1 Introduction
Regular physical activity, that is, “any bodily movement produced by the skeletal muscles that results in a substantial increase over the resting energy expenditure” [
1], is an important and modifiable health factor for all age groups, and there is evidence that “exercise is medicine” for a wide range of diseases and conditions [
2]. In contrast to physical activity, which can be performed in varying domains, exercise training is a form of physical activity “usually performed on a repeated basis over an extended period of time with a specific external objective such as improvement of fitness, physical performance or health” [
1]. The 2018 Physical Activity Guidelines Advisory Committee Report concluded that health benefits of regular physical activity include reduced risk of excessive weight gain, improved cognitive function, reduced risk of dementia and reduced risk of cancers in various sites [
3]. In addition, physical activity reduces the risk of progression of some chronic conditions, such as osteoarthritis, hypertension, and type 2 diabetes. Hence, staying physically active throughout the lifespan is of great importance for health and well being.
This message is clearly heard by women, as more and more participate in sports than ever before and the proportion of women athletes at the most recent Olympic Games was nearly 50%. Women and men differ in key areas of anatomy and physiology relevant to sports training, but perhaps of greatest difference is the often over-looked pelvic floor.
Indeed, the pelvic floor in women may be the only area of the body where the positive effect of physical activity has been questioned. The pelvic floor consists of muscles and connective tissues (ligaments and fascia) that need to work together to form a structural support for the pelvic organs to prevent urinary leakage or protrusion of the pelvic organs. Pelvic floor dysfunction may lead to common conditions such as urinary incontinence (UI), anal incontinence (AI) and pelvic organ prolapse (POP) [
4]. Given that these conditions affect between one in three to one in four women [
5,
6], understanding whether physical exercise might predispose to, or prevent, dysfunction of the pelvic floor, and thus these conditions, is important. Additionally, the conditions themselves, especially UI, may cause women to stop exercising or be one of many barriers to continuing lifelong regular physical activity [
7,
8]. As a consequence, women and society bear the cost of inactivity and UI, both substantial [
9,
10].
Known risk factors for pelvic floor disorders are pregnancy and vaginal childbirth, older age and obesity [
11]. Strenuous work or exercise has also been widely debated as a possible risk factor. Indeed, the very definition of stress urinary incontinence [SUI], “complaint of involuntary loss of urine during effort or physical exertion, or during sneezing or coughing” [
12], highlights the fact that leakage occurs during physical activity. Adequate function of the pelvic floor including the pelvic floor muscles (PFM), connective tissue and nervous system, is crucial in counteracting the increases in intra-abdominal pressure (IAP) and ground reaction forces that occur during physical activity, and well-functioning PFM may compensate for weak connective tissue.
In 2004, Bø described two possible and opposing hypotheses on the effect of physical activity on the pelvic floor [
13]:
1.General exercise training strengthens the pelvic floor. The theories behind this hypothesis are that the impacts that occur during physical activity may stretch and fatigue the PFM, leading to a training effect, and/or that impacts during exercise could lead to a co-contraction of the PFM, creating an acute indirect training effect. This may reduce the levator hiatus area by causing hypertrophy and shortening of the surrounding muscles, thereby lifting the pelvic floor and the internal organs into a higher pelvic location. Theoretically, such morphological changes could reduce the risk of UI, AI and POP. On the other hand, it is also theoretically possible that these changes could negatively impact labor and childbirth by making it more difficult for the fetus to descend with pushing.
2.General exercise training overloads, stretches and weakens the pelvic floor. This hypothesis is based on the fact that physical activity increases IAP, and if the pelvic floor muscles are not able to co-contract quickly or strongly enough to counteract this increased pressure or withstand the ground reaction forces, the levator hiatus could become wider, stretching and weakening the muscles. According to this theory, overload of the PFM may increase the risk of UI, AI and POP, but on the other hand, should also result in easier childbirth.
The aim of this narrative literature review is to describe and discuss the evidence supporting or refuting these two hypotheses, including how exercise influences PFM strength, muscle fatigue, pelvic floor morphology, pelvic floor disorders, and labor and birth variables.
4 Discussion
To summarize key findings of this review, exercising women may have similar or stronger PFM and larger levator ani muscles than non-exercising women, but this does not have a greater risk of obstructed labor or childbirth. Women that perform PFM training antenatally also are not more likely to have outcomes associated with obstructed labor (such as prolonged second stage of labor or cesarean delivery), but rather, appear to have shorter first and second stages of labor; this conclusion is limited by lower quality data. In terms of pelvic floor dysfunction, urinary incontinence is common amongst exercising women, with exercise increasing the odds 2.5–3 times and with greater prevalence rates associated with higher impact activities. Mild-to-moderate physical activity, such as walking, may decrease the risk of future UI. Scant research suggests that strenuous activity in younger women does not predispose to UI in later life, though a large volume of strenuous activity during the teen years might. The few studies available to assess the association between exercise and POP and AI are inconsistent in their conclusions. The rigor of these findings is limited by inconsistencies in the literature in defining each of the pelvic floor disorders and in methods of assessing and characterizing pelvic floor muscle strength, morphology, and intra-abdominal pressure.
In terms of IAP, variability amongst women is high. Some exercises thought to be associated with higher IAP are in fact not, and many generate lower IAPs than common daily activities. However, whether higher IAPs directly affect the pelvic floor is not known. In addition to the actual load on the pelvic floor, repetitions of loading and dynamic activities may impact the pelvic floor differently than the actual load.
Measuring PFM function during physical activity and specific exercises is very challenging. Results from studies attempting this are difficult to compare. Electromyography (EMG) may provide a means of assessing the kinematic aspects of PFM contraction, and not simply the pressures obtained, during exercise. Whether methods such as surface EMG are valid in assessing PFM activity is debated given input from other muscle groups during multitask activities such as running and jumping [
92]. EMG is not a measure of muscle strength, but of activation of muscle fibers. Strength and activation are correlated, but measure different features. Vaginal pressure transducers designed to measure PFM strength may be impacted by IAP increases. Measurement devices to be kept inside the urethra, vagina or rectum during exercise may move, impacting results. Even in the laboratory setting, measuring PFM strength can be challenging. It is vital to ensure that women are indeed contracting the PFM correctly. We are not aware of commercially available instruments that measure increases in IAP and function of the PFM simultaneously, but new developments are being investigated [
28,
33]. However, given the location of the PFM inside the pelvis, their close connection to other muscle groups and their inclusion in the abdominal canister that responds to all IAP and ground reaction forces, it may prove difficult to differentiate between opposing forces occurring during physical activity.
Studies investigating PFM strength and exercise are cross-sectional, and thus causality cannot be established. That one session of strenuous exercise results in acute PFM fatigue does not provide any evidence about whether such fatigue might cause stronger or weaker PFM in the long term [
34]. The effect of IAP on pelvic floor function requires not only consideration of the PFM, but also of other co-existing factors. We suggest that there may be an individual threshold of IAP related to each individual’s harm/benefit ratio. Most women can tolerate huge increases in IAP without leaking. Hence, the connective tissue and the PFM are adequately counteracting this increased pressure and corresponding downward movement. In others, only small amounts of IAP may move the pelvic floor downwards [
93], widen the levator hiatus [
94] and decrease the maximal urethral closure pressure, causing leakage or descent.
Most of the literature about exercise and pelvic floor dysfunction is limited to UI with few studies about AI and POP, and is generalizable to recreational exercisers with scant data about strenuous exercisers and elite athletes. Most large epidemiological studies on physical activity and UI are cross-sectional. The inherent selection bias with this design, especially for a condition like UI, may be large, as women with UI tend to stop exercising. These study designs cannot ascertain whether women are exercising because they are dry or whether they are dry because they exercise. Many epidemiological studies do not control for obesity, which is important as obesity is a risk factor for UI and exercising women are more likely to be within normal weight distribution, thus reducing their risk of UI [
11].
Although exercisers may have stronger PFM than non-exercisers, the PFM may still be too weak or too slow to counteract the IAP or ground reaction forces during strenuous activities [
21]. It is reasonable for athletes to consider focused strength training of the PFM, particularly if they have symptoms of UI, given the grade A recommendation for PFM training as first line treatment for UI in the general female population [
95]. Furthermore, pregnant continent women who exercise the PFM (primary prevention) are 62% less likely to experience UI in late pregnancy and have a 29% lower risk of UI 3–6 months postpartum [
96]. In women with POP, supervised PFM training results in lift of the pelvic floor, a smaller levator hiatus area and PFM hypertrophy [
97].
Whether such training effects are found in elite athletes remains to be investigated. Several small uncontrolled studies suggest that athletes and soldiers demonstrate improvements in symptoms or PFM strength after PFM training [
98‐
100]. In a more recent RCT, in 32 volleyball players, UI was more likely to improve after PFM training than after written information only [
101]. However, young, nulliparous women in general, and athletes in particular, have low level of knowledge about the pelvic floor and little knowledge about how to train the PFM [
102].
Elite athletes are accustomed to regular training and are highly motivated for exercise. Adding 3 sets of 8–12 close to maximum PFM contractions, 3–4 times a week to their regular strength-training programs is feasible [
101,
103]. Proper strength training of the PFM seems to be important as one RCT found that simply contracting the PFM during daily activities does not itself seem to improve PFM function and prevent UI [
104]. While a minority of athletes performed a correct PFM contraction at their first assessment in one study, all learned proper technique after individual instruction by a physiotherapist [
69]. We are not aware of studies investigating whether exercising women are more likely to adhere to PFM training.
Because most elite athletes are nulliparous, one does not expect damage to ligaments, fascia, muscle fibers or peripheral nerves. Therefore, the effect of PFM training in athletes might be equal to or better than that of other women. On the other hand, the increases in IAP and ground reaction forces that must be counteracted automatically by the athlete’s PFM are higher than that required in the general population. The pelvic floor, therefore, probably needs to be much stronger and respond more quickly to forces in elite athletes than in the lay public.
The prevalence of UI during sports is high. Given the numerous health benefits of physical activity, no one should be recommended to stop exercising, and this is certainly not an option for elite athletes. However, athletes should be provided with the same information and advice to train the PFM as the rest of the female population. Athletes do not talk about UI with their coaches or trainers [
78,
81,
105]. Education directed at coaches has potential to improve UI in athletes.