Assessing Muscle Length

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Original Editors - Naomi O'Reilly and Jess Bell

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Introduction

Muscle length refers to the ability of a muscle crossing a joint to lengthen, allowing one or more joints to move through the full available range of motion. The lengthening of a muscle fiber begins with the sarcomere, the basic unit of contraction in the muscle fiber. As the sarcomere contracts, the area of overlap between the thick and thin myofilaments increases. As it lengthens, this area of overlap decreases, allowing the muscle fiber to elongate. Maximal muscle length therefore is the greatest extensibility of the muscle-tendon junction. Muscle length testing is done to determine whether the muscle length is limited or excessive, i.e., whether the muscle is too short to permit normal range of motion, or stretched and allowing too much range of motion.

Muscle Length Tests are are performed to determine whether the range of muscle length is normal, limited, or excessive i.e., whether the muscle is too short to permit normal range of motion, or lengthened and allowing too much range of motion and is used to identify if these changes in muscle extensibility may be contributing to movement impairment and/or symptoms. Muscle length testing consist of movements that increase the distance between the origin and insertion, thereby lengthening muscles in directions opposite to the muscles actions, while assessing its resistance to passive lengthening.

Precise testing requires that one of the bony attachments of the muscle be in a fixed position while the other bony attachment is moved passively in the direction of lengthening the muscle. This means that to assess and measure the length of a muscle we need to passively stretch or lengthen the muscle across the joint or joints crossed by that muscle. For the best accuracy and precision, muscle length testing should be performed when the patient is not in acute pain in order to avoid pain inhibition and muscle guarding. The purpose of muscle length testing is to help determine whether reduced or increased range of motion at a joint is caused by the length of the muscle being tested or if it is caused by other structures.

Structure of Muscles

Skeletal muscles are made up of striated muscle fibres. A muscle connects to bones or joint capsules by connective tissue structures, such as tendons or aponeuroses. Muscle fibres contain smaller units called myofibrils, which in turn 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, which pulls the origin and insertion of the muscle-tendon complex closer together, so the muscle shortens during a contraction. When sarcomeres contract, the amount of overlap between thick and thin myofilaments increases. As it lengthens, the amount of overlap decreases, so the muscle fibre can lengthen.

Kruse and colleagues state that “the force exerted actively by a muscle can be expressed as a function of muscle length.” The length where muscles don’t 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 two is the length range of active force exertion.

Factors Effecting Muscle Length

Gender

In general, biological females tend to be more flexible than biological males, PAWAR - research looking specifically at hamstrings length has found that females can have up to 8 degrees more range in their passive SLR (Youdas et al.) and 12 degrees in the knee active knee extension test (Corkery et al).

Age

Force production capacity is impacted by reduced muscle fibre length and altered pennation angles FRAGALA (definition: angle between the longitudinal axis of the muscle and its fibers.)

Older adults experience increased fibrosis, sarcopenia, decreased force production and a general reduction in flexibility. These changes can impact independence and lead to an increase in morbidity and mortality ZOTZ.

There may also be an association between range of motion and muscle length of the lower limb and balance performance in older adults with foot deformities JUSTINE https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4842465/

However, as Reese and Bandy point out, there is not much research on age-related changes in muscle length using direct measurement tests. And one study on hamstring length by Youdas and colleagues using 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. A common posture seen in clinical practice is a forward head position. GUOHAO

In a forward head position, an individual’s chin tends to come forward. They have flexion of their mid-lower cervical spine and extension of their upper cervical spine. This posture is often linked to time spent sitting at desks on computers, laptops and mobile phones. KHAYATZADEH

There is research to suggest that a forward head posture can be associated with vestibular deficits, decreased proprioception, abnormal muscle activity, and altered breathing patterns GUOHAO and it has a bearing on muscle length. 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. KHAYATZADEH

Measurement Methods

Muscle length can be measured through two methods,

Composite Method

We can also assess muscle length using composite tests, which look at movement across more than one muscle or joint. Common composite tests are the Apley’s Test or the Sit and Reach Test. While these tests are commonly used research suggests that these composite tests do not provide accurate measurements of muscle length as they assess combinations of movements across several joints involving 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

Muscles are characterised by the number of joints they cross. We have one-joint muscles, two-joint muscles and multi-joint muscles. Muscles that pass over one joint only, the range of motion and range of muscle length will measure the same but 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

As their name suggests, one-joint muscles cross just one joint. We can determine the length of a one-joint muscle by measuring the passive range of motion of the joint that it crosses. But 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. So to determine their length, we measure the passive range of hip abduction.

To assess and measure the length of a muscle that crosses one joint, the joint crossed by the muscle is positioned so that the muscle is lengthened across the joint and the position of the joint is measured and this represents an indirect measure of the muscle length.

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

Two-Joint Muscle

Unlike one-joint muscles, muscles that cross two or more joints typically do not allow full range of motion across all the joints they cover, which is known as passive insufficiency. To assess and measure the length of a two-joint muscle, position one of the joints crossed by the muscle so as to lengthen the muscle across the joint. Then move the second joint through a passive range of motion until the muscle is placed on full stretch and prevents further joint motion. Assess and measure the final position of the second joint; the joint position represents an indirect measure of the muscle length.

For example, 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 this muscle, we would position our patient in supine. The starting position is shoulder extension, with the elbow flexed and supinated. We then extend the elbow and measure the range of elbow extension to determine muscle length of biceps. If we want to measure elbow joint extension, we would position the shoulder joint in neutral, to prevent passive insufficiency of biceps brachii affecting our results. We can also compare the results to see the difference in range when the muscle isn’t on full stretch.

Similarly, if we are considering rectus femoris length vs knee flexion range of motion. Rectus femoris crosses the hip and knee, flexing the hip and extending the knee. So to measure knee flexion, we must have the hip flexed to avoid passive insufficiency of rectus femoris. If we want to determine the length of this muscle, we can position the patient in prone, which puts the hip in some extension and then we measure the amount of knee flexion allowed by rectus femoris. If the hip flexes during the movement, we know there are length limitations in this muscle. This is also known as the 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 one 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.

For 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 it is also a wrist flexor. To assess this muscle (and the other multi-joint finger flexors), we position the patient in sitting with their forearm in pronation on a table. The hand rests over the table. We move the elbow and finger joints into extension and then passively extend the wrist. We measure the amount of wrist extension to assess the length of this muscle

Measurement Tools

Measurement of muscle length include can be completed through use of one or more of the following tools.

1. Goniometer; A goniometer is a device that measure an angle or allow an object to be rotated into a specific angular position.
2. Inclinometer; An inclinometer or clinometer is an instrument used for measuring angles of slope, elevation, or depression of an object with respect to gravity's direction
3. Tape Measure; A tape measure if one of the simplest measurement tools that can be used, which measure in centimetres (cm) or inches (in). These can be cloth or metal.

Principles of Measurement

Positioning

Ensure maximal lengthening of the muscle from origin to insertion.

Stabilisation

Firmly stabilise one end of the muscle, typically this is at the origin or proximal aspect of the muscle.

Speed of Movement

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.

Determining End Feel

The most valuable clinical information is the muscular end feel and the location of the range of motion end feel. When the muscle is on full stretch, the end feel will be firm, and the patient will report a pulling sensation or pain in the region of the muscle.

Aligning Measurement Device

Documentation

Resources

References