Analysis of pelvic floor muscle electromyography parameters in women with or without sexual dysfunction

Introduction

Female sexual dysfunction (FSD) is an umbrella term that encompasses several facets of sexuality in terms of desire, arousal, orgasm and/or sexual pain. FSD can occur during any phase of the sexual response cycle, and reduces the quality of life of women.¹ Pelvic floor dysfunction (PFD) refers to a group of disorders related to abnormal positioning or function of the pelvic organs due to a variety of factors, such as defects/weakness, injuries and dysfunctions of pelvic floor support. PFDs typically include prolapse of the uterus, cervix, vagina, bladder and rectum, and incontinence. PFD can afflict women both physically and psychosomatically.² The prevalence of FSD ranges from 30% to 50% in the general population, but the incidence of FSD can be as high as 50–83% in women with PFD.³

Electromyography (EMG) is an electromyographic mode that can be used to assess muscle fatigue.⁴ The amplitude of EMG signals is a measure of the strength of the muscles being activated. It is relatively small when the muscles are relaxed, but will be elevated when there are more muscle contractions.⁵ Pelvic floor muscle (PFM) EMG is a measurement of intravaginal bioelectrical signals collected from a series of muscles of the pelvic floor through contractions and rest according to the Glazer protocol,⁶ involving the following phases: pre-baseline rest, phasic contractions, tonic contractions, isometric contractions for muscle endurance evaluation and post-baseline rest. The level of contraction potential represents the degree of myoelectric fatigue, whereas the recovery latency represents the reduced capacity of muscle after contractions. It has good validity in assessing PFM function.⁷

Evidence suggests that women with greater PFM strength have better sexual function.⁸ Sexually active women have better PFM endurance than inactive women.⁹ However, it has also been reported that PFM strength is not correlated with sexual function.¹⁰ Here, we assessed female PFM function with EMG, and explored the relationships between EMG parameters and FSD, in an attempt to provide an objective basis for FSD diagnosis and treatment.

Participants and methods

Results

Discussion

The Glazer protocol is a commonly used tool in EMG devices for assessing PFM bioelectric activity.⁶ PFM EMG has shown good reliability in the quantification of PFM activity patterns.¹³ In the present study, among the PFM EMG parameters, the peak EMG amplitude during the phasic (flick) contractions and the recovery latency during the tonic contractions were closely related to FSD. Multivariate linear regression suggested that the peak EMG amplitude during the phasic (flick) contractions was 5.39 μV higher in the FSD group than in the non-FSD group, whereas the recovery latency during the tonic contractions was 0.29 s shorter.

EMG is the sum of the areas surrounded by the curve per unit time after the measured EMG signals are processed. It indicates the total number of discharges of motor units at a given time instant when the muscle is involved in an activity, reflecting the strength of the muscle’s electromyographic activity during this period. The muscle EMG value reflects the size of each motor unit’s discharge and the number of muscle fibres involved in muscle contraction during exercise. Typically, greater EMG amplitude indicates increased fatigue. Thus, EMG amplitude is a crucial indicator for assessing muscle fatigue.¹⁴ At a muscular level, fatigue posits the reduced capacity of muscle fibres to produce force, even in the presence of motor neuron excitation via either spinal mechanisms or electric pulses applied externally. Prior to decreased force, when sustaining physically demanding tasks, alterations in the muscle electrical properties take place. These alterations are termed myoelectric manifestation of fatigue.¹⁴ In the present study, the FSD group had greater peak EMG amplitude during the phasic (flick) contractions than the non-FSD group. This finding suggests that the pelvic floor muscles in women with FSD may be more susceptible to fatigue.

EMG records electrical activity generated by muscle fibres during muscle contraction, and the properties of the recorded activity depend on the fibre membrane potential and the nerve activation signals sent from motor neurons to the muscle. EMG has been widely used in various kinesiological studies.¹⁵ Skeletal muscle consists of three fast fibres and one slow fibre, among which types I and IIa are slow fibres that contain the highest mitochondrial content and fatigue resistance. Despite fibre type differences in the degree of fatigability, the contractile properties undergo characteristic changes with the development of fatigue that can be observed in whole muscles, single motor units and single fibres. In the absence of muscle fibre damage, the prolonged relaxation time associated with fatigue is reduced. The central nervous system senses this condition and reduces the alpha-motor nerve activation frequency as fatigue develops.¹⁶ The recovery latency of slow fibres indicates the speed of their response and correlates with muscle excitability. In the present study, the FSD group had a shorter recovery time during tonic contractions, suggesting that the coordination of pelvic floor muscles may be poorer in FSD women.

The relationship between PFM strength and sexual function remains controversial. PFD women also experience a significant burden of sexual dysfunction,¹⁷ Pasqualotto et al.¹⁸ found that FSD occurred less frequently in women with higher PFM strength, which was consistent with our finding: women in the FSD group had a shorter recovery latency during the tonic contractions, along with poorer muscular stability and coordination. However, it has also been proposed that female sexual function is not related to the pelvic floor. In a study evaluating PFM strength in different body positions in healthy women and its correlation with sexual activity, it was concluded that there was no correlation between PFM strength and orgasm.¹⁹ No evidence has demonstrated any possible association between PFM function and sexual satisfaction.²⁰ In contrast, our study revealed that the presence or absence of FSD correlated with the peak EMG amplitude during the phasic (flick) contractions and the recovery latency during the tonic contractions among all the PFM EMG parameters, possibly due to differences in the study population.

Our findings may provide insights into the treatments of pelvic floor disorders in PFD women, facilitating the timely adjustment of the treatment regimens. PFM EMG is a reliable tool for early risk detection and preventive interventions of PFD.²¹ It has been suggested that the strength of at least two types of muscles (either abdominal or PFM muscles) can be improved if progress is observed in the sexual function; thus, EMG measurement is a potential technique to quantify the changes in female sexual function.²²

To date, EMG has been widely used to study muscle coordination, although it is not able to measure deeper muscles. EMG relates primarily to nerve output from the spinal cord and, therefore, is associated with the number of activated motor units and their discharge rates.²³ However, it is not clear whether the association between PFM strength and sexual function is due to the fact that women with stronger muscles have better sexual function or whether better sexual function leads to increased PFM strength.⁸ There may be an anatomical correlation between the PFMs and female sexual function.²⁴ The somatic nerves innervating the PFMs are branches of sacral spinal nerves S3, S4 and S5. The pudendal nerve originates from the sacral plexus (S2–S4), and contains sensory and motor nerve fibres. It finally spreads into three main terminal branches: the motor branch of the inferior rectal nerve innervates the lower rectum and the external anal sphincter; and the sensory branch senses the skin anteriorly and laterally to the anus. The perineal nerve originates from the middle part of the pudendal canal, and terminates by bifurcating into motor and sensory branches to innervate the external urethral sphincter and perineal muscles. The terminal branch of the pudendal nerve is the dorsal nerve of the clitoris, which is responsible for clitoral sensation.²⁵ Sexual arousal is primarily achieved through spinal cord reflexes. The congestion of the clitoris, labia,and vagina caused by stimulation of the autonomic nerves of the vagina and cavernous bodies of the clitoris is also a spinal cord sexual reflex.²⁶ PFMs and sexual function are innervated by the same nerves. Pelvic floor muscle EMG parameters may change in women with FSD. Further research with larger sample sizes is needed to explore specific patterns of these changes.

However, whether EMG signals can be used to accurately assess muscular coordination remains unclear.²³ FSD cannot be better attributed to a specific condition, and its aetiology can be explained using a biopsychosocial model that includes biological, psychological and sociocultural factors, as well as interpersonal influences.²⁷ Although our study showed that PFM EMG parameters showed certain changes among women with FSD that can be used as auxiliary indicators for the clinical treatment of FSD, these parameters alone are insufficient for diagnosing sexual dysfunction in women, because it is not possible to explain the abnormal signal variations through the physiological processes. Therefore, more research is required to demonstrate the correlations between FSD and PFM EMG parameters.