Assessing Muscle Length

Original Editors - Naomi O'Reilly and Jess Bell

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

Introduction[edit | edit source]

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.[1][2] A muscle's ability to lengthen is essential for functional activities,[3] so rehabilitation professionals need to consider muscle length when assessing and treating patients. This article discusses the general principles of the muscle length assessment.

Structure of Muscles[edit | edit source]

Muscles connect to bones or joint capsules by connective tissue structures, such as tendons or aponeuroses.[4]

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.[5]

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.[4] When sarcomeres contract, the overlap between thick and thin myofilaments increases. The amount of overlap decreases as it relaxes so that the muscle fibre can elongate. Maximal muscle length is, therefore, the greatest extensibility of the muscle-tendon junction.[6]

Kruse and colleagues[4] state: "The force exerted actively by a muscle can be expressed as a function of muscle length.”[4]

  • 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[4]

For a detailed discussion of these principles, please see Kruse et al. Stimuli for adaptations in muscle length and the length range of active force exertion - a narrative review.

Muscle Length Assessment[edit | edit source]

The muscle length assessment helps 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.[7]

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.[7] For example, to measure the length of the hip flexors, we position the hip in extension.

Measurement Methods[edit | edit source]

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

Composite Tests[edit | edit source]

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 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 give a general idea of flexibility rather than an exact measurement of a single muscle’s length.[2]

Direct Measurement[edit | edit source]

In the direct measurement method, the distance between adjacent joint segments is measured. When using this method, we must 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[edit | edit source]

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.[2][7] 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[2][8][7]

Two-Joint Muscles[edit | edit source]

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."[9]

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 lengthened position
  • Move the second joint passively until the muscle is on full stretch and prevents further motion at the joint
  • Measure the final position of the second joint to determine muscle length[2][8]

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[7]

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 can flex during the movement, which prevents passive insufficiency of rectus femoris from affecting results.

Multi-joint Muscles[edit | edit source]

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 with the tested muscle in a lengthened position. We then move the remaining joint crossed by the muscle passively until the muscle is on full stretch and prevents further motion at the joint. We assess and measure the final position of this joint to determine muscle length.[2][8]

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 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[2]

Measurement Tools[edit | edit source]

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[edit | edit source]

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.[1] 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.[10] [11] We can improve the validity and reliability of goniometric measurements by:[12][13][14]

  • 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

The following video demonstrates how to measure muscle length in the lower limb using a goniometer.


Inclinometer[edit | edit source]

An inclinometer (or clinometer) is used to measure the angles of slope, elevation, or depression of an object. Most inclinometers are calibrated or referenced to gravity, which means that the starting position of the inclinometer can be consistently identified and repeated.[2] Inclinometers have a circular disc which is filled with fluid and a bubble or weighted needle to point to a number (in degrees) on a protractor scale.[2]

Evidence suggests that the hand-held inclinometer is a valid and reliable instrument for assessing muscle length.[16] [17] Boyd[16] found that intra-rater reliability for a hand-held inclinometer to measure a straight leg raise was excellent. Romero-Franco et al.[17] found that both intra-tester and inter-tester reliability were good when an inclinometer was used to assess motion around the knee.

Tape Measure[edit | edit source]

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.[2]

Rosa et al.[18] 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.[18]

Principles of Assessment[edit | edit source]

Following these principles can help improve the accuracy of your muscle length assessment.[2]

Level of Skill: Every clinician must be skilled at using measurement tools to improve the reliability of a muscle length test. Clinicians should practise using an instrument until they achieve a high level of intra-rater reliability.[2]

Communication: Patients should be given detailed information before an assessment so they understand what will happen and what they need to do. Before starting your muscle length assessment, explain what you will do and why. You can show your patient the measurement tool that you will use. Make sure you do not use medical jargon to ensure understanding - e.g. avoid terms like supine and prone or proximal and distal.[2][1]

Positioning: In a muscle length test, we are measuring the final, elongated position of the muscle - we do not need to measure the starting position.[1] Remember to ensure that the tested muscle is lengthened across all the joints it crosses.[6]

Isolating Muscle: We need to isolate the muscle being tested as much as possible (across one or two joints).[2]

Stabilisation: During a test, ensure that you fix / stabilise one end (usually the proximal bony segment of the joint being moved) and passively move the other end to lengthen the muscle.[6] Substitute movements at other joints may occur without adequate stabilisation, which can affect results.[2]

Speed of Movement: The clinician should move a joint slowly through the available motion to determine muscle length. This means you are more likely to avoid eliciting a quick stretch of the muscle spindle, and inducing a twitch response and muscle contraction.[19]

Determining End-feel: End feel refers to the resistance to movement at a joint's normal end range of motion.[2] Common end feels include hard (bony), firm (capsular, muscular, and ligamentous) and soft (soft tissue approximation). We expect a firm end feel when assessing muscle length because the muscle should be on full stretch. The patient may also report a pulling sensation, stretch or pain in the region of the muscle being lengthened.[2][6] We can compare sides to get an understanding of what is "normal" for the patient. We can also look at muscle length normative values for each muscle. For examples of normative values for some muscles please see: Muscle Length Normative Values.

Aligning Measurement Tool: Bony landmarks are usually used to align our measurement tools when assessing muscle length. This is because bony landmarks are more stable and less likely to experience a change in position from factors like oedema. You will usually need to find three landmarks to align a goniometer:

  1. One landmark is used to align the stationary arm - this arm is usually aligned with the midline of the stationary segment of the joint.
  2. One landmark is used to align the moving arm - this arm is usually aligned with the midline of the moving segment of the joint.
  3. One landmark is used to align the fulcrum of the goniometer - the fulcrum is positioned over a point near the joint's axis of rotation.[2]

Documentation: Record the results of your muscle length test (and the testing position) in the patient's notes.

Acute Pain: If a patient has acute pain, muscle length testing may need to be delayed - otherwise, muscle guarding or pain inhibition might impact results.[7]

Factors Affecting Muscle Length[edit | edit source]

Gender[edit | edit source]

Evidence suggests that biological females tend to be more flexible than biological males.[20] [21] Research looking specifically at hamstring length has found that females can have up to 8 degrees more range in their passive straight leg raise[22] and 12 degrees more range during an active knee extension test than males.[23]

Age[edit | edit source]

Our muscles also change with age. Older adults experience increased fibrosis, sarcopenia, decreased force production and a general reduction in flexibility.[24] Justine et al.[25] 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.[25]

However, as Reese and Bandy[2] 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.[22][2]

Posture[edit | edit source]

Posture can have an impact on muscle length because our muscles and tissues adapt to how they are used. For example, in a forward head posture, the cervical flexors and occipital extensors have been found to shorten compared to a neutral spine while the cervical extensors and occipital flexors lengthen.[26]

Summary[edit | edit source]

Having adequate muscle length enables optimal movement. Thus, assessing muscle length is an important piece in the clinical puzzle. When performing your assessment, please remember the following:

  • The key to muscle length testing is to ensure that the tested muscle is in its lengthened position across all the joints it crosses.
  • There are different testing options to assess muscle length, and your choice of test may be influenced by factors such as the acuity of your patient's injury, their age, co-morbidities and pain levels.
  • When we observe changes in muscle length, 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.

References [edit | edit source]

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  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 Reese NB, Bandy WD. Joint Range of Motion and Muscle Length Testing-E-book. Elsevier Health Sciences; 2016 Mar 31.
  3. Tomalka A. Eccentric muscle contractions: from single muscle fibre to whole muscle mechanics. Pflugers Arch. 2023 Apr;475(4):421-435.
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  26. Khayatzadeh S, Kalmanson OA, Schuit D, Havey RM, Voronov LI, Ghanayem AJ, Patwardhan AG. Cervical spine muscle-tendon unit length differences between neutral and forward head postures: a biomechanical study using human cadaveric specimens. Physical therapy. 2017 Jul 1;97(7):756-66.