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.
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.
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. 
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. 
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. 
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. 
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.
It has been proposed that intensive care unit (ICU)-acquired weakness (ICUAW) should be assessed using the sum of manual muscle strength test scores in 12 muscle groups (the sum score). This approach has been tested in patients with Guillain-Barré syndrome, yet little is known about the feasibility or test characteristics in other critically ill patients. We studied the feasibility and interobserver agreement of this sum score in a mixed cohort of critically ill and injured patients.
Manual muscle testing (MMT) during critical illness was not possible for most patients because of coma, delirium and/or injury. Among patients who were able to participate in testing, we found that interobserver agreement regarding ICUAW was good, particularly when evaluated after ICU discharge. MMT is insufficient for early detection of ICU-acquired neuromuscular dysfunction in most patients and may be unreliable during critical illness.
Systematic strength assessment and the definition of ICUAW according to the MRC sum score is becoming more common in research  and has been recommended for both research and clinical practice . However, little is known about the feasibility or test characteristics of manual muscle testing (MMT) or about ICUAW as a dichotomous diagnosis on the basis of the MRC sum score for the general population of patients with critical illness. There are two groups for whom studies of interobserver agreement of MRC sum scores have been described: patients with Guillain-Barré syndrome  and ICU survivors after hospital discharge . Compared with these groups, ICU general population patients are less likely to be able to cooperate with volitional strength assessment and more likely to have limited access to their extremities because of trauma, burns and treatment involving medical devices. We sought to determine the feasibility of assessment and interobserver agreement regarding the diagnosis of ICUAW and the MRC sum score in a mixed cohort of critically ill and injured patients.
Critically ill patients were consecutively screened for eligibility after 48 hours in the ICU. Inclusion criteria included age at least 18 years, at least 3 days of mechanical ventilation for acute respiratory failure and the expectation that the patient would be able to follow complex commands. We excluded patients with spinal cord injury, stroke, injury preventing the evaluation of six or more muscle groups, inability to follow complex commands during the follow-up period, inability to understand English and inability to provide informed consent.
Eligible patients were screened 5 days each week for attention and comprehension on the basis of their responses to five orders as described by De Jonghe et al. . Screening was coordinated with daily interruption of sedation according to institutional protocol. Once the patient was able to follow at least three orders consistently, two observers performed the structured MMT: BKL, a senior medical resident, and CLH, an attending critical care physician. Prior to this study, both observers completed multistep training in performance of MMT that included the creation of a detailed MMT instruction manual, didactic teaching of each other and other healthcare professionals and supervised practice in and out of the ICU setting (with a standardized patient as well as practice patients in the ICU). The order of observer assessment was random (determined by coin flip). Examinations were performed independently within 30 minutes of each other. The second observer was blinded to the results of the first observer's evaluation.
Each observer repeated the attention screen and then performed the 12 muscle group strength assessment: bilateral shoulder abduction, elbow flexion, wrist extension, hip flexion, knee extension and foot dorsiflexion. The patient was positioned in either the sitting or supine position, depending on the patient's clinical situation. Strength in each muscle group was scored according to the six-point MRC system, in which a score of 0 was no contraction, 1 was a flicker of contraction, 2 was active movement with gravity eliminated, 3 was active movement against gravity, 4 was active movement against gravity and resistance and 5 was normal power . If the patient would not or could not perform the test for an individual muscle group, no score was recorded and data were indicated as missing. 153554b96e