1.1 Subjects and design
The present study was performed in a urogynecology clinic and laboratory in Seoul, Korea, from August to December 2018. Subjects were randomized into ES and control groups and the investigators were blinded to the group assignments. The sample size required for a power of 0.80and effect size of 0.682, at an α level of 0.05, was calculated a priori using G*Power software (version 3.1.3; University of Kiel, Kiel, Germany), with reference to pilot data (n = 3 subjects per group) and with a main outcome variable of lumbopelvic stability (one-leg lowering test). The required sample size was at least 16 subjects per group. Subjects were recruited by advertisements that provided a telephone contact; all volunteers were invited to visit us and were evaluated in terms of the inclusion and exclusion criteria. The inclusion criteria were 1) SUI diagnosed by a urogynecologist, 2) leakage episode recorded more than once a week, 3) Body mass index < 30 kg/m2, 4) age between 30 and 60 years, 5) not addicted to alcohol or drugs and 6) successful completion of the medical screening questionnaire. The exclusion criteria were 1) aversion to electrical stimulation, 2) cardiac pacemaker implanted, 3) device implanted in the pelvis or hip joint(s), 4) pregnant/planning to become pregnant, 5) pelvic or abdominal surgery within the last 6 months, 6) concomitant treatment for SUI during the trial period and 7) neurological or psychiatric disease.
Table 1 shows the characteristics of the subjects. A total of 33 subjects who met the inclusion criteria were divided into the control and ES groups using a list of random numbers (www.randomization.com) (Figure 1). Before the study, we explained all procedures, and all subjects signed a written informed consent form approved by the Institutional Review Board. Approval for the present study was obtained from the Institutional Review Board of Yonsei University, Wonju (1041849-201806-BM-056-02). The work was also registered by CRIS under the code KCT0003357 (granted on 11th November 2018) and the registration timing was retrospective. The authors confirmed that all ongoing and related trials for this intervention are registered.
1.2 Electrical stimulation
The ES device (EasyK7, Alphamedic Co., Ltd., Daegu, Korea) employs three transcutaneous electrodes, which are placed in both the perivaginal (two electrodes) and sacral regions (one electrode) to stimulate the PFM and surrounding structures [20]. The three electrodes create an electromagnetic field that stimulates the PFM over a wide area when the subject sits on the device. ES is applied as biphasic asymmetric impulses at 25 Hz, with pulse and rest periods of 11 s each. The mean current intensity was 17.63 ± 7.47 mA (range: 2.5–30 mA). Each ES session was 15 minutes in duration.
1.3 Intervention
Subjects in the ES group were provided with an ES device and underwent their first session in our laboratory, where they were taught how to use, manage, and clean the device. Subjects were asked to use the device once a day (15-min session, as set by the manufacturer), on 5–6 days a week for 8 weeks, according to the training frequency outlined previously [20]. In addition, all participants underwent ES sessions to capture any increases in stimulation amplitude. Adherence to this schedule was checked by telephone twice per week.
The control group walked for over 20 minutes daily. Both groups were assessed before and after 8 weeks of training.
1.4 Outcomes
1.4.1 Pelvic floor muscle function
PFM assessments were performed by a urogynecologist using a vaginal pressure measurement device with all participants in the hook-lying position [21]. We used a VVP-3000 perineometer (QLMED Ltd., Gyeonggi-do, Korea) and a vaginal probe 115 mm in length and 24 mm in diameter. The baseline value was recorded in mmHg and the device was then zeroed at rest. PFM strength was measured from baseline to peak effort over 2 s, and is reported in mmHg as the mean pressure rise during two maximal voluntary contractions (MVCs) [21]. To measure PFM power, all subjects were asked to contract the PFM as rapidly as possible. PFM power was defined as peak pressure/time to MVC (mmHg/s) [20].
1.4.2 Lumbopelvic control: one- and double-leg lowering tests
The one- and double-leg lowering tests were used to measure the subject’s lumbopelvic control during movement of the lower limbs [9, 14, 22-24]. In the supine position, the subject flexed the hips and knees to 90°. A Smart KEMA pressure sensor (KOREATECH Co., Ltd., Seoul, Korea) was set to 40 mmHg and placed below the lordotic curvature of the spine between L1 and S1, with the hips and knees in 90° of flexion (Figure 2) [14, 24]. Using its strap, the Smart KEMA motion sensor (KOREATECH Co., Ltd.) was attached to the thigh between the greater trochanter and knee joint. During performance of the abdominal drawing-in maneuver, the pressure on the sensor was increased by 10 mmHg. Subjects were asked to hold the lumbopelvic position to maintain a pre-set pressure of 50 mmHg, by contracting the abdominal muscles while slowly lowering one or both legs to the supporting surface [14]. One- and double-leg lowering (hip extension) angles were measured with a motion sensor, and lumbopelvic control was defined as the moment when the pressure sensor reading decreased below 50 mmHg (Figure 2). As the core muscles are necessary for lumbopelvic control and stabilization during leg motion, a larger leg-lowering angle indicates greater lumbopelvic control [24].
1.4.3 Abdominal muscle thickness and contraction ratio
A real-time ultrasound scanner (A35; Samsung Medison, Seoul, Korea) was used to measure the thickness of the TrA, internal oblique abdominis (IO), and external oblique abdominis (EO) muscles on the right side of the abdominal wall in M-mode, using a 4.5-cm, 3–16 MHz linear probe (LA3-16A) connected to a screen that showed the image. Calipers were used to measure muscle thickness in centimeters. Three trials were performed for each task.
To obtain resting thickness measurements, all subjects were placed in the supine position, with the examiner on the subject’s dominant side. To standardize the location of the transducer, the hyperechoic interface between the TrA and the thoracolumbar fascia was positioned on the dominant side of the ultrasound image [25]. All images were taken at the end of expiration.
In addition, abdominal muscle thickness was measured during the active straight leg raise (ASLR) maneuver on the dominant side, performed with the subject lying in the supine position on a standard plinth with the lower extremities straight and hands resting on the chest. The feet were positioned 20 cm apart prior to asking the subject to raise the dominant lower extremity 5 cm off the plinth without bending the knee [25].
Contraction ratios (TrA contraction ratio = TrA in ASLR/TrA at rest; IO contraction ratio = IO in ASLR/IO at rest; EO contraction ratio = EO in ASLR/EO at rest) were calculated based on the abdominal muscle thickness, while resting and in the ASLR position [25].
1.5 Statistical analysis
All statistical analyses were performed using SPSS software (ver. 18.0; SPSS Inc., Chicago, IL, USA). In all analyses, p < 0.05 was taken to indicate statistical significance. The Kolmogorov–Smirnov Z-test was used to assess the normality of the data. Two-way repeated measures analysis of variance (ANOVA) was used to examine time × group interaction effects on PFM functions, lumbopelvic control, abdominal muscle thickness at rest and during ASLR, and the contraction ratio. Whenever a significant interaction was observed, the paired t-test was used to determine within-group differences, and the independent t-test was used to determine differences between the ES and control groups.