Principles of Manual Muscle TestingOverview
An important component of an orthopedic examination is the assessment of muscle strength. The Guide to Physical Therapist Practice lists both manual muscle testing (MMT) and dynamometry as appropriate measures of muscle strength.
Manual muscle testing is a procedure for the evaluation of the function and strength of individual muscles and muscle groups based on the effective performance of a movement in relation to the forces of gravity and manual resistance.
Dynamometry is a method of strength testing using sophisticated strength measuring devices (e.g., hand-grip, hand-held, fixed, and isokinetic dynamometry).
When performing strength testing a particular muscle or muscle group is first isolated, then an external force is applied. Resistance applied at the end of the tested range is termed a 'break test'. Resistance applied throughout the range is termed a 'make test'. The results of the strength testing differ depending on the method used. The isometric hold (break test) shows the muscle to have a higher test grade than the resistance given throughout the range (make test). The handheld devices used in dynamometry can help quantify the "breaking force" necessary to depress a limb held in a specific position by the patient.
Whichever testing method is used, the resistance should be applied and released gradually to give the patient sufficient time to offer resistance. Following the manual muscle test, the muscle tested is said to be "weak" or "strong" based upon the muscle's ability to resist the externally applied force over time. A number of grading systems exist for manual muscle testing (Table 1).
Table 1: Comparison of MMT grades
Medical Research Council
Daniels and Worthingham
Kendall and McCreary
Holds test position against maximal resistance
Holds test position against moderate to strong
Holds test position against moderate resistance
Holds test position against slight to moderate
Holds test position against slight resistance
Holds test position against gravity
Gradual release from test position
Moves through partial ROM against gravity
Moves through complete ROM gravity eliminated
and holds against pressure
Able to move through full ROM gravity eliminated
Moves through partial ROM gravity eliminated
No visible movement; palpable or observable
tendon prominence/flicker contraction
No palpable or observable muscle contraction
In the Medical Research Council scale, the grades of 0, 1, and 2 are tested in the gravity-minimized position (contraction is perpendicular to the gravitational force). All other grades are tested in the anti-gravity position. The Daniels and Worthingham grading system is considered the more functional of the three grading systems outlined in Table 1 because it tests a motion that utilizes all of the agonists and synergists involved in the motion.  The Kendall and McCreary approach is designed to test a specific muscle rather than the motion, and requires both selective recruitment of a muscle by the patient and a sound knowledge of anatomy and kinesiology on the part of the clinician to determine the correct alignment of the muscle fibers. Choosing a particular grading system is based on skill level of the clinician while ensuring consistency for each patient, so that coworkers who may be re-examining the patient are using the same testing methods.
It must be remembered that the grades obtained with MMT are largely subjective and depend on a number of factors including the effect of gravity, the manual force used by the clinician, the patient's age, the extent of the injury, and cognitive and emotional factors of both patient and clinician.[7-10]
The techniques for manual muscle testing demonstrated on the video clips are based on those described by Kendall and McCreary. Consistency with the testing procedure is critical. For example, changing the point of force application affects the length of the lever arm and therefore the muscle torque - shorter lever arms will provide higher testing scores when compared to using longer lever arms. 
It is important that the clinician provides instructions to the patient. For example, the following statements may be used:
"I'm going to test the strength of one of the muscles that bends your elbow"
"This is the movement pattern I want you to do. Do it first on your uninvolved side."
The patient and the part to be tested should be positioned comfortably on a firm surface in the correct testing position. The correct testing position ensures that the muscle fibers to be tested are correctly aligned. The patient is properly draped so that the involved body part is exposed as necessary.
Stabilization, which helps to prevent substitute movements and adds validity to the muscle test, can be provided manually or through the use of an external support such as a belt. The stabilization is applied to the proximal segment using counter pressure to the resistance.
Active Range of Motion
The patient moves through the test movement actively against gravity (if using the Daniels and Worthingham grading system, the clinician passively moves the patient's joint through the test movement). The clinician palpates the muscle for activity and also notes any adaptive shortening (slight to moderate loss of motion), substitutions or trick movements (weakness or instability), or contractures (marked loss of motion). The joint is then returned to the start position. If the patient is unable to perform the muscle action against gravity, the patient is positioned in the gravity-minimized position. Generally speaking, testing the muscle in the transverse plane can minimize the effects of gravity.
The test should be completed on the uninvolved side first to ascertain normal strength before being repeated on the involved side. The patient is instructed to complete the test movement again and then hold the segment in the desired position. The clinician alerts the patient that resistance will be applied and then applies resistance in the appropriate direction and in a smooth and gradual fashion. The proper location for the application of resistance is as far distal as possible from the axis of movement on the moving segment without crossing another joint. Resistance should never cross an intervening joint unless the integrity of the joint has been assessed as normal.  The resistance is applied in a direction opposite the muscle's rotary component and at right angles to the long axis of the segment (opposite the line of the pull of the muscle fibers). The test is repeated three times and the muscle strength grade is determined. Fatigue with three repetitions may be suggestive of nerve root compression. Note: For the sake of brevity, the video clips demonstrate the technique only once.
The clinician grades the test (Table 1).
Guide to physical therapist practice, Phys Ther, 2001. 81: p. S13-S95.
Wintz, M.N., Variations in current manual muscle testing. Phys Ther Rev, 1959. 39: p. 466-475.
Palmer, M.L. and M. Epler, Principles of Examination Techniques, in Clinical Assessment Procedures in Physical Therapy, M.L. Palmer and M. Epler, Editors. 1990, JB Lippincott: Philadelphia. p. 8-36.
Frese, E., M. Brown, and B. Norton, Clinical reliability of manual muscle testing: Middle trapezius and gluteus medius muscles. Phys Ther, 1987. 67: p. 1072-1076.
Daniels, K. and C. Worthingham, Muscle Testing Techniques of Manual Examination. 5 ed. 1986, Philadelphia: WB Saunders.
Kendall, F.P., E.K. McCreary, and P.G. Provance, Muscles: Testing and Function. 1993, Baltimore: Williams & Wilkins.
Bohannon, R.W., Measuring knee extensor muscle strength. American Journal of Physical Medicine & Rehabilitation, 2001. 80: p. 13-18.
Ottenbacher, K.J., et al., The reliability of upper- and lower-extremity strength testing in a community survey of older adults. Archives of Physical Medicine & Rehabilitation, 2002. 83: p. 1423-7.
Escolar, D.M., et al., Clinical evaluator reliability for quantitative and manual muscle testing measures of strength in children. Muscle & Nerve, 2001. 24: p. 787-93.
Bohannon, R.W. and D. Corrigan, A broad range of forces is encompassed by the maximum manual muscle test grade of five. Perceptual & Motor Skills, 2000. 90(3 Pt 1): p. 747-50.
Caruso, W. and G. Leisman, A force/displacement analysis of muscle testing. Perceptual & Motor Skills, 2000. 91: p. 683-92