Volume 89, Issue 9 , Pages 1741-1747, September 2008
Pelvic Floor Muscle Activity in Different Sitting Postures in Continent and Incontinent Women
Article Outline
Abstract
Sapsford RR, Richardson CA, Maher CF, Hodges PW. Pelvic floor muscle activity in different sitting postures in continent and incontinent women.
Objective
To determine whether resting activity of the pelvic floor muscles (PFMs) and abdominal muscles varied in different sitting postures in parous women with and without stress urinary incontinence (SUI).
Design
PFM and abdominal muscle activity was recorded in 3 sitting postures: slump supported, upright unsupported, and very tall unsupported. Spinal curves were measured in slump supported and upright unsupported.
Setting
A research laboratory.
Participants
Women (N=17) with a history of vaginal delivery, 8 who were symptomatic of SUI and 9 who were asymptomatic.
Interventions
Not applicable.
Main Outcome Measures
Electromyographic activity of (1) the resting PFM recorded per vaginam with surface electrodes and (2) superficial abdominal muscles using surface electrodes. Changes in spinal curves were measured with a flexible ruler.
Results
Electromyographic activity of the PFM increased significantly from slump supported to upright unsupported postures in both groups (P<.001) but with lower levels of activity in women with SUI (P<.05). PFM activity increased further in very tall unsupported sitting in comparison with slump supported sitting (P<.001). Obliquus internus abdominis electromyographic activity was greater in upright unsupported than in slump supported sitting (P<.05), and electromyographic activity of other abdominal muscles was greater in very tall unsupported than slump supported. Women with SUI had a trend for greater activity in the abdominal muscles in upright unsupported than asymptomatic women. Asymptomatic women had a greater depth of lumbar lordosis in upright unsupported sitting than women with SUI (P=.04).
Conclusions
More upright sitting postures recruit greater PFM resting activity irrespective of continence status. Further investigation should consider the effect of sitting posture in rehabilitation.
Key Words: Abdominal muscles, Pelvic floor, Rehabilitation, Urinary incontinence
List of Abbreviations: ANOVA, analysis of variance, BMI, body mass index, PFM, pelvic floor muscle, SUI, stress urinary incontinence
THE PELVIC FLOOR MUSCLES are a multilayered muscle complex that forms the base of the abdomino-pelvic cavity. These muscles contribute to support of the pelvic contents, postural control, intra-abdominal pressure, and continence.1, 2, 3, 4, 5, 6 Optimal function requires continuous activity of the PFM in upright postures.7, 8 Continuous PFM electromyographic activity has been reported in lying, sitting, and standing at rest. This activity increases as people move into more upright positions,7, 8 yet is absent in some women.7 It is also possible that PFM activity may vary with changes in sitting posture.
Different sitting positions, supported or unsupported, change the lumbopelvic curve. Lumbar flexion has been suggested to alter the subjective awareness of a voluntary PFM contraction in healthy women; a sense of decreased activation of the PFM is reported in a flexed position.9 Clinically, women have reported a sensation of pelvic floor descent during coughing in a flexed lumbar position, but not when sitting upright with a lumbar lordosis. Although it has been argued that a normative lumbar curve influences the direction of forces in the abdominal cavity,10 it is perhaps more likely that passive and active support for the abdominal contents is affected by a change in lumbar and pelvic position.
Recent data suggest that PFM activity varies with the lumbar curve in healthy women.11 PFM activity is greater when sitting with a lumbar lordosis than a slumped position. Abdominal muscle activity is also affected by lumbopelvic posture: abdominal muscle activity is decreased in lumbar flexion.12 This relationship between the changes in abdominal and PFM activity is not surprising considering the well-known association between the activity of these 2 muscle groups.6, 9, 13, 14 Whether the PFM and abdominal muscles are affected by sitting posture in women with SUI has not been studied.
This study aimed to investigate the effect of sitting postures, 2 of which are common in everyday life, on electromyographic activity of the PFM and abdominal muscles in women with reported SUI, and to compare the findings with those of a group of women asymptomatic of SUI. A further aim was to compare spinal posture between women with and without SUI.
Methods
Participants
Seventeen parous women with or without SUI were recruited from volunteers and patients. Subjects were excluded if they could not comprehend English, were pregnant or had been pregnant in the last 12 months, had a BMI greater than 30kg/m2, had acute or chronic low back pain or significant postural abnormality (eg, significant scoliosis or kyphosis), had a neurologic condition, had a chronic or acute respiratory condition at the time of testing, had chronic constipation or symptomatic vaginal prolapse, had a previous history of vaginal surgical repair, or were currently or recently involved in an intensive abdominal fitness program involving sit-ups and resisted abdominal flexion routines. A completed comprehensive questionnaire about their obstetric history, general health, history of back pain, urinary control, and constipation was used to confirm their suitability for the study. All women had a history of vaginal delivery and gave written consent to participation in the study. The study was approved by the institutional medical research ethics committee.
Subjects were divided into the 2 groups. Asymptomatic women (n=9) had a mean age of 45 years (range, 32–66y), parity of 2.89 (range, 1–4), and BMI of 23.44kg/m2 (range, 19.55–26.32kg/m2). Women with self-reported symptoms of SUI (n=8) had a mean age of 41.8 years (range, 33–55y), parity of 1.63 (range, 1–2), and BMI of 22.8kg/m2 (range, 19.02–26.20kg/m2). Neither group of subjects had received formal PFM rehabilitation. Data for the subjects in the asymptomatic group have been presented previously using different analysis methods.11
Electromyography
PFM electromyographic activity was recorded using vaginal surface electrodes (Periform probe).a The curved shape of the probe enabled it to be held firmly in close contact with the lateral vaginal walls to record activity in the adjacent PFM. With all surface electromyographic recordings, there is potential for cross-talk from activity of adjacent muscles. However, in this study, it was anticipated that activity of adjacent hip and trunk muscles would be of low intensity, and cross-talk has been shown to be minimal in such circumstances.2 Pairs of disposable Ag/AgCl surface electrodesb were placed over obliquus externus abdominis bilaterally (positioned inferior to tenth rib in a line joining the most inferior point of the costal margin and the contralateral pubic tubercle) and over obliquus internus abdominis and the underlying transversus abdominis bilaterally (electrodes positioned immediately medial to the anterior superior iliac spine along a line joining both spines).15 Electrodes were also placed over rectus abdominis lateral to the umbilicus. A ground electrode was positioned either over the tibial tuberosity or on the ulna just inferior to the olecranon. Standard skin preparation was performed to reduce electric impedance.16 Electromyographic data were band-pass–filtered between 20 and 1000Hz and sampled at 2KHz using an Amlab data acquisition systemc and stored on computer for later analysis.
Procedures
To confirm the functional integrity of the neural innervation to the PFM, the ability of all subjects to contract the PFM voluntarily was tested prior to the study. The subject placed a hand over the vagina and perineal body while lying with hips and knees flexed and feet resting on the couch. She was asked to contract her PFM and report her findings. This initial test gave the subject an opportunity to familiarize herself with the required action. The first author repeated this test and confirmed the subject's reported activity. All subjects were able to achieve a concentric PFM contraction with perineal movement in a cephalad direction with contraction, and movement in a caudal direction with PFM relaxation.17 Subjects emptied the bladder at the start of the experiment to standardize the bladder volume, followed immediately by self-insertion of the water-lubricated vaginal Periform probe.
Subjects sat on a broad-topped stool, with feet well supported on the floor. Seat height was adjusted for comfort. The back support was provided by a padded support, 19cm deep, positioned at the level of the mid thoracic spine, and attached to an adjustable wall bracket (fig 1). The firm, noncompressible surface of the seat allowed a stable sitting posture. A cut-out area in the seat surface prevented pressure on the vaginal probe. None of the subjects reported any sensation of pressure from the vaginal probe in the sitting position.
Fig 1.
Diagrammatic representation of the upright unsupported (A) and the slump supported (B) sitting postures used in the study.
Recordings of PFM and abdominal muscle electromyographic activity were made for 10 seconds in 3 different sitting postures: slump supported sitting (flexed spine resting against the back support), upright unsupported sitting (sitting upright without support; see fig 1), and, in 13 subjects, very tall unsupported sitting (sitting in an extreme upright position with the chest elevated with greater extension at the thoracolumbar junction). This position was added for the last 13 subjects in the study to assess the effect of a more extreme change to the spinal curvature. There was no specific attention to spinal curvature. Five subjects with SUI and 8 asymptomatic control subjects performed the very tall sitting posture. The order of positions was randomized (selection of opaque envelopes containing trial order) for each participant. During recordings in slump supported and upright unsupported positions, subjects were asked to look directly ahead without moving or talking, while breathing normally. Subjects were not given detailed instructions, nor was their posture corrected. The slump supported and upright unsupported postures were chosen because they are similar to those used in daily life. In the very tall position, subjects were asked to sit tall “like a dancer.” Although participants knew that recordings were to be made from the PFM, they were not given any instructions for PFM activity and were not informed of the hypothesis related to the effect of position.
The shape of the thoracic and lumbar curvatures from C7 to S2 was recorded for all subjects using a long flexible ruler18,d that could be fitted to both curves in the slump supported and upright unsupported postures. The flexible ruler is a malleable strip of lead, encased in rubber, which can be molded to the curve and retains that shape when removed from the curve. Measurements of the spinal curve made with a flexible ruler have been shown to be reliable.19, 20 Tracings of the shape of the curve, taken prior to electromyographic recording, were made on graph paper (fig 2). The very tall unsupported sitting posture was not monitored because it was maintained for a shorter period.
Fig 2.
Tracing of spinal curves from C7 to S2, in slump supported sitting and upright unsupported sitting, in an asymptomatic subject. Measurements of the depth of (a) thoracic and (b) lumbar curves from the vertical alignment were made as indicated by the horizontal lines. Legend: a1, depth of the thoracic kyphosis in upright unsupported sitting; a2, depth of the thoracic kyphosis in slump supported sitting.
Data Analysis
A modified turning point filter was used to remove the heart beat artefact from the electromyographic recordings.21 Root mean square electromyographic amplitude was calculated for 5 seconds during each trial. The data from 3 trials were averaged for the slump supported, upright unsupported, and very tall unsupported positions.
Electromyographic data were analyzed as raw electromyographic activity. Although there are issues associated with comparison of raw electromyographic activity because of differences in the placement of the electrode and differences in filtering by soft tissues, there are also issues associated with comparison of electromyographic data that have been normalized to activity recorded during maximal efforts and standardized maneuvers. For instance, women with SUI are unlikely to perform maximal voluntary efforts of the PFM, and if the recruitment of the PFM is changed in women with SUI, the activity recorded during a standardized maneuver is likely to differ between women with and without incontinence.22 Because we did not expect large differences in soft tissue between groups and electrodes were placed over specific landmarks, we argue that this is unlikely to affect electromyographic amplitudes systematically. Comparison of unnormalized data is likely to yield more valid data than that normalized to attempted maximal voluntary contraction efforts or standardized tasks, during which levels of muscle activity may vary between groups.22
Differences in the spinal curves were defined using the criteria set by Milne and Lauder and others.19, 23 Changes in the 2 sitting postures, slump supported and upright unsupported, were analyzed by measuring the depth of the thoracic and lumbar curves. Curve depth was quantified as the distance between the apex of the lumbar and thoracic curve and a line drawn between C7 and S2 (see fig 2). The averages of the depths for each position were calculated for each subject group.
Statistical Analysis
In the primary analysis, repeated-measures ANOVA was used to compare the changes in raw electromyographic activity between the 2 groups of subjects (independent factor: asymptomatic and symptomatic) for 2 sitting postures (repeated measures: slump supported and upright unsupported). In the secondary analysis, changes were compared between the 3 sitting postures—slump supported, upright unsupported, and very tall unsupported—for the 13 subjects who performed all 3 postures using repeated-measures ANOVA. We used t tests for independent samples to compare the depth of the lumbar lordosis and the thoracic kyphosis between the groups.
Results
When the asymptomatic and symptomatic subjects moved from slump supported to upright unsupported sitting, there was a significant increase in electromyographic activity of the PFM (P<.001) and right obliquus internus and transversus abdominis (P<.05). There was also a trend for increased activity of right obliquus externus abdominis (P=.12) and left obliquus internus and transversus abdominis (P=.10) in both groups. There was no change in rectus abdominis electromyographic activity (P=.28). Comparisons of raw electromyographic activity between groups indicated that women with SUI had reduced PFM electromyographic activity in both sitting postures (P<.05) than the control subjects. This was associated with a trend for increased activity in the rectus abdominis (P=.059) in both positions (fig 3).
Fig 3.
Changes in electromyographic (EMG) muscle activity in 3 different sitting postures—slump supported, upright unsupported, and very tall sitting—in asymptomatic (black box) and symptomatic subjects (white box). Abbreviations: LOE, left obliquus externus abdominis; LOI, left obliquus internus abdominis and transversus abdominis; RA, rectus abdominis; ROE, right obliquus externus abdominis; ROI, right obliquus internus abdominis and transversus abdominis. *P<.05 comparison between postures; †P<.05 for comparison between groups.
Comparison of the 3 postures in the secondary analysis of these 2 groups indicated that PFM activity (P<.001) was increased in all subjects in the very tall unsupported posture compared with the slump supported posture. However, there were no differences between groups in this smaller subset of subjects. Electromyographic activity of the left obliquus externus abdominis (P=.003), left obliquus internus and transversus abdominis (P=.046), right obliquus externus abdominis (P=.005), and right obliquus internus and transversus abdominis (P=.002) was significantly greater in very tall unsupported than the slump supported position. There was no change in rectus abdominis activity (P=.12) (see fig 3).
Observation of the Flexiruled traces of the spinal curves showed that all subjects had a complete loss of lumbar lordosis in the slump supported position (see fig 2). The depth of the lordosis for each group and the depth and changes in the kyphosis from the upright unsupported to slump supported positions are shown in table 1. Asymptomatic women had significantly greater lumbar lordosis than women with SUI (P=.04).
Table 1. Depth of Thoracic and Lumbar Curves Measured From Vertical in Asymptomatic Women and Women With Symptomatic SUI in Slump Supported Sitting and Upright Unsupported Sitting
| Women | Lordosis Upright Unsupported (b) (mm) | Kyphosis Upright Unsupported (a1) (mm) | Lordosis Slump Supported (mm) | Kyphosis Slump Supported (a2) (mm) |
|---|---|---|---|---|
| Asymptomatic | 7.9 | 21.9 | 0.0 | 58.4 |
| Symptomatic | 2.9 | 37.1 | 0.0 | 72.8 |
Discussion
The results of this study indicate that activity of the PFM is affected by sitting posture in women with and without SUI. Although the group of symptomatic women, most of whom had severe SUI,22, 24 had less PFM electromyographic activity and a tendency toward greater activity of the superficial abdominal muscles than the continent women, activity of the pelvic floor and obliquus internus and transversus abdominis muscles increased in both groups when moving from a slumped to an upright unsupported sitting posture. The lower activity of the PFM was associated with reduced lumbar lordosis. These data support the hypothesis that lumbopelvic posture affects the activation of the PFM.
Effect of Sitting Posture on PFM Activity in Women With and Without SUI
In women with and without SUI, PFM activity increased when they moved from a slumped sitting posture to upright sitting. This is consistent with recent data reported for the same group of healthy women using different analyses.11 That study involved analysis of electromyographic activity normalized to maximum voluntary contraction, which is reasonable for healthy continent women but not for between-group comparison. Of the abdominal muscles, only activity of the obliquus internus and transversus abdominis was significantly increased in upright sitting compared with slumped sitting. This agrees with earlier data that showed increased obliquus internus and transversus abdominis activity but no change in rectus abdominis activity in more upright seated postures in healthy subjects.12
In the subgroup of women who also adopted a very tall sitting posture involving thoracolumbar extension, there was a significant increase in activity of the PFM, obliquus internus and transversus abdominis, and obliquus externus abdominis compared with the slump sitting position. However, probably because of the small number of subjects in this subanalysis, there was no group difference.
Changes in PFM activity may be explained by the effect of different trunk postures on trunk muscle activity. Although activity of the paraspinal and abdominal muscles is minimal in slumped sitting, with posture maintained by passive structures, activity is increased when sitting upright.12 Increased activity of the PFM may be required to accommodate increased intra-abdominal pressure, which occurs with greater abdominal muscle activity.1, 25 In addition, previous studies suggest that voluntary activation of the abdominal muscles is associated with increased PFM activity.13 This coordination of different muscle groups is supported by animal studies that have shown pelvic floor and abdominal muscles receive common projections from the brainstem in cats and rhesus monkeys,26, 27 and stimulation of S3 nerve roots in humans evokes activity in both abdominal and PFM motor cortical areas.28
Activity of the Pelvic Floor and Abdominal Muscles in Women With and Without SUI
In all sitting postures, women with severe symptoms of SUI had less activity of the PFM and a tendency for increased activity in the rectus abdominis. This is similar to recent data that showed greater obliquus externus abdominis electromyographic activity in standing in women with a similar severity of symptoms.22 These changes in muscle activity would have the tendency to increase the challenge for the PFM to maintain continence because of the potential for greater intra-abdominal pressure associated with abdominal muscle activity. Because of the small number of subjects, it was not possible to evaluate whether the PFM electromyographic activity was associated with the severity of SUI symptoms.
Although it is acknowledged that both fascia and muscle contribute to pelvic organ support and function,4, 29 PFM strength and activity are considered to be important factors for the maintenance of urinary continence. Both the voluntary strength and endurance of the PFM have been shown to be decreased in women with prolapse and with SUI.7, 30, 31, 32 The current findings of decreased tonic activity in the PFM in sitting provide further evidence of compromised activity of this muscle group in women with SUI. However, there are several additional issues to consider.
A key issue highlighted by the present study is that when women with SUI adopted an upright sitting posture, they did not achieve the same degree of lumbar lordosis as the asymptomatic controls. Because the present data confirm that lumbo-pelvic position influences PFM activity, this points to a possible link between posture and symptoms of incontinence. Although it is unclear why lumbar lordosis is smaller in women with SUI, an abnormal spinal curve in standing (which normally involves a greater depth of the lumbar lordosis than sitting33) with loss of the lumbar lordosis and an increase in thoracic kyphosis have been shown to be associated with an increased incidence of vaginal prolapse.23, 34 Mattox et al23 suggested that their findings were consistent with the proposal of Zacharin10 that a normative lumbosacral curve may protect the pelvic floor from direct intra-abdominal forces by deflecting them onto the anterior abdominal wall and the flattened pubic bone. However, pressure is distributed equally in a fluid-filled cavity (although slightly greater at the inferior aspect because of hydrostatic pressure). Perhaps a more likely explanation is provided by the present data suggesting that PFM activity is reduced in a slumped position, hence decreasing the support for the pelvic organs. Thus incontinence is likely to be a complex issue associated with decreased PFM activity, changes in posture, and increased abdominal muscle activity as well as the damage to muscle and supporting structures that results from childbirth.35
Study Limitations
In this study, recordings of PFM activity were made with surface electrodes. This technique records activity from part of the pelvic floor and has the potential for cross-talk from adjacent muscles. The laterally oriented electrodes on the vaginal probe probably recorded from the different sections of the pubovisceralis muscle,36 but it is not possible to conclude that the findings apply to all muscles of the PFM complex. Although there is a possibility of cross-talk from other muscles in the vicinity of surface electrodes, according to a recent study, it is unlikely that activity of adjacent muscles was of sufficient intensity to affect pelvic floor muscle recordings.2
Recordings of abdominal muscle activity with electrodes placed over the lower abdominal wall are likely to record activity from the obliquus internus abdominis and the underlying transversus abdominis, the deepest abdominal muscle. The transversus abdominis is a postural muscle that contributes to lumbopelvic stability and intra-abdominal pressure25, 37 and has been shown to have a close association with PFM activity.9 Recordings of transversus abdominis electromyographic activity with intramuscular electrodes would be required to confirm the response of this muscle to changes in posture.
The accuracy of recording of the shape of the spinal curves in this study relied on close approximation of the malleable ruler along the spinous processes and careful transposition of the curve onto graph paper. This method of monitoring spinal curves, while not having the accuracy of radiographs, has been used by others to provide a simple assessment of the shape of the curves.18, 19, 23
In this study, raw electromyographic data were analyzed. Although there is potential for error to be introduced using this technique, we argue that analysis of data normalized to maximal or standardized submaximal activity would introduce substantial errors. It would be unlikely that women with SUI could contract the PFM or abdominal muscles maximally or effectively.
Clinical Implications
Maintenance of an upright sitting position with a lumbar lordosis requires endurance in all the postural muscles of the trunk, including the PFM.2 Such activity of the PFM contributes to bladder inhibition38 and ongoing pelvic organ support29 and may provide prophylaxis against the development of vaginal prolapse and urinary urgency and incontinence. This may also facilitate rapid muscle recruitment during coughing and sneezing. However, this requires further investigation.
If women habitually sit in a slumped posture, this has the potential to be associated with reduced activity of the PFM and perhaps deconditioning, which may contribute to pelvic floor dysfunction. Workplace activities and lifestyles have changed dramatically over the last half century. Today more women work in paid employment, often with many hours in sitting; spend more time seated in cars; and perform less physically onerous household chores. In some regions of the world, an increased prevalence of pelvic floor problems has become apparent over 1 generation.39, 40
Conclusions
These data point to the importance of spinal posture for control of the PFM and abdominal muscles. As more upright sitting postures recruit greater PFM resting activity, use of these trunk postures at leisure and in the workplace may be an effective strategy to enhance coordination and control of the PFM and abdominal muscles. The application of upright postures as a component of exercise programs could be beneficial in enhancing PFM recruitment during rehabilitation and may also contribute to prevention of pelvic floor problems. The efficacy of such an approach should be tested in clinical trials of women with urinary incontinence.
Suppliers
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Supported by the National Continence and Women's Health Group of the Australian Physiotherapy Association, and the National Health and Medical Research Council of Australia.
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated.
PII: S0003-9993(08)00425-5
doi:10.1016/j.apmr.2008.01.029
© 2008 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved.
Volume 89, Issue 9 , Pages 1741-1747, September 2008
