The Effect of Different Standing and Sitting Postures on Trunk Muscle Activity in a Pain-Free Population

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1 The Effect of Different Standing and Sitting Postures on Trunk Muscle Activity in a Pain-Free Population SPINE Volume 27, Number 11, pp , Lippincott Williams & Wilkins, Inc. Peter B. O Sullivan, PhD,* Kirsty M. Grahamslaw, M Manip Ther, PT, Michelle Kendell, M Manip Ther, PT, Shaun C. Lapenskie, M Manip Ther, PT, FCAMT, Nina E. Möller, M Manip Ther, PT, and Karen V. Richards, M Manip Ther, PT Study Design. A normative, single-group study was conducted. Objective. To determine whether there is a difference in electromyographic activation of specific lumbopelvic muscles with the adoption of common postures in a painfree population. Summary of Background Data. Clinical observations indicate that adopting passive postures such as sway standing and slump sitting can exacerbate pain in individuals with low back pain. These individuals often present with poor activation of the lumbopelvic stabilizing musculature. At this writing, little empirical evidence exists to document that function of the trunk and lumbopelvic musculature are related to the adoption of standardized standing and sitting postures. Methods. This study included 20 healthy adults, with equal representation of the genders. Surface electromyography was used to measure activity in the superficial lumbar multifidus, internal oblique, rectus abdominis, external oblique, and thoracic erector spinae muscles for four standardized standing and sitting postures. Results. The internal oblique, superficial lumbar multifidus, and thoracic erector spinae muscles showed a significant decrease in activity during sway standing (P 0.027, P 0.002, and P 0.003, respectively) and slump sitting (P 0.007, P 0.012, and P 0.003, respectively), as compared with erect postures. Rectus abdominis activity increased significantly in sway standing, as compared with erect standing (P 0.005). Conclusions. The findings show that the lumbopelvic stabilizing musculature is active in maintaining optimally aligned, erect postures, and that these muscles are less active during the adoption of passive postures. The results of this study lend credence to the practice of postural retraining when facilitation of the lumbopelvic stabilizing musculature is indicated in the management of specific spinal pain conditions. [Key words: abdominal muscles, back muscles, electromyography, lumbar spine, posture, sacroiliac joint, stability] Spine 2002;27: From the *School of Physiotherapy, Curtin University of Technology, Shenton Park, Western Australia, the Taylor Physiotherapy and Sports Injury Clinic, Edinburgh, Scotland, United Kingdom, SportsMed Murdoch, Murdoch, Western Australia, D. Freer and Associates Physiotherapy, Barrie, Ontario, Canada, the Haukeland University Hospital, Bergen, Norway, Ruth Jones Physiotherapy, Southampton, Hampshire, United Kingdom. Acknowledgment date: May 15, First revision date: September 17, Acceptance date: October 24, The manuscript submitted does not contain information about medical devices or drugs. No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Postural retraining has traditionally been an integral part of physiotherapeutic intervention in the treatment of low back pain (LBP). However, the relation between posture and LBP is largely unknown. 11 It has been proposed that different postures that superficially appear similar may lead to altered muscle activation. This results in the generation of very different forces in the skeletal system. 5 Postural training works on the assumption that an optimally aligned skeletal system reduces stress in its structures. Commonly adopted relaxed postures often are passive in nature, with a predisposition toward sway standing and slump sitting. 15 It is proposed that these postures rely on the passive lumbopelvic structures for the maintenance of an upright position against gravity. As a result, the requirement for muscle activity is diminished. It has been reported clinically that these passive postures frequently exacerbate LBP, 27,38 and that the adoption of these postures is associated commonly with motor dysfunction of spine-stabilizing muscles such as the lumbar multifidus muscle and the deep abdominal musculature. 27 Bergmark 9 proposed that the muscles controlling the trunk could be classified into two groups. The first group includes muscles attached directly to the lumbar vertebrae that can provide spine segmental stability. The lumbar multifidus, transversus abdominis, and internal oblique muscles are part of this group. The second group consists of large torque-producing muscles with no segmental attachment to the lumbar spine. These muscles control gross trunk movement and provide general trunk stability. They include the rectus abdominis, external oblique, and thoracic erector spinae muscles. It is recognized that cocontraction of spine-stabilizing muscles, specifically the lumbar multifidus, transversus abdominis, and internal oblique muscles, has the ability to enhance lumbopelvic stability. The lumbar multifidus muscle is unique in its ability to enhance lumbar segmental stability 14,41 while dynamically stabilizing the sacroiliac joint through its control of sacral nutation. 36 The transversus abdominis and internal oblique muscles provide a stabilizing influence on the lumbar spine via the thoracolumbar fascia and control of intraabdominal pressure. 13,20,39 The anteroinferior portion of the internal oblique and transversus abdominis muscles is capable of generating compression and hence of increasing the stability of the sacroiliac joints. 32,

2 Effect of Different Postures O Sullivan et al 1239 At this writing, no empirical evidence exists to document that the function of the stabilizing and torqueproducing muscles in the lumbopelvic region is related to the adoption of standardized standing and sitting postures in subjects with or without LBP. The function of the lumbar spine muscles during standing and sitting has been examined previously using electromyography (EMG), but there has been an overwhelming focus on the erector spinae muscle group to the exclusion of the stabilizing musculature. 1,15,40 The methods used to analyze posture in the existing literature often are found to be complex and difficult to reproduce in a clinical setting, 17,42 and in other studies, postural standardization has not been quantified. 35,37,38 The comparability and clinical application of these studies is reduced by the lack of postural standardization. In light of these limitations, this study was performed to determine whether a relation existed between activation of specific lumbopelvic muscles and the adoption of common standing and sitting postures in subjects without LBP. It was hypothesized that the internal oblique and superficial lumbar multifidus muscles acting as stabilizers of the lumbopelvic region would show greater activation in active postures (erect standing and sitting) than in passive postures (sway standing and slump sitting). Methods Participants. This study recruited 20 adult subjects (10 men and 10 women) from the Perth metropolitan region. Their mean age was 27.8 years (range, years), mean height 174 cm (range, cm), and mean weight 67.7 kg (range, kg). Ethical approval and written informed consent were obtained. Subjects were excluded if they were pregnant, had a body mass index greater than 30, had any reports of LBP (requiring medication or consultation with a health professional) in the preceding 6 months, had any known spine disorders, or experienced pain in the test postures. Equipment and Procedures. Markers were placed on the right lateral tip of the acromion, the midpoint of the greater trochanter, the tip of the lateral malleolus, and the L1 spinous process by the same investigator. The skin was prepared to reduce skin impedance below 5 k. Two Ag/AgCl surface electrodes (3M Health Care Products, London, Canada) were placed unilaterally 25 mm apart and parallel to the muscle fibers over the following muscles: rectus abdominis (5 cm inferior to the xiphoid process and 3 cm lateral to midline), external oblique (lower edge of eighth rib), thoracic erector spinae (5 cm lateral to T9 spinous process), internal oblique (medial to the anterior superior iliac spine), 26 and superficial lumbar multifidus (2 cm lateral to midline at the L4 L5 interspinous space). 33 The internal oblique electrode also may have picked up activity of the transversus abdominis muscle, which lies directly beneath it. A common earth electrode was placed over the iliac crest. The EMG equipment consisted of a Bortec (Bortec Electronics, Calgary, Canada) and a Medelec amplifier (Oxford Instruments, Surrey, UK) connected to a Power Macintosh computer Figure 1. Erect standing posture: 177 angle between markers. (Apple Computer, Cupertino, CA). The gain setting was 3 K, and the signal was sampled at 1000 Hz. Data were processed and stored on Labview 5.1 Virtual Instruments Software (National Instruments, Austin, TX). Raw data were demeaned and band-pass filtered at 6 to 400 Hz) using a fourth order zero lag Butterworth filter (National Instruments, Austin, TX). The use of two EMG amplifiers had no effect on the outcome analysis because the comparison was made only of individual muscle activation between postural conditions. Electromyographic recordings of thoracic erector spinae muscles were acquired for 15 subjects because of missing data. During data collection, the study participants were barefoot with arms relaxed and lightly clasped in front of their body and feet positioned 20 cm apart. They were instructed to focus straight ahead at a designated point. An adjustable height treatment table was used for all the sitting positions. The hips and knees were flexed to 90. Using standardized instructions, the participants were positioned by the same investigator for all the trials. Four standardized postures were examined: erect standing, sway standing, erect sitting, and slump sitting. The ability to reliably position subjects in the test postures was determined in a pilot study using five subjects undergoing two trials. Intraclass correlation values were between 0.86 and 0.94, showing excellent reliability. Erect standing was defined as a position in which the markers on the acromion, greater trochanter, and lateral malleolus lined up to form an angle of approximately 180 (Figure 1). To move into sway standing, the participants relaxed and let the pelvis translate anteriorly relative to the trunk (Figure 2). The angle between the markers then was remeasured. In this study, it was deemed necessary for the angle between erect and sway standing to differ by a minimum of 15.

3 1240 Spine Volume 27 Number Figure 2. Sway standing posture: 151 angle between markers. Figure 3. Erect sitting: 132 angle between markers. Figure 4. Slump sitting: 93 angle between markers. Erect sitting was defined as a position in which the subject had a neutral pelvic tilt, neutral lumbar lordosis, and neutral thoracic kyphosis (Figure 3). To move into slump sitting, the trunk was relaxed into flexion and the pelvis rotated posteriorly (Figure 4). The angle between the acromion, L1 spinous process, and greater trochanter was measured. In this study it was deemed necessary for the angle between erect and slump sitting to differ by a minimum of 20. To measure the postures objectively, a digital camera (Intel, Santa Clara, CA) was placed perpendicularly to the participant 275 cm from the proximal foot for the standing postures and 200 cm for the sitting postures. The same investigator took a digital photograph, which was imported into an IBM 1460i laptop computer (IBM, New York, NY) running Scion Image (Scion, Fredrick, MD), an image-processing program that measures angles between manually marked positions on a digital image. Surface EMG measurements were amplitude normalized to two standardized activities designed to elicit a stable submaximal voluntary contraction. This was performed because normalization to a maximal voluntary contraction reportedly is unreliable, 23 reducing the ability to detect small changes in levels of motor activity during the performance of postural tasks. 38 The first standardized activity was the crook-lying bilateral leg raise, in which the heels were held 5 cm above the plinth, during which activity of the internal oblique, rectus abdominis, and external oblique muscles was measured for 3 seconds. The second activity was prone-lying bilateral active knee flexion, in which the dorsum of the feet was held 5 cm above the plinth, during which the activity of thoracic erector spinae and superficial lumbar multifidus muscles was measured for 3 seconds. The positions were tested in the following order: supine lying (used as a reference position), crook-lying bilateral leg raise, prone-lying bilateral active knee flexion, erect standing, sway standing, erect sitting, and slump sitting. After a 15-

4 Effect of Different Postures O Sullivan et al 1241 Figure 5. Error bars indicate standard deviation. SLM superficial lumbar multifidus; IO internal oblique; EO external oblique; RA rectus abdominis; TES thoracic erector spinae second delay to ensure stable conditions, three 3-second trials were recorded for each position. A rest period of 1 minute was given between test postures to eliminate fatigue. The EMG data of the three trials were visually inspected. To reduce the effects of postural sway, the trial with the least number of spikes was selected for analysis. In the case of three consistent traces, the first trace was chosen to avoid bias. The electrocardiographic artifact then was extracted and the root mean square of the EMG data calculated. Statistical Analysis. A sample of 20 subjects was determined to be sufficient for detecting an effect of moderate size with an alpha level of 0.05 and a power level of 80%. Statistical analysis was performed using SPSS statistical analysis software, Version 10.0 (SPSS, Chicago, IL). Data were analyzed for normality, and parametric analysis was performed. A paired t test was used to detect a difference in muscle activation between erect and sway standing for all five muscles. The test was repeated to detect a difference in muscle activation between erect and slump sitting. Results The mean and standard deviation of muscle activities (expressed as a percentage of submaximal voluntary contraction) for erect versus sway standing and erect versus slumped sitting are presented in Figures 5 and 6, respectively. Standing Postures There was a significant decrease in superficial lumbar multifidus (t[19] 3.631; P 0.002), internal oblique (t[19] 2.400; P 0.027), and thoracic erector spinae (t[14] 3.526; P 0.003) activation as subjects moved from erect standing into sway standing. Conversely, the rectus abdominis muscle (t[19] 3.142; P 0.005) Figure 6. Error bars indicate standard deviation. SLM superficial lumbar multifidus; IO internal oblique; EO external oblique; RA rectus abdominis; TES thoracic erector spinae

5 1242 Spine Volume 27 Number was significantly more active, and the external oblique muscle (t[19] 1.490; P 0.153) showed a trend toward increased activity in the sway standing position, but the latter was not statistically significant. Sitting Postures There was a significant decrease in superficial lumbar multifidus (t[19] 2.796; P 0.012), internal oblique (t[19] ; P 0.007), and thoracic erector spinae (t[14] 3.634; P 0.003) activation as the participants moved from erect sitting into slump sitting. Neither the rectus abdominis muscle (t[19] 0.436, P 0.668) nor the external oblique muscle (t[19] 0.842; P 0.410) showed any significant difference in activity between the two sitting postures. Discussion The results of this study show there is a relation between the activity of specific muscles in the lumbopelvic region and the adoption of common standing and sitting postures. Specifically, activation of the superficial lumbar multifidus, internal oblique, and thoracic erector spinae muscles decreased in passive sitting and standing postures, but increased in erect postures, indicating a postural stabilizing role for these muscles. An early objective of this study was to devise a method of standardizing postural alignment and measurement. The standardized postures in this study closely represent commonly observed postures and were found to be reliable, quantifiable, and capable of being reproduced in a clinical setting. Standing Postures A key finding of this study was the decrease in activation of the superficial lumbar multifidus, internal oblique, and thoracic erector spinae muscles and the concurrent increase in rectus abdominis activation observed during sway standing as compared with erect standing. It has been proposed that as the pelvis shifts anteriorly to the thorax when the subject moves from erect to sway standing, the line of gravity moves posteriorly to the lumbar vertebral bodies. The result is extension of the low lumbar spine and lumbosacral junction. 4 Maintenance of an upright position during sway standing then is achieved mainly through activation of the anterior abdominal musculature. 4 This was demonstrated in the current study by the significantly increased activation of the rectus abdominis muscle in the sway standing posture. The decrease in superficial lumbar multifidus and thoracic erector spinae activity likely resulted from a lumbar extensor moment acting through the trunk in the sway standing posture, resulting in trunk flexor activation. Interestingly, an increase in rectus abdominis activity was associated with a decrease in internal oblique muscle activity. Individual muscles of the abdominal wall have different functions with regard to supporting upright postures, highlighting the fact that these muscles do not act as a homogeneous group. This notion is supported by previous research reporting regional differences in the function of the external oblique muscle. 22 Perhaps the internal oblique muscle functions more to support erect postures through its influence on the lumbar spine via its control of intraabdominal pressure and its attachment to the thoracolumbar fascia, 13,39 and to provide stability to the sacroiliac joint in weightbearing through the force closure mechanism. 37 Increased activation of the thoracic erector spinae muscle during erect standing is in agreement with previous findings, 1 reflecting the muscle activity required to maintain the thoracic spine in a neutral kyphosis against gravity. It has been previously reported that the back muscles show slight to continuous activity, 2,16,40 moderate levels of activity, 1,40 or no activity 16 in erect positions. The inconsistency in these previous findings may reflect variance in subjects natural posture and the lack of postural standardization within the study design. Sitting Postures Electromyographic activity of the superficial lumbar multifidus, internal oblique, and thoracic erector spinae muscles was significantly lower during slump sitting than during erect sitting. A reduction in EMG activity of the erector spinae during slump sitting has been reported consistently in the literature, indicating a flexion relaxation response of the back muscles. 1 3,15,35 It appears that postural muscle activity decreases as the lumbopelvic region becomes dependent on its passive structures to maintain the position against gravity at end-range spine flexion. Snijders et al 38 also documented this relaxation phenomenon in the internal oblique muscle when comparing supported upright sitting with crossed leg sitting. It was proposed that the reduced activity of the internal oblique muscle observed in the crossed leg posture reflected enhanced passive system stability in the sacroiliac joints, with less need for the dynamic stability provided by the internal oblique muscle. 38 In the current study, the activity levels of the external oblique and rectus abdominis muscles did not differ significantly between the two sitting postures, demonstrating the limited role that these muscles play in lumbopelvic stability under low load conditions such as sitting. Clinical Relevance The current findings clearly show a link between activity of the lumbopelvic postural stabilizing muscles and the maintenance of erect upright postures. These findings suggest a close relation between the adoption of passive postures and reduction in activity of the lumbo-pelvic stabilizing muscles. The authors speculate that the neural control system may adjust levels of motor activity in the lumbopelvic stabilizing muscles, depending on posture and degree of load sharing with the passive lumbopelvic structures. This speculation warrants further investigation. The authors also speculate that individuals who habitually adopt passive postures deactivate and potentially decondition the stabilizing muscles of the lumbopelvic region. A decrease in trunk muscle efficiency has

6 Effect of Different Postures O Sullivan et al 1243 been shown to increase load on the lumbar discs and ligaments. 18 This may leave the lumbopelvic region vulnerable to strain, instability, or injury. 12 A positive correlation has been shown between the presence of LBP and time spent watching television in large studies conducted with adolescents. 6 8 The authors of these studies proposed that this association might be the result of prolonged sitting as well as poor posture or physical inactivity. Clearly, further research is required to clarify the relation between sustained passive postures, muscle activity, and back pain. Cholewicki and McGill 12 reported that lumbar multifidus activity comprising no more than 3% of maximal voluntary contraction was sufficient to ensure segmental stability of the lumbar spine, with lower levels potentially compromising this stability. In the current study, the change in superficial lumbar multifidus activity observed between erect and passive postures was slightly less than 35% of submaximal voluntary contraction. Although a direct comparison cannot be made between the two studies, the authors speculate that the changes in muscle activity observed between erect and passive postures in the current study reflect a substantial change in motor activity that may have clinical significance. The relation of posture to trunk muscle activity in upright postures, and LBP is not well established. 10,25,30 One reason for this may be previous lack of postural standardization and classification of back pain conditions in the literature. It is recognized that postural stabilizing muscles such as the lumbar multifidus, internal oblique, and transversus abdominis muscles have an important stabilizing influence on the lumbopelvic region, 32,39,41 reducing stress on the passive structures. 18 Specific motor dysfunction of these muscles in the presence of chronic LBP also is well documented. 19,21,29,31 Further investigation is required to determine whether a relation exists between an individual s habitual adoption of passive postures and dysfunction of these muscles. With LBP, in which the passive structures of the lumbopelvic region already are sensitized, the authors speculate that adopting passive postures may result in exacerbation of pain. Adopting erect postures, which facilitate key lumbopelvic stabilizing muscles, may result in more effective load sharing with the active system, reducing focal end range stress on the sensitized passive structures. Conversely, if a pain disorder is related to increased extensor muscle activity, adopting more passive postures may relieve pain. These hypotheses are the focus of ongoing research. The results of this study also may have implications for motor retraining among individuals with specific lumbopelvic pain syndromes. Historically, considerable emphasis has been placed on muscle retraining in nonweightbearing or nonfunctional positions. 24,34 This study showed that erect postural alignment in weightbearing positions facilitates the stabilizing musculature of the lumbopelvic region. In contrast, facilitation of muscle activity in nonweightbearing or poorly aligned positions may hinder the transfer of improved lumbopelvic muscle function into everyday activities. This supports specific retraining of the stabilizing musculature, with concurrent postural reeducation, in the treatment of specific LBP conditions In the current study, commonly expressed postures observed in clinical practice were studied to verify whether postural change leads to different patterns of motor activity. The findings support the conclusion that the lumbopelvic stabilizing musculature is active in maintaining erect postures, and that these muscles are less active during the adoption of passive postures. Key Points The superficial thoracic erector spinae muscle and the anteroinferior portion of the internal oblique muscle are preferentially activated in erect standing and sitting postures and less active in passive trunk postures. The neural control system appears to adjust the levels of motor activity in the lumbopelvic stabilizing muscles, depending on trunk posture. The findings support the conclusion that adopting passive postures during standing and sitting has an inhibitory affect on the key stabilizing muscles in the lumbopelvic region. The findings support the conclusion that training of erect standing and sitting postures has a facilitatory affect on the key stabilizing muscles in the lumbopelvic region. Acknowledgments The authors acknowledge with appreciation the valuable contributions of Dr. Marie Blackmore (statistician), Geoff Strauss (senior lecturer), Dr. Kathy Henderson (senior lecturer), and Paul Davey (research assistant) of the Curtin University of Technology. References 1. Andersson B, Jonsson B, Ortengren R. Myoelectric activity in individual lumbar erector spinae muscles in sitting: A study with surface and wire electrodes. Scand J Rehabil Med Suppl 1974;3: Andersson B, Ortengren R, Nachemson AL, et al. The sitting posture: An electromyographic and discometric study. 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