Muscle Strength Testing

Original Editor - The Open Physio project.

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Introduction

An assessment of muscle strength is typically performed as part of a patient's objective assessment and is an important component of the physical exam that can reveal information about neurologic deficits. It is used to evaluate weakness and can be effective in differentiating true weakness from imbalance or poor endurance. It may be referred to as motor testing, muscle strength grading, manual muscle testing, or any other synonyms. Muscle strength can be assessed by a number of methods-manually, functionally, or mechanically. Strength depends on the combination of morphological and neural factors including muscle cross-sectional area and architecture, musculotendinous stiffness, motor unit recruitment, rate coding, motor unit synchronization, and neuromuscular inhibition

Function

The function of muscle strength testing is to evaluate the complaint of weakness, often when there is a suspected neurologic disease or muscle imbalance/weakness. It is an important part of the assessment in many client groups including

• patients with stroke, brain injury, spinal cord injury, neuropathy, amyotrophic lateral sclerosis, older people group and a host of other neurologic problems.
• rehabilitation after sporting injuries eg ACL repair
• after fractures and joint replacements eg TKR
• gait and balance problems in the older adult
• falls risk assessment

Muscle Interactions and Joint Dynamics in Limb Movement

The coordinated contraction of agonist and antagonist muscle groups is essential for generating movement around limb joints. During any movement, the muscle antagonists undergo simultaneous changes in length, moving in opposite directions. The dynamic properties of these muscles are significantly influenced by the direction of length change, leading to complex interactions that drive joint dynamics. Understanding these intricate muscle interactions is crucial for comprehending the complexities of limb movement and joint dynamics. When examining a group of muscle actions, it's important to differentiate between the agonist, which creates the movement, and the antagonist, which relaxes or decreases its tone to facilitate the movement created by the agonist.

The synergist muscle contracts in coordination with the agonist to produce the desired movement. An illustrative example of this dynamic interplay can be observed in elbow flexion, where the biceps brachii acts as the agonist, driving the primary movement, while the triceps brachii functions as the antagonist, providing the necessary resistance. In this scenario, the brachialis muscle acts as the synergist, working in concert with the agonist (biceps brachii) to facilitate and optimise the elbow flexion movement. This coordinated effort among the agonist, antagonist, and synergist muscles exemplifies the intricate and synchronised nature of muscle actions in generating controlled and purposeful movements.

The Oxford Scale

The most commonly accepted method of evaluating muscle strength is the Oxford Scale (AKA Medical Research Council Manual Muscle Testing scale). This method involves testing key muscles from the upper and lower extremities against the examiner’s resistance and grading the patient’s strength on a 0 to 5 scale accordingly:

1. Flicker of movement
2. Through full range actively with gravity counterbalanced
3. Through full range actively against gravity
4. Through full range actively against some resistance
5. Through full range actively against strong resistance

Commonly tested muscles include the shoulder abductors, elbow flexors, elbow extensors, wrist extensors, finger flexors, hand intrinsic, hip flexors, knee extensors, dorsiflexors, great toe extensor, and plantar flexors. These muscle groups are commonly chosen, so that important spinal nerve roots are assessed systematically eg testing the strength of the elbow flexors, elbow extensors, wrist extensors, finger flexors, and hand intrinsic allow for a methodical evaluation of the C5 to T1 nerve roots.

This short mute video is a good introduction.

There are a number of limitations to the usefulness of the Oxford scale. These include:

• Poor functional relevance;
• Non-linearity i.e. the difference between grades 3 and 4 is not necessarily the same as the difference between grades 4 and 5;
• A patient's variability over time i.e. alternating between grades due to fatigue;
• Intra-rater reliability;
• Only assesses muscles when contracting concentrically;
• The difficulty of applying the Oxford Scale to all patient's in clinical practice (so that strength is rarely assessed throughout the full range as many patients assessed by physiotherapists do not possess full range due to their respective pathology).

Due to these shortcomings, physiotherapists commonly use modified versions of the Oxford scale in clinical practice.

Performing Manual Muscle Tests

The following links demonstrate Manual Muscle Testing of specific joints and movements:

Upper Extremities	Lower Extremities
Shoulder Flexion	Hip Flexion
Shoulder Extension	Hip Extension
Shoulder Abduction	Hip Abduction
Shoulder Horizontal Adduction	Hip Adduction
Scapula Elevation	Hip External Rotation
Scapular Retraction/ Adduction	Hip Internal Rotation
Elbow Flexion	Knee Flexion
Elbow Extension	Knee Extension
Wrist Flexion	Plantarflexion
Wrist Extension	Dorsiflexion
	Ankle Eversion
	Ankle Inversion

Dynamometer

Distal strength can be semiquantitatively measured with a handgrip ergometer (or with an inflated BP cuff squeezed by the patient) to record grip strength. Requires specialized equipment, most commonly dynamometers. Dynamometry is a more precise measurement of the force that a muscle can exert and can allow for differences in strength to be recorded over time. Expensive versions exist as do cheap versions as found on internet searches.

Functional Testing

Often provides a better picture of the relationship between strength and disability. As the patient does various manoeuvres, deficiencies are noted and quantified as much as possible (eg 30 seconds sit to stand test, TUG.).

• Rising from a chair or stepping onto a chair tests proximal leg strength, walking on the heels and on tiptoe tests distal strength.
• Pushing with the arms to get out of a chair indicates quadriceps weakness.
• Swinging the body to move the arms indicates shoulder girdle weakness.
• Rising from the supine position by turning prone, kneeling, and using the hands to climb up the thighs and slowly push erect (Gowers sign and seen in Duchenne gait) suggests pelvic girdle weakness.

Clinical Significance

Muscle strength testing can help diagnose many problems in which weakness plays a role. Careful technique is important for ensuring valid and reproducible results.

• The Oxford Scale is commonly accepted and does not require special equipment, and demonstrates reasonable interrater reliability. More precise methods of measurement, such as hand-grip dynamometry, are less subjective and provide a quantifiable measurement that can be tracked over time. However, a cross-sectional study (2020) shows a lower correlation between the handgrip strength and standard strength measures of the lower limbs (hip, knee, and ankle extensor/flexor muscles), and functional capacity in older women.
• Functional assessment of strength focuses on how independently patients are able to perform their activities of daily living and whether strength is a limiting factor.

• Effect of Protein supplements on muscle-For untrained individuals, consuming supplemental protein likely has no impact on lean mass and muscle strength during the initial weeks of resistance training. Protein supplementation may accelerate gains in both aerobic and anaerobic power as the frequency, duration, volume of resistance training increase.