The State of Pelvic Floor Muscle Dynamometry: A Critical Literature Review

El-Sayegh B1, Sawan M2, Dumoulin C3

Research Type

Basic Science / Translational

Abstract Category

Continence Care Products / Devices / Technologies

Abstract 25
Novel Techniques and Approaches in Basic Science
Scientific Podium Short Oral Session 3
Wednesday 4th September 2019
09:45 - 09:52
Hall G3
Pelvic Floor Female Rehabilitation
1.Polystim Neurotech Laboratory, Department of Electrical Engineering, Polytechnique Montréal/ Centre de recherche, Institut Universitaire de Gériatrie de Montréal, 2.Polystim Neurotech Laboratory, Department of Electrical Engineering, Polytechnique Montréal, 3.School of Rehabilitation, Faculty of Medicine, Université de Montréal, Montréal, Canada/ Centre de recherche, Institut Universitaire de Gériatrie de Montréal

Batoul El-Sayegh



Hypothesis / aims of study
Women’s pelvic floor muscle (PFM) integrity is of high importance to maintain urogynecological health [1]. PFM dynamometry is defined by the IUGA and the ICS joint report on the terminology for female pelvic floor dysfunction as an instrument measuring PFM resting and contractile forces using strain gauges mounted on a speculum, which is inserted into the vagina [2]. Dynamometry measures PFM forces in Newton units (N = 1 kg × m/s2) [2]. In the past 20 years, research and development of PFM dynamometers has produced many prototypes for both research and commercial use. There is currently a need to review existing literature on dynamometry development to understand which specific aspects warrant further research. The aim of this study is to review the state of the literature on PFM dynamometry and to discuss the advantages and limitations of available PFM dynamometers in both research and clinical fields.
Study design, materials and methods
In this critical literature review, we conducted a search of four databases (initiated in September 2018): MEDLINE, Compendex, Derwent Innovation Index and Web of Science. Through Boolean logic, we used the search terms “dynamometer”, “instrumented speculum” and related terms, combined with “pelvic floor muscle” and related words. A search strategy was developed with the help of two experienced librarians in engineering and medical fields. Additional literature was identified by searching the reference lists of relevant scientific articles. A final search update was performed on December 17th, 2018. The literature search was restricted to articles in the English language and excluded conference papers. Furthermore, only studies on the development or psychometric evaluation of original PFM dynamometry devices were included in this review.
Fourteen PFM dynamometers were identified in the literature. PFM dynamometers were grouped into two categories: research PFM dynamometers (ten), used exclusively for research purposes, and clinical PFM dynamometers (four) commercially available for clinical purposes only. Clinical PFM dynamometers were further divided into two subcategories according to the targeted customers: personal PFM dynamometers meant to be used by patients at home and the clinical PFM dynamometers meant to be used by physiotherapists in a clinical setting.
Specific characteristics, advantages and disadvantages of each dynamometer subgroup are presented separately in Figure 1 and Figure 2.

Research PFM Dynamometer
All 10 PFM dynamometers were compared in terms of measurement direction inside the vagina, whether the dynamometer base was fixed or hand held, and if it had a fixed or variable aperture. Aperture range and psychometric properties were also assessed for each dynamometer. Advantages and limitations in the usefulness of the measurements taken with each dynamometer are detailed in Figure 1. 

Over time, the material used for the research PFM dynamometers have shifted from aluminum and stainless steel to, more recently, 3D-printed plastic, which allows more possibilities in device design. Of the ten dynamometers, 50% measured PFM forces in the antero-posterior direction, 10% in the latero-lateral direction, 20% in both directions, and 20% measured in either antero-posterior or latero-lateral directions by turning the device by 90 degrees. The reason for measuring PFM forces in different directions appears to be related to measurement needs. The antero-posterior direction is often used to measure the active/maximal PFM forces with regard to urinary incontinence. In contrast, the latero-lateral measurements are used to evaluate the passive/elastic properties of the vaginal muscles before birth. The aperture of the PFM dynamometers can either be fixed (40%) or variable (60%). The apertures of different prototypes vary from a minimum of 5 mm (inside the instrument branches) to a maximum of 70 mm. By measuring PFM forces using different apertures, the length-tension curve of PFM muscles can be documented. Recent advances include motorized adjustment of the aperture, which can further improve validity and comfort of measurements. A total of 60% of the dynamometers are hand-held, which allows movement of the device with PFM contractions. However, validity and test-retest reliability of measurements are dependent on the evaluator’s technique.

We identified four common drawbacks for PFM dynamometers. First, few research groups conducted and published psychometric assessments of the measuring instrument. Without validity and repeatability of studies, accuracy and usefulness of obtained data may be questioned. Second, only 30% (Dumoulin et al., Verselet et al., Ashton Miller et al.) conducted studies to quantify the effect of the intra-abdominal pressure on their measurements. Third, although 60% of dynamometers can be used in the standing position, no studies reported psychometric properties specific to this position. Finally, only one study (1/10) reported the use of strain gauge’s differential arrangement (Dumoulin et al.), which ensures consistent values wherever force is measured on the dynamometer branch. Otherwise, the measure can be influenced by the lever arm and repeatability may be questioned.

As dynamometry is an emerging field, more work is needed to create a standard research dynamometer that can distinguish pelvic floor muscle contractions from other artifacts (e.g. deep abdominal pressure). Additional research should also be undertaken measuring intravaginal forces in the standing position, as urinary incontinence and pelvic organ prolapse symptoms usually occur in this position.
Clinical PFM Dynamometer: Figure 2 shows the findings for the different dynamometers. The dynamometers are noticeably heterogeneous in shape and weight. All dynamometers intended for personal use have silicone as a cover material, whereas the only dynamometer developed for clinical settings is made of steel. Price range varies significantly between the cheapest and the most expensive dynamometer (x1.5). Among the existing clinical PFM dynamometers, only 25% have conducted reliability and validity studies. This is of importance as, without proper psychometric assessments, measurements using clinical PFM dynamometers may be misleading. For example, women may be falsely informed that they are performing PFM contractions correctly. Another major drawback is that 75% of clinical PFM dynamometers are not adapted to different vaginal sizes.
Interpretation of results
Our critical literature review highlighted that further research is needed on psychometric evaluation, size and position of use of existing PFM dynamometers used in research and clinical settings. We excluded abstract papers and papers written in languages other than English, which could affect the completeness of the published information. This is a limitation of our study.
Concluding message
This review summarizes the available PFM dynamometers in both research and industry and provides a complete analysis of the advantages and limitations of each. Our results clearly demonstrate that this discipline is still in the ascent phase. More work is needed to reach the full technological maturity of PFM dynamometers in both research and clinical fields.
Figure 1 Figure 1. Summary of Findings; NR= not reported.
Figure 2 Figure 2. Summary of Findings: Wt= weight: L= length; W= width; D= diameter; NR= not reported.
  1. Haylen B, de Ridder D, Freeman R, et al. An International Urogynecological Association (IUGA)/International Continence Society (ICS) joint report on the terminology for female pelvic floor dysfunction. Neurourol Urodyn. 2010;29:4–20.
  2. Haylen B, Maher C, Barber M, et al. An International Urogynecological Association (IUGA)/International Continence Society (ICS) joint report on the terminology for female Pelvic Organ Prolapse (POP): International Urogynecological Association (IUGA). Neurourol Urodyn. 2016;35:137–168.
<span class="text-strong">Funding</span> This research was supported by the AGE-WELL NCE Trainee Award Program. <span class="text-strong">Clinical Trial</span> No <span class="text-strong">Subjects</span> None