Introduction

Muscle strength testing 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 many other synonyms. The muscle strength evaluation may be performed by nurses, physicians, physical therapists, occupational therapists, chiropractors, and other practitioners.

Function

The function of muscle strength testing is to evaluate the complaint of weakness, often when there is a suspected neurologic disease. It is an integral part of the neurologic exam, especially for patients with stroke, brain injury, spinal cord injury, neuropathy, amyotrophic lateral sclerosis, and a host of other neurologic problems.

The most commonly accepted method of evaluating muscle strength is the 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:

• 0 No muscle activation
• 1 Trace muscle activation, such as a twitch, without achieving full range of motion
• 2 Muscle activation with gravity eliminated, achieving full range of motion
• 3 Muscle activation against gravity, full range of motion
• 4 Muscle activation against some resistance, full range of motion
• 5 Muscle activation against examiner’s full resistance, full range of motion

Commonly tested muscles include the shoulder abductors, elbow flexors, elbow extensors, wrist extensors, finger flexors, hand intrinsics, 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; however, further muscles can be tested to evaluate individual peripheral nerves. For example, testing the strength of the elbow flexors, elbow extensors, wrist extensors, finger flexors, and hand intrinsics allow for a methodical evaluation of the C5 to T1 nerve roots. However, one could more specifically test the thumb abductors to evaluate the median nerve and the abductor digiti minimi to evaluate the ulnar nerve. [1][2][3]

Issues of Concern

Proper technique must be employed during testing to ensure valid results. Tight or restrictive clothing should be removed so that the examiner can visualize the muscles being tested and observe for muscle twitch. The examiner should also stabilize the joint and ensure that other muscles do not provide assistance. Muscles should first be tested with gravity eliminated by positioning the patient, so that muscle contraction is perpendicular to gravity, such as along an examining table or bed. If the patient is unable to engage the muscle with gravity eliminated, the examiner should place a hand on the muscle and ask the patient to contract his or her muscles again. This allows the examiner to feel for a muscle twitch, even if a twitch is not visible. This observation would differentiate a score of 0 from a score of 1. When the patient demonstrates the full range of motion with gravity eliminated, the test should be repeated against gravity for the full range of motion. If this is successful, the patient should be challenged by the addition of a small degree of resistance, then maximal resistance by the examiner. The unaffected or less affected side should be tested first to gauge contralateral strength for comparison; all four limbs should be tested for completeness and to help guide the differential diagnosis based on patterns of weakness, such as upper extremity only, lower extremity only, or proximal muscles rather than distal. [2]

The Medical Research Council Manual Muscle Testing method is very common, easy to perform, and does not require any specialized equipment. Despite these advantages, it also has its limitations. Scoring is subjective based on the examiner’s perception. There is variability between examiners for the maximal resistance they are able to apply, as some examiners are stronger than others. The test does not account for musculoskeletal conditions that may make testing painful or difficult to tolerate, such as tendinopathy or arthritis. The test is dependent on patient effort, which may be poor in some patients, owing to pain, proper comprehension of instructions, psychological causes, or secondary gain. Finally, the grading system classifies strength level but does not directly quantify strength. [4]

The Alternatives to the Medical Research Council Manual Muscle Testing system aims to quantify strength directly in terms of pounds, Newtons, or other units. This requires specialized equipment, most commonly dynamometers. Dynamometry provides a more precise measurement of the force that a muscle can exert and can allow for differences in strength to be tracked over time that an examiner may not subjectively notice when using the MRC scale. Hand-grip dynamometry is a popular example, in which the patient squeezes a handle that records the force being applied. Limitations of dynamometry include the need for costly or specialized equipment, limited muscle groups that can be tested, and limited availability of testing equipment to clinicians across specialties or settings. [5]

Another approach to muscle strength testing involves testing functional movements instead of quantifiable strength. Examples of functional tests include squatting or rising from a chair. Functional strength tests provide information about whether the patient is strong enough to perform essential daily activities, a limitation of both the Medical Research Council Manual Muscle Testing method and dynamometry. However, functional strength tests do not provide a grade or numeric quantity that can be tracked over time to gauge improvement. [5]

Clinical Significance

Muscle strength testing can help a practitioner diagnose neurologic problems in which weakness is a prominent deficit. The muscles targeted for testing should be methodically chosen based on suspected diagnoses and for complete characterization of the strength deficit in various limbs. Careful technique is important for ensuring valid and reproducible results. The Medical Research Council Manual Muscle Testing method is commonly accepted, performed across several disciplines, 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. 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.

In patients with fictitious or hysterical weakness, the initial resistance to movement may appear normal, followed by a sudden giving away. Or the individual may not be using the adjacent or other supportive muscles in an appropriate fashion.

Other Issues

Limitations of the Grading Scale

• Muscle being tested may have no clinical relevance
• There may be individual variation in reporting
• Only assesses muscles which are contracting in a concentric manner
• The scale may not be applicable in all patients

Enhancing Healthcare Team Outcomes

Muscle strength testing 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 many other synonyms. The muscle strength evaluation may be performed by nurses, physicians, physical therapists, occupational therapists, chiropractors, and other practitioners.