Manual Muscle Testing (MMT Grades)

Manual muscle testing (MMT Testing) is the most common method by which clinicians evaluate muscle strength.

MMT utilizes a standardized grading system that is determined by the ability of the tested muscle act against gravity or against resistance applied by the examiner.

Manual Muscle Testing Grades (MMT Grades)

In 1916, Lovett and Martin first described the method of manual muscle testing in newborns with infantile paralysis. Since then, abundant research has been conducted regarding its various applications, including modifications of the original grading scale used to describe muscular strength.

Despite these modifications, the scale that is most widely accepted is very similar to the original proposal by Lovett and Martin and was devised by the Medical Research Council (MRC) in 1943.

The MMT Grades has six levels (0–5):

1. Grade 0: No visible or palpable contraction.
2. Grade 1: Visible or palpable contraction without motion.
3. Grade 2: Full range of motion, gravity eliminated.
4. Grade 3: Full range of motion against gravity.
5. Grade 4: Full range of motion against gravity, moderate resistance.
6. Grade 5: Full range of motion against gravity, maximal resistance.

Although the MMT grades is still widely used in clinical practice due to its low cost and rapidity, there are several limitations that must be noted:

1. The first limitation is that the MMT scale is subjective in nature and the score depends on the clinician’s judgment.
2. The second limitation of MMT is the inability of the scale to detect small, between-level differences in strength. This is largely due to the stepwise design of the scale and has spurred the development of other scales that have more diagnostic levels.
3. Third, the MMT scale has been criticized for not being capable of detecting clinically relevant differences in muscle strength.

MMT Grades was originally developed to measure strength improvements in patients treated with paralytic disorders and muscular dystrophies. Thus, the application of MMT to a variety of clinical settings is probably due to tradition rather than sound scientific rationale. As a result of the subjectivity and reported inaccuracy of MMT, many clinicians (and insurers) prefer to measure strength with more objective means that are more sensitive, such as with handheld dynamometers.

See Also: Hip Muscles Anatomy

MMT Testing Reiability

The inter- and intra-observer reliabilities of MMT in the evaluation of various pathologies resulting in muscle weakness range from 0.82 to 0.97 and 0.96 to 0.98, respectively, according to reports dating back to 1954. However, only a few studies have specifically examined the reliability of manual muscle testing for the evaluation of patients with various shoulder pathologies.

Dynamometry

A dynamometer is a device used to determine the mechanical force generated by a contracting muscle. While these measurements of force are generally given in Newtons or kilograms, torque can be calculated by simply multiplying Newtons or kilograms by the distance (in meters) between the dynamometer and the center of rotation of the involved joint.

Dynamometers first appeared in 1763 and, since then, numerous modifications have been made. Currently, dynamometers come in a large variety of shapes, sizes, and functional mechanisms that produce the desired force measurements. Isokinetic dynamometers are large machines capable of generating numerous values including peak muscular force, power, and endurance among numerous other measurements. Isokinetic testing has been used as a standard method of muscle strength test over the past 40 years since it has been found to be reliable, reproducible, and valid on numerous occasion. As a result, isokinetic devices have also been used as reference standards for the evaluation of newer devices that test muscle strength.

See Also: Grip Strength Test

Electromyography

Electromyography (EMG) has been used extensively over the past century to evaluate the utility of various manual muscle tests. An electromyogram is obtained by placing an EMG electrode on the skin over the muscle being tested (i.e., surface EMG) or, alternatively, a thin wire can be placed directly into the muscle of interest (i.e., intramuscular EMG). When the muscle is stimulated, the electrical potential that is produced by the muscle travels through the electrode and towards the connected electromyograph which interprets and displays the signal through an oscilloscope.

It is important to remember that Electromyography readouts with higher amplitude do not necessarily indicate that the muscle is generating greater force. As an example, an eccentrically contracting muscle produces similar amplitude as a concentrically contracting muscle; however, the force produced by the eccentric contraction may be much less than that produced by the concentric contraction. EMG is an important tool for the evaluation of skeletal muscle activity; however, its interpretation can be influenced by several factors that must be taken into account. Features of the surface electrode such as width, diameter, and electrical properties can influence the signal output.

In the case of surface EMG, increased distance or increased soft-tissue interposition between the surface electrode and the muscle being tested can also significantly influence signal interpretation. The primary drawback of thin-wire EMG is that the sample size is limited to the surface area of the small electrode whereas surface EMG can obtain measurements over an expanded area of muscle tissue and is also easier to implement; however, this can also introduce unwanted noise due to soft-tissue interposition and contributions from surrounding musculature. In addition, the amplitude or morphology of the EMG readout may be affected by the type of muscle being tested (fast-twitch versus slow-twitch).

See Also: Forearm Muscles Anatomy

Muscle Strength Assessment Studies

A large number of studies have evaluated the inter- and intra-rater reliability using handheld dynamometry to assess muscular strength:

A systematic review by Stark et al. identified 19 studies in which the authors compared handheld dynamometry to isokinetic muscle strength assessment. In that review, all but two studies demonstrated either good to excellent correlation with isokinetic testing or good to excellent intra-class correlation coefficient (ICCs).

In general, clinical dynamometry is performed with handheld devices due to their portability, simplicity, low cost, and reported excellent reliability and validity when compared to isokinetic dynamometry. Although there are numerous such devices that have been reported as both accurate and reliable for the measurement of muscular force, most handheld dynamometers fall into one of two categories depending on the mechanism of measurement. These include spring scale and strain gauge dynamometers.

Spring scale dynamometers work simply by measuring the deformation (lengthening) of a spring as a force is applied, this deformation distance is converted to kilograms and is based on the stiffness (spring constant) of the inserted spring. Strain gauge dynamometers are more complex and work by detecting changes in electrical signals caused by the deformation of an electrical insulator by an outside force (e.g., the force of muscle contraction).

In contrast, the strain gauge dynamometers measured forces that were much closer to the actual applied force. Hayes and Zehr evaluated the reliability of MMT, a manual spring scale dynamometer and a digital strain gauge dynamometer to measure rotator cuff strength using a random effects statistical model. In this group of patients with symptomatic rotator cuff disease, they found that the digital strain gauge dynamometer was the most reliable method of measuring rotator cuff strength.

There are several other potential limitations of digital handheld dynamometry. The first is that these handheld devices are of minimal use when testing large muscle groups that can produce a much larger force than the examiner can resist. This is particularly true for large, high-output lower extremity muscles that may overcome the strength of the examiner’s upper extremity. A second limitation is that an inability to adequately stabilize the device while the subject applies maximal force is quite difficult to achieve. As a result, handheld dynamometers placed in a fixed apparatus have gained popularity to eliminate the effect of examiner strength and stabilization on the reliability of strength measurements.

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