Pelvic floor muscle tone, which includes active contributions from muscle activity and passive contribution from viscoelastic properties, is argued to be increased in various pelvic health conditions, particularly those involving pain. This is often referred to as “overactive” pelvic floor muscles. This study aimed to systematically review the evidence for increased pelvic floor muscle tone in pelvic health conditions, with emphasis on the quality of the evidence and measures used.

This was a systematic review with narrative synthesis. The review followed PRISMA guidelines and was pre-registered on Prospero, the international prospective register of systematic reviews (CRD42020161957). 

Three databases (Medline, CINAHL, EMBASE) were searched using standardised search terms. Papers were included if they; (1) evaluated a pelvic health condition; (2) included a measure of pelvic floor muscle tone; Papers were excluded if they were; (1) animal studies; (2) Systematic reviews (which did not add additional primary data). Papers were entered into Covidence and screened by title, abstract and full-paper as necessary. Two reviewers screened the initial 1000 papers to confirm agreement and then screened the remaining papers individually. 

Data were extracted using a standardized form. This included reference details, condition group, comparison group/data, method, any identified methodological issues, outcome measures, finding and term(s) used to reflect overactivity. Study quality was analysed using an adapted version of the ROBINS-I tool [1] (adapted to consider the specific features of methods used to study pelvic floor muscle tone), and a scoring system that we developed to reflect whether the design was adequate to draw convincing conclusions. This score considered whether the study; (1) included a comparison group, (2) used a validated method, and (3) involved no methodological issues that precluded interpretation (e.g., no normalisation of EMG [2]). Only studies that met all three criteria were included in the final analysis of evidence for greater pelvic floor muscle tone in pelvic health conditions.

The search identified 6,084  studies from which 121 were included. These studies reported 7 different methods with 76 unique outcome measures that were used 177 times in varying combinations across the 121 papers. The most common condition studied was pelvic pain (various definitions n=12) followed by urinary, then bowel disorders. Most studies were cross-sectional. Few studies included a control group for comparison. Most studies used methods that have not been validated or were performed in a manner that preclude interpretation. Ten papers using 12 methods (e.g., normalised EMG; vaginal dynamometry/manometry) provided evidence that was considered to be convincing according to our criteria. Of these, 7 papers using 9 measures demonstrated greater tone in the pelvic health condition (mostly pain) compared to controls, and 3 papers using 3 methods showed no significant difference in tone. Of the affirmative papers, 4 used methods to measure cases with vestibulodynia/vestibulitis, 3 methods were used for other chronic pelvic pain, 1 method for vaginismus and 1 method for mixed diagnostic group. Of the negative papers, 1 studied pelvic girdle pain, 1 studied voiding dysfunction and 1 studied prostatodynia.  Of the remaining 111 papers with 165 measures that could be considered to give inconclusive evidence, 14 (8%) measures were interpreted as no difference in tone and the remaining 151 (92%) measures were interpreted to as demonstrating greater tone.

Although there is evidence for increased pelvic floor muscle tone in a range of pelvic health conditions, the overall quality of the evidence was poor and methodological issues precluded convincing conclusions to be drawn from in 165 (93 %) of the measures. Other issues include; inaccurate use of terminology, and limited investigation of male, transgender and paediatric populations. Careful consider of measurement method and study design is required to generate interpretable outcomes.

The comprehensive review found evidence for increased pelvic floor muscle tone in pelvic health conditions, but the majority of studies use methods that preclude convincing interpretation. To advance evidence in this field, future work should consider inclusion of clearly defined control/comparison groups (preferably individuals without pelvic health complaints or other major health conditions), use of validated outcome measures, and careful application of methods.

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