The present study is the first one to report on the impact of different body positions on the electromyographic activity of pelvic floor muscles during electrical stimulation in women with stress urinary incontinence.

Stress urinary incontinence (SUI) is the involuntary loss of urine on effort, on physical exertion including sporting activities, or on coughing or sneezing. The goal of physical therapy in the treatment of SUI is to improve pelvic floor muscle (PFM) function by increasing strength, coordination, speed, and endurance. Electrical stimulation (ES), the application of electrical current to stimulate the pelvic viscera and their nerve supply, is widely used for inducing PFM contractions and thereby improving their function in the treatment of SUI. 

Several authors have studied the relationship between body position and resting/voluntary PFM activation. Some of them have suggested that the pressure on structures of the abdomen and lesser pelvis increases in the standing position due to gravitational forces. Increased pressure on the bladder and urethra, in turn increases the PFM tone. In supine, the gravitational force does not affect the PFM as much [1-2].

Similar anatomical factors are expected to be in action during ES. Resulting forces from such factors are expected to interact with forces generated by ES and subsequently lead to varying PFM activation levels across different positions. While numerous studies have reported that ES can be effective in the treatment of SUI, no study till date has investigated the effect of body positions on PFM activation during ES.

Subjects: Total 16 sessions were conducted with 08 female subjects with SUI (identified using the Questionnaire for Urinary Incontinence Diagnosis), aged between 18 and 60 years (35.6±13.6), weight 55±7.12kg, parity of 0.6±0.8, and 5.6±2.6 years since onset of symptoms.

Subjects with one or more of the following medical conditions were excluded from the study: Current urinary tract infection, current menstrual cycle, pregnancy, cardiac pacemaker or metal-based implants, impaired cognition, neurogenic bladder dysfunction or cystocele beyond the introitus, previous treatment of UI, hormone therapy, cognitive or neurological disorder, uncontrolled hypertension, cancer, radical prostatectomy, pelvic or abdominal surgery, menstrual  abnormalities, atrophic vaginitis, vaginal infections, intrinsic sphincteric deficiency, pelvic irradiation.

Electrical stimulation: Interferential therapy (4EL90° Trapezoid) was administered using Vectrostim Plus (TechnoMed Electronics) to all subjects. The output frequency consisted of one channel with 2000Hz and the second channel with 2000Hz–2150Hz. Four carbon rectangular electrodes (4X6cm) each were placed according to Laycock four-pole technique after cleaning the skin at site: two posterior electrodes placed medial to the ischial tuberosities on each side of the anus, and two anterior electrodes placed lateral to the symphysis pubis. Electrodes were secured with adhesive tapes.

Electromyography: Electromyographic signals were acquired using the 2-channel configuration on BIOPAC MP36 (BIOPAC EMG Solutions) with 4.4cm diameter circular surface electrodes of the self-adhesive, disposable Ag/AgCl type (3M™). The two channels were each used for the assessment of PFM and abdominal activity. For the former, electrodes were placed adjacent to the mucocutaneous line of anus bilaterally, and the reference electrode on the inner thigh [3]. For the latter, electrodes were placed on the right TrA/IO, and the reference electrode on the anterior superior iliac spine.

Body Positions: Six position order combinations of the three selected body positions (supine - SU, sitting - ST, standing - SD) were defined: SU-ST-SD, SU-SD-ST, ST-SU-SD, ST-SD-SU, SD-SU-ST, and SD-ST-SU. Supine position was assumed with legs straight and a thin pillow under the head, sitting was assumed with the feet well supported on a stool but without any back support, and standing was assumed with a neutral straight posture.

Protocol: Testing was performed in a private room allocated for this study. All subjects were explained in detail all procedures and were educated in PFM contractions. The electrodes attachment for electromyography was tested by recording increased activity during voluntary PFM contractions. 

Each session was allocated a position order combination such that no subject starts in the same body position in more than one session. The ES was administered for a total of 20 minutes, during which three electromyographic recordings of 5 minutes each were made for the three positions. Before starting such recording, the current intensity was increased up to the maximum tolerable by the subject. The subject was instructed to assume the allocated body position in a comfortable manner, to be able to maintain it with stability; to look straight without moving or talking but breathing normally. After completion, ES was stopped, and electrodes were removed. All procedures were performed by the same expert physiotherapist across all sessions. All data captured was stored securely on a designated computer system.

Signal processing and analysis: The stimulation output was separately captured on a digital oscilloscope for a fixed load. This output was analyzed and subsequently, a signal processing technique was designed to remove the stimulation signals from the electromyography data.

Then, the electromyography data of abdominal muscles was inspected to select windows with unusually high electrical activity. Corresponding windows were removed from the PFM electromyographic data before signal processing. 

To counter the 100-150 Hz interference of the ES, an IIR low pass filter with a passband frequency of 50Hz was used. Then, a bandstop filter was used to eliminate the 3.6Hz and its corresponding third harmonic 10.8Hz frequency data. An example filtered data is shown in Figure 1.

The square roots of variance values of the filtered PFM electromyography data were calculated. The square root of variance values for PFM activity in supine position were used for normalization of the corresponding values in all positions. This normalization scaling was selected over the conventional normalization with respect to maximal voluntary contractions to address the inter-subject variability caused by different stimulation current intensities. The normalized values were then used to analyze the differences across body positions through t-tests with the statistical threshold at p-value <0.05.

A comparison of the PFM electromyographic data showed that the normalized PFM activity in both sitting (1.35±0.58mV) and standing position (1.54±0.64mV) was higher than that in supine position (Figure 2). The normalized PFM activity in supine position was significantly different from that in sitting (p=0.028 (<0.05)) and standing (p=0.006 (<0.01)) positions. No statistically significant differences were seen between sitting and standing positions.

Several studies in the past have shown that volitional PFM recruitment can be influenced by different body positions and postures. The results of the present study show that PFM electromyographic output and thereby, PFM activation can also be influenced by different body positions during ES. This leads one to ask: How is the effectiveness of PFM contraction affected when body position is changed during an ES session? What are the long-term implications of such changes in the body positions on the clinical effectiveness of ES treatment? Further investigation is required to address these questions, particularly towards optimization of ES treatment for administration outside clinical settings where continuous control over body position is tedious.

The author notes the small number of subjects, and investigation of only one ES mode (interferential therapy) as limitations of the present study.

Results of the present study highlight the importance of body position during an ES session. Future studies should look at the long-term clinical effects of administering ES treatment for SUI with changing body positions.

Chmielewska, D., Stania, M., Sobota, G., Kwasna, K., Blaszczak, E., Taradaj, J. and Juras, G., 2015. Impact of different body positions on bioelectrical activity of the pelvic floor muscles in nulliparous continent women. BioMed Research International, 2015.
Lee, K., 2019. Investigation of electromyographic activity of pelvic floor muscles in different body positions to prevent urinary incontinence. Medical Science Monitor: International Medical Journal of Experimental and Clinical Research, 25, p.9357.
Krhut, J., Zachoval, R., Rosier, P.F., Shelly, B. and Zvara, P., 2018. ICS educational module: electromyography in the assessment and therapy of lower urinary tract dysfunction in adults. Neurourology and Urodynamics, 37(1), pp.27-32.