Strength Testing in Pain Assessment
Assessment of motor strength is an important concern in pain evaluation. Weakness may be one of a claimant’s more prominent complaints. The current standards published in the American Medical Association Guides to the Evaluation of Permanent Impairment (the Guides) are the most commonly used source for rating pain and disability. The Guides, Fifth Edition,1 continues to rely on manual muscle testing (MMT) in assigning ratings in disorders involving the peripheral nervous system, neuromuscular junction, and muscular system (section 13.9), the upper extremities (chapter 6), and the lower extremities (chapter 17). However, there are a multitude of reasons why MMT is a poor method of strength measurement, and its use may result in false impairment ratings. Alternatives to MMT should be considered. These include handheld dynamometry, cable tensiometry, and computerized isokinetic dynamometry. In the evaluation of impairment of the extremities, the AMA Guides use range of motion testing. In contrast to muscle strength testing, which has continued to use techniques from the previous century, there has been progress in the scientific determination of range of motion (see Figure 3). Visual methods gave way to goniometers. At the present time, inclinometers are the only recognized reliable method spinal range of motion measurement.
Sample cases of erroneous evaluation when utilizing manual muscle testing are presented as case studies below.
Case Study #1
Mrs. X, a 35-year old female, injured her arm and neck in 1988. She has had problems with her neck and left arm since the incident occurred. An MRI revealed that Mrs. X had bulging discs in the cervical spine. EMG studies were negative. While seeking treatment, Mrs. X was evaluated by a number of physicians who determined that there was weakness of the left arm. She was then sent for an Independent Medical Examination (IME) which was performed by a neurologist. By using MMT, this specialist found no weakness in the left arm and hand, and determined that there was no objective evidence of impairment. It was asserted that the weakness documented by five other physicians over the previous decade was nonorganic. However, no objective basis for this conclusion was given. It was suggested that she seek a second opinion. In using more quantitative, objective testing—by means of a dynamometer—there was ample evidence of decreased strength and sensation in the left arm and hand. Based on this loss of strength, an appropriate rating was opined.
Case Study #2
Mr. Y, a 42-year old male, had undergone a partial left medial meniscectomy. A year after surgery he complained of weakness of the left leg. He was sent for an IME doctor who opined that he had “give away weakness” of the left leg, and assigned a 1% impairment to the lower extremity based on Table 64 of the AMA Guides, Fourth Edition.2 He was sent for another IME. By using a strain gauge dynamometer to evaluate Mr. Y’s leg strength, it was found that the force generated by his left quadriceps muscle was 30 lbs. These measurements were reproducible and the coefficient of variation was less than 10%. The right quadriceps muscle was 50 lbs. He was assigned a 5% whole person impairment based on Table 39 (on page 77) of the Fourth Edition.
Review of the Literature
The method of manual muscle testing was initially devised by Lovett3 in 1912. MMT assigns a number on an ordinal scale, with a corresponding verbal descriptor as a measure of strength. The MMT grades are as follows: Five (5) is “normal” or full motion of the joint upon which the muscle of interest acts with full resistance. Four (4) is “good,” or full joint motion against gravity with partial resistance. Three (3) is “fair,” or full motion against gravity only. Two (2) is “poor,” or full motion possible, but only if gravity is eliminated by testing in an entirely horizontal motion. One (1) is “trace, or evidence of muscle contraction, but with no detectable motion. Zero (0) is no detectable muscle contraction. Grades zero to three are totally objective, as scoring merely requires observation by the examiner without active participation. However, making the distinction between grades four and five is totally subjective. In routine disability determination, we are primarily dealing with grades four and five only, and thus the degree of subjectivity is quite influential.
In 1939, Kendall and Kendall4 proposed more precise numerical equivalents of the five strength grades with values of 0, 25, 50, 75, and 100 percent of normal strength for grades zero through five, respectively. This system of percentages is still used in the AMA Guides1 with some modification. The Guides speak in terms of “percent motor deficit,” where strength ranges from Class 1 (normal), or 0% motor deficit, to Class 6 (the equivalent of Kendall grade zero), or 100% motor deficit. Also, the Guides allow for ranges of percentages within each class, so that Class 5 (the equivalent of grade 4) can be scored anywhere in the range of 1% to 25%. The Guides note that grade 4 covers the wide range of minimal weakness to what is ordinarily considered severe weakness, when only minimal resistance can be overcome. The only guidance regarding the method of scoring within each grade is “The examiner must use clinical judgment to estimate the appropriate percentage …”1
Manual muscle testing has been shown to be an unreliable method for assessing strength in several clinical studies. Beasley5 found that skilled examiners performing MMT often rated strength as normal in patients who had as much as 50 percent strength loss, as measured by quantitative testing. Krebs6 found that manual muscle testing was unable to detect weakness associated with femoral neuropathy when the strength deficit was less than 50% on quantitative testing. Frese et al.7 studied interrater consistency (the extent to which two or more examiners agree) in MMT performed on 110 patients, each of which was examined by two of eleven participating physical therapists. They found that in four muscles tested, the two examiners agreed on the grade assigned only 28% to 47% of the time.