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7 Articles in Volume 6, Issue #8
Hyoid Bone Syndrome
Minimally Invasive Interventional Spine Treatment – Part 1
Mobile MRI—Imaging on Wheels
On the Role of Primary Care Within a System of Integrative Multi-disciplinary Pain Management
Pediatric Headaches
Practical Applications of Low Level Laser Therapy
Strength Testing in Pain Assessment

Strength Testing in Pain Assessment

Quantitative, objective measurement of muscle strength in the extremities is suggested as a means of improving the validity and reliability of strength measurements 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

Figure 1. . Manual muscle testing of the biceps. From Aids to the Investigation of Peripheral Nerve Injuries, Medical Research Council, London: Her Majesty’s Stationery Office, 1943.

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.

As an examiner-dependent subjective evaluation, MMT is fraught with problems. There is more than one way of performing MMT. Therapists may use either the standard techniques of Daniels and Worthingham8 or those of Kendall and McCreary.9 There are “make” tests and “break” tests. In a “make” test, the examiner applies resistance to the tested muscle which is equal or nearly equal to the force generated by the subject, beginning early or in the middle of the range of motion of the joint acted on by the respective muscle. The examiner instructs the subject to “Push against me as hard as you can.” The examiner then makes a subjective judgment of the amount of resistance required, relative to “full resistance,” to stop or slow the joint motion, based on his/her clinical experience. The examiner uses an internal basis for comparing test results, adjusting for age, sex, and the particular muscle being tested. This judgment can be quite variable. In a “break” test the examiner places the joint in a starting position, then instructs the subject to “Hold, hold, don’t let me move you” while applying a force that overcomes (“breaks”) the force generated by the subject. A subjective judgment is then made as to whether the amount of force required to break is normal or less than normal. This test is generally done with the joint position either at neutral or at the end of its range of motion. This reaction by the subject has been termed “give away.”10 It is ironic that this same term is more commonly used with a quite different connotation. Whenever an examiner is confronted with a patient with some questionable degree of weakness, there is a tendency to state that there is so-called “give away weakness,” implying that the claimant is somehow faking. However, all weakness—whether feigned or not—may be defined “give away weakness” when the weakness is evaluated with a break test.

Nicholas et al.11 have shown that examiners unknowingly interpret muscle strength based more on the total amount of effort they exert (which is influenced by the length of time that the force is exerted) rather than on the actual peak force. Another source of error is the variability in strength between different examiners. Examiners with relatively weak upper limbs often will be unable to overcome contractions of muscle groups in the subject’s lower extremity, while other examiners can break this force. In this situation, a subject will be considered as having 0% motor deficit when, in fact, there may be true weakness. Another error may occur if the examiner applies force too quickly. In this situation, a subject may “break” but the same force, applied more slowly, would not overcome the subject. This would result in an underestimation of strength. Wakim et al.12 observed that the degree of stabilization of the patient is an important factor. If a muscle is not adequately stabilized, then it will be incapable of generating its maximal force of contraction. Thus the strength of inadequately stabilized muscles will be underestimated in this situation. Stabilization can be affected by the position of the subject, the ability of the subject to contribute to stability by using stabilizing muscles, the firmness of the surface on which the patient is sitting or laying, and the efforts of the examiner. For example, the knee extensors cannot generate as much force when the subject is seated on a soft cushion. Alternatively, inadequate fixation of the trunk can also cause overestimation of strength, as stronger proximal muscles may substitute for weaker distal muscles during a test. The joint position at the beginning of the test can also have a significant impact on the estimation of strength, since muscle strength changes greatly as joint position changes. Varying joint positions results in changes in the mechanical advantage of the skeletal lever system, and also changes the length of the muscle, thereby changing its position in the length-tension relationship. The problems of using MMT led Sapega to state: “It is probably not an exaggeration to compare the manual muscle testing of muscular strength to auscultation of the heart without a stethoscope.”13

In handheld dynamometry, the maximal isometric force generated by the subject is transmitted via an electronic or mechanical transducer and is then quantified in a digital or analog display. Examples are the Nicholas Manual Muscle Tester, using an electronic transducer, and the Jamar dynamometer, using a mechanical, hydraulic transducer. In computerized isokinetic dynamometry, the maximal isometric rotational force generated by the subject is measured throughout the joint range of motion, rather than at a standard angle. The joint angle varies at a fixed speed. The results of computerized isokinetic dynamometry are graphic, rather than a single number.

The coefficient of variation (CV) of repeated tests in one subject has been used to assess the reliability of subject effort, i.e. whether the effort is sincere or feigned. In general, CVs for true maximal effort tend to be lower than CVs if weakness is feigned. However Dvir14 has shown that CVs cannot be used to detect malingering, because no cutoff point for CV can be identified which separates true maximal effort from feigned effort. The two distributions of CV values for feigned and maximal effort have too great a degree of overlap. Furthermore, Simonsen15 found that the average CVs in sincere effort varied across diagnoses. Nevertheless, very low values of CV can exclude malingering.

Figure 2. Force time graphs of biceps strength test using a computerized device.

Dynamometry has been demonstrated to be reliable, both when comparing multiple measurements made by one examiner, and also when comparing multiple measurements among different examiners. It has been demonstrated to be reliable in both healthy subjects and in subjects with disabilities. It has also been demonstrated to be reliable when using either make tests or break tests. Scott et al.16 studied the reliability of dynamometers in assessing hip strength when using break tests. They found reliability for flexion, abduction, and extension. They further found that a dynamometer anchored for stability was more reliable than a handheld dynamometer (HHD) in assessment of hip extension. Agre, et al.17 demonstrated reliability of the HHD in assessment of the upper extremities, including elbow flexion and extension, and shoulder flexion, using make tests. Hsieh and Phillips18 demonstrated HHD reliability for shoulder internal rotation, hip flexion, and hip external rotation. Whereas the above three studies were conducted on healthy subjects, Bohannon and Andrews19 studied patients with medical disorders affecting strength—primarily cerebrovascular accidents. They found that HHD testing of three muscle groups in both the upper and lower extremities was reliable. Wang, et al.20 found that HHD was reliable for lower limb testing in elderly fallers with a variety of diagnoses. Ottenbacher et al.21 showed that HHD was reliable for upper and lower extremity strength testing when performed by trained non-therapist lay examiners. Computerized isokinetic dynamometry has also been shown to be reliable in stroke patients.22

Discussion

In the case of Mrs. X, five different physicians—over the course of a number of years—had determined that there was a valid strength deficit present in the left arm. Furthermore, a functional capacity evaluation had determined that she could do only sedentary and light work. Why then did the IME neurologist find that there was no strength deficit in the left arm? While it is obviously in the financial interest of the carrier to find that there is no impairment—therefore, no need for monetary compensation— it is hoped that very few dishonest physicians would give opinions biased in favor of the interests of the referral source. A more likely cause of reporting Mrs. X’s strength as normal is the desire by an examining physician, such as this neurologist, to be precise and not report an abnormality where there is no conclusive evidence. We commonly observe in medical practice that the examiner, consciously or unconsciously, uses approximately a 95% probability threshold for reporting an abnormality. In other words, he/she does not report an abnormality unless he/she is at least 95% certain that it is truly present. While on the other hand, under civil litigation it is necessary that there be only a minimum of a 51% probability that a proposition is true in order for it to be considered true.

Some physicians believe that if there are not both positive EMG findings and abnormal findings on an MRI scan which are consistent with these EMG findings, then radiculopathy should not be diagnosed. This is a fallacy. In fact, radiculopathy is often present in the absence of abnormal EMG’s. Radiculopathy may be diagnosed clinically by the presence of characteristic findings in a careful history and physical examination alone. Zambelis et al.23 found that positive sharp waves and/or fibrillations were found in only 21.2% of patients with known chronic lumbosacral radiculopathy. It is assumed that the physiologic basis for Mrs. X’s possible strength deficit is a possible cervical radiculopathy. It follows that if the neurologist feels that radiculopathy cannot be diagnosed due to negative EMG studies, then it leads to a conclusion that there is no basis for a strength deficit. If it appears that there is no medical explanation for a strength deficit, then an examiner may be biased against finding one. However, the putative cause of weakness should not be a factor in the determination of motor impairment. Weakness is weakness. The only sure way to insure absence of bias is to use a totally objective means of measuring strength, such as a dynamometer. Another cause of underreporting of strength deficits in independent medical examinations is the general prejudice in the medical community regarding disability applicants. Without definitive test results, there is a tendency to believe that complaints of weakness are “functional,” and to label the claimant a malingerer. If objective measurements of strength, such as a dynamometer or other similar instrument were used, then it is likely that the neurologist’s conclusion about Mrs. X would have been different.

Figure 3. Comparison of different evaluation techniques.

Practical Aspects

How can one more precisely measure strength in extremities? Figure 1 presents a diagram of the measurement of strength in the biceps brachii muscle which is commonly described in neurology textbooks. It is possible to get a good idea of muscle weakness problems by testing the biceps brachii (C5,C6), triceps (C7,C8), quadriceps (L2,3,4), and gastrocnemius (L5,S1). Generally speaking, the strengths of these muscles should be equal.

As noted in Figure 1, placing one’s hand at the forearm is the way most clinicians assess motor strength. Placing a dynamometer instead at this position would give a numerical reading of maximum force. This would significantly improve the accuracy of the measurements. The lever arm length between the attachment point of the muscle and the transducer at the forearm must be the same between repetitions. It is important that the starting point for breaking action is at the same range of motion position for each test and repetition. This is needed because of the wide variations in force the muscles can exert depend on the starting point of the breaking action. The resistance should be built up smoothly until reaching the break point (over 2 – 3 seconds).24

An even better method would be to use computerized apparatus that would provide a force time curve instead of merely a reading of the maximum force. An example of this tracing is presented in Figure 2. In doing these tests, it is important that at least three readings be taken. Coefficients of variability can be calculated to help determine reliability.

Conclusion

Pain patients often complain of weakness in the extremities. The choice of strength assessment technique which is performed on a claimant can have a significant impact on the ultimate impairment rating. While the AMA Guides recommend using manual muscle testing in determining strength deficit, this is quite problematic particularly if the deficit is 25% or less. Research studies have shown that this testing can miss a 50% strength loss. As such, patients may be labeled as having “give away weakness” which implies malingering. Objective methods, using various types of equipment, have been more recently developed and should be used instead. Much as inclinometry has replaced visual assessment for better determination of range of motion deficits, quantitative measurement techniques—such as a dynamometer—are indicated for more precise strength evaluation (see Figure 3). These more objective methods should be used in preference to manual muscle testing, especially since the diagnostic instrumentation for muscle strength testing is not necessarily expensive.

Test Your Knowledge

1. Research studies have revealed that manual muscle testing can miss a ____% strength loss in the extremities.
A. 10%
B. 20%
C. 30%
D. 40%
E. 50%

2. Which nerves are assessed when testing the triceps muscle?
A. C5, C6
B. C6, C7
C. C7, C8
D. C6–C8
E. None of the above

3. Nerves involved in quadriceps muscle testing:
A. L2, L3, L4
B. L3, L4
C. L5, S1
D. None of the above

4. Research studies have revealed that 2 examiners agree at most on the grade assigned for manual muscle strength testing which percent of the time?
A. 91%
B. 74%
C. 60%
D. 54%
E. 47%

5. True statement(s) regarding the coefficient of variability:
A. very low values can exclude malingering
B. there is only a small overlap in CV values for
feigned and maximal efforts
C. is equal to sum of the values divided by the
square root of the sum of the values
D. all of these statements are true

6. True statement(s) regarding manual dynamometry:
A. found to be reliable in both healthy subjects and
those with impairment
B. reliable in both break and make tests
C. dynamometer anchored for stability more reliable
than a handheld dynamometer
D. all of the above

7. Under civil litigation it is necessary that there be only a minimum of a _____% probability that a proposition is true, for it to be considered true.
A. 51%
B. 61%
C. 71%
D. 81%
E. 91%

Answers: 1-E, 2-C, 3-A, 4-E, 5-A, 6-D, 7-A

 

Last updated on: December 28, 2011
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