Acute effects of local vibration and visual feedback on the pelvic floor muscle training in Japanese healthy adults: A cross-over study

Abstract

BACKGROUND:

Those undergoing pelvic floor muscle training (PFMT) often experience difficulty in perception; therefore, an easier PFMT method should be devised.

OBJECTIVE:

To determine the effectiveness of combining PFMT with either vibration stimulation or visual feedback provided by a branded cushion (not a prototype) in increasing PFM muscle activity. Since PFM does not involve large joint movements, muscle activity was a suitable indicator.

METHODS:

Twenty healthy adults were included in this study. All participants performed PFMT under three conditions using a branded cushion: vibration stimulation, visual feedback, and a control. All three conditions were provided separately. PFM activity of the midline of the perineum at two points was recorded as the root mean square measured using PFM electromyography, measured twice for each condition. Muscle activity ratio was obtained by calculating maximum voluntary contraction of PFM in pre- and post-PFMT conditions.

RESULTS:

PFM activity and muscle activity ratio were both significantly higher following PFMT under vibration stimulation and visual feedback conditions ( $p<$ 0.01, $p<$ 0.01, respectively).

CONCLUSIONS:

PFMT accompanied by vibration stimulation or visual feedback could immediately raise PFM muscle activity. We believe this study contributes to improving PFMT efficiency by suggesting more suitable PFMT methods.

Keywords

Pelvic floor muscle muscle activity urinary incontinence

1. Introduction

Urinary incontinence (UI) is a common and serious problem affecting people of all age groups and sexes; however, because of pregnancy and childbirth, women are more likely to have symptoms [1]. The number of young patients with UI is increasing, with 12.5% of reported cases affecting young people (aged 18–23) [2]. Additionally, according to an epidemiological study of UI in different age groups, 55.5% of the people with a mean age of 51.7 $\pm$ 9.5 reported having UI symptoms [3]. Thus, many people experience UI symptoms, and its prevalence increases with age. Interestingly, a previous study in participants over 21 years having UI showed that more than half of the respondents did not receive any treatment because a small amount of urine leakage was considered to be a natural phenomenon, with sufferers unaware that treatment would improve their symptoms [4]. In this regard, lack of preventive measures such as strengthening of the pelvic floor muscles (PFM) or treatment of UI will increase the risk of its occurrence in the future. However, to date, most studies on UI have been conducted primarily in older adults [5] and do not often include young adults in the age range of 20–30 years. Even in young adults, UI can be induced by various factors, such as excessive abdominal pressure on the pelvic floor due to sports activities [6]. Therefore, it is necessary to consider young people when exploring ways to prevent UI symptoms.

There has been considerable discussion about UI prevention strategies. Continuous pelvic floor muscle training (PFMT) is necessary, given that a trained PFM has been shown to be useful for preventing and managing UI [7, 8]; the minimum recommended duration of this training is 3 months [9]. However, contraction of the PFM is difficult to recognize because it does not involve large joint movements, and PFMT is challenging for patients with UI because of difficulty in physically sensing the contraction themselves. Therefore, PFMT is unsustainable, and many people fail to turn it into a habit because they experience difficulty in noticing the effects.

In recent years, various methods have been devised to train patients to contract the PFM correctly. A previous study in which PFMT was performed using a cushion to provide either vibration stimulation or visual feedback showed an improvement in the activity of the PFM under both conditions [10]. The cushion had an embedded 40-Hz vibration terminal and a pressure sensor designed to detect contraction of PFM. Participants could obtain visual feedback by checking the hand-held pressure gauge [10]. Indeed, the training of skeletal muscles has shown to increase muscle output when combined with vibration stimulation [11, 12]. However, research targeting the PFM is lacking. In particular, there is no simple method established for visual feedback to promote PFM contraction, because a previous study inserted a probe into the vagina and measured its pressure [13]. Therefore, if training with vibration stimulation or visual feedback using a cushion increases PFM activity, this would help acquire PFMT as a habit due to its simplicity. However, the study describing this strategy included only 10 male subjects [10]. As the prevalence of UI is generally higher in women as a result of childbirth and pregnancy [1], we believe that this approach needs to be validated in young women in order to test whether the same effects can be expected regardless of sex.

The main purpose of this study was to confirm whether PFMT with vibration stimulation or visual feedback was effective in increasing PFM activity. Note that visual feedback combined with vibration stimulation could cause inaccurate pressure detection. Moreover, the current specifications for the cushion’s usage do not allow simultaneous use; thus, vibration stimulation and visual feedback were implemented separately. The other objective was to find out whether there was any difference between sexes regarding the effectiveness of PFMT with vibration stimulation or visual feedback using a cushion, compared to a control intervention. The hypotheses were that PFM activity would be higher following PFMT task with vibration stimulation and visual feedback than after normal PFMT, and that there would be no differences between sexes in the rate of increase in muscle activity after different PFMT conditions.

2. Methods

2.1 Study design

Figure 1.

Flowchart of study participants. ICIQ-SF: International consultation on incontinence questionnaire – short form.

This crossover study examining immediate changes in muscle activity after PFMT was conducted in a laboratory setting. The study protocol met the requirements of the Declaration of Helsinki and was approved by the Ethical Committee for Epidemiology of Hiroshima University (approval number E-2005-1). The participants were fully informed of the details of the study by using the Consent Explanation Document, and verbal consent was obtained.

The PFM activity was measured pre- and post-intervention under three conditions: control, vibration stimulation, and visual feedback with cushions; comparisons were performed among these groups. In the control condition PFMT was performed while sitting on a cushion without any further stimulation. The vibration stimulation condition involved participants contracting the PFM along with the vibration generated by the terminals embedded in a cushion. In the visual feedback condition, PFMT was performed by holding the remote control in one hand and checking the pressure created in the cushion area.

2.2 Participants

Table 1
Basic information of the participants in this study

	Age (years)	Height (cm)	Body weight (kg)	BMI (kg/m ${}^{2}$ )
Total ( $n=$ 20)	23.3 $\pm$ 0.9	165.9 $\pm$ 8.4	56.3 $\pm$ 9.3	20.4 $\pm$ 2.0
Femails ( $n=$ 10)	22.9 $\pm$ 0.6	159.3 $\pm$ 5.0	48.7 $\pm$ 4.0	19.2 $\pm$ 1.3
Males ( $n=$ 10)	23.7 $\pm$ 1.0	171.8 $\pm$ 5.9	63.1 $\pm$ 7.1	21.4 $\pm$ 2.0

Mean $\pm$ SD. BMI: Body mass index.

Figure 2.

Randomization of the measurement order of this study.

A total of 24 individuals applied for the study after being recruited through a poster display. Figure 1 shows the flowchart of study participant selection. The inclusion criteria were as follows: (1) age, 20–30 years and (2) absence of UI symptoms. The exclusion criteria were as follows: (1) International Consultation on Incontinence Questionnaire-Short Form (ICIQ-SF) score higher than 1, which measures the presence of UI, (2) a history of urologic disease and (3) pregnant or possibly pregnant women. Four participants were excluded for having an ICIQ-SF score of 1 or more; thus 20 healthy individuals, 10 women and 10 men were finally included in the study. Basic information on the participants is presented in Table 1. We confirmed that at least 15 participants in each group were needed to produce an appropriate effect size when G*power was used (version 3.1.9.2, Heinrich-Heine-University Düsseldorf, Düsseldorf, Germany), according to a previous study that used an effect size of 0.81, an alpha error of 0.05, a mean power of 0.80, and two tails [10]. The study implemented a cross-over design, and there was no shortage of participants as the 20 participants performed PFMT in all three conditions. Although the number of participants was higher than the minimum number required for the study considering to the risk of dropouts, this did not have any detrimental effects on study results. The number of participants in this study was justified because the difference obtained in the previous study was large enough to be detected with less than 30 participants.

2.3 Intervention of PFM exercise

All participants performed PFMT in the three conditions: control, vibration stimulation, and visual feedback. The order of the trials was randomized using the www.randomization.com website to reduce bias as shown in Fig. 2.

The protocol used in this study is shown in Fig. 3. After providing consent to participate in the study, participants practiced PFMT in the three conditions, with each condition lasting for 5-min. Three days later, participants performed PFMT again and measurements were taken. Before the measurements, participants sat in a resting position for 10-min. Subsequently, muscle activity in the pre-PFMT period was measured. Each PFMT lasted 5-min and consisted of 10-s of PFM sustained contraction followed by 40-s of rest, repeated a total of six times [10]. Subsequently, post-PFMT muscle activity was measured. It should be noted that after concluding the first trial and taking additional 10-min rest, the participants continued PFMT in the remaining conditions [14].

Figure 3.

Protocol for measurements and PFMT in this study. PFMT: pelvic floor muscle training.

Figure 4.

Posture during pelvic floor muscle training and seated cushion.

During PFMT, participants maintained a seated posture on a cushion (Kyuttoburu, Dream Inc., Nagoya, Japan) with the hip and knee joints flexed at approximately 90 ${}^{\circ}$ (Fig. 4). A vibration terminal of 40 Hz was embedded in this cushion. In addition, the center of the cushion is raised like a mountain and has a pressure sensor embedded in it that bites into the perineum when the participant sits down (Fig. 4). Pressure is sensed by pinching of the pressure sensor during contraction of the PFM, and the participant could obtain visual feedback by checking the pressure gauge on the hand. The scale of the five-step gauge increased when the pressure gauge contacted the PFM and detected pressure (Fig. 5).

Figure 5.

Pressure gauge for visual feedback condition using a cushion.

Figure 6.

Comparison of RMS of the pelvic floor muscle pre and post exercise in each condition. RMS: root mean square.

Figure 7.

Comparison of muscle activity ratio of the pelvic floor muscle in each condition.

2.4 Measurement of maximum voluntary contraction (MVC) of the PFM

The MVC of the PFM was measured for 5-s before and after each exercise task, and the activity of the PFM was recorded using surface electromyography (P-EMG plus, Oisaka Electronic Equipment Ltd., Hiroshima, Japan). Electromyography (EMG) was performed by attaching an electrode (P-00-S, Ambu, Ballerup, Denmark) to the midline of the perineum at two points, on the right and left sides, to record the activity of the PFM. This targeted the transverse perineal muscle involved in the opening and closing of the urethral opening and the adjacent external anal sphincter [10, 14, 15]. One male and one female examiner each placed the subject’s electrodes. The measurements for each condition were taken twice to minimize the effects of fatigue. The measurement was filtered within 10–200 Hz and digitized at a sampling rate of 1000 Hz. The root mean square (RMS) of the mean amplitude was analyzed in 3-s, excluding the first and last seconds of the unstable waveform. Furthermore, the reproducibility of this measurement was confirmed through a preliminary experiment involving 20 healthy adults. The intraclass correlation coefficients (ICC) (1.1) values for MVC measurement showed excellent reliability (ICC $=$ 0.955) [16].

Table 2
Comparison of muscle activity ratio of the pelvic floor muscle in each condition between sexes

Muscle activity ratio (post/pre)	Females	Males	$p$ value	Effect size ( $r$ )
Control	1.00 $\pm$ 0.29	1.01 $\pm$ 0.13	0.927	0.02
Vibration stimulation	1.83 $\pm$ 0.57	1.54 $\pm$ 0.51	0.218	0.29
Visual feedback	1.44 $\pm$ 0.26	1.39 $\pm$ 0.30	0.684	0.10

Mean $\pm$ SD.

2.5 Statistical analysis

The observations were analyzed using IBM SPSS Statistics version 28.0 for Windows, (IBM Japan Co Ltd, Tokyo, Japan).

For comparison of the RMS between pre- and post-exercise tasks in each condition, the corresponding $t$ -test was performed when normality was followed, and the Wilcoxon signed rank-sum test was performed when normality was not followed. The normality test was positive in the vibration stimulation condition but not in the control and visual feedback conditions. Comparison of the muscle activity ratio (post/pre) among the three conditions was performed using the Kruskal–Wallis and Bonferroni post-hoc tests. The Mann–Whitney U test was performed to confirm the differences in the muscle activity ratio between women and men. The effect size ( $r$ ) was calculated for the RMS pre- and post-PFMT in each condition. Effect sizes for the t-test were calculated from t-values and degrees of freedom, and those for the Wilcoxon signed rank-sum test and the Mann–Whitney U test were determined from standardized test statistics and number of participants. The level of significance was set at $p<$ 0.05.

3. Results

Figure 6 shows the results of the PFM activity for pre- and post-exercise in each condition, represented as the RMS of the mean amplitude. When comparing the activity of the PFM pre and post exercise task, there was a significant increase of 62.69% in the vibration stimulation condition and of 41.44% in the visual feedback condition (vibration stimulation: pre 23.05 $\pm$ 11.81 mv, post 37.50 $\pm$ 19.00 mv, $p<$ 0.01, effect size $r=$ 0.79; visual feedback: pre 23.53 $\pm$ 14.74 mv, post 33.28 $\pm$ 22.12 mv, $p<0.01$ , effect size $r=$ 0.60). In the control condition, no significant changes in PFM activity were detected pre and post exercise task (pre 24.26 $\pm$ 12.92 mv, post 25.02 $\pm$ 16.33 mv, $p=$ 0.904, effect size $r=$ 0.02).

Figure 7 shows the results of the muscle activity ratio post/pre (control: 1.00 $\pm$ 0.22; vibration stimulation: 1.68 $\pm$ 0.54; visual feedback 1.41 $\pm$ 0.28). When comparing the conditions, the muscle activity ratio was significantly higher in the vibration stimulation condition than in the control condition ( $p<$ 0.01). The visual feedback condition also showed a higher muscle activity ratio than the control condition ( $p<$ 0.01).

Table 2 shows the differences between women and men in the muscle activity ratio of the PFM for each condition. As shown, there were no significant differences between sexes in any of the three conditions ( $p>$ 0.05).

4. Discussion

This is the first study to show that PFMT with vibration stimulation or visual feedback can cause an increase in PFM activity in young adults. The main findings were as follows: (1) with vibration stimulation, PFM activity increased by approximately 60% after exercise, and the muscle activity ratio of this condition increased significantly compared with that in the control condition and (2) PFM activity increased by approximately 40% with visual feedback and a significant increase was observed in the muscle activity ratio compared with that in the control condition. In addition, we confirmed that there were no sex-based differences in muscle activity ratios of PFM pre- and post-exercise in the control, vibration stimulation, and visual feedback conditions.

We observed an increase in the PFM activity after exercise with vibration stimulation. The tonic oscillatory reflex induced by vibration stimulation has been reported to be involved in the increase in muscle activity during exercise [17] and an increase in muscle output after exercise [12]. When vibration stimulation is applied to skeletal muscles, $\alpha$ -motoneurons are excited, and muscle contraction is observed owing to centrifugal impulses generated by these neurons. Simultaneously, centrifugal impulses generated by $\gamma$ -motoneurons cause afferent impulses in the fibers of muscle spindles, which pass through the spinal monosynaptic reflex circuit and induce muscle activity via $\alpha$ -motoneurons [18]. As the shallow PFMs measured in this study are rhabdomyosarcoma muscles innervated by the somatic nerve [19, 20], we expected that they might have been activated by the same mechanism when PFMT with vibration stimulation was performed. The Meissner corpuscles are skin receptors that detect pressure and vibration stimulus information. These sensory receptors particularly respond to vibration frequencies between 10 and 50 Hz. Therefore, it is possible that the use of vibration stimulation, with a vibration frequency of 40 Hz, facilitated awareness of the PFM group during vibration [21]. So far, not enough evidence of PFMT with local vibration stimulation has been reported. Therefore, we believe that these results showing an immediate 60% increase in PFM activity with this method provide new information in terms of providing novel and effective PFMT options.

Muscle activity also increased after PFMT with visual feedback. Various descending nerve tracts are involved in voluntary muscle control [22]. The corticospinal tract, one of the descending nerve tracts, is related to voluntary control of skeletal muscles, such as the PFM, and can integrate visual feedback more efficiently, improving performance and learning of visuomotor tasks [23]. In the corticospinal tract, motor control is better when visual feedback is present [23]. In addition, the cerebellum functions to integrate centrifugal information of the intended movement and afferent feedback [24], and motor control by visual information occurs when information is input via the cerebellum to the premotor cortex [25, 26]. In this study, it is possible that PFMT with visual feedback might have facilitated the control of movement by the corticospinal tracts and cerebellum and increased the PFM activity ratio pre- and post-PFMT. A previous study of an exercise intervention in patients with UI, performed using a probe inserted into the vagina to provide visual feedback of pressure during PFM exercises, found that PFM activity increased after exercise intervention [13]. Palpation is also used in clinical practice to monitor the movement of PFMs. However, these methods could cause a high psychological burden and create sanitary problems in certain cases. In this study, easy visual feedback by checking the pressure gauge of the cushion helped to smoothly perform PFMT. Therefore, providing cushions that allow for visual feedback could contribute to the implementation of ongoing training at home.

In this study, there were no significant differences in the PFM activity ratios pre- and post-PFMT between sexes in any of the three conditions. In previous studies, men were less affected by distal vibration because of their larger body size as opposed to women [27]. Therefore, in this study, local stimulation was applied directly to the PFM, and muscle activity ratios were used to compare between men and women to avoid the influence of differences in body size. Regarding visual feedback, the literature has different views on sex differences [28, 29] and there is no consistent conclusion. Therefore, as predicted, there were no sex differences in PFMT using vibration or visual feedback. Importantly, the pelvic structures of both sexes are unique, and the muscles present in the deeper layers of the pelvis are different for women and men; however, the muscles present in the shallower layers are the same [30], such as the transverse perineal muscle involved in the opening and closing of the urethral opening targeted in this study and the adjacent external anal sphincter. This is one of the biggest reasons that vibration stimulation and visual feedback could increase PFM activity, regardless of sex. Therefore, this study should help establish an effective method of conducting PFMT for both sexes.

This study has some limitations. First, the frequency of the vibration cushion was only 40 Hz. Vibration frequencies between 25 and 50 Hz had been used in whole-body vibration training, and between 8 and 300 Hz in local vibration training [12, 31, 32, 33], and hence we cannot discard the possibility that there may be more appropriate vibration frequencies for PFMT. However, it is meaningful that this study found PFMT with 40 Hz local vibration stimulation to be useful. Second, the correlation between the pressure detected by the sensor installed on the raised seat surface of the cushion and the activity of the PFM could not be verified. However, we expect that clarifying these correlations will help to establish a visual feedback method more appropriate than conventional feedback, such as the insertion of a probe into the vagina. Third, combined therapy with vibration stimulation and visual feedback was not possible in this study, because the cushion would not allow simultaneous operation of these methods. Besides, it was difficult for the researchers to change the functionality of the equipment. Moreover, providing visual feedback in combination with vibration stimulation could provide inaccurate pressure sensing, and could not be used considering the machine’s current capabilities. In the future, we plan to investigate the long-term effects of PFMT with vibration stimulation and visual feedback and to determine whether the same effects can be expected in subjects with UI symptoms.

5. Conclusions

The results of this study show that exercise interventions such as vibration stimulation and visual feedback immediately increase PFM activity. In addition, the muscle activity ratio (post/pre PFMT) increases compared with the control condition in vibration stimulation and visual feedback conditions. These exercise interventions for the PFM with a cushion providing vibration stimulation and visual feedback may be effective methods to stimulate the PFM and induce longer training sessions.

Author contributions

CONCEPTION: Rami Mizuta, Noriaki Maeda and Makoto Komiya.

PERFORMANCE OF WORK: Rami Mizuta, Honoka Ishihara, Tsubasa Tashiro, Mitsuhiro Yoshimi and Sakura Oda.

INTERPRETATION OR ANALYSIS OF DATA: Rami Mizuta and Noriaki Maeda.

PREPARATION OF THE MANUSCRIPT: Rami Mizuta, Tsubasa Tashiro and Mitsuhiro Yoshimi.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Noriaki Maeda and Makoto Komiya.

SUPERVISION: Yukio Urabe.

Ethical considerations

The study protocol met the requirements of the Declaration of Helsinki and was approved by the Ethical Committee for Epidemiology of Hiroshima University (approval number E-2005-1). The participants were fully informed of the details of the study by using the Consent Explanation Document, and verbal consent was obtained.

Funding

The authors report no funding.

Footnotes

Acknowledgments

This work was supported by JST, the establishment of university fellowships towards the creation of science technology innovation, Grant Number JPMJFS2129.

Conflict of interest

The authors have no conflicts of interest to report. Given his role as an Editorial Board Member, Noriaki Maeda had no involvement nor access to information regarding the peer review of this article.

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