical meaningfulness to such a small difference (3% of MVIC [X=11% [+ or -] 8% of MVIC for first half, X = 14% [+ or -] 10% of MVIC for second half]). As with the erector spinae musculature during lowering, critical timing is apparently not important for oblique abdominal muscular support

Limitations

Because the lifting event was operationally defined as beginning when the crate left the floor, as indicated by the pressure-sensitive switch, the EMG activity prior to this time was not quantified. The erector spinae musculature, in particular, often exhibited a substantial amount of EMG activity prior to lifting, as illustrated in Figure 1. Thus, the beginning of the lifting activity, as defined, did not co-incide with the onset of the EMG activity A case can be made for die EMG activity occurring prior to the lifting activity somehow modulating the EMG activity occurring during lifting, thereby confounding the interpretation of our results.

By defining the lifting event with the pressure-sensitive switch, we were able to standardize the beginning of the lifting activity We did not, however, have a standardized way of measuring the point at which some form of meaningful EMG activity began, as it varied considerably among the subjects. For example, a substantial amount of EMG activity sometimes occurred while the subject was descending into the squat position, and the activity continued until the crate left the floor. At other times, the EMG activity did not notably increase until the subject was already m the full squat position, but had not yet lifted the crate. A suggestion for further research in this area would be to develop and standardize an operational definition for this prelifting period, analyze the EMG activity that occurs during it, and relate it to the EMG activity that occurs during lifting.

An additional concern for our study relates to the fact that we did not monitor the raw EMG records to rule out artifact. This leaves us with the possibility that the averaging of the EMG signal may have included further support addition to the true EMG signal.

Conclusion

The findings of this study further support our belief that the optimal position for handling loads is the squat style in which the lumbar spine is aligned in its normal lordosis and the pelvis is aligned in an anterior tilt. Our results provide a stronger foundation on which to base our recommendation of the AT lift for minimizing low back injury.

(dagger) Dr Rose died on Apr 4, 1909. (8) Beckman Instruments Inc, 3900 River Rd, Schiller Park IL 60176. (dagger) Gould Inc, Instruments Div, 3631 Perkins Ave, Cleveland, OH 44114. (double dagger] Model HP-85, Hewlett-Pachard Co, Corvallis Div, 1000 NE Circle Blvd, Corvallis, OR 97330.

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