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 to occur. 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:

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

Some of these factors will be discussed in more detail below.

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”

• Isometric contractions are 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: 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.

Length of Muscle

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
relative slow contraction 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 are 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 to the muscle.
• Certain factors can impact the integrity of connective tissues at any part of this pathway, and this is evident in force production and overall muscle strength.
  • Pain has been shown to impact the production of muscle force. It results in a reduction in maximal voluntary contraction and endurance time during submaximal contractions.
  • There is also correlation between pain intensity and reduced muscle strength in individuals with chronic pain, with increased pain intensity resulting in decreased muscle strength and force production.
  • Similarly, inflammation can impact force production, with research suggesting higher levels of circulating inflammatory markers are significantly associated with lower skeletal muscle strength and mass.
  • Many diseases, including neuromuscular diseases, cancer, chronic inflammatory diseases, and acute critical illness are associated with skeletal muscle atrophy, muscle weakness, and general muscle fatigue, which is associated with increased morbidity and mortality and decreased quality of life.

Age

• Ageing effects all body organs and systems in the skeletal muscle.
• As we age, our muscles undergo progressive changes. These primarily result in loss of muscle mass and strength. Muscle mass decreases approximately 3–8% per decade after the age of 30 and 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 thereafter. 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
• Active range of motion or resistance work are contraindicated (e.g. post-operative protocols etc)
• 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 or pulmonary disease
• Prolonged immobilisation
• Extreme debility

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 are a wide range of scales available for completing manual muscle testing including:
  • Modified 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

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 the end of the available range of motion. It is called the break test because the patient is trying 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 during muscle testing is essential in ensuring that the appropriate muscle is being tested. It also helps prevent substitution by other muscles.
  • Aim to isolate the action of a specific muscle to minimise the influence of other muscles when testing
    • Place the body part that the muscle acts on in the starting position.
    • As a rule, this will typically be in the mid-range of the muscle, so it can produce maximum force during the test.
    • If there is any variance to the patient's position from standard assessment position 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 the elbow flexion strength.
  • 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 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’s body needs to be placed in a stable position - the joint that the muscle acts on must be firmly fixed in place.
  • Stabilisation comes initially from the effect of gravity and the weight of the patient on the treatment table or chair.
  • Hand placement of the rehabilitation professionals on the limb to be assessed offers 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 they are assessing to increase accuracy.

• Application of Resistance
  • Apply resistance at the end of the range in one-joint muscles. This allows for consistency in procedure.
  • Two-joint muscles should be tested in mid-range. Length tension is more favourable in this range.
  • Aim to test muscles and muscle groups at optimal length-tension. However there are situations where a rehabilitation professional will not be able to distinguish between Grade 5 and 4 without putting the patient at a mechanical disadvantage.
  • Ensure to apply resistance slowly and gradually at the distal end of the limb with pressure opposite the line of pull of the muscle to be tested. Typically a lumbrical grip is most comfortable for the patient.

• Application of Grades
  • Always start with 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 ensuring to isolate muscle or muscle group to be tested.
  • If the patient cannot move through any part of the range of motion against gravity, re-position the patient 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
    • muscle grade allocated
    • symptoms experienced that may have impacted on 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 be utilised to 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 on muscle strength are also important in order to clinically reason why the person is experiencing loss of strength. Manual muscle testing with the Oxford Scale is the most commonly used grading scale, which 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, but can be more time consuming to complete and require access to more expensive equipment.

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 quick to complete, does not require special equipment and exhibits reasonable inter-rater reliability. However, it is a clinical skill that needs to be practised on a variety of patients to acquire the necessary skills and experience.