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 or joints to lengthen, thus allowing the joint or joints 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.

Muscle Length Assessment

The muscle length assessment is performed to determine whether a muscle's length is normal, decreased or increased. This assessment can help identify if changes in muscle extensibility are contributing to a movement impairment and/or symptoms or if other structures are involved.

To test muscle length, we must position the muscle so that the distance between its origin and insertion increases - i.e. we lengthen the muscle in the direction opposite to its action (e.g. to measure the length of the hip flexors, we position the hip in extension).

During the test, the examiner assesses the muscle's resistance to passive lengthening.

• During a test, ensure that you fix / stabilise one end (usually the origin) and passively move the other end to lengthen the muscle
• Ensure that the muscle being tested is 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 might impact results

Measurement Methods

There are two main methods used to assess muscle length: composite tests and direct measurement.

Composite Tests

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, 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

In the direct measurement method, the excursion between adjacent segments of one joint is measured. When using this method, 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.

One Joint Muscles

One-joint muscles cross just one joint. They typically allow full passive range of motion at the joint they cross. If a one-joint muscle is short and limits the range of motion, you will notice a firm end feel caused by muscle tension. To determine the length of a one-joint muscle, we measure the passive range of motion of the joint that it crosses in the direction opposite to its action.

Example: To measure the length of adductor longus, brevis and magnus:

• Position the hip joint so that these muscles are in a lengthened position - i.e. hip abduction
• Measure the degree of passive hip abduction achieved

Two-Joint Muscles

Muscles that cross two or more joints typically do not allow full range of motion across all the joints they cross. 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, we must:

• Position one of the joints so that the muscle is in a lengthend position
• Move the second joint passively until the muscle is on full stretch, preventing further joint motion
• Measure the final position of the second joint to determine muscle length

Example #1: Measuring biceps brachii length vs elbow extension range of motion.

Biceps brachii crosses the shoulder and the elbow. It flexes and supinates the elbow and is a weak shoulder flexor. To test the length of biceps brachii:

• Position the patient in a supine
• Place the arm in shoulder extension, elbow flexion and supination
• Passively extend the elbow and measure the elbow extension range to determine the length of biceps brachii

To measure extension of the elbow joint, position the shoulder joint in neutral to prevent passive insufficiency of biceps brachii from affecting your results. You can compare your results to see the difference in elbow extension range when biceps brachii is and isn't fully lengthened.

Example #2: Measuring rectus femoris length vs knee flexion range of motion.

Rectus femoris crosses the hip and knee. It flexes the hip and extends the knee. To test the length of rectus femoris:

• Position the patient in prone
• Passively flex the knee and measure the angle of knee flexion to determine the length of rectus femoris
• NB: If the hip flexes during the movement, we know there are length limitations in rectus femoris - this is also known as Ely's Test

To measure flexion of the knee joint, position the patient in supine and assess knee flexion range (actively and passively). In this position, the hip is able to flex during the movement, which prevents passive insufficiency of rectus femoris from affecting results.

Multi-joint Muscles

We follow the same principles when measuring multi-joint muscles. To measure the length of a multi-joint muscle, all but one of the joints are positioned so that the muscle is in a lengthend position. We then move the remaining joint crossed by the muscle passively until the muscle is on full stretch, and is preventing further motion at the joint. We assess and measure the final position of the joint to determine muscle length.

Example: Measuring flexor digitorum superficialis length.

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, and it is a wrist flexor. To assess the length of this muscle (and the other multi-joint finger flexors):

• Position the patient in sitting with their forearm in pronation on a table
• The patient's hand rests over the edge of the table
• Move the elbow and finger joints into extension and then passively extend the wrist
• Measure the amount of wrist extension to determine the length of flexor digitorum superficialis.

Measurement Tools

Three main measurement tools are used to assess muscle length: the universal goniometer and its variants, the inclinometer and its variants, and linear forms of measurement such as a tape measure.

Goniometer

Figure 1. Goniometer

Goniometers measure angles. All universal goniometers have a central "body" with a protractor and fulcrum, which you centre over a patient's joint, and two "arms" to align with the patient's body parts.

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-rater reliability. We can improve the validity and reliability of goniometric measurements by:

• using standardised positions
• stabilising the body part proximal to the joint being tested
• aligning the goniometer with bony landmarks
• having the same therapist conduct repeated tests

Inclinometer

An inclinometer or clinometer is used to measure angles of slope, elevation, or depression of an object. 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. Inclinometers have a circulr disc which is filled with a fluid, and a bubble or weighted needle to point to a number (in degrees) on a protractor scale.

Evidence suggests that the hand-held inclinometer is a valid and reliable instrument for assessing muscle length. Boyd found that intra-rater reliability for a hand-held inclinometer to measure a straigth leg raise was excellent. Romero-Franco et al. found that both intra-tester and inter-tester reliability were good when an inclinometer was used to assess motion around the knee.

Tape Measure

Fig.2 Tape Measure

Tape measures are one of the simplest measurement tools that can be used. They typically measure in centimetres (cm) or inches (in). A tape measure is inexpensive, easy to use and readily available in most clinics.

Rosa et al. looked at assessing the muscle length of pectoralis minor using a tape measure and found it has good reliability and good intra-rater reliability (ICC 0.82 0.87) with same day testing.

Principles of Measurement

Every clinician must be skilled at using measurement tools in order to improve the reliability of our muscle length tests. It is important that clinicians practise using an instrument until they achieve a high level of intra-rater reliability.

Instructions for the Patient

Patients should be given information prior to an assessment so they have an understanding of what is going to happen. Before starting a muscle length assessment, it is important to explain what you will be doing and why. 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.

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 are not many studies on age-related changes in muscle length that use direct measurement tests. But at least one study on hamstring length which does use this method found no significant difference in muscle length with increasing 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 compared to a neutral spine, whereas the cervical extensors and occipital flexors lengthen.

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.