Hypothesis / aims of study
According to the International Continence Society, OAB is a symptom-defined condition characterised by urinary urgency, usually accompanied by frequency and nocturia, with or without urgency urinary incontinence, in the absence of urinary tract infection or other obvious pathology(1). Electromyography (EMG) of the Pelvic Floor Muscles (PFM) is widely used to increase our understanding of pelvic floor (dys)function. Changes in EMG of the PFM can be related to changes in muscle performance and motor control.
The aim of this study is to determine the differences in EMG activity of the individual pelvic floor muscles during contractions and relaxation in patients with OAB to healthy volunteers.
Study design, materials and methods
In this retrospective case-controlled study, a comparative study design was used.
Participants were asked to perform three consecutive tasks according to a standardized protocol: one-minute rest, ten maximum voluntary contractions (MVC’s), and three endurance contractions. The MAPLe® probe was used for EMG registration of the pelvic floor musculature, with a grounding electrode on the spina iliac anterior superior. The MAPLe is a probe with a matrix of 24 electrodes enabling the measurement of EMG activity from the different sides and layers of the PFM(2). Mean raw EMG values per electrode (24 in total) were calculated for tone at rest, for every single MVC and every single endurance contraction.
Comparisons of the average of the individual electrodes between the groups were made for the mean value of tone at rest, the mean value of the 10 MVC’s and the mean value of the 3 endurance contractions, using Students-T-test. Linear mixed effect models were performed to analyse the relationship between groups and the EMG activity of repeated contractions.
As fixed effects the consecutive number of contractions and the OAB group into the model were used, next to an intercept. As random effects intercepts were used for participants and groups, as well as by-participants and by-groups random slopes for the effect of the consecutive contractions. Statistical analyses were performed using R.
In this study 65 women, 50 women with complaints of OAB and 15 healthy pelvic floor physiotherapists, were assessed. Despite the fact of the difference in the number of participants, the groups were comparable for all key parameters except for age and menopause. EMG signals of the PFMs in patients with OAB showed a significant decrease in EMG activity in compared to healthy volunteers. For tone at rest, a decrease in EMG signals was found between 0.8-44.3%, (Fig.IA). MVC showed a decrease in in EMG signals between 30.5-64.9 %, (Fig. IB). Endurance contraction showed a decrease in EMG signals between 31.9-67.6 %, (Fig. IC). Regarding the analyses of repeated measures with linear mixed effect models, the OAB patients showed a significantly lower EMG-activity for nearly every electrode from the first to the last contraction, for both, the 10 consecutive MVCs and the 3 consecutive endurance contractions, compared to the healthy volunteers. Repeated measures of MVC contractions showed a reduction in EMG signals between 28.7-64.9%, endurance contractions showed a reduction of EMG signals between 32.3-68.3%. In the last step, interaction term between the OAB group and the number of contractions was entered to examine differences in rate of decrease (slope) of EMG activity from the first to the last contraction in groups. No significant interaction for any of the single electrodes was found, there is no faster decline in EMG activity from the first to the last contraction in the OAB group compared to the healthy group. Reduction of EMG activity found in the T-test were almost comparable to the reduction of EMG activity calculated with repeated measurements.
Interpretation of results
The results of this study show an overall decrease of activity of the PFM in patients with complaints of OAB compared to healthy volunteers in MVC’s and endurance contractions as well as tone at rest. At first glance, looking at the Grids, it appears that reduced activity is located in especially in the electrodes nearest to the puborectal muscle. Together with the absence of interaction between repeated contractions and OAB group, gives the impression that besides strength, both motor control and coordination, the inability to contract the pelvic floor muscle effectively, play important roles in patients with OAB.
The great reduction of EMG activity in this study is likely to be triggered by complaints of OAB. It is well known that PFMT with urge impression techniques, improves OAB complaints, include minimal side effects, however adherence rate is low(3). From our point of view, looking at the results in our study, it could be of clinical relevance, that motor control and coordination are the major cause of PFM dysfunction in OAB patients. This could result in simplifying PFM exercises, strength and adherence will be of less importance.