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.