Pelvic Prime's Anatomy Academy Series
Measuring Pelvic Floor Muscle Strength
Role of Physical Therapists
Many women struggle to contract their pelvic floor muscles correctly because the muscles are inside the pelvis and not visible. Studies show that over 30% of women do not contract their pelvic floor muscles correctly even after instructions. Men, as well, can have difficulty properly contracting their pelvic floor muscles.
Common mistakes include contracting the wrong muscles or breath holding. This is why proper assessment and feedback are crucial with pelvic floor muscle training.
Physical therapists aim to improve pelvic floor dysfunction by specifically assessing the ability of pelvic floor muscle contractions including the quality of muscle strength, endurance, and relaxation ability. Location, muscle bulk, elasticity, and connective tissue all influence the quality of pelvic floor contractions. A strong and well-positioned pelvic floor may not need to contract during activities that increase abdominal pressure, unlike a weak and stretched pelvic floor.
During pelvic floor treatments, it's important to measure pelvic floor muscle function before and after the training to see if there are any improvements. If no changes are observed, the training program might need adjustments in intensity, frequency, or duration. In some cases, underlying conditions might make improvement difficult.
Eight (8) Tools for Assessing Pelvic Floor Muscles
Various tools are used to assess pelvic floor muscle function, such as clinical observation, vaginal or rectal palpation, electromyography, manometry, ultrasound, and MRI. These tools can measure different stages of pelvic floor muscle contractions, including resting activity, increased tone, voluntary contractions, and automatic contractions during physical activities.
Measuring pelvic floor muscle strength can be tricky as it requires isolating the muscles and giving adequate instructions. Plus, the testing situation might not reflect real-life activities. For physical therapists, it’s essential to specify the equipment, testing procedure, and motivation given during testing to ensure accurate results and to understand what treatment was effective.
Did you know?
In 1948, Kegel explained that a correct pelvic floor muscle contraction feels like a squeeze around the urethra, vagina, and anus, and a lifting sensation. He estimated this lift to be about 30 to 40 mm when lying down. But recent studies using MRI and ultrasound show the lift is actually smaller with an average lift of 10.8 mm in a sitting position, and 11.2 mm while lying down. Studies have shown that women without incontinence exhibit better coordination between their pelvic floor and abdominal muscles. Notably, 25.5% of women with stress urinary incontinence were unable to contract their pelvic floor muscles.
1. Visual Observation
Visual observation usually involves looking at the outer perineal muscles, assuming the deeper levator ani muscles behave similarly. However, this isn't always the case. Visual observation can miss deeper muscle contractions and may not always indicate effective urethral closure.
Most pelvic floor physical therapists start with a visual observation alone to assess how well the pelvic floor muscles contract. However, there isn't much research on how reliable or valid this method is. Studies have found that intra-tester reliability can be high, but inter-tester reliability is low for visual observation, so it's best if only one tester is involved.
Visual observation is good for a first impression of pelvic floor muscle contractions but shouldn't be used for detailed measurements. It is expected that ultrasound will become the preferred method in the future as it's better for biofeedback and teaching.
The Recommended Procedure Physical Therapists Use for Pelvic Floor Muscle Assessment Using Visual Observation:
1. Explain the procedure to the patient.
2. Use models and drawings to teach the patient how to contract the pelvic floor muscles.
3. Have the patient lie down with knees bent and hips apart, covered with a towel.
4. Support the patient's legs and allow practice time.
5. Ask the patient to breathe normally, lift the perineum inwards, and squeeze around the openings without moving the pelvis or using other muscles.
6. Observe the contraction and note if it's correct, non-existent, or straining.
7. If there is a visible contraction, give positive feedback and prepare for deeper muscle palpation.
2. Vaginal Palpation
Vaginal palpation is used to:
Check the ability to contract and relax the pelvic floor muscles
Measure pelvic floor muscle strength by feeling the squeeze and lift.
Assess endurance and ability to repeat contractions.
Evaluate muscle tone, relaxation, coordination with abdominal muscles, symmetry, scarring, adhesions, pain, injuries, and muscle recruitment.
Different methods and scales are used for vaginal palpation. The Oxford grading system, a 6-point scale, is commonly used by physical therapists. Studies show mixed results on the reliability of vaginal palpation. Some find high inter-rater reliability, while others report poor agreement. The modified Oxford scale, though commonly used, has been questioned for its ability to differentiate between varying strengths of pelvic floor muscle contractions.
Modified Oxford Grading System
The modified Oxford grading scale is a way to measure how well pelvic floor muscles work. It uses a 6-point scale, but half-points can be added to show when a muscle contraction is between two levels. This expands it to a 15-point scale when using + and - signs:
0 = no contraction
1 = small flicker
2 = weak
3 = moderate (with lift)
4 = good (with lift)
5 = strong (with lift)
The Oxford grading system is based on the Medical Research Council scale from 1943, but it was changed because the old scale wasn't very good at measuring small differences. One problem with the modified Oxford scale is that it gives one score for two things—how much the muscles close (occlusion) and how much they lift. It can be hard for the person doing the test to tell the difference between these two actions with their fingers.
Difference between Inter & Intra Reliability:
Inter-rater reliability and intra-rater reliability are used to assess the consistency of measurements or ratings.
1. Inter-rater reliability: Measures the extent to which different people give consistent estimates of the same phenomenon.
Example: If two teachers grade the same set of student essays independently and give similar scores, their grading shows high inter-rater reliability.
2. Intra-rater reliability: This measures the consistency of a single person over time.
Example: If a teacher grades the same set of essays at two different times and gives similar scores each time, their grading shows high intra-rater reliability.
Muscle tone refers to the continuous and passive partial contraction of muscles, even when they are not actively being used. This slight tension helps maintain posture and ensures that muscles are ready for action. Essentially, muscle tone keeps muscles firm and ready to respond to movement or resistance.
Hypertonicity means increased tone and is often associated with a muscle being overly tight.
Hypotonicity means decreased tone and is often associated with a muscle being sluggish or slow to respond.
Accurate measurement of muscle tone, especially for pelvic floor muscles, is complex and usually involves instruments like dynamometry. MRI and ultrasound are preferred for assessing pelvic floor muscle injuries. Studies show that palpation alone is less reliable for detecting levator ani muscle trauma compared to ultrasound.
Comparing vaginal palpation with vaginal squeeze pressure shows varying levels of agreement. While palpation can be good for assessing lift, other muscle functions are better measured with electromyography. Pelvic floor muscle function is often measured while lying down, but leakage is more common in upright positions. Studies suggest high reliability in different positions but prefer lying positions for standardization. For clinical practice, visual observation and palpation can give a good first impression of pelvic floor muscle function, but for accurate and reliable measurements MRI and ultrasound are preferred.
Dynamometry explained:
Dynamometry is a method of measuring force, power, or strength. It involves using a device called a dynamometer, which can assess the strength of muscles or the force of movements. For example, hand grip dynamometers measure how strong someone's grip is, while other types can measure leg or arm strength. Dynamometry is commonly used in physical therapy, sports, and medical evaluations to track muscle strength and recovery.
Electromyography explained:
Electromyography (EMG) is a technique used to measure and record the electrical activity produced by muscles. Small sensors, called electrodes, are placed on the skin or inserted into the muscle. These electrodes detect the electrical signals that muscles generate when they contract. EMG helps diagnose muscle and nerve disorders, monitor muscle health, and guide treatment plans in medical and physical therapy settings.
3. Manometry: Understanding Vaginal Squeeze Pressure Measurement
Vaginal squeeze pressure is a way to measure the strength of the pelvic floor muscles. To check how strong these muscles are, doctors ask patients to squeeze their pelvic muscles as hard as they can, hold the squeeze, or repeat the squeeze multiple times. A device called a manometer is used to measure the pressure in millimeters of mercury (mmHg). This device can be placed in the urethra, vagina, or rectum to get readings.
Measuring vaginal squeeze pressure can be tricky. Some tools provide consistent and reliable results, while others vary based on the user and the method. The position of the patient and the size of the measuring device can also affect the results. Additionally, increased abdominal pressure from straining can lead to false readings. To get accurate pelvic floor muscle measurements, these co-contractions need to be controlled.
The Recommended Manometry Procedure for Physical Therapists
For accurate results, healthcare providers should:
Fully inform patients about the procedure.
Ensure patients are in a consistent position for each test.
Start with observing and feeling the pelvic floor muscle contraction.
Be cautious and use standardized methods to ensure reliability.
4. Dynamometers
Accurate measurements of pelvic floor muscle function (like strength, speed, endurance, and coordination) are important for understanding muscle problems and checking if physical therapy is working. Dynamometers are tools that measure muscle power and force without relying on an evaluator's opinion. A pelvic floor muscle dynamometer measures pelvic floor resting and contractile forces using strain gauges attached to a speculum inserted into the vagina.
Pelvic floor muscle dynamometers are useful for assessing pelvic floor muscle function and predicting treatment outcomes. However, more research and standardized protocols are needed for widespread clinical use.
Pelvic Floor Muscle Function for Women With Stress Urinary Incontinence
Researchers studied the differences in pelvic floor muscle function between women who were able to control their bladder and those who had stress urinary incontinence (where they leak urine during activities like coughing or sneezing).
The women with stress incontinence had
lower passive force and endurance
lower rate of force development and fewer rapid contractions.
less stiffness at maximum aperture
lower maximal force.
less anteroposterior active strength.
The studies showed that continent women generally have stronger and more effective pelvic floor muscles, which help them control their bladder better.
The Recommended Procedure Physical Therapists Use for Pelvic Floor Muscle Assessment Using Dynamometers:
Explain the Tool and Procedure:
- Describe the dynamometer and how it will be used.
- Use anatomical models and drawings to explain pelvic floor muscle contractions.
2. Patient Preparation:
- Ask the patient to lie on a treatment table with knees bent and feet flat.
- Insert the dynamometer gently, ensuring it is placed correctly for accurate measurements.
3. Measurement Process:
- Allow the patient to get used to the device.
- Standardize commands and provide encouragement during measurements.
- Record strength, endurance, and coordination of the pelvic floor muscles.
4. Post-Evaluation:
- Discard used materials and disinfect the device.
5. Surface Electromyography
Electromyography (EMG) records the electrical signals produced when muscles contract. There are two main uses for EMG.
Motor Unit EMG helps diagnose nerve and muscle problems like nerve damage or muscle disease by using needles to study the muscle’s inner workings.
Surface EMG (sEMG) uses electrodes placed on the skin and studies how muscles behave and move.
Muscle Fibers & Motor Units
To understand EMG signals, it's important to know how muscles produce electrical currents. Muscles are controlled by motor units, not just individual muscle fibers.
A motor unit is made up of a motor neuron (from the spinal cord) and the muscle fibers it controls.
When a muscle is activated, the electrical signal travels from the motor neuron --> through its axon --> to the muscle fibers --> causing them to contract. This electrical activity creates action potentials that spread along the muscle fibers.
The action potential is the result of sodium ions (Na+) moving into the muscle fiber followed by potassium ions (K+) moving out. The EMG signal captures the electrical activity from these motor units. As the muscle exerts more force, the EMG signal gets stronger in two ways:
by activating more motor units
by increasing the firing rate of the active motor units.
Smaller motor units produce less force and therefore are activated first. Larger, more powerful motor units are activated next.
Electromyography for Neurophysiology Assessment
Intramuscular EMG is used in neurophysiology to study individual motor unit potentials (MUPs). This helps in understanding how muscles and motor units work and diagnosing issues like nerve damage and muscle disease. It involves using needle or wire electrodes to measure the electrical activity in muscles. These electrodes can be used on muscles like the urethral sphincter, pelvic floor muscles, and external anal sphincter. The external anal sphincter is the most commonly tested because it's easy to evaluate and has plenty of reference data.
During an EMG exam, the first step is to observe the muscle's spontaneous activity when the electrode is inserted. Normally, a brief burst of activity is seen due to the muscle's response to the electrode. If no activity is observed, it could indicate severe nerve damage. Another important observation is the muscle's resting activity. Normally, the external anal sphincter shows a low, constant discharge of motor units.
Pathological activity, such as irregular wave patterns, can indicate muscle problems. EMG can analyze these patterns in two ways:
by looking at the overall activity of multiple motor units
by examining the specific characteristics of individual MUPs.
These analyses are compared to standard data to diagnose muscle and nerve issues. Recent studies suggest that analyzing individual MUPs is the most accurate method.
Kinesiological Surface Electromyography
Kinesiological sEMG, unlike needle EMG, studies muscle behavior during rest and movement. It uses surface electrodes placed on the skin or inserted probes to measure muscle activity. Different types of electrodes are used depending on the muscle being studied, such as vaginal probes for pelvic floor muscles and anal probes for the external anal sphincter. sEMG can evaluate muscle activation levels, timing and patterns of activation, frequency content, and evoked activation.
The level of muscle activation is measured by the amplitude of the sEMG signal, showing how much electrical activity the muscle generates. This is useful for assessing muscle behavior during rest and various activities, like maintaining continence or responding to physical challenges.
The timing of muscle activation can be analyzed to understand how muscles prepare for actions, such as coughing or movement. Incorrect activation patterns, such as the habit of contracting instead of relaxing, can also be identified.
Frequency content analysis & power spectrums
Frequency content analysis helps in understanding muscle fatigue by examining changes in the power spectrum of the sEMG signal. A power spectrum is a representation of the distribution of power contained within a signal as a function of frequency. It essentially shows how the power of a signal is distributed across different frequency components.
In the context of electromyography (EMG), the power spectrum is used to analyze the frequency content of the EMG signal. By examining the power spectrum of an sEMG signal, researchers can gain insights into various characteristics of muscle activity, such as muscle fatigue. When muscles fatigue, the frequency content of the EMG signal often shifts, which can be observed as changes in the power spectrum. This analysis helps in understanding the underlying physiological processes occurring during muscle contractions and fatigue.
Evoked Activation Assessment via Magnetic Stimulation
Magnetic stimulation, often through methods like Transcranial Magnetic Stimulation (TMS), involves using magnetic fields to non-invasively stimulate specific areas of the brain. By applying this technique, researchers can observe and measure the brain's influence on the pelvic floor muscles, such as:
1. Muscle Coordination: Understanding how the brain coordinates the activities of the pelvic floor muscles, which are crucial for functions like bladder control, bowel movements, and sexual function.
2. Neuroplasticity: Observing how different types of brain stimulation can affect muscle activity, which can shed light on the brain's ability to reorganize and adapt, a concept known as neuroplasticity.
3. Therapeutic Effects: Exploring the potential therapeutic effects of brain stimulation techniques on improving muscle function in individuals with pelvic floor disorders. This could lead to new treatments for conditions like incontinence or pelvic pain.
4. Neuromuscular Pathways: Gaining a deeper understanding of the neuromuscular pathways that link the brain to the pelvic floor muscles, which can help in diagnosing and treating neurological disorders affecting these pathways.
Key Factors in sEMG Measurements
Not all EMG recordings are the same. The way recordings are set up and analyzed can change the quality of the results. EMG measures electrical activity from muscles, but understanding and interpreting these signals correctly requires technical knowledge and experience.
Pelvic floor muscles fibers are complicated with different origins, layers, and fiber directions, making it challenging to record sEMG accurately. Electrodes should be placed along the muscle fibers and away from areas like tendons. Commercial probes often have designs that don’t match this need, causing potential errors.
Cross-Talk occurs when nearby muscles' activity is picked up by the electrodes, leading to possible misinterpretation.
Artifacts and noise are unwanted signals that come from sources like movement or other body functions. Different shapes and placements of electrodes affect the quality of the signal. For example, cylindrical and pear-shaped probes are commonly used, but they can cause movement artifacts or discomfort. These issues can interfere with the sEMG signal, especially during activities like jumping or running. Proper skin preparation and careful electrode placement can help reduce these artifacts.
Confounding Factors like skin condition, fat tissue, and humidity can affect sEMG recordings. To compare results accurately between people, the sEMG amplitude often needs to be normalized.
Reliability of sEMG depends on the quality of the equipment used. When the probe is not removed, intra-session reliability (within the same session) is usually excellent. However, between-session reliability (different days) can vary. Controlling factors like probe positioning is crucial for consistent results.
6. Urethral Pressure Measurements
Controlling urination depends on the forces inside and outside the urethra while the bladder fills up. If the pressure in the abdomen (like when you cough or lift something heavy) becomes greater than the pressure in the urethra, urine can leak out.
Four Types of Catheters to Measure Urethral Pressure
1. Fluid Perfusion: A catheter (thin tube) is slowly pulled out of the urethra while fluid is pumped through it, measuring pressure.
Catheter Size: Must be appropriate; too large can give false readings.
Perfusion Rate: Rate of fluid flow affects accuracy.
Withdrawal Speed: Speed of pulling the catheter affects the results.
2. Microtip / Fiberoptic Small sensors on catheters measure quick changes in pressure but can be affected by their position and flexibility.
3. Balloon: These are better at avoiding positional errors but must be properly sized to avoid false readings due to stretching the urethra.
4. Air-Filled: Newer technology but requires more research to be reliable.
Factors Affecting Urethral Pressure
Bladder Volume: Pressure can change as the bladder fills.
Patient Position: Standing or lying down can affect pressure.
Pelvic Floor Activity: Muscle contractions can change pressure readings.
Normal Urethral Pressure Profiles
Men: Generally, two peaks are seen in the pressure profile.
Women: Pressure decreases with age, especially after menopause.
Key terms used when measuring urethra pressure
Urethra: the tube that carries urine from the bladder out of the body
Urethral Pressure: The force needed to open the urethra.
Urethral Pressure Profile (UPP): A graph showing pressure along the urethra.
Maximum Urethral Pressure (MUP): The highest pressure recorded.
Maximum Urethral Closure Pressure (MUCP): The highest difference between urethral and bladder pressure.
Functional Urethral Length: Part of the urethra where pressure is higher than in the bladder.
7. Ultrasound
Ultrasound is becoming more common for checking the shape and function of the pelvic floor muscles. Ultrasound allows us to see these muscles without being invasive. There are three main locations for transducer placement for pelvic floor ultrasound:
Transabdominal has been used to study levator activity but it provides only an indirect and limited view.
Internal vaginal can show static anatomy and measurements, but having an instrument inside the vagina makes it hard to see how the muscles work during maneuvers like Valsalva or pelvic floor muscle contractions.
Translabial or transperineal is the only method that allows direct assessment of both the structure and function of the levator muscles.
Transabdominal Ultrasound
The transducer is placed on the abdomen, above the pubic bone.
Can be done in any position.
Bladder filling is optional.
Internal Vaginal Ultrasound
The transducer is placed inside the vaginal canal.
Typically done lying down.
Bladder filling is optional.
Translabial / Transperineal Ultrasound
The transducer is placed on the perineum.
Can be done lying on the back or standing.
Bladder filling is optional.
Parting the labia can improve image quality.
Anal Canal & Anal Sphincter
Anal sphincter tears can impact both the internal anal sphincter (IAS) and the external anal sphincter (EAS).
External anal sphincter appears as a bright ring. These tears appear as dark defects in the bright ring.
Internal anal sphincter appears as a darker ring. These tears also appear as defects in a dark ring.
External anal sphincter defects are linked to anal incontinence. The extent of trauma is quantified by counting the number of slices showing an EAS defect, with four to six abnormal slices considered a "residual defect."
Forceps are the main risk factor for both levator and sphincter trauma.
A cranioventral (diagonally upward) shift of pelvic organs in a sagittal midline orientation indicates levator contraction.
Levator Activity & Avulsion
Ultrasound can also be used for pelvic floor muscle exercise training by providing visual biofeedback. This technique has validated the concept of "the knack," which is the contraction of the levator muscles immediately before increases in intra-abdominal pressure from activities like coughing. The levator ani muscle plays a crucial role in pelvic support. When it is injured, such as through avulsion (tearing away), it can lead to an enlargement and asymmetry of the levator hiatus. This can sometimes be seen during a Valsalva maneuver, where a person tries to exhale forcefully with a closed mouth and nose.
Recent studies have identified risk factors and consequences of avulsion. Factors like birth weight, the length of the second stage of labor, and the size of the fetal head can increase the risk of avulsion. However, the use of forceps during delivery is the most significant modifiable risk factor, and their use should be minimized except in emergencies.
The first vaginal delivery often causes the most significant changes, including tears and alterations in the levator muscle's ability to support pelvic organs. Avulsion can have medium- and long-term consequences, such as pelvic organ prolapse, especially in the anterior and central compartments. Larger defects increase the likelihood of prolapse symptoms. Avulsion also leads to abnormal distensibility of the levator hiatus, contributing to prolapse.
Avulsion and ballooning are linked with early rectal prolapse. Recognizing avulsion and ballooning is essential for planning prolapse surgeries, as these conditions are major risk factors for prolapse recurrence. The impact of avulsion on urinary and fecal incontinence is less clear. While weak pelvic floors are often associated with urinary incontinence, some evidence suggests that major levator avulsion defects might not correlate with stress urinary incontinence.
Childbirth can affect urinary continence through other mechanisms, such as nerve damage or injury to the urethral sphincter muscles. Pressure transmission through ligaments and tissues might also play a role. The relationship between levator trauma and fecal incontinence is mixed; some studies found no link, while others identified levator avulsion as a risk factor for fecal incontinence.
8. MRI (Magnetic Resonance Imaging)
Just as a shoulder injury limits arm movement, damage to pelvic muscles affects pelvic function. Modern imaging like MRI and ultrasound now allows us to pinpoint such injuries, improving treatment success by tailoring therapy to each person's needs. These technologies reveal variations in muscle size and shape among individuals, influenced by genetics, daily activities, and exercise. This variation impacts pelvic function; someone with naturally robust muscles may still function well after losing muscle bulk.
Birth: A Major Event Causing Pelvic Floor Dysfunction
Vaginal birth can increase the chances of pelvic floor problems. During childbirth, the levator ani muscle and pelvic floor stretch significantly to allow the baby's head to pass through. Pelvic muscle exercises are essential for recovery after giving birth. These exercises help reduce incontinence and improve muscle function faster than if no exercises are done. Imaging technology has shown us how these muscles heal and return to their normal state.
Right after birth, the pelvic floor sags, and the opening is wider than usual. Over the first six months, most women’s pelvic muscles recover and return to their normal position and strength. However, between 13% and 36% of women experience injury to the levator ani muscle during childbirth. This injury often affects the pubovisceral muscle and sometimes the iliococcygeal muscle. Difficult births, use of forceps, long labor, and large baby head sizes increase the risk of muscle injury. In fact, forceps deliveries are linked with a 63% injury rate. While vacuum-assisted births and epidurals do not increase the risk, the extent of muscle damage varies.
With MRI we can view chemical changes, like increased fluid in the muscle, to see how the muscles heal after birth. MRI has found that some women lose muscle bulk on both sides, while others only on one side.
Mechanisms of Levator Injury
Injury to the levator ani muscle can happen in several ways, such as nerve damage, muscle tears, or stretching. Studies using MRI have shown muscle tears at the pubic bone attachment and muscle edema caused by stretching. Computer models have confirmed that the parts of the muscle that stretch the most are the ones that get injured. The pubovisceral muscle is the most commonly injured, followed by the iliococcygeal muscle.
Clinical Implications of Levator Ani Muscle Injury
Injury to the levator ani muscle is a major cause of pelvic organ prolapse. Studies have found that women with significant muscle defects are more likely to experience prolapse. Also, women with muscle injuries are more likely to suffer from fecal incontinence.
Knowing the type of pelvic floor injury is crucial for proper treatment. Imaging has shown different injury patterns among women, but it is still unclear whether birth-induced muscle injury is due to nerve damage or muscle rupture. Even so, pelvic muscle training can improve muscle use and strength.
References: Bo, K., Berghmans, B., Mørkved, S., & Van Kampen, M. (Eds.). (2023). Evidence-Based Physical Therapy for the Pelvic Floor (3rd ed.). Elsevier.