Muscle Strength Scale in Physical Therapy

Muscle strength grading is a system used by physical therapists (PTs) to determine how a muscle or group of muscles is working. Your PT may test your muscle strength during your initial evaluation and assessment and at regular intervals thereafter to determine your progress during rehabilitation.

Dynamometer Hand Grip Strength Test
 BanksPhotos / E+ / Getty Images

Measuring muscle strength may be an important component of your rehabilitation plan, particularly if the PT feels that muscle weakness is contributing to your pain and limited mobility. There are several ways to measure muscle strength, that provide both objective and observational results.

Defining Strength

Muscle strength is defined as the ability of a muscle to contract and produce force in a single effort. Muscle strength differs from muscle endurance, the latter of which is defined by the amount of time a muscle can perform a specific task before failure. With that said, muscle strength and endurance are both needed to achieve optimal physical function and mobility.

There are many things that can limit muscle strength, including:

If referred to a PT, muscle strength will almost invariably be considered whatever condition you have. This not only involves evaluating larger muscles like the bicep or hamstrings but also smaller muscles like those of the wrist and hand if undergoing carpal tunnel surgery.

Other tests commonly performed during the evaluation will measure your flexibility, gait, range of motion, balance, coordination, and mobility. These baseline results help the PT track your progress during rehab.

There are two methods used by PTs to measure muscle strength, called manual muscle testing and dynamometric testing.

Manual Muscle Testing

Manual muscle testing (MMT) is the most popular way to test muscle strength. For this test, the PT will push on your body in specific directions while you resist the pressure. A score or grade is then assigned, depending on how much you were able to resist the pressure.

Muscle strength is measured with an MMT on a five-point scale:

  • 0/5: A 0/5 grade means that you are unable to create any noticeable contraction in a specific muscle. This can occur when a muscle is paralyzed, such as after a stroke, spinal cord injury, or cervical or lumbar radiculopathy. Sometimes, pain can prevent a muscle from contracting at all.
  • 1/5: A grade of 1/5 occurs when muscle contraction is noted but no movement occurs. In this case, the muscle is not strong enough to lift the particular body part against gravity or move it when in a gravity-reduced position. A small contraction may be detected with palpation (physical touch) but not enough to effect movement.
  • 2/5: A 2/5 grade is assigned when a muscle can contract but cannot move the body part fully against gravity. However, when gravity is reduced or eliminated with a change in body position, the body part will be able to move through its full range of motion.
  • 3/5: With a 3/5 grade, you are able to fully contract a muscle and body part through its full range of motion against the force of gravity. But when resistance is applied, the muscle is unable to maintain the contraction.
  • 4/5: A 4/5 grade indicates that the muscle yields to maximum resistance. The muscle is able to contract and provide resistance, but, when maximum resistance is exerted, the muscle is unable to maintain the contraction.
  • 5/5: A 5/5 grade means that the muscle is functioning normally and is able to maintain its position even when maximum resistance is applied.

Although the manual muscle test relies on subjective observation, the criteria and definitions are considered distinct enough to yield relatively reliable results.

Occasionally, a PT may grade your strength in half increments, using the + or - sign. For example, a grade of 4+/5 indicates that your muscle yielded to maximum resistance but was able to provide some resistance during the test. A 4-/5 grade means that your muscle was not on the verge of collapse during testing.

MMT is popular because it is inexpensive and readily available. It is easy to perform and does not require any special equipment. With that said, the method is less reliable within the good (4/5) to normal range (5/5), with results often varying considerably between one PT and the next.

Dynamometric Testing

Another method of measuring muscle strength is called dynamometry and involves a handheld device known as a dynamometer. Dynanometric testing evaluates the length-tension relationship of the muscle, meaning now much tension a muscle exerts during an isometric contraction in relation to the length of a muscle.

The test is performed by placing the body part in a position where it is not influenced by gravity. After the dynamometer is positioned against the muscle, the individual exerts pressure against it for several seconds. A reading in pounds or kilograms is then displayed. Some devices are digital, and others are spring-loaded.

To quantify your relative muscle strength, the dynamometric reading is compared to the reference (expected) values for a person of your age and sex. These readings are used to track performance as you undergo physical therapy.

In addition to standard isokinetic dynamometers used to measure key muscle groups, such as those of the elbow, hip, shoulder, or knee, there are handheld dynamometers that can measure grip strength and even pinch strength.

A Word From Verywell

If you are experiencing muscle weakness resulting in the loss of functional mobility, speak with your doctor about exploring the possible causes. You may be referred to an orthopedic surgeon if the cause is believed to be musculoskeletal or a neurologist if the muscle weakness is believed caused by a nerve disorder.

It is only by getting a proper diagnosis that a physical therapist can perform the appropriate evaluation and draw up an effective rehabilitation program.

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