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Muscle strength is defined as the maximum force a muscle or muscle group can generate at a specified velocity. It is the ability of skeletal muscle to develop force to provide stability and mobility within the musculoskeletal system. Muscle strength testing is an important component of the physical exam that can reveal information about neurological deficits. The most commonly accepted method of evaluating muscle strength is the Medical Research Council Manual Muscle Testing scale, which involves testing key muscles from the upper and lower extremities against the examiner's resistance and grading the patient's strength on a 0-5 scale.
| Characteristics | Values |
|---|---|
| Definition | Muscle strength is the maximal force a muscle or muscle group can generate at a specified or determined velocity |
| Factors | Morphological and neural factors, including muscle contraction |
| Muscle Contraction | Occurs when tension-generating sites within the muscle cells are activated |
| Type of Contraction | Defined by changes in the length of the muscle during contraction |
| Isometric Contraction | No movement, and no external work is done by the muscle |
| Signal | A signal is generated in the brain and travels through nerve cells in the brain stem and spinal cord, to the peripheral nerves and the muscle |
| Pain | Affects muscle force production |
| Muscle Strength Testing | Evaluates the complaint of weakness, especially with suspected neurologic disease or muscle imbalance/weakness |
| Muscle Groups | Elbow flexors, extensors, finger flexors, hand intrinsics, hip flexors, knee extensors, toe extensors, etc. |
| Functional Testing | Provides a better picture of the relationship between strength and disability |
| Muscle Strength Grading | Scored on a scale of 0-5 |
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What You'll Learn
Muscle strength testing
The function of muscle strength testing is to evaluate the complaint of weakness, often when there is a suspected neurological disease. It is an integral part of the neurological exam, especially for patients with stroke, brain injury, spinal cord injury, neuropathy, amyotrophic lateral sclerosis, and other neurological problems. The most commonly accepted method of evaluating muscle strength is the Medical Research Council Manual Muscle Testing scale, which involves testing key muscles from the upper and lower extremities against the examiner's resistance. The patient's strength is then graded on a scale from 0 to 5. Commonly tested muscles include the shoulder abductors, elbow flexors, wrist extensors, finger flexors, hip flexors, knee extensors, and plantar flexors.
To ensure valid results, proper technique must be employed during testing. Restrictive clothing should be removed so that the examiner can visualise the muscles being tested and observe for muscle twitch. The examiner should also stabilise the joint and ensure that only the muscle being tested is providing movement. Muscles should first be tested with gravity eliminated, and if the patient is unable to engage the muscle, the examiner should place a hand on the muscle and ask the patient to contract again to feel for a muscle twitch. The test should then be repeated against gravity, and resistance should be added to challenge the patient.
Functional testing can also provide a better picture of the relationship between strength and disability. As the patient performs various maneuvers, deficiencies are noted and quantified. For example, rising from a squatting position or stepping onto a chair tests proximal leg strength, while walking on the heels and tiptoes tests distal strength. Pushing with the arms to get out of a chair indicates quadriceps weakness, and swinging the body to move the arms suggests shoulder girdle weakness.
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Manual muscle testing
MMT involves testing key muscles from the upper and lower extremities against the examiner's resistance and grading the patient's strength on a standardised scale. Commonly tested muscles include the shoulder abductors, elbow flexors, elbow extensors, wrist extensors, finger flexors, hand intrinsics, hip flexors, knee extensors, dorsiflexors, great toe extensors, and plantar flexors. These muscle groups are chosen to assess important spinal nerve roots systematically.
During MMT, the examiner stands to the side of the patient, who is sitting upright and positioned to allow full movement of the joint against gravity. The examiner demonstrates the desired movement, then requests the patient to repeat it. If the patient can move through the desired range of motion against gravity, the examiner attempts to apply resistance while instructing the patient to resist the movement. The examiner grades the patient's strength based on their ability to tolerate resistance.
MMT can be performed with the patient sitting upright in bed and following simple commands. It is a reliable method for assessing strength in patients with acute respiratory distress syndrome (ARDS) and other critical illnesses, who often experience generalised weakness, reduced exercise tolerance, and persistent nerve and muscle impairments.
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Functional testing
Functional tests involve testing essential movements rather than quantifiable strength. Examples of such tests include squatting, rising from a chair, or stepping onto a chair, which tests proximal leg strength. Walking on the heels and tiptoes tests distal strength. Pushing with the arms to get out of a chair indicates quadriceps weakness, while swinging the body to move the arms suggests shoulder girdle weakness. These tests provide valuable information about whether the patient has the strength to perform daily activities, but they do not yield a grade or numeric value that can be tracked over time to gauge improvement.
When conducting functional testing, it is important to choose the muscles to be tested based on suspected diagnoses and the character of symptoms, including the exact location, time of occurrence, and any associated factors. For example, in patients with suspected deltoid weakness, the examiner observes the patient's movement to see if the shoulder and head tilt towards the weak deltoid, indicating a lack of effort. Subtle weakness may be indicated by decreased arm swing while walking, decreased spontaneous use of a limb, or impairment of fine dexterity, such as buttoning a shirt.
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Factors affecting muscle strength
Muscle strength is influenced by a combination of physiological, neurological, and mechanical factors. Physiological strength depends on muscle size, the cross-sectional area of the muscle, and responses to training. The larger the muscles, the stronger the person, which is why most men are stronger than most women. This is because muscle size is increased by the presence of testosterone, the male sex hormone. However, it is important to note that while gender influences the quantity of muscle tissue, it does not affect the quality.
Neurological strength refers to the strength of the signal that tells the muscle to contract. Mechanical strength refers to a muscle's pulling force and how those forces can be leveraged using bones and joints. The power of muscles refers to how quickly they can transfer energy when contracting or stretching to move a load. The energy for muscle contraction comes from glucose transported by the blood and deposited in muscle tissues.
Muscle strength is also influenced by the point of tendon insertion. For example, if two people have the same arm and muscle length, but one person's bicep tendon attaches to the forearm further from the elbow joint, that person has a biomechanical advantage and can lift more weight in bicep exercises.
Other factors that influence muscle strength include muscle composition and quality, bone strength, age, limb length, and psychological and cognitive factors. Muscle strength can be improved through a safe and effective strength training program, although the rate of strength and muscle gain is typically greater from ages 10 to 20, the years of rapid growth and development. It is important to vary training programs and allow for adequate rest between sessions to continue improving muscle strength over time.
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Muscle strength grading
Muscle strength is defined as the maximum force a muscle or muscle group can generate at a specified velocity. It is the ability of skeletal muscle to develop force to provide stability and mobility within the musculoskeletal system, which is necessary for functional movement.
Muscle strength testing is an important component of the physical exam that can reveal information about neurological deficits. It is used to evaluate weakness and can help differentiate between true weakness and imbalance or poor endurance. It is often referred to as muscle strength grading, motor testing, manual muscle testing, or other synonyms. The evaluation may be performed by nurses, physicians, physical therapists, occupational therapists, chiropractors, and other practitioners.
The most commonly accepted method of evaluating muscle strength is the Medical Research Council Manual Muscle Testing scale, which involves testing key muscles from the upper and lower extremities against the examiner's resistance. The patient's strength is then graded on a scale from 0 to 5. Commonly tested muscles include the shoulder abductors, elbow flexors, wrist extensors, finger flexors, and hip flexors. These muscle groups are chosen to assess important spinal nerve roots systematically.
Another approach to muscle strength testing involves testing functional movements instead of quantifiable strength. Examples of functional tests include squatting, rising from a chair, or walking on heels and tiptoes. Functional strength tests provide information on whether the patient can perform essential daily activities, but they do not provide a grade or numeric quantity that can be tracked over time.
It is important to note that muscle strength testing requires careful technique to ensure valid and reproducible results. The examiner should observe the patient's movement, stabilize the joint, and ensure that other muscles do not provide assistance. The patient's clothing should also be considered, as tight or restrictive garments may hinder the examiner's ability to visualize the muscles being tested.
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Frequently asked questions
Muscle strength is the maximal force a muscle or muscle group can generate at a specified or determined velocity. 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.
Muscle strength can be assessed in three ways: manually, functionally, or mechanically. Manual muscle testing (MMT) is a hands-on test that 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. Functional testing involves observing the patient as they perform various maneuvers, noting and quantifying any deficiencies. Mechanical testing involves measuring the force of a muscle contraction with a handheld device, such as a handgrip ergometer or a handheld dynamometer.
Muscle strength depends on a combination of morphological and neural factors. The ability to contract a muscle is dependent on a signal generated in the brain, which travels through nerve cells in the brain stem and spinal cord, to the peripheral nerves and the muscle. Pain has been shown to affect muscle force production, reducing the maximal voluntary contraction and endurance time during submaximal contractions.
Muscle strength testing is an important component of the physical exam, especially for patients with neurological disorders or injuries. It can help evaluate the complaint of weakness, differentiate true weakness from imbalance or poor endurance, and aid in the design of safe and appropriate treatment plans.
