Hypothesis / aims of study
We aimed to evaluate the feasibility of a standardized 38-minute in-lab treadmill-based pad test to evaluate of the presence and severity of urine leakage among female runners. Our objectives were to describe the distribution of pad weight gain values observed among female runners who report running-induced stress urinary incontinence (RI-SUI) and those who do not, to compare pad weight gain between runners with RI-SUI and continent runners, and to investigate the sensitivity and specificity of pad weight gain as an objective measure of urine leakage.
Study design, materials and methods
The study was approved by the local ethics board (H-06-18-759) and all participants provided written, informed consent prior to engaging in any study activities. Female runners over 18 years of age, who ran at least 5 km in under 50 min, twice a week, and who had maintained an average running distance over 10 km per week for a minimum of 1 year were invited to participate in this cross-sectional, observational study. Runners with female pelvic anatomy who reported no urine leakage while exercising or during activities of daily living and those who frequently experienced UI while running (self-reporting one or more episodes per week) were recruited. Participants were ineligible if they had undergone any major urogenital surgery, had pelvic organ prolapse greater than POP-Q stage 2, had a body mass index greater than 30kg/m2, were pregnant or had given birth in the past year, or if they were uncertain about whether they leaked or not. Runners with UI were excluded if they reported leakage associated with urgency during running or if they experienced ≥1 episode of urine leakage per month not associated with exercise.
Transperineal ultrasound imaging was performed as part of a larger protocol not related to this report. Trans‐abdominal ultrasound was then used to standardize bladder volume to between 100 and 200 mL before beginning the protocol. Wearing a pre-weighed incontinence pad, participants performed a standardized in-lab running protocol on a treadmill (NordicTrack Commercial 2450) which included a 6-minute warm-up at incremented speeds starting at 5 km/h and increasing to a maximum of 15 km/h, depending on the participant's performance, followed by 30 minutes of running at a steady self-selected speed at an intensity that they deemed to be somewhat hard (level 13–14 on the Borg Scale), and a 1-2 minute cool down. Participants were asked to report any instances of perceived urine leakage while running. The pad was weighed by a research assistant who was not otherwise involved in the study as soon as possible after participants finished the running protocol. Pad weight gain was tested for normality using the Shapiro-Wilk test and was compared between groups using the Mann-Whitney U test. A receiver operating characteristic (ROC) curve was used to evaluate the sensitivity and specificity of different cut-off points for pad weight gain to identify those who experienced UI during the protocol.
Interpretation of results
Pad weight gain during the treadmill protocol was highly variable among runners both with and without RI-SUI, with skewed distributions. While pad weight gain was significantly higher among female runners with RI-SUI than those without, the distributions overlapped. The ROC analysis suggests that a pad weight gain of 9.36 g provides a reasonable threshold for pad weight gain to identify UI during running, however the sensitivity at this cut-off is low; it is likely to produce many false negatives (27.3%). While the next step should involve testing the model with a new sample, these results already suggest that a standardized, treadmill-based pad-test protocol may not have great utility to classify the presence of UI experienced during exercise.
Pad weight gains of up to 4g during general exercise (1) and up to 8g while running (2) have been suggested as potential cut-off values in previous studies, yet the findings here suggest that a higher cut-off may be needed. Several studies have used exercise-based pad tests as an outcome for intervention research, where the mean pad weight gain among participants with exercise-induced UI has been much smaller than the 9.36g cutoff determined here, some lower than 2g (1). It is therefore not clear whether participants in these studies leaked urine on the pad test. Other factors including the type and intensity of exercise, and the ambient temperature and humidity of the environment may affect pad test outcomes during exercise. While it is possible in the current study that remnants of ultrasound gel in some participants may have influenced pad weight gain, attempts were made to wipe away all gel before the pad was inserted, and the weight of such remnants would be minimal relative to the pad weight gains observed.
While the ROC analysis did not produce a viable cut-off score with optimal sensitivity and specificity, it is unlikely that a treadmill-based pad test is needed for diagnosis, since participants most often can feel urine leakage and thus subjective reporting is normally sufficient. We did, however, screen two potential participants who were unsure if they leaked urine during exercise- they saturated pads but could not determine if the wetness was due to urine or not. In those cases, oral phenazopyridine, which stains the urine bright orange, is likely a better approach that a pad test to confirm RI-SUI.
It remains possible that a standardized treadmill-based pad test has adequate test-retest reliability to evaluate changes over time, but this has not yet been established.