Comparison of integrated electromyographic activity and lumbar curvature during standing and during sitting in three chairs.

Comparison of Integrated Electromyographic Activity and Lumbar Curvature During Standing and During Sitting in Three Chairs Low back pain (LBP) is a common ailment affecting 70% to 80% of the population in industrialized countries. [1] Sitting is often the position of greatest discomfort. Pain during seated activities such as eating, reading, or typing can influence the quality of life and work productivity of people with LBP. [1,2] Maintenance of proper posture during sitting may help to alleviate LBP and serve as a prophylactic measure in the development of law back problems because the structures of the low back receive minimal stress when the back is well aligned.

The back is a complex system of anatomical structures. Compromise of any of these structures can lead to the development of LBP. [3-5] The integrity of the nervous tissue depends on the orientation of the vertebrae; disruptions of the bony structures will impinge on the nerves, resulting in pain. Compromise of the ability of the intervertebral disks to hydrostatically absorb and distribute load forces may also result in LBP. [3]

Two schools of thought have developed concerning the influence of sitting posture on low back structures. Some researchers support the idea that flexed postures, which reduce or eliminate the normal lumbar lordosis, will decrease stress on the posterior portions of the vertebrae and disks. These flexed positions may improve disk nutrition, slow the degenerative process, and consequently result in a reduction of LBP. [4,5] Others claim that maintenance of lumbar lordosis during sitting decreases both intradiskal pressure and myoelectric activity of the posterior paraspinal muscles. [6,7] These differences may be important in development of therapeutic programs to relieve or prevent LBP.

Several studies have examined the relationship between various chair characteristics and sitting posture to determine which factors contribute most to stress reduction and, thereby, to low back comfort. Andersson and associates investigated the effect of backrest inclination and lumbar support on paraspinal muscle activity and disk pressure. [8,9] Postural changes induced by alterations in thigh-trunk angle and thoracic support have been examined by Keegan, [6] and seat inclination has been studied by Mandal [10] and by Bendx and Biering-Sorenson. [11] These researchers found that backrest inclination, lumbar support, and a thigh-trunk angle of 110 to 130 degrees were of critical importance in the maintenance of good sitting posture.

Recently, Frey and Tecklin compared the lordosis achieved while sitting in a conventional chair with that achieved in a newly developed chair, the Balans[R] Multi-Chair (*1) (BC). [12] This new chair, commonly called a "computer chair," "kneeling chair," or "back chair," has no back support and consists of a slightly forward-tilting seat with a padded rest for the lower legs (Fig. 1A). The Bc was designed to preserve the lumbar lordosis while sitting and consequently prevent or reduce LBP. [13,14] Frey and Tecklin documented this effect in their study and showed that subjects assuemd a lordosis more similar to that of relaxed standing while writing at a desk using the BC. [12] Despite the commercial success of this chair and others like it, relatively little research has been done to substantiate its theoretical benefits. The present study was undertaken to examine low back muscular activity and lumbar curvature (LC) during use of the BC in sitting, typing, and writing as compared with two types of standard chairs. The null hypotheses for this study were 1) There will be no differences in activity of lumbar erector spinae (ES) muscles or inLC during relaxed or erect sitting in the three chairs, 2) there will be no differences in activity of lumbar ES muscles or in LC during writing or typing in the three chairs, and 3) there will be no differences in activity of lumbar ES muscles or in LC achieved in standing as compared with that achieved in sitting in the three difference chairs.

Method

Subjects

Twenty healthy subjects (12 women, 8 men), ranging in age from 22 to 37 years (X = 25 [+ or -] 3.7 years), volunteered for this study. Eighteen subjects were right-handed, and 2 subjects were left-handed. These subjects had no history of LBP or knee pain, did not use a BC or similar chair on a regular basis, and were able to type using the standard method of touch typing. An informed consent form was read and signed by each subject prior to participation in the study. The research protocol was approved by Boston University's Sargent College of Allied Health Professions Clinical Research Review Committee.

Procedure

Four pairs of Beckman silver-silver chloride miniature electrodes (*2) were placed bilaterally over the subjects' lumbar ES muscles, 3 cm lateral to the spinous processes at L2-3 and L5-S1. These sites were chosen as representative of upper and lower lumbar musculature. [15] Paired electrodes were placed 2 cm apart, center to center. The skin over these areas was prepared prior to electrode application to reduce skin resistance to below 20,000 [Omega].

Integrated electromyographic activity was recorded on a Grass Model 7D polygraph (*3) with four Model 7P3 and four Model 7P10 integrators. (*4) The amplifiers had an input impedance of 10,000 M[Omega] with a common mode-rejection ratio of 1,600:1 (90 dB). The frequency band for the integrators was set for half amplitude at 10Hz and 20 kHz, with full-wave rectification. Integrators recorded the EMG activity as a cumulative ramp function with an amplitude reset.

Following placement of the electrodes, each subject was asked to perform a maximal voluntary contraction (MVC) of the lumbar ES muscles using a modified manual muscle test technique. [16] Subjects were positioned prone, arms and their sides, and asked to extend their trunk, without using their arms, as strongly as possible. Manual resistance was given over the upper back in an anterior direction. The IEMG data recorded during the MVC were used as a control for normalization of experimental data.

Lumbar curvature was measured using a flexible ruler. (*4) The spinous processes of L2 and S1 were marked, and the flexible ruler was molded to the subjects' lumbar curve between these two points. Figure 2 shows our procedure for flexible ruler measurement of the lumbar spine with the subject in the sitting position. We used the procedure illustrated in Figure 2 of Frey and Teckin's article [12] for flexible ruler measurement of the lumbar spine with the subject in the standing position. The shape of the curve was transcribed to paper for mathematical analysis using the formula:

[theta] = 4 arctan (2 H/L)

where L represents the length of the line drawn between the points L2 and S1 and H represents the length of the perpendicular from the midpoint of this line to the molded curve (Fig. 3). [17] This technique is a modification of the method validated by Hart and Rose, who recommended measuring the distance between L1 and S2. [17] We measured the distance from L2 and S1 because of interference from the chair seat, which blocked full extension of the ruler. Rather than risk compromise of reliability because of this inteference, we chose to measure a slightly shorter arc. The arctan formula ssumes that the apex of the lordosis is located at the midpoint of the curve. [17,18] Because the apexes of both curves are at L4, both computations of the formula should produce the same result. The shorter arc, therefore, should still accurately represent the lumbar curvature. All curvature measurements were made by one researcher (AS). Intrarater reliability for flexible ruler measurements was established at .79 using an intraclass correlation coefficient [(ICC[2,1]).sup.19]

Integrated electromyographic activity was measured in both relaxed and erect postures while standing and while sitting in the BC, and office chair (OC), and a straight-back chair (SBC) (Fig. 1). Subjects were also tested while writing and typing in each chair. The OC and the SBC measured 45 cm from floor to seat, which established hip and knee angles of approximately 90 degrees. The BC was adjusted so that the rear portion of the seat was 45 cm from the floor. This adjustment allowed for direct comparison to the OC and the SBC and established a mean thigh-trunk angle of 119 [+ or -] 3.7 degrees. Keegan found this range to be optimal for comfortable sitting. [6]

Lumbar curbes were measured in the relaxed and erect postures of standing and sitting in the BC and the SBC. It was not possible to measure LC in the OC because the backrest support blocked access to the low back. Lumbar curvature was also measured during the activities of writing and typing in the BC and the SBC.

Relaxed standing was defined as standing with both feet flat on the floor, shoulder-width apart, weight evenly distributed, and trunk and arms relaxed. Erect standing was defined as a "military" posture; subjects were instructed to "stand up straight." Relaxed sitting in the OC or the SBC was defined as sitting with feet flat on the floor, sacrum against the back of the chair, and forearms resting on thighs. Relaxed sitting in the BC was defined as the subject's most comfortable position, with hands resting on thighs. For erect postures in all three chairs, the subjects were instructed to "sit up straight."

For writing and typing activities, subjects were instructed to adjust each chair to their most comfortable position in front of a desk that was a standard 75 cm from the floor. For writing, all subjects were given the same sentence to copy repeatedly until a 10-second EMG recording and a LC measurement were taken. For typing, all subjects were given the same page to type, again continuing for the 10-second EMG recording and LC measurement. The typewriter used was an electric touch-type model, and subjects were instructed not to correct mistakes, but to type as smoothly as possible.

Data Reduction and Analysis

Integrated electromyographic activity was measured for the middle four seconds of the MVC, the relaxed and erect standing and sitting postures, and during writing and typing in each chair. Each of the test values was then expressed as a percentage of MVC for each muscle. For three-way analyses of variance (ANOVAs) for repeated factors were used to examine IEMG activity in relaxed postures, erect postures, and typing and writing. The independent variables were chair (sitting in OC, SBC, BC and standing), side (right vs left), and muscle (L2 vs L5). A two-way ANOVA with two repeated factors was used to compare LC across chairs and postures (relaxed vs erect). Duncan multiple range tests were used where appropriate to identify sources of differences at the .05 level.

A two-tailed, paired t test compared LC during typing on the SBC and the BC, and another compared LC during writing in the two chairs. Typing and writing were not compared with each other because they represent different functional activities. We believed that a statistical comparison would not have beeen meaningful. To avoid an increased chance of Type I error attributable to the use of multiple t tests, the acceptable level of significance for these tests was lowered to .01.

Results

Relaxed Postures

The analysis of IEMg activity during relaxed postures demonstrated a significant interaction effect between chair and muscle (F = 8.kkf df = 3,114; p = .0003) (Tab. 1). Integrated electromyographic activity during standing was significantly greater than during sitting in the OC and the SBC, but not in the BC. Muscle activity at L5 in standing was significantly higher than at L2 in standing and greater than both L2 and L5 during sitting in all chairs (Fig. 4).

Erect Postures

In the erect postures, the main effect of chair was significant (F = 3.75; df = 3,114; p = .013), with the IEMG activity in standing greater than during sitting in the OC (Tab. 2). The interaction effect of side and muscle was also significant (F = 9.63; df = 1,38; p = .003). Muscle activity at L5 was greater on the left side than on the right side for all chairs (Fig. 5).

Typing and Writing

Analysis of IEMG acitivity during typing demonstrated a significant intereaction between chair and side (F = 4.43; df = 2,76; p = .01) (Tab. 3). Muscle activity was greater on the left side than on the right side while typing in the OC and the BC (Fig. 6). There was no significant difference in IEMG activity between sides in the SBC.

For writing, only the main effect of side was significant (F = 7.54; df = 1,38; p = .009) (Tab. 4). Again, IEMG activity was consistently greater on the left side than on the right side (Fig. 7).

Lumbar Curvature

Analysis of the LC showed a significant intereaction betweeen chair and posture (F = 5.48; df = 2,36; p = .008). Lumbar curvature in both relaxed and erect standing was significantly greater than in all seated postures. There was no significant difference, however, between relaxed and erect postures in standing. There was also no significant difference between relaxed na derect LC during sitting in the BC. In relaxed sitting, LC was greater in the BC than in the SBC. Lumbar curvature in the SBC was significantly greater in the erect as compared with the relaxed posture (Tab. 5, Fig. 8). Analysis of the LC during activities showed significant differences across chairs in both typing (t = 3.5, df = 18, p = .002) and writing (t = 3.5, df = 18, p = .003), with LC greater in the BC than in the SBC (Tab. 6, Fig. 9).

Discussion

Intensity of Muscle Activity

Less than 10% of the IEMG activity of the MVC was recorded in any of the postures of activities. Other researchers also have shown that maintenance of standing and sitting postures requires low-level muscle activity. [8,9,20] Our findings are consistent with those of Andersson et al, [8] who found less ES muscle activity in supported sitting than in standing. Similarly, both Miller [21] and Andersson et al [22] have shown than less muscle activity is present in quiet sitting postures as compared with standing. The smaller amount of IEMG activity seen during relaxed sitting in the SBC and the OC may be attributed to the chairs' backrests, which subjects used for support. Andersson and colleagues have performed several studies to examine ES muscle activity during sitting in a chair with a forward-tilting seat. [8,9,22] Subjects in these studies sat with feet flat on the floor, creating a thigh-trunk angle of more than 90 degrees. They found an increase in ES muscle activity, which was necessary to prevent the trunk from failling forward. The BC is similarly designed to tilt the pelvis anteriorly and maintain LC. The BC, however, is also designed to support the lower legs. This support reduces the forward flexion moment and may decrease the back muscle activity needed to hold the trunk upright. This consideration may explain why similar ES muscle activity is seen in standard chairs and the BC during relaxed sitting.

Significantly greater IEMG activity occurred in standing at L5 than at L2. The ES muscles at the two levels probably perform similar functions. Because of its relative proximity to the pelvis, however, the ES muscle at L5 may hve a larger role than the ES muscle at L2 in maintenance of pelvic position during standing. The difference between L2 and L5 may also be attributable to EMG crosstalk from other muscles, such as the quadratus lumborum and the multifidi muscles. These muscles may tend to be more active near L5, where greater muscle activity would be necessary to maintain proper pelvic position, than near L2.

Postural Differences in Integrated

Electromyographic Activity

In the erect postures, all chairs produced similar levels of IEMG activity, indicating that ES muscle activity is a function of the posture and not of backrest of thigh-trunk angle. In standing and all three chairs, greater IEMG activity occurred on the left side at L5 in the erect posture (Fig. 5). This finding is contrary to that of Jonsson, [23] who found no difference in ES muscle activity between sides in symmetrical activities, such as prone trunk extension, flexion in sitting, and sitting and standing with loads anterior to the trunk. Jonsson, however, did not take any recordings in erect postures nor did he integrate his EMG data as we did. Rather, he graded the EMG activity on a scale of 0 to 3. Our method of data reduction may have allowed us to detect differences in EMG activity that his method could not.

During typing, there was greater IEMG activity on the left side than on the right side in the BC and the OC, a

pattern similar to that seen in erect postures. In these chairs, subjects may have assumed a posture close to that of erect sitting than relaxed sitting. This finding may account for the similar muscle patterns seen in erect sitting and typing. The difference between sides was not found in the SBC, perhaps because subjects used the backrest for support, which would simulate a posture closer to that of relaxed sitting.

For writing, greater IEMG activity was again seen on the left side of the body than on the right side in all chairs. This trend is similar to that of typing and erect sitting. During writing, however, subjects tended to lean forward and support their upper extremities on the desk, a posture obviously different from that of typing or erect sitting.

Whether these findings are related to the majority of subjects being right-handed is unclear. Most research on low back EMG activity has concentrated on only one side of the body at a time. Research studying right- and left-handed populations separately should be conducted to examine the possibility of asymmetrical patterns related to dominance.

Standards of Lumbar Curvature

Several researchers have based their studies on the assumption that the LC in standing is an ideal reference for proper sitting posture. [6,10,12] Our results show that standing produced a greater LC than sitting in either the BC or the SBC, regardless of posture (Fig. 8). The maximum LC achieved in any of the three chairs was only half that of relaxed or erect standing. Andersson has shown that more of the decrease in lordosis from standing to unsupported sitting is due to backward pelvic rotation than to decreased lumbar extension. [8] Because pelvic position in standing is markedly different than in sitting, therefore, standing LC may not be a valid standard for evaluation of ideal sitting LC.

Lumbar curvature measurements in the SBC and the BC in relaxed postures show that the BC inherently places a person in a position of greater LC. This greater LC is likely a function of the BC's forward-sloping seat, which decreases the backward pelvic rotation that occurs with sitting. Interestingly, voluntary actions to sit erect in the BC had minimal effect on LC (Fig. 8). Although a decreased LC was evident inthe SBC when subjects relaxed, the chair did not prevent them from attaining the same degree of LC as in the BC when sitting erect.

Subjects were able to maintain a greater LC in the BC than in the SBC while they performed typical office activities. This finding further supports the claim that the BC inherently promotes a lordotic posture, requiring little conscious effort to maintain it. This finding may have implications for persons who would benefit from lumbar extension.

Reliability

Although intrarater reliability for measurement of LC in our study was good, other studies using the flexible ruler have reported higher values. [17,18] Lumbar curvature measurements in those studies, however, were taken with subjects only in relaxed standing or in full forward flexion. When measuring LC in relaxed standing only, intrarater reliability in the present study was higher (ICC[2,1] = .86) than the composite score. Frey and Tecklin measured lordosis in seated postures, but did not report intrarater reliability. [12] Further studies on the flexible ruler technique are necessary. Intrarater and interrater reliability need to be established for various standing and seated postures.

Questions of Disk Pressure

Proponents of extension for low back care claim that the maintenance of lordosis results in decreased intradiskal pressure. [3,6,8,9] Nachemson has found that unsupported sitting, especially with forward flexion, results in increased disk pressure. [3] To perform activities such as typing and writing, subjects in this study leaned forward in the OC and the SBC into an unsupported flexed sitting posture, theoretically increasing disk pressure. Andersson et al have found that anterior sitting with a lordosis, as in the BC, results in decreased disk pressure. [9] Before any assumptions can be made about the effect of sitting in the BC on disk pressure, however, specific studies investigating this relationship are necessary.

Further Research

The role of muscle activity in postural maintenance should be clarified. Our results indicate that the levels of IEMG activity seen in the BC and the SBC are similar, although a greater LC was seen in the BC (Fig. 10). This finding suggests that IEMG activity is not a good indicator of changes in lumbar posture. Other passive mechanisms may be responsible for the maintenance of lumbar extension. The anterior pelvic tilt produced by the BC will facilitate this posture.

Only the ES muscles, which are primarily mobility muscles, were monitored in this study. Although minimal IEMG activity was recorded in these muscles, deeper muscles such as the multifidi may play a more active role in postural maintenance. [23] Our study focused on the lumbar spine only. Further research should also be undertaken to investigate the impact of the BC on other body segments. Its effect on the thoracic and cervical areas should be examined in addition to its effect on the lower extremities. Individual leg length may influence comfort and posture maintenance in the BC.

Conclusions

These findings support the following conclusions:

1. Lumbar curvature and IEMG activity in standing were greater than in sitting in any of the three chairs.

2. Greater muscle activity occurred on the left side than on the right side in erect sitting, during typing and writing in all three chairs, and in erect standing.

3. A greater LC was found in the BC than in the SBC in relaxed sitting and during typing and writing.

4. There was no difference in ES muscle activity across chairs.

5. Minimal ES muscle activity was necessary to maintain standing and sitting postures.

6. Changes in IEMG activity of the ES muscles did not occur with changes in LC.

Many people currently spend the majority of their time sitting. This static posture, with the spine either flexed forward or maintained in some extension, increases the mechanical load placed on the spinal structures. Higher loads have been linked to higher levels of LBP. [3] Any improved chair design that can prevent or decrease LBP by reducing some of the load placed on the back, therefore, will obviously be a critical adjunct in physical therapy management of patients with LBP. Many persons report being able to sit for longer periods in the BC than in conventional chairs, with less discomfort in the low back. Our study has shown that LC can be maintained during activities in this chair without increased ES muscle activity. These factors may indeed play a role in prevention or reduction of patients' LBP. We would recommend, however, that more research be conducted into various aspects of the BC. Its biomechanical impact on the entire spine and lower extremity must be determined before use of the BC in a low back program can be fully advocated.

(*1) Westnofa USA, Inc, 7040 N Austin, Niles, IL 60648.

(*2) SensorMedics Corp, 1630 S State College Blvd, Anaheim, CA 92806.

(*3) Grass Instrument Co, 101 Old Colony Ave, Quincy, MA 02169.

(*4) American Black School, 1514 13th St, PO Box 1193, Ashland, KY 41105-1193.

References

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[14] Westnofa Balans Seating: Lumbar Curvature and Back Strain. Niles, IL, WEstnofa Balans Seating, Div Westnofa USA, Inc, 1986

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D Bennett, MSPT, PT, is Staff Physical Therapist, New England Deaconess Hospital, 185 Pilgrim Rd, Boston, MA 02215. Address all correspondence to Ms Bennett at 1471 Beacon St #10, Brookline, MA 02146 (USA).

D Gillis, MSPT, PT, is Staff Physical Therapist, Colorado Sports and Spine center, Colorado Springs, CO 80919.

LG Portney, MS, PT, is Research Therapist, Motor Control Laboratory, Tufts University/New England Medical Center, Boston, MA 02111.

M Romanow, MSPT, PT, is Staff Physical Therapist, The Greenery, Brighton, MA 02135.

A Sanchez, MSPT, PT, is Staff Physical Therapist, Virginia Mason clinic, Seattle, WA 98111.

Ms Bennett, Ms Gillis, Ms romanow, and Mr Sanchez were students in the Master of Science in Physical Therapy Program, Sargent College of Allied Health Professions, Boston University, Boston, MA 02215, when this study was completed in partial fulfillment of the requirements for their master's degree. Ms Portney was their research advisor.

This article was submitted August 10, 1987; was with the authors for revision for 64 weeks; and was accepted April 7, 1989.