Assessing Muscle Strength

Original Editors - Naomi O'Reilly and Wanda van Niekerk

Top Contributors - Naomi O'Reilly, Wanda van Niekerk, Jess Bell, Lenie Jacobs, Kim Jackson and Angeliki Chorti      

Introduction

Muscle strength is defined as the maximal force a muscle or muscle group can generate at a specified or determined velocity. Essentially, it is the ability of skeletal muscle to develop force in order to provide stability and mobility within the musculoskeletal system, which is necessary for functional movement. The muscle strength assessment is integral to the objective examination as it provides valuable information on strength and neurological deficits.

Muscle strength decreases with age, and many pathologies can reduce muscle strength and control. For example, it can be impaired following injury, infection, major surgery or in many medical conditions including but not limited to stroke, cerebral palsy, muscular dystrophy, metabolic syndromes, spinal cord injury, motor neuron disease, multiple sclerosis, Parkinson's, COPD, heart failure, and arthritis. Muscle strength can be a predictor of mortality, hospital length of stay, and hospital readmission.

Factors Determining Muscle Strength

Strength depends on a combination of morphological and neural factors, including:

• type of muscle contraction
• cross-sectional area of muscle
• muscle architecture
• stiffness of the musculotendinous structure
• motor unit recruitment, rate coding and motor unit synchronisation
• neuromuscular inhibition
• speed of contraction

Types of Muscle Contraction

A muscle contraction occurs when tension-generating sites within the muscle cells are activated. The type of contraction is defined by changes in the length of the muscle during contraction.

Isometric Contractions

Greek, isos: “equal” and metron: “measure”

• An isometric contraction is a static contraction with variable/accommodating resistance that does not result in changes in muscle length. Tension is generated in the muscle, but the distance between the muscle attachments remains the same. In an isometric contraction, cross bridges form, disengage and reform. There is no movement, and no external work is done by the muscle.

Please note that "cross bridge" refers to the attachment between the myosin and actin filaments. Read more about cross bridges and the sliding filament theory here: Sarcomere.

Isotonic Contractions

Greek, isos: “equal” and tonos: “straining”)

Figure 1 Types of Muscle Contractions

In an isotonic contraction, tension remains the same, but the muscle's length changes. There are two types of isotonic contractions: concentric and eccentric contractions.

Concentric Contraction

• During a concentric contraction, there is a shortening of the muscle, so the origin and insertion of the muscle move closer together.
• A muscle performs a concentric contraction when it lifts a load or weight that is less than the maximum tetanic tension it can generate.
• The muscle shortens, movement occurs, and external work is done.

Eccentric Contraction

• During an eccentric contraction, the muscle lengthens as it gives in to an external force that is greater than the contractile force exerted by the muscle.
• In reality, the muscle does not lengthen. Instead, it returns from its shortened position to its normal length.
• The muscle lengthens, movement occurs, and external work is done.

Muscle Length

Muscle length is an important factor in governing force and tension. The full range in which a muscle can work = the range between the position of maximal stretch to the position of maximal shortening. As shown in Table 1, full range is divided into three parts.

Table 1. Three Parts of Muscle Length Range

Outer Range	Inner Range	Mid Range
muscle working in maximally stretched position moves between longest length and mid-point of range least overlap of actin and myosin fewer cross bridges formed less tension produced	muscle working in maximally shortened position moves between the shortest length and mid-point of range actin and myosin overlap decreased number of sites available for cross bridge formation less force generated	muscle working between mid-point of outer range and mid-point of inner range optimal overlap of actin and myosin optimal number of sites for cross bridge formation maximum tension generated

Muscle Fibre Type

• There are three types of muscle fibres.
  • These can be classified based on how fast the fibres contract relative to other fibres and how the fibres regenerate adenosine triphosphate (ATP) (i.e. the source of energy for muscles).
  • Muscle fibre type can also be influenced by training.
    • People who do well at endurance sports tend to have a higher number of slow-twitch fibres.
    • People who are better at sprint events tend to have higher numbers of fast-twitch muscle fibres.

Table 2. Muscle Fibre Types

Type I / Slow Twitch / Slow Oxidative	Type IIa / Fast Twitch / Oxidative-Glycolytic	Type IIb / Fast Twitch / Glycolytic
relatively slow contractions use aerobic respiration (oxygen and glucose) for ATP production produce low-power contractions over long periods and are slow to fatigue high aerobic capacity, efficient at working isometrically, useful in maintaining posture and joint stabilisation	fast contractions primarily use aerobic respiration respond quicker than Type I but also fatigue more quickly because they may switch to anaerobic respiration (glycolysis)	fast contractions primarily use anaerobic glycolysis quickest response but fatigue rapidly and have a relatively slow recovery rate

Read more: Muscle Fibre Types, Sliding Filament Model of Contraction, The Muscle Contraction Process

Neural Factors

• Neural factors influence the tension-developing capacity of the muscle, which determines the extent to which a muscle is activated.
• Tension is influenced by neural input through two mechanisms:
  • Motor unit recruitment
  • Modification of the firing frequency of motor units

Integrity of Connective Tissue

• For a person to intentionally contract a muscle, they must generate a signal in their brain. This signal travels from the brain, through nerve cells in the brain stem and spinal cord, to the peripheral nerves and the muscle.
• Various factors can impact the integrity of connective tissues at any part of this pathway and, thus, affect force production and overall muscle strength.
  • Pain has been shown to affect muscle force production
    • pain reduces the maximal voluntary contraction and endurance time during submaximal contractions.
  • There is a correlation between pain intensity and reduced muscle strength in individuals with chronic pain
    • increased pain intensity results in decreased muscle strength and force production.
  • Inflammation can impact force production
    • research suggests that higher levels of circulating inflammatory markers are significantly associated with lower skeletal muscle strength and mass.
  • Many conditions, including neuromuscular disorders, cancer, chronic inflammatory diseases, and acute critical illness are associated with skeletal muscle atrophy, muscle weakness, general muscle fatigue, increased morbidity and mortality and decreased quality of life.

Age

• As we age, our muscles progressively change. These changes primarily lead to reduced muscle mass and strength.
• Muscle mass decreases approximately 3–8% per decade after the age of 30. This rate of decline is even higher after the age of 60.
• The total number of muscle fibres reduces with age, beginning at around 25 years and progressing at an accelerated rate from then on. This leads to reduced muscle cross-sectional area and reduced muscle power.
• There is also a decrease in the number of functional motor units. This is associated with an enlargement of remaining motor units (these remaining units also experience "reduced neuromuscular junction transmission stability".
• Overall, these changes in the muscle mass, muscle fibre and cross-sectional area of the muscle during the ageing process are important clinically as they lead to reduced muscle strength.

Read more: Muscle Function: Effects of Ageing

Contraindications

Muscle strength assessments are typically contraindicated when a muscle contraction or motion of the tested part of the body could disrupt the healing process, cause injury or worsen the condition. Some instances where a muscle strength assessment may be contraindicated include:

• Unhealed fracture
• Dislocation or unstable joint
• Situations where active range of motion or resistance work are contraindicated (e.g. post-operative protocols etc)
• If pain limits participation
• Severe inflammation
• Severe osteoporosis
• Haemophilia
• Cognitive concerns / decreased ability to complete the test

Precautions

During a muscle strength assessment, ensure you respect pain and consider patient comfort. Specific precautions include:

• Abdominal surgery or hernia
• Bony ankylosis
• Haematoma
• Cardiovascular disease
• Pulmonary disease
• Prolonged immobilisation
• Cases where fatigue may be harmful or exacerbate the person's condition ( for example lower motor neuron disease, chronic obstructive pulmonary disease, multiple sclerosis)

Measuring Muscle Strength

Muscle strength testing is used to determine the capability of the muscle or muscle group to produce force. It provides information that is useful in differential diagnosis, prognosis and management of neuromuscular and musculoskeletal disorders. While there are many methods of assessing muscle strength, there are three key approaches described in the literature and used clinically (see Table 3): isokinetic, isotonic, and isometric testing.

Table 3. Key approaches to muscle strength testing

Isotonic	Isokinetic	Isometric
tests muscle strength using a constant external resistance involves the use of free weights or resistance machines testing techniques such as the one-repetition maximum (1-RM) are used: 1-RM = the maximum weight a patient can lift against gravity through an entire range of motion involves adjusting the weight with repeated lifting until the individual can only lift it once sufficient rest is necessary between attempts to avoid fatigue time-consuming testing method gross strength testing of muscle groups rather than individual muscles read more on 1-RM	tests muscle strength with specialised equipment (isokinetic dynamometers) where movement velocity remains constant during a muscle contraction the isokinetic dynamometer generates an isokinetic torque curve the highest point of the curve indicates the strength of the muscle or muscle group tested provides an objective and quantitative assessment of muscle strength isokinetic machines allow: isolation of specific joints - this allows for targeted testing of particular muscle groups evaluation of muscle strength across differing speeds, ranges of motion comparison of left and right sides reliable testing (if testing protocols are followed), but can be cost prohibitive gross strength testing of muscle groups rather than individual muscles	type of muscle testing where the muscle generates force (at a specific joint angle) against an immovable resistance so that muscle length remains the same throughout the test most commonly used methods for isometric muscle testing: manual muscle testing (MMT) handheld dynamometry (HHD) both are inexpensive and highly portable with MMT requiring no equipment other than the examiner’s hands

Manual Muscle Testing (MMT)

• Manual muscle testing helps to determine the extent and degree of muscle weakness resulting from disease, injury or disuse to provide a basis for planning therapeutic procedures.
• It is used to evaluate the function and strength of an individual muscle or muscle group, based on the effective performance of a movement in relation to the forces of gravity or manual resistance through the available range of motion.
• There is a wide range of scales available for completing manual muscle testing, including:
  • Medical Research Council (MRC) Scale - also known as the Oxford Scale
  • Daniels and Worthinghmans Manual Muscle Testing Scale
  • Kendall Muscle Testing Scale
  • Note that some scales use tests based on actions (e.g. elbow flexion) rather than tests of individual muscles (e.g. biceps brachii). In these cases, the grade will represent the performance of all muscles contributing to the specific action.

Table 4. Medical Research Council Scale (Oxford Scale)

Grade	Description
0	No contraction
1	Flickering contraction
2	Full range of motion with gravity eliminated*
3	Full range of motion against gravity
4	Full range of motion against gravity with minimal resistance
5	Full range of motion against gravity with maximal resistance

* please note that we now try to avoid the term "gravity eliminated" as this is only possible in a zero-gravity environment, so we use the term "gravity minimised".

As per Daniels and Worthington's 'Muscle Testing: Techniques of Manual Examination and Performance Testing', there are two different methods used for manual muscle testing:

1. Break Test: resistance is applied to the body part at or near the end of the available range or at the point in the range where the muscle is most strongly challenged. It is called the break test because the patient tries to stop the therapist from "breaking" the muscle hold when resistance is applied.
2. Active Resistance Test: resistance is applied to the body part through the available range of motion. This type of manual muscle testing requires skill and experience and is not the recommended practice.

Dynamometry

Dynamometry is a more precise and objective measurement of the force that a muscle can exert. It allows the assessor to compare strength on each side and measures strength changes during a rehabilitation programme. It typically uses the same positioning as manual muscle testing but provides more quantifiable data.

Benefits of dynamometry:

• more sensitive than manual muscle testing
• norms available

Principles of Assessment

Some overall guiding principles when assessing muscle strength are as follows:

• Compare the unaffected side with the affected side
  • Where possible, assess the unaffected limb's active range of motion first.
    • This shows the patient's willingness to move and provides a baseline for normal movement of the joint being tested.
    • It also shows the patient what to expect, which increases patient confidence and reduces apprehension when testing the affected side.
• Any movements that are painful should be completed last. This helps to minimise the risk of overflow of painful symptoms to the next movement.

• Preparation
  • Determine whether there are contraindications or precautions and what joints, muscles and motions need to be tested.
  • Organise the testing sequence by body position to minimise changes in positioning.

• Communication
  • Briefly explain the procedure for manual muscle testing to the patient.
  • Explain and demonstrate the movement to be performed and/or passively move the patient’s limb through the test movement.
  • Explain and demonstrate the examiner’s and  patient's roles and confirm the patient understands and is willing to participate.

• Expose the Area
  • Explain and demonstrate anatomical landmarks and why they need to be exposed.
  • Adequately expose the area and drape the patient as required.

• Positioning
  • Proper positioning of the patient ensures that the appropriate muscle is tested. It also helps prevent substitution movements/actions by other muscles.
  • Aim to isolate the action of a specific muscle to minimise the influence of other muscles when testing
    • Place the patient in the starting position.
    • Make sure the patient is comfortable and properly supported.
    • The muscle or muscle group tested can be placed in full outer range when testing strength through range. In cases where strength is tested isometrically, the muscle or muscle groups should be placed in the appropriate test position. This is often in mid-range, so that it can produce maximum force during the test. When using isometric testing note that strength varies throughout range of motion. A better picture of the muscle's ability will be formed if the muscle is tested isometrically in inner-, mid- and outer range. Whichever position or method you decide to use, it is crucial to be consistent with testing and reassessments; documentation of the test positions and types of testing is key.
    • If there is any variance to the patient's position from the standard assessment positions outlined in our technique videos, ensure you make a note of this in your documentation.
      • For example, if a patient cannot achieve full elbow extension, record the starting angle before measuring strength of the elbow flexors.
  • Tables 5 and 6 provide information on patient positioning for testing:

Table 5. Guide to Upper Limb Positioning for Manual Muscle Testing

Body Region	Muscle Action	Patient Position in Relation to Grade Being Tested
		Grade 0 and 1	Grade 2	Grade 3, 4 and 5
Shoulder	Extension	Prone	Side Lying	Prone
	Flexion	Supine	Side Lying	Supine
	Abduction	Supine	Supine	Side Lying or Standing
	Adduction	Supine	Supine	Side Lying or Standing
	External Rotation	Prone	Supine	Sitting - Hips and Knees at 90°
	Internal Rotation	Supine	Supine	Sitting - Hips and Knees at 90°
Elbow	Extension	Prone	Side Lying or Sitting	Prone or Sitting
	Flexion	Supine	Side Lying or Sitting	Supine or Sitting
	Supination	Supine or Sitting	Difficult to eliminate gravity in full range of motion (FROM)	Supine or Sitting Grade 3 - Difficult to complete FROM against gravity
	Pronation	Supine or Sitting	Difficult to eliminate gravity in FROM	Supine or Sitting Grade 3 - Difficult to complete FROM against gravity
Wrist	Extension	Supine or Sitting	Supine or Sitting Forearm in Mid Position	Supine or Sitting Forearm Pronated
	Flexion	Supine or Sitting	Supine or Sitting Forearm in Mid Position	Supine or Sitting Forearm Supinated
	Ulnar Deviation	Supine or Sitting	Supine or Sitting Forearm Pronated	Supine or Sitting Forearm Pronated
	Radial Deviation	Supine or Sitting	Supine or Sitting Forearm Pronated	Supine or Sitting Forearm in Mid Position

Table 6. Guide to Lower Limb Positioning for Manual Muscle Testing

Body Region	Muscle Action	Patient Position in Relation to Grade Being Tested
		Grade 0 and 1	Grade 2	Grade 3, 4 and 5
Hip	Extension	Prone	Side Lying	Prone
	Flexion	Supine	Side Lying	Supine
	Abduction	Supine	Supine	Side Lying or Standing
	Adduction	Supine	Supine	Side Lying or Standing
	External Rotation	Prone	Supine	Sitting - Hips and Knees at 90°
	Internal Rotation	Supine	Supine	Sitting - Hips and Knees at 90°
Knee	Extension	Supine	Side Lying	Sitting
	Flexion	Prone	Side Lying	Prone or Standing
Ankle	Plantarflexion	Prone	Side Lying	Prone or Standing
	Dorsiflexion	Supine	Side Lying	Supine or Sitting
	Eversion	Supine	Supine	Side Lying
	Inversion	Supine	Supine	Side Lying

• Stabilisation
  • The patient needs to be in a stable position - the joint that the muscle acts on must be firmly fixed in place.
  • The effect of gravity and the weight of the patient on the treatment table or chair provides initial stabilisation. The therapist's hand placement on the patient's limb provides additional stabilisation of the proximal joints while the resistance is placed distally.
  • Substitute movements at other joints may occur without adequate stabilisation, which can affect results. Rehabilitation professionals should know and recognise the possible substitute movements at each joint to increase accuracy.

• Application of Resistance
  • One-joint muscles: apply resistance at the end of range for consistency.
  • Two-joint muscles: apply resistance in mid-range as length-tension is more favourable in this range.
  • Aim to test muscles and muscle groups at optimal length-tension. However, there may be situations where you cannot distinguish between Grade 5 and 4 strength unless you put the patient at a mechanical disadvantage.
  • Ensure that you apply resistance slowly and gradually at the distal end of the limb. Pressure should be applied opposite to the line of pull of the muscle being tested. Typically, a lumbrical grip is most comfortable for the patient.

• Application of Grades
  • Always start strength testing in a position against gravity (Grade 3 in MRC Scale) to determine if the patient can move through the full range of motion against gravity. Ensure you isolate the muscle or muscle group being tested.
  • If the patient cannot move through any part of the range of motion against gravity, re-position them so that the resistance of gravity is eliminated for the test movement (i.e. the patient performs the movement in the horizontal plane).
  • In this case, it may be necessary to support the weight of the limb on a relatively friction-free surface or manually.

• Documentation
  • Documentation of manual muscle testing should list:
    • the muscle being tested
    • the muscle grade allocated
    • symptoms experienced that may have impacted strength
    • changes needed to positioning to complete the test. e.g. right quadriceps 4/5 with pain, performed in supine.

Clinical Significance

Muscle strength testing can help determine if there is a loss in muscle strength. Careful and consistent technique is important to ensure valid and reproducible results. Understanding the factors that may impact muscle strength is also important in order to clinically reason why an individual is experiencing strength loss. The MRC Scale is the most commonly used grading scale. It is quick to complete and does not require special equipment, and while it is a subjective measure, it demonstrates reasonable inter-rater reliability. More precise methods of measurement, such as dynamometry, are less subjective and provide a quantifiable measurement that can be tracked over time. However, they can be more time-consuming and require access to more expensive equipment.

The coordinated contraction of agonist, antagonist, and synergist muscles is crucial for generating movement around limb joints. Comprehending these interactions is essential for understanding limb movement, joint dynamics, and conducting muscle strength testing.

Isolating individual manual muscle tests may not elucidate the underlying reasons for specific functional limitations. Combining these tests into functional muscle testing, such as the action of standing up from a chair, can provide a more comprehensive understanding.

Summary

• Consistency in techniques is important for valid and reliable results
• Having a good understanding of the factors that influence muscle strength will enhance your clinical reasoning skills
• Manual muscle testing is a clinical skill that needs to be practised on a variety of patients to acquire the necessary skills and experience