Introduction- Measurement of pelvic floor muscle function and strength and pelvic organ prolapse

The International Classification of Impairments, Disabilities and Handicaps (ICIDH) (1997), lately changed to International Classification of Functioning, Disability, and Health (ICF) (2002), is a World Health Organization (WHO)-approved system for classification of health and health-related states in rehabilitation science. According to this system, the causes of a non-optimally functioning pelvic floor (e.g. muscle and nerve damage after vaginal birth) can be classified as the pathophysiological component. Nonfunctioning pelvic floor muscles (PFM) (reduced force generation, incorrect timing or coordination) are the impairment component, and the symptom of pelvic floor dysfunction (e.g. urinary leakage, fecal incontinence, or pelvic organ prolapse) is a disability. How the symptoms and conditions affect the women’s quality of life and participation in fitness activities is an activity or participation component.

Physiotherapists working to prevent or treat pelvic floor dysfunction aim to improve disability and activity/ participation components by improving PFM function and strength. Hence, it is important to measure all ICF components. In this chapter we deal only with the pathophysiological and impairment component with a focus on assessment of ability to contract the PFM and measurement of PFM strength. The main reasons for physical therapists to conduct high-quality assessment of ability to contract the PFM and PFM strength are as follows.

1. Without proper instruction, many women are unable to volitionally contract PFM on demand. This may be because they are situated at the floor of the pelvis and are not visible from the outside. In addition the muscles are seldom used consciously. Several studies have shown that more than 30% of women do not contract their PFM correctly at their first consultation, even after thorough individual instruction. The most common errors are to contract the gluteal, hip adductor, or abdominal muscles instead of the PFM. Some women also stop breathing or try to exaggerate inspiration instead of contracting the PFM. Some studies have demonstrated that many women strain, causing PFM descent, instead of actively squeezing and lifting the PFM upward. For proper contraction of the PFM, it is mandatory that women receive precise training with appropriate monitoring and feedback. Hay-Smith found that in the reports of only 15 of 43 RCTs they reviewed did the authors state that a correct PFM contraction was checked before training began.

2. In intervention studies evaluating the effect of PFM training, the training is the independent variable meant to cause a change in the dependent variable (e.g. stress urinary incontinence [SUI] or pelvic organ prolapse. Thus, measurement of PFM function and strength before and after training is important to determine whether the intervention has made significant changes. Even in the presence of tissue pathology (e.g. europathy), if there is no change in PFM function or strength after a training programme commensurate with that pathology, the training programme has been of insufficient dosage (intensity, frequency or duration of the training period) or the participants have had inadequate adherence. It is likely that such programmes have not followed muscle training recommendations.

In this chapter we describe different measurement tools such as clinical observation, vaginal palpation, electromyography (EMG), vaginal squeeze pressure measurement (manometry), urethral pressure measurement (stationary and ambulatory), dynamometry, ultrasonography and magnetic resonance imaging (MRI) in use for assessment of the PFM. This can be either assessment of unconscious co-contraction of the PFM during an increase in abdominal pressure or ability to volitionally perform a correct contraction. A correct voluntary contraction is described as an elevation and squeeze around the pelvic openings.

Muscle strength has been defined as ‘the maximum force that can be exerted against an immovable object (static or isometric strength), the heaviest weight which can be lifted or lowered (dynamic strength), or the maximal torque which can be developed against a preset rate-limiting device (isokinetic strength). Maximum strength is often referred to as the maximum weight the individual can lift once. This is named the one repetition maximum or 1RM. Maximum strength is measured through a maximum voluntary contraction. Maximum voluntary contraction refers to a condition in which a person attempts to recruit as many fibers in a muscle as possible for the purpose of developing force. The force generated is dependent on the cross-sectional area of the muscle and the neural components (e.g. number of activated motor units and frequency of excitation. Hence, PFM strength is a surrogate for underlying factors that will change with regular strength training.

Muscle power is the explosive aspect of strength and is the product of strength and speed of movement [power = (force + distance)/time]. Muscle force is reduced with speed of the contraction. Power is the key component of functional application of strength. Speed, however, changes little with training, thus power is changed almost exclusively through gains in strength.

Muscular endurance can be classified as:

1. ability to sustain near maximal or maximal force, assessed by the time one is able to maintain a maximum static or isometric contraction;

2. ability to repeatedly develop near maximal or maximal force determined by assessing the maximum number of repetitions one can perform at a given percentage of 1RM. Muscle strength measurement may be considered an indirect measure of PFM function in real-life activities. Women with no leakage do not contract voluntarily before coughing or jumping. Their PFM contraction is considered to be an automatic co-contraction occurring as a quick and effective activation of an intact neural system. Other important factors for a quick and effective contraction are the location of the pelvic floor within the pelvis, the muscle bulk, stiffness/elasticity of the pelvic floor and intact connective tissue.

A stretched and weak pelvic floor may be positioned lower within the pelvis compared with a well-trained or non-injured pelvic floor. The time for stretched muscles to reach an optimal contraction may be too slow to be effective in preventing descent against increased abdominal pressure (e.g. sneeze), thereby allowing leakage to occur. In general, when measuring muscle strength it can be difficult to isolate the muscles to be tested, and many test subjects need adequate time and instruction in how to perform the test. In addition, the test situation may not reflect the whole function of the muscles, and the generalizability from the test situation to real-world activity (external validity) has to be established. Therefore, when reporting results from muscle testing, it is important to specify the equipment used, position during testing, testing procedure, instruction and motivation given, and what parameters are tested (e.g. ability to contract, maximum strength, endurance).

When testing the PFM, additional challenges are present because muscle action and location are not easily observable. Whether a measurement tool should be used in clinical practice or in research depends on its responsiveness, reliability and validity. These terms are used slightly different in different research areas and have somewhat different defi nitions in different textbooks of research methodology. The definitions given below are the ones we have chosen to use in this textbook.

• Responsiveness: the degree or amount of variation that the device is capable of measuring; the ability of a tool to detect small differences or small changes.

• Reliability: consistency or repeatability of a measure. The most common way to establish stability of a test is to perform a test–retest. Intratest reliability is conducted by one researcher measuring the same procedure in the same subjects twice. Inter-test reliabilityis conducted when two or more clinicians or researchers are conducting measurement of the same subjects.

• Validity: degree to which a test or instrument measures what it is supposed to measure.

• Logical (face) validity: condition that is claimed when the measure obviously involves the performance being measured (e.g. squeeze and elevation of the PFM can be felt by vaginal palpation).

• Content validity: condition that is claimed when a test adequately samples what it should cover (few methods measure both squeeze pressure and elevation of the PFM).

• Criterion validity: the degree to which the scores on a test are related to some recognized standard, or criterion (e.g. clinical observation of inward movement of the perineum during attempts to contract the PFM compared with ultrasonography).

• Concurrent validity: involves a measuring instrument being correlated with some criterion administered at the same time or concurrently (e.g. simultaneous observation of inward movement during measurement of PFM strength with manometers and dynamometers).

• Predictive validity: degree to which scores of predictor variables can accurately predict criterion scores.

• Diagnostic validity: ability of a measure to detect differences between those having a diagnosis/problem/condition/symptom with those not.

• Sensitivity: the proportion of positives that are correctly identified by the test.

• Specificity: the proportion of negatives that are correctly identified by the test.

It is important for physiotherapists (PTs) who treat patients with pelvic floor dysfunction to understand the qualities and limitations of the measurement tools they use. This chapter will provide the information needed for PTs to understand the application of each tool to the measurement of the PFM. In many instances the PT may need thorough supervised instruction from other professionals before starting to use new equipment. In most cases, when available, receiving results from assessment of PFM activity from other professionals (e.g. radiologists) provides the best results.