Dominic Stone
Muscle performance is a topic of interest to many, from students wanting to improve their fitness to athletes looking to enhance their athletic abilities. The three key factors influencing muscle performance are strength, power, and endurance. Muscle strength is determined by a combination of physiological, neurological, and mechanical factors. Physiological strength depends on muscle size, cross-sectional area, and response to training, while neurological strength assesses the signal strength that triggers muscle contraction. Mechanical strength refers to a muscle's pulling force and how it can be manipulated using bones and joints as levers. Muscle strength testing is a common method for evaluating muscle performance and can be done manually, functionally, or mechanically. Dynamometry, for instance, uses machines to measure force production, while handheld isometric dynamometry (HHD) is a more recent, highly accurate, and reliable standard.
| Characteristics | Values |
|---|---|
| Muscle strength testing | To evaluate weakness, often when there is a suspected neurological disease or muscle imbalance/weakness |
| Manual muscle testing (MMT) | A widespread, nearly universal test used by physical therapists to determine muscle strength and weakness in their patients |
| Dynamometry | Using large, expensive machines to measure force production |
| Handheld isometric dynamometry (HHD) | Handheld devices that are very accurate and have high levels of inter-rater and intra-rater reliability |
| Distal strength | Can be semi-quantitatively measured with a handgrip ergometer or with an inflated BP cuff squeezed by the patient |
| Muscle quality (MQ) | Calculated by measuring strength per unit of lean mass |
| Muscle power | How quickly the muscles can transfer energy |
| Muscle endurance | Ability to perform over extended periods without fatiguing |
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Manual muscle testing
During MMT, each muscle group is tested bilaterally. The examiner stands on the side of the patient being tested, who is sitting upright and positioned to allow full movement of the joint against gravity. The examiner then demonstrates the desired movement against gravity, and the patient is asked to repeat the motion. If the patient can move through the desired range of motion against gravity, the examiner attempts to apply resistance in the testing position, instructing the patient to "hold it" and not let them push it down or bend it. The patient's strength is then graded on a scale of 3 to 5, depending on the amount of resistance tolerated. If the patient cannot move against gravity, the extremity is repositioned to allow movement with gravity eliminated.
MMT can also be used to test distal strength with a handgrip ergometer or an inflated BP cuff squeezed by the patient. This requires specialised equipment, most commonly a dynamometer, which provides a precise measurement of the force a muscle can exert and allows for differences in strength to be recorded over time.
The most commonly accepted method of evaluating muscle strength is the Oxford Scale (AKA 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 to 5 scale. Commonly tested muscles include the shoulder abductors, elbow flexors, wrist extensors, and hip flexors.
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Dynamometry
There are two main types of dynamometry: handheld dynamometry (HHD) and fixed dynamometry. HHD is a portable device that is easy to use for patients and convenient for clinicians. It is held by the therapist to derive an objective measurement of force. The patient exerts a maximum isometric force while the dynamometer is held stationary. The therapist can match the force and provide verbal commands to encourage the patient to push as hard as possible. Each trial is performed over a 3-4 second hold, with 2-3 trials performed. HHD has been shown to have high levels of inter-rater and intra-rater reliability, with above 90% agreement between two practitioners testing the same patient, as well as one practitioner testing the same patient on two different occasions. However, HHD may be less reliable when used for testing certain body areas, and repeatability can vary when compared to fixed assessment devices.
Fixed dynamometry, on the other hand, is a complex piece of equipment rarely seen outside of a research setting. It is typically a large, expensive machine used to measure force production. Common makes include BioDex or KinCom isokinetic dynamometers. Fixed devices tend to be more reliable and valid when determining a patient's strength compared to handheld tools.
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Muscle strength grading
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 is also known as motor testing, muscle strength grading, manual muscle testing, or other synonyms. The muscle strength 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. 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. However, functional strength tests do not provide a grade or numeric quantity that can be tracked over time to gauge improvement.
Muscle strength can be assessed by a number of methods: manually, functionally, or mechanically. Distal strength can be semiquantitatively measured with a handgrip ergometer (or with an inflated BP cuff squeezed by the patient) to record grip strength. This requires specialised 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. The Oxford Scale is also commonly accepted and does not require special equipment, and demonstrates reasonable inter-rater reliability.
The 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. 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.
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Muscle quality
MQ encompasses the ability of skeletal muscle to perform various functions effectively, including force production, contraction and relaxation, metabolism, substrate turnover and storage, heat generation, myokine production, and electrical conduction. The term "muscle quality" is ambiguous due to the lack of a universal definition, and technical challenges arise when measuring muscle quantity and quality in a clinical setting. However, MQ is an essential component of human physiology, referring to the integration of functional and structural characteristics of the muscle. It includes factors such as strength, power, endurance, and flexibility, which collectively determine an individual's physical capacity.
From a morphological perspective, muscle quality considers muscle composition, architectural properties, and structural properties. It involves the relationship between contractile tissue, which generates force, and non-contractile tissue, which includes connective tissue and fat. The ratio of contractile to non-contractile tissue influences muscle strength and quality, with higher proportions of contractile tissue contributing to better muscle quality. Additionally, biological and adaptive changes associated with aging must be considered when interpreting muscle quality assessments in clinical or research contexts.
Assessing muscle quality is valuable for optimizing physical performance and preventing injuries. It is important to recognize that muscle size alone does not determine strength; instead, it is the interplay of muscle quantity and quality that influences overall performance. Advancements in technology have made it possible for individuals to monitor their muscle quality at home using devices like the Tanita RD-953 Body Composition Monitor. Furthermore, clinical assessments of muscle strength, such as dynamometry and manual muscle testing, play a crucial role in diagnosing neurological diseases, muscle imbalances, and weaknesses. By understanding and improving muscle quality, individuals can enhance their physical capabilities and overall muscle health.
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Muscle power
There are various methods to assess muscle power, including manual, functional, and mechanical testing. Manual muscle testing can be done with a handgrip ergometer or an inflated BP cuff squeezed by the patient to record grip strength. Functional testing can be done through a repeated sit-to-stand (STS) task, which is chosen for its similarity to many daily activities. Mechanical testing can be done through dynamometry, which is a precise measurement of the force a muscle can exert.
Despite the existence of these methods, there is still a lack of consensus on the ideal testing protocols for muscle power, especially in older adults. Further research is needed to fully understand the mechanisms involved in the age-related loss of muscle power and to develop optimal training programs.
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Frequently asked questions
The three major factors that affect muscle performance are strength, power, and endurance.
Muscle strength can be divided into three types: physiological, neurological, and mechanical. Physiological strength depends on factors such as muscle size and the cross-sectional area of the muscle. 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 changed using bones and joints as levers.
Muscle strength can be assessed manually, functionally, or mechanically. Manual muscle testing (MMT) is a widespread test used by physical therapists to determine muscle strength and weakness in their patients. Mechanical tests include dynamometry, which uses machines to measure force production.
The Timed Up and Go test and the Community Balance and Mobility (CBM) scale are commonly used tests of balance and muscle strength.
Muscle performance can be improved through strength training, which involves performing multiple sets with ample rest time in between. The weight should be set so that the muscle fatigues/fails between six and twelve repetitions.
