Pelvic floor muscle (PFM) dysfunctions play a crucial role in the pathophysiology of various pelvic floor disorders. Conservative treatments targeting these dysfunctions are recognized as a first-line treatment and efficacy has been shown in high-quality randomized clinical trials. However, the objective evaluation of the PFMs, more specifically the assessment of PFM tone or stiffness, poses an ongoing challenge. Shear wave elastography (SWE) is a non-invasive and real time technique that appears promising for the evaluation of muscle stiffness. This technology has been widely used clinically to discriminate changes in tissue stiffness in breast cancer masses, and has been increasingly used to study stiffness/stress of muscles (limb and trunk) at rest and during contraction ][1]. The use of this innovative technology for assessing muscle stiffness requires a comprehensive validation process given that anisotropy may influence measurement (i.e. properties of the measure depend on direction, and thus, measures are influenced by the orientation of the ultrasound beam relative to the muscle fibres) [2]. No study has investigated the validity of SWE for evaluating stiffness properties of the puborectalis muscle. As stiffness increases with muscle activation, consistent with similar studies for limb muscles, validity of SWE could be interpreted from the relationship between changes in stiffness and muscle activation during contractions of different intensity.

This study aimed to examine the relationship between changes in muscle stiffness (evaluated with SWE) and changes in of PFM activation during contractions of the puborectalis muscle at different target intensities.

Fifteen parous/nulliparous women participated in the study. Women were excluded if they reported any pelvic floor disorders, vulvo-vaginal pain, previous urogynecological surgery or major neurological/muscular conditions. Participants were convened to a 90-min session for PFM assessment. An experienced physiotherapist instructed women about PFM anatomy and physiology and verified whether women performed adequate PFM contraction using digital palpation.

A small intra-vaginal electromyography (EMG) suction electrode (diameter equivalent to a urinary catheter), positioned on the puborectalis muscle, was used to assess PFM levels of activation. Maximum voluntary contractions (MVC) of the PFMs were performed to calculate the maximum muscle activation level and to set targets for sub-maximal contractions. Women performed a 6-s sustained PFM contraction at 10%, 20%, 30%, 50%, or 75% of MVC in random order while receiving real-time visual feedback on a computer screen, showing both the target and current EMG amplitude. Each contraction intensity was repeated twice and followed by a rest period of at least 10 s. Left and right side were assessed in separate trials.

During each contraction, stiffness of the puborectalis muscle was assessed using SWE (Aixplorer, SuperSonic Imagine) with a linear transducer (frequency range: 2-10 MHz) placed on the perineum (Figure 1). The transducer was placed lateral to the vagina and aligned so that muscle fascicles/connective tissues of puborectalis could be visualised. Stiffness was evaluated by selecting a region of interest (ROI) that corresponds to muscle fibres perpendicular to the ultrasound beam. For each contraction, 2-3 frames with stable SWE maps were selected (frame rate: 1Hz). The average shear elastic modulus (kPa) within the ROI was calculated and averaged across frames and across the two repetitions. The root mean square (RMS) EMG was calculated over the same time windows. The SWE and RMS EMG from each participant were normalized to the maximal value recorded during MVC.

Linear regression analyses were performed to assess the relationship between normalized puborectalis stiffness (SWE) and normalized RMS EMG. The coefficient of determination (r²) was calculated to quantify the goodness of fit of the regression line using data from each participant, and a group average was obtained by pooling data from all participants together.

Women were aged 35.8 ± 7.2 (range 23-47), had mean parity of 1.2 ± 1.4 (range 0-4) and a mean body mass index of 22.2 ± 2.7 kg/m² (range 1.9-3.0). Our results showed that the normalized SWE and RMS EMG were highly correlated, i.e. puborectalis stiffness progressively increase with increasing intensities of PFM activation. The average coefficient of determination across individual participants was r² = 0.90 ± 0.08 and r² = 0.87 ± 0.15 for the left and right sides, respectively. When data from all participants were included in the regression analysis, a coefficient of determination of r² = 0.81 and r² = 0.72 were obtained for the left and right sides, respectively (both p < 0.001, see Figure 2).

Our study confirms that the puborectalis muscle can be visualised lateral to the vagina using a transperineal approach. The results concur with those of previous studies for limb muscles that report a strong linear relationship (correlation of determination r² ≥ 0.8) between stiffness and muscle activation for other skeletal muscles (e.g. shoulder muscles) [2,3].

These findings provide evidence to support the validity of SWE as a valid, non-invasive method to quantify puborectalis stiffness. SWE measures were found to be strongly associated with the intensity of PFM contraction and therefore, it may be used to indirectly assess the level of activation of the puborectalis muscle without the use of more invasive techniques. Moreover, this forms solid basis to further investigate the clinical relevance of SWE for evaluating the involvement of increased PFM tone in the pathophysiology of pelvic pain.

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