Assessing Muscle Length

Original Editors - Naomi O'Reilly and Jess Bell

Top Contributors - Naomi O'Reilly, Jess Bell and Ewa Jaraczewska      

Introduction

Muscle length refers to the ability of a muscle crossing a joint/s to lengthen, thus allowing the joint/s to move through their full available range of motion. A muscle's ability to lengthen is essential for fuctional activities, so it is important for rehabilitation professionals to consider muscle length when assessing and treating patients. This article discusses general principles of the muscle length assessment.

Structure of Muscles

Muscles connect to bones or joint capsules by connective tissue structures, such as tendons or aponeuroses.

Skeletal muscles are made up of striated muscle fibres. These muscle fibres contain smaller units called myofibrils, which are made of thick and thin myofilaments. These filaments are organised longitudinally into units called sarcomeres, which is the basic contractile unit of the muscle fibre.

A muscle belly generates force when the sarcomeres contract. This pulls the origin and insertion of the muscle-tendon complex closer together, thus shortening the muscle. When sarcomeres contract, the overlap between thick and thin myofilaments increases. The amount of overlap decreases as it relaxes, so the muscle fibre can elongate. Maximal muscle length is, therefore, the greatest extensibility of the muscle-tendon junction.

Kruse and colleagues state that: "The force exerted actively by a muscle can be expressed as a function of muscle length.”

• The length at which muscles do not actively generate force is known as active slack length
• The length at which muscles are able to generate their maximal active force is known as optimum muscle length
• The difference between the active slack length and the optimum muscle length is the length range of active force exertion

For a full discussion of this, please see Kruse et al. Stimuli for adaptations in muscle length and the length range of active force exertion—a narrative review, and Figure 1 in particular.

Factors Affecting Muscle Length

Gender

Evidence suggests that biological females tend to be more flexible than biological males. Research looking specifically at hamstring length has found that females can have up to 8 degrees more range in their passive straight leg raise and 12 degrees more range during an active knee extension test than males.

Age

Our muscles also change with age. Older adults experience increased fibrosis, sarcopenia, decreased force production and a general reduction in flexibility. Justine et al. note that there may be an association between range of motion and muscle length in the lower limb and balance performance in older adults with foot deformities.

However, as Reese and Bandy point out, there is not much research on age-related changes in muscle length which use direct measurement tests. But at least one study on hamstring length which does use this method found no difference in muscle length associated with age.

Posture

Posture can have an impact on muscle length because our muscles and tissues adapt to how they are used. For example, in a forward heat posture, the cervical flexors and occipital extensors have been found to shorten in a forward head position compared to a neutral spine, whereas the cervical extensors and occipital flexors lengthen.

Muscle Length Assessment

A muscle length assessment is performed to determine whether a muscle's length is normal, limited, or excessive. This assessment can help identify if changes in muscle extensibility is contributing to movement impairment and/or symptoms or if other structures are involved. To test muscle length, we must position the muscle in a way that increases the distance between its origin and insertion - i.e. we lengthen the muscle in the direction/s opposite to its action. During the test, the examiner assess the muscle's resistance to passive lengthening. During a muscle length test, it is important to consider the following:

• Ensure that you fix one end (usually the origin) and passively move the other end to lengthen the muscle.
• The muscle must be passively stretched or lengthened across all the joints it crosses.
• If a patient has acute pain, muscle length testing may need to be delayed - otherwise muscle guarding or pain inhibition may impact results.

Measurement Methods

A review of the literature identifies two methods that are often used to assess muscle length.

Composite Measurement

Muscle length is commonly assessed through the use of composite tests (e.g. Apley’s Test or the Sit and Reach Test). These tests look at movement across more than one muscle or joint. However, while they are frequently used as a measure of muscle length, research suggests that they do not provide accurate measurements of muscle length because they assess combinations of movements across several joints and involve several muscles. Thus, they tend to provide a general idea of flexibility rather than an exact measurement of a single muscle’s length.

Direct Measurement

Direct measurement of muscle length is the alternative method where excursion between adjacent segments of one joint is measured. When using direct measurement techniques, we need to consider that muscles are characterised by the number of joints they cross, i.e., one-joint, two-joint, and multi-joint muscles. This will impact the measurement of muscle length.

• The range of motion and muscle length will measure the same for muscles that pass over one joint only.
• For muscles that pass over two or more joints, the normal range of the muscle will be less than the total range of motion of the joints over which the muscle passes.

One Joint Muscle

One-joint muscle cross just one joint. It typically allows a full passive range of motion at the joint it crosses. If a one-joint muscle is short and limits the range of motion, you’ll notice a firm end feel caused by muscle tension.

We can determine the length of a one-joint muscle by measuring the passive range of motion of the joint that it crosses, by positioning the joint where the muscle is lengthened across the joint, and the position of the joint is measured. This represents an indirect measure of muscle length.

Effectively we measure the range in the direction opposite to its action. For example, the hip adductors (adductor longus, brevis and magnus) are one-joint muscles. To determine their length, we measure the passive range of hip abduction.

Two-Joint Muscle

The muscles that cross two or more joints typically do not allow a full range of motion across all the joints they cover. This is known as passive insufficiency.

"Passive insufficiency occurs when a multi-joint muscle is lengthened to its fullest extent at both joints, thereby preventing the full range of motion of each joint it crosses."

To assess and measure the length of a two-joint muscle, do the following:

• Position one of the joints crossed by the muscle so as to lengthen the muscle across the joint.
• Move the second joint through a passive range of motion until the muscle is on full stretch, preventing further joint motion.
• Assess and measure the final position of the second joint.

This represents an indirect measure of muscle length.

Example #1: The biceps brachii crosses the shoulder and the elbow. It flexes and supinates the elbow and is a weak shoulder flexor. Test the length of this muscle as follow:

• Position the patient in a supine.
• The starting position is shoulder extension, with the elbow flexed and supinated.
• Extend the elbow and measure the elbow extension range to determine the biceps' muscle length.

To measure elbow joint extension, position the shoulder joint in neutral to prevent passive insufficiency of the biceps brachii from affecting our results. You can compare the results to see the difference in range when the muscle isn’t on full stretch.

Example #2: Measuring rectus femoris length vs knee flexion range of motion. Rectus femoris crosses the hip and knee, flexing the hip and extending the knee.

Measure knee flexion as follow:

• Flex the hip to avoid passive insufficiency of rectus femoris.
• To determine the length of this muscle, position the patient in a prone, which puts the hip in some extension
• Measure the amount of knee flexion allowed by rectus femoris.

If the hip flexes during the movement, we know this muscle has length limitations. This is also known as Ely's Test.

Multi-joint Muscle

Measurement of multi-joint muscles follows the same principles as measuring a two-joint muscle. To assess and measure the length of a multi-joint muscle, position all but one of the joints crossed by the muscle so that the muscle is lengthened across the joints. Then move the remaining joint crossed by the muscle through a passive range of motion until the muscle is on full stretch and prevents further motion at the joint. Assess and measure the final position of the joint; the joint position represents an indirect measure of the muscle length.

An example: The flexor digitorum superficialis crosses the elbow, wrist and hand and inserts into the middle phalanges of digits 2-5. It primarily flexes digits 2-5 at the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints but is also a wrist flexor.

Assess this muscle (and the other multi-joint finger flexors) as follows:

• Position the patient sitting with their forearm pronation on a table.
• The hand rests on the table.
• Move the elbow and finger joints into extension and then passively extend the wrist.
• Measure the amount of wrist extension to assess the length of this muscle.

Measurement Tools

Measurement of muscle length is assessed using three primary types of instruments, which include the universal goniometer and the variations of this measurement tool, the inclinometer and its variations, and linear forms of measurement such as the tape measure.

Goniometer

Fig.1 Goniometer

A goniometer (Fig.1) is a device that measures angles. All universal goniometers have a central "body" with a protractor and fulcrum to centre over the patient's joint and two "arms" to align with the patient's body parts.

Generally, goniometers have been shown to have good to excellent reliability, depending on the motion and joint being measured, with intra-rater reliability higher than inter-rate reliability. The use of standardised positions, stabilisation of the body part proximal to the joint being tested, use of bony landmarks to align the goniometer, and repeated testing conducted by the same therapist all help to improve the validity and reliability of goniometric measurements.

Inclinometer

An inclinometer or clinometer, consisting of a circular, fluid-filled disk with a bubble or weighted needle that indicates the number of degrees on the scale of a protractor, is an instrument used for measuring angles of slope, elevation, or depression of an object with respect to gravity's direction. The majority of inclinometers are calibrated or referenced to gravity, which means that the starting position of the inclinometer can be consistently identified and repeated.

Evidence suggests that the hand-held inclinometer is valid and reliable for assessing muscle length. Intra-rater reliability for the hand-held inclinometer during SLR testing was excellent (ICC, 0.95 to 0.98) with standard error of measurement between 0.54° and 1.22° and the minimal detectable change was between 1.50° and 3.41°. While both intra- and inter-tester reliability were good during the assessment of motion around the knee.

Tape Measure

Fig.2 Tape Measure

A tape measure is one of the simplest measurement tools that can be used, which typically measures on a scale with centimetres (cm) or inches (in). The tape is inexpensive, easy to use and readily available in most clinics.

The tape measure has also shown good reliability for muscle testing with good intra-rater reliability (ICC 0.82 0.87) shown within the same day to measure pectoralis minor muscle length.

Principles of Measurement

Regardless of the measurement tool being used, the individual employing the instrument must become skilled in the use of the measurement tool to improve reliability in assessing muscle length. Practice in using an instrument should continue until the user has established a high level of intra-rater reliability. Many of the steps involved in measuring muscle length are the same as though used for measuring joint range of motion. The following principles provide the basic framework for muscle length measurement.

Table 1. Principles of Range of Motion and Muscle Length Assessment

Description
Determine the type of measurement to be performed
Explain the purpose of the procedure to the patient
Position the patient in the preferred position for the measurement
Stabilise the proximal joint segment.
Instruct the patient in the specific motion that will be measured
Move the patient's distant joint segment passively through the available range
Determine the patient's end-feel at the end of the range
Return the patient's distal joint segment to the starting position
Palpate bony landmarks for measurement device alignment
Align the measurement device with the appropriate bony landmarks
Move the patient's distant joint segment passively to the end range
Read the scale of the measurement device and note the reading
Document Muscle Length

Instructions

Patients should be provided with instructions prior to performing any assessment technique so they have an understanding of what is going to happen. Before beginning the muscle length assessment, describe to the patient what will take place and why the measurement is being completed. Show the patient the measurement tool, and explain, in simple terms, its purpose and how it will be used. Show the patient the position they are to assume, again using simple terms and avoiding medical terminology such as supine or prone.

Positioning

The muscle to be measured should be placed in the fully elongated position, ensuring maximal lengthening of the muscle from origin to insertion. The examiner is concerned about the final, elongated position of the muscle and not the measurement of the starting position, as would be appropriate for the measurement of the joint range of motion.

The muscle being measured should be isolated across one joint. When measuring two or multi-joint muscles, move the final joint through a passive range of motion until the muscle is on full stretch and prevents further joint motion.

Stabilisation

To ensure accurate measurement of muscle length, firmly stabilise one end of the bony segment of the joint being measured, typically at the origin or proximal aspect of the bone. Without adequate stabilisation and isolation of the muscle being measured, the patient may substitute motion at another joint resulting in measurement error.

Speed of Movement

Once the patient is positioned and the proximal joint segment is adequately stabilised, the examiner should passively move the joint and lengthen the muscle through the available range of motion. The elongation of the muscle should be performed slowly to avoid eliciting a quick stretch of the muscle spindle and subsequently inducing a twitch response and muscle contraction.

By moving the limb slowly through the range of motion to be measured, the patient is aware of the exact movement to be performed and can cooperate more fully and accurately with the procedure. The examiner can also get an estimation of the patient's available range of motion prior to completing the final measurement, which can provide a check to minimise the possibility of gross measurement error.

Determining End Feel

Each joint has a characteristic feel to the resistance encountered at the end of the normal range of motion. Typical end-feels encountered at the end of the normal range of motion are hard (bony), firm (capsular, muscular, and ligamentous) and soft (soft tissue approximation). When assessing muscle length, we are typically looking for a firm end feel when the muscle is on full stretch, and the patient will report a pulling sensation, stretch or pain in the region of the muscle being lengthened.

Align Measurement Device

Precise alignment of the measurement device relies on accurate palpation of landmarks. Bony landmarks are typically used to align the measurement device since bony structures are more stable and less subject to change in position.

Three landmarks are typically used to align the goniometer, with two landmarks to align the arms of the goniometer:

1. One landmark for the stationary arm aligned with the midline of the stationary segment of the joint.
2. One landmark for the moving arm aligned with the midline of the moving segment of the joint.
3. One landmark for the fulcrum of the goniometer aligned with a point near the axis of rotation of the joint.

Priority should be given to the alignment of the stationary and moving arms of the goniometer for accurate alignment.

Documentation

The examiner is most concerned about the final, elongated position of the muscle, with the measurement taken in this final position.

Summary

Muscle length is really important for our function, with full range of motion required to enable optimal movement. Thus, assessing muscle length is an important piece in the clinical puzzle. There are different testing options for muscle length, and your choice of test may be influenced by different factors depending on your patient population and each individual’s presentation, including their age, co-morbodities, pain levels and more. Once you decide on a test, you need to be sure to set up your space, so that your patient is able to move through their full range without anything getting in the way, such as pillows. Regardless of the type of test, the key to muscle length testing is to ensure that the tested muscle is in its lengthened position across all the joints it crosses.

If you observe changes in muscle length during testing, it’s important to recognise that this could be because of changes between the agonist and antagonist muscles, so we must always consider our findings in the context of the rest of our assessment, including our movement analysis, as well as posture, range of motion, muscle strength, tone, neural tests and more, and apply our clinical reasoning skills to what we find. Short or tight muscles might be weak or strong, and long muscles might also be weak or strong, so there is no one principle for all muscles. We need to be thorough and consider all aspects of our assessment to be able to go on and select the most appropriate treatment.