Intra-vaginal dynamometry yields objective and reliable measures of pelvic floor muscle (PFM) strength and stiffness [1]. Through motor learning, a single assessment session results in improvements in force generating capacity of some skeletal muscles [2] and as such, a familiarization session may be necessary to reduce bias in the measurement of PFM force outcomes.  The objective of this study was to investigate the impact of task familiarization on active and passive PFM forces  measured in nulliparous women using an automated intra-vaginal dynamometer. Results from this study will provide a basis for clinical protocol development using intra-vaginal dynamometry.

This study received research institutional ethics board approval. Nulliparous women were recruited, provided written informed consent, and attended three laboratory assessments at one-week intervals. Immediately prior to the first assessment, the physiotherapist provided instructions and tactile cuing to ensure that women could perform a proper voluntary PFM contraction. In a standardized supine position, the arms of an automated dynamometer [1] (Figure 1) were inserted into the vagina and were held at a comfortable angle. Active PFM properties were measured during maximal voluntary contractions with the arms open to two different anteroposterior diameters, 25mm and 35mm. Women were asked to contract their PFMs as quickly and forcefully as possible, to hold the contraction for 5s and then to relax their PFMs. Passive PFMs forces were measured while women were instructed to keep their PFMs relaxed as the arms of the dynamometer opened from an initial diameter of 15mm to a maximum diameter of 40 mm at two different speeds, 25mm/s and 50mm/s. Tissue elongation was held for 5s and then the arms returned to their initial position. Three repetitions of each task were performed. Women were asked to perform three sets of ten pelvic floor contractions daily between visits. Force data, sampled at 100Hz, were filtered using a second order, dual pass, low pass filter with a cut-off of 5Hz. Baseline force (N), peak force (N), relative peak force (N), and rate of force development (N/s) were computed for each task. Trials with evidence of reflex PFM activation were removed prior to analysis. Between-trial and between session effects were tested for each outcome using one-way repeated measure ANOVAs (α=0.05).

Twenty nulliparous women (mean age = 35±15 years; BMI= 23.47±4.04kg/m2) participated. There were no between trial effects for MVC outcomes, but resistance to passive stretch was higher for trial 1 than for trials 2 and 3 at both opening speeds, thus between session effects were computed using trials 2 and 3 only for passive tasks. No significant differences were found across the three visits for any force outcomes studied (Table 1).

There is no evidence of motor learning on dynamometric measures of active or passive PFM forces in nulliparous women measured when measurements sessions are repeated three times at one week intervals.

A formal familiarization session may not be required when both active and passive PFM forces are measured using intravaginal dynamometry. Ensuring that women perform a correct pelvic floor contraction prior to intravaginal dynamometry seems adequate for this population. Future studies should aim to validate these results in women reporting symptoms associated with pelvic floor dysfunction such as urinary incontinence, pelvic pain and pelvic organ prolapse.

Bérubé et al. (2018). An Automated Intravaginal Dynamometer: Reliability Metrics and the Impact of Testing Protocol on Active and Passive Forces Measured from the Pelvic Floor Muscles. Neurourology and Urodynamics. In press.
Ploutz-Snyder et al. (2001). Orientation and familiarization to 1RM strength testing in old and young women. The Journal of Strength & Conditioning Research, 15(4); p. 519-523.