In the early stages of pelvic floor muscle training (PFMT), the activation of pelvic floor muscles, such as the puborectalis, striated urethral sphincter, anal sphincter and bulbocavernosus, is often difficult due to a limited muscle awareness [1]. Real-time biofeedback devices are adopted to gain awareness of pelvic floor muscle and their contraction, but their reliance on physiotherapists and invasiveness (e.g., intra-anal insertion) represent significant limitations [2, 3]. Alternatively, physiotherapists could provide verbal, non-invasive feedback on pelvic floor muscle contractions while palpating synergistic core muscles such as the multifidus and the obliquus internus (OI) abdominis muscles. The volumetric and shape variation of the bulbocavernosus muscle (BC) during its contraction, synergic to the pelvic floor activation, could offer the opportunity to indirectly assess pelvic floor muscle recruitment by an external pressure sensor placed beneath the perineum. Nevertheless, the current literature does not investigate pelvic floor muscle contraction in men using such external pressure sensors. Moreover, it is unclear if the pressure variations detected by external devices could reliably correspond to pelvic floor muscle activation. Hence, the following study aimed to 1) ascertain if an external pressure device could effectively monitor pelvic floor muscle contraction using ultrasound and 2) explore the synergistic roles of OI and multifidus muscles during pelvic floor muscle contraction through surface electromyography (sEMG) in healthy men.

An experimental repeated measures study design was adopted. Recruitment was voluntary and participants were required to read an informative note, provide their informed consent, and drink at least 500ml of water before the experiment. The inclusion criteria required to be healthy men with no history of any clinical conditions. The experimental setup (Figure 1) involved the co-registration of multiple signals acquired from different interconnected systems to ensure data synchronisation: 1. A sensorised inflatable system measured BC contraction through pressure variations. The device consisted of an air chamber and a pressure sensor controlled by a programmable board. Data visualisation occurred wirelessly through a mobile interface, synchronised with other systems via coaxial connections. 2. A sEMG setup with two pairs of electrodes placed bilaterally on the multifidus and OI abdominis muscles. sEMG signals were acquired in single differential mode. 3. A Wireless convex ultrasound probe positioned under the suprapubic zone (in a sitting position), coupled with a tablet application, recorded bladder dynamics during pelvic floor contractions. 4. A silicone-coated switch was used to synchronise sEMG and ultrasound images. Pressing the switch generated identifiable artefacts in both ultrasound images and voltage signals, serving as a trigger. 5. An Arduino board provided a structured temporal trigger for participants, aiding the data segmentation and analysis processes. Participants underwent preparation for sEMG electrode placement and were familiarised with pelvic floor muscle contraction commands using both the pressure device and ultrasound guidance. While seated on the external pressure device, participants performed three pelvic floor contractions lasting 2 seconds with 2 seconds of rest, for a total of three series. Data collection involved acquiring time-varying signals of sEMG activity (mV), bladder movements via ultrasound in motion mode, and pressure variations (kPa) from the external device. Data were processed in MATLAB: sEMG signals were filtered and normalised on their average value; pressure sensor data were low-pass filtered; ultrasound images were processed using automated routines to track bladder movements over time. Both pressure variations and bladder movements were normalised between 0 and 1. We adopted a multivariate mixed-effect model for repeated measures to analyse whether there was a simultaneous increase in activity during contraction (ON phase) compared to relaxation (OFF phase) across various signals. We aimed to determine if the pressure signal reflected pelvic floor muscle contraction in accordance with ultrasound evidence. With this model, we considered the correlation between signals in each subject (random effect) and the correlation within the trials of the same subject (repeated measures). After estimating the normalised amplitude (NA) for different signals, within each subject we assessed the consistency between signals in the differences between ON-OFF phases. We reported the mean and the 95% confidence intervals (CI) for the NA of our sample in the ON and OFF phases.

We conducted an initial analysis on 11 healthy participants (mean ± standard deviation [26 ± 2.24 years old]). The NA means of the bladder base movements during the OFF and ON phases were 0.21 (CI [0.14-0.28]) and 0.7 (CI [0.63-0.77]), respectively. For the left multifidus sEMG signals, the NA means during the OFF and ON phases were 1.03 [0.96-1.10] and 1.10 [1.03-1.17], respectively, while for the right one 1.05 [0.98-1.12] and 1.11 [1.04-1.18]. The NA means of the left OI sEMG signals during the OFF and ON phases were 0.93 [0.86-0.99] and 1.19 [1.12-1.25], respectively, while for the right one 0.90 [0.83-0.97] and 1.11 [1.04-1.18]. The NA means of the pressure signals during the OFF and ON phases were 0.24 [0.17-0.31] and 0.58 [0.52-0.65]. Figure 2 shows the graphical representation of results presented above.

The pressure and ultrasound signals exhibited similar and synchronic amplitude variations, proving that an external pressure device could be an effective biofeedback for monitoring pelvic floor muscle contraction. Such a system offers various advantages, including its potential utility as a non-invasive biofeedback tool in the initial phases of PFMT to enhance muscle awareness and holds promise for paediatric interventions. Moreover, the analysis revealed significant differences in the amplitude of OI signals between pelvic floor muscle contraction (ON phase) and relaxation (OFF phase). This pattern could offer valuable feedback for physiotherapists during subsequent phases of PFMT, such as exercises performed in positions where it is difficult to palpate the perineum (i.e. standing). Regarding the multifidus muscle, our experimentation yielded inconclusive results. This could be attributed to the challenges associated with sEMG signal acquisition from deep back muscles like the multifidus, where a needle EMG assessment could be more appropriate.

An external pressure device positioned beneath the pelvic floor, in contact with the bulbocavernosus muscle, could indirectly assess pelvic floor activation and support pelvic floor muscle training in men. The simultaneous co-activation of the OI during pelvic floor contraction found by assessing sEMG signal, offers an additional opportunity of a non-invasive biofeedback for PFMT also in a position where the perineum is not easily accessible, like in a standing position.

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Continence 12S (2024) 101630
DOI: 10.1016/j.cont.2024.101630