Identification of suboptimal muscular effort is an issue of distinct importance for the effective evaluation of functional capacity and the resolution of claims relating to injury-based muscular weakness. On one hand, because of the huge economic cost of musculoskeletal injuries (which in the United States alone was estimated at approximately $70 billion1) and, on the other, because of the availability of dedicated instruments and novel testing protocols, this subject has attracted considerable interest in recent years.
The scientific exploration of suboptimal effort involves two distinct protocols. In one experiment, the subject is directed to produce a force that corresponds to a prescribed percent of the maximal effort (ME). In the other experiment, the subject is asked to feign effort, a situation that simulates what is known among clinicians as malingering. 2 The traditional approach to the detection of either type of suboptimal effort was based on force (strength) variability. Based on the coefficient of variation (CV) = standard deviation/mean × 100, force scores derived from a set of repeated MEs were expected to exhibit lower variation compared with suboptimal exertions performed under the same conditions. Underlying this approach was the assumption that ME required less motor planning in terms of the feedforward and feedback loops and, hence, resulted in lower intercontraction variations. Studies of suboptimal performance using CV have been reported in various muscle systems, 3-6 mainly those operating in hand grip. 7-10 It is likely that focusing on the hand was a result of the availability of a standard instrument for measuring grip strength,11 the role grip strength deficiency has in determining the degree of impairment, 12 and the ability of individuals to effectively reproduce submaximal grip forces.13
Based almost exclusively on static protocols, namely, where strength was measured during isometric contraction of the grip musculature, various cut-off values for differentiating between maximal and suboptimal effort levels were suggested. 14 However, more recent research has seriously challenged the validity of the isometric-based CV in detecting optimal effort. 8, 10 The main concern was that, although the CVs associated with MEs were largely smaller, the extent of overlapping in the respective distributions was considerable. This inevitably resulted in sensitivities and specificities that were unacceptable, both clinically and medicolegally.8, 15, 16
Dynamic muscle strength is commonly measured using isokinetic dynamometry, and it refers to the torque-generation capacity of muscles during either concentric (muscle shortening) or eccentric (muscle lengthening) contractions. The variability of dynamic strength has been reported in a few studies, 17-20 and the results, in terms of the extent of overlapping between maximal and suboptimal efforts, were quite similar to those derived from static measurements. Only one study compared CVs derived from ME and 50% of ME in both dynamic and static strength of the same muscle group.21 It was indicated that although the dynamic CVs, which were based on the average torque and the slope-to-peak torque, were effective in differentiating maximal from submaximal effort, the isometric strength-based CVs were not.
In a recent study, grip strength was measured using isokinetic dynamometry, both in concentric and eccentric contraction modes,22 and the resulting length-tension and force-velocity curves supported the validity of this method. In view of the above-mentioned features of grip force (i.e., its role in rating hand impairment and the ease with which it can be feigned in static measurements), the main objective of this study was to examine the validity of using static and dynamic CVs to identify feigned grip effort.
SUBJECTS AND METHODS
Seventeen female health professional students, aged 20 to 25 yr, volunteered to participate in this study. None had suffered previous trauma or other pathologic injury to the muscles or joints of the upper limb. Eleven of the subjects were right hand-dominant. All participants signed an informed consent form.
Static grip strength was measured using a Jamar hand dynamometer (Therapeutic Enterprise Inc., Clifton, NJ). Isokinetic grip strength was measured using a special attachment22 that was harnessed to a KinCom 125E+ dynamometer (Chattanooga Instruments, Chattanooga, TN). The system was calibrated on a weekly basis using weights of 50, 100, and 200 N.
Testing was performed in two sessions. During the first session, subjects were asked to perform three maximal isometric contractions at each of the five rungs of the Jamar dynamometer. These contractions were performed intermittently by the right and left hands, with an intercontraction period of 5 s. The dominant hand was tested first. During the second session, which took place 2 wk later, both static and dynamic measurements were performed. Starting with the static test, the subject was first asked to produce maximal grip at the rung that, during the first session, corresponded to the maximal strength value. This part of the test was performed bilaterally with the dominant hand first and an intercontraction period of 5 s. After 10 min, the subject was asked to feign grip effort at the same rung with the following instructions: "Imagine that 2 yr ago you were involved in an accident in which your hand was injured. Although now completely recovered, you are suing the insurance company for reason of weakness of grip (in other words, you are trying to gain unlawfully). Please do your best to convince me (the examiner) that your claim is sincere." No mention as to either the level of effort relative to the maximal or to the consistency of performance was made so that the test conditions would resemble, as closely as possible, those of the real situation. This part of the test was performed bilaterally, with the dominant hand first and an intercontraction period of 1 min.
The dynamic measurements took place 15 min after the static tests. The subject sat on the system's adjustable position seat. The distance between the back support of the seat and the grip attachment was individually adjusted to result in 80° of flexion at the elbow. Subjects were further stabilized using a pelvic strap. Two contraction modes were measured. In the concentric (Con) mode, the movable fingers handle (attached to the lever arm) was positioned to result in an initial position of maximal finger joint extension. In the eccentric (Ecc) mode, the handle's initial position corresponded to maximal finger joint flexion. In both contraction modes, subjects were told to actively squeeze the handle, overcoming a bias force of 10 N. However, in the Con mode, fingers underwent flexion, whereas in the Ecc mode, extension took place. Two velocities were used: 4°/s and 16°/s.
Subjects were tested bilaterally with the dominant hand first. Familiarization and warm-up consisted of three submaximal, followed by one maximal, Con-Ecc contraction cycles, all performed at 4°/s. Maintaining this velocity, grip strength was measured approximately 2 min later. Subjects were instructed to squeeze as hard and as fast as they could. Measurements consisted of three consecutive maximal Con-Ecc contraction cycles that were performed with intercontraction and intercycle pauses of 5 s. Concentric (eccentric) strength was defined as the average of the three contractions, in which the average value of the strength curve at each of these contractions was used as the base parameter. After a 30-s pause, the same protocol was repeated at 16°/s. A 5-min pause was allowed, and the same protocol was used for measuring the dynamic grip strength of the contralateral hand. No verbal or visual (screen-based) feedback was given. In the second part of the study, which followed 15 min later, subjects were asked to feign grip effort under the same instructions given for the isometric effort and repeating the above protocol.
The average grip strength scores were read from the screen using KinCom dedicated software. Results were analyzed using the SAS statistical software package (version 6.1) (SAS Institute, Cary NC); they included descriptive statistics and analysis of variance with repeated measures.
No significant differences were found with respect to laterality, and therefore, the findings from both hands were pooled. The mean standard deviations and CVs for the optimal rung in maximal and feigned effort are outlined in Table 1 and depicted in Figure 1. The subjective value of the feigned effort relative to the maximal effort was approximately 30%. Analysis of variance with repeated measures revealed that the mean value of the feigned effort was significantly lower than that of the maximal effort (F1,66 = 209.73; P = 0.0001; r2 = 0.76). The CV for the feigned effort was significantly higher that that of the maximal effort (F1,66 = 38.59; P = 0.0001), but the relationship was substantially weaker (r2 = 0.37).
No significant differences were found with respect to laterality; therefore, the findings from both hands were pooled. The means, standard deviations, and CVs for the efforts are outlined in Table 1 and depicted (for the Con effort at 16°/s) in Figure 2. The subjective value of the feigned effort relative to the maximal effort ranged from 20 to 30%, with one exception relating to the Con effort at 16°/s (13%). Analysis of variance with repeated measures revealed that the mean of the isokinetic strength in the feigned effort was significantly lower than that of maximal strength in all test situations (F1,66 = 334.52, P = 0.0001, r2 = 0.83 (for Con at 4°/s); F1,66 = 262.71, P = 0.0001, r2 = 0.80 (for Con at 16°/s); F1,66 = 294.41, P = 0.0001, r2 = 0.82 (for Ecc at 4°/s); F1,66 = 202.8, P = 0.0001, r2 = 0.75 (for Ecc at 16°/s)). The CV for the isokinetic feigned effort was significantly higher that that of the maximal effort (F1,66 = 18.79, P = 0.0001, r2 = 0.22 (for Con at 4°/s); F1,66 = 18.82, P = 0.0001, r2 = 0.22 (for Ecc at 4°/s); F1,66 = 16.95, P = 0.0001, r2 = 0.21 (for Ecc at 16°/s)), except during Con at 16°/s (F1,66 = 0.01, P = 0.93, r2 = 0.00). All associated r2 values were low.
Comparison of Static- and Dynamic-Based CVs
Application of the general linear model indicated a significant but weak relationship between the variables of grip strength, type of contraction (static v dynamic), performing hand, and effort (maximal v feigned) (F3,132 = 18.13; P = 0.0001; r2 = 0.30). This was mainly because of the drastic effect of the effort variable. In all isokinetic tests, the performing hand (right or left) was not significant. With respect to contraction type, the Ecc strength-based CVs were significantly different from the static ones (P = 0.03 and P = 0.046 at 4°/s and 16°/s, respectively); however, their Con counterparts were not (P = 0.28 for both velocities).
A multivariate prediction model that is based on tolerance limits is presented in Table 2. It enables the determination of effort sincerity in terms of individual rather than group performance scores and is based on the use of a dichotic variable whose value is either 0 (in maximal effort) or 1 (in feigned effort). From this model, acceptance or rejection zones (corresponding to sincerity or insincerity, respectively) are derived. For the identifying parameter to be effective, there must be a distinct separation between the zones. However, it seems that the extent of overlap between the CVs corresponding to maximal and feigned effort is considerable. For instance, using the 99% confidence level, a subject could have a CV as high as 34% in a static test and still be proclaimed sincere. This number covers approximately two-thirds of the range prescribed for the corresponding feigned-based CV (50.98%). It is only on exceeding the latter figure that a true-positive (insincerity) can be confidently declared. As is clearly evident, the overlapping zones between dynamic maximal and feigned CVs are compatible with those characterizing their static counterparts. Consequently, although overlapping is reduced on lowering the level of confidence to 95%, the resulting proportions of false-negatives are still very high, leading to a test sensitivity of 44% in the feigned static efforts and one even lower in the feigned dynamic efforts: 8.8, 5.9, 20.6, and 14.9% for the 4, 16, 4, and 16°/s Con and Ecc, respectively.
This study compares the static and dynamic CV-based variability of grip strength using two levels of effort. The findings unequivocally indicate that, with respect to grip strength and irrespective of the method applied, CV is not a valid indicator of feigned effort.
Feigning effort is a complex phenomenon that involves neurophysiologic factors as well as cognitive and, possibly, affective ones. As such, it may differ from a situation in which the effort exerted is linked to some prescribed percent of ME, although it cannot be ruled out that subjects do adjust the level of muscular contraction to some internal quantitative cue. Furthermore, in the present study, the instruction was to feign effort, without reference to the consistency of the performance or feedback, either verbal or another type. Hence, the findings were expected to be as close as possible to those from a real testing situation.
It should be pointed out that the internal order of the experimental conditions in both the static and dynamic tests was based on maximal followed by submaximal contractions. In a recent study focusing on knee extension effort, the effect of order was examined under isokinetic conditions; it did not affect the results.23 However, at this stage, it is impossible to firmly state whether the external order, namely, static followed by dynamic efforts, had any effect on the final outcome. Given the relatively long period between the two tests (15 min), it would not seem unreasonable to assume that such an effect, if at all present, was indeed minimal.
Analysis of the findings relating to the magnitude of the CV in maximal static effort reveals close similarity with that from previous studies, namely, approximately 5%. 10, 15, 24-26 CV values relating to static submaximal efforts are difficult to interpret because, in almost all grip effort studies, subjects were instructed to produce a percent of the maximum rather than being explicitly told to feign effort. However, the CVs for these efforts ranged between 107, 25 and 20%. 11 In the present study, the corresponding CV value was somewhat higher (21.4%), but it was within the expected range.
In terms of the dynamic CVs, existing knowledge relates to muscle groups other than those responsible for grip and, therefore, comparison is not valid. It would, however, be of interest to note that in two independent studies17, 21 relating to concentric isokinetic knee extension efforts, the CV for the ME ranged between approximately 5 to 13% and 6 to 9%, respectively. On the other hand, the CVs for 50% of the maximum ranged between 6 and 11% and 16 and 19%, respectively. These values should be compared with 9 to 15% and 15 to 22% values for the maximal and feigned Con efforts, respectively, in the present study. Given that the CVs depend on the specific muscle system and speed of testing, this resemblance is quite striking. Another intriguing result relates to the fact that, between the two dynamic modes, only the Ecc-based CVs were significantly different from their static counterparts. This may point to different strength modulation mechanisms operating at the level of the central nervous system, which controls concentric and eccentric force production.19
Not surprisingly, the results point to a significant reduction in force output during feigned effort. However, among the distinct situations, there was a conspicuous deviation from the general proportional reduction in strength (20-30%), which was associated with the high-velocity Con effort. Findings indicated that most subjects exerted minimal force on the handle, which on average, was approximately 13% that of the maximum. In addition, instead of manifesting a typical inverted U shape, corresponding to a single maximum near the middle of the range of motion, the strength curves were usually flat throughout. This finding agrees with similar observations based on feigned effort in the five-rung static Jamar test.27 Indeed, the loss of the typical bell-shape form in a static test is commonly interpreted by clinicians as indicative of malingering. It would seem, therefore, that submaximal high velocity Con efforts are particularly suited for exposing feigned efforts, a fact borne out by previous studies.17-19, 21
As pointed out in a former article,10 the determination of effort optimality is largely a statistical question. In other words, whether a subject or a patient is performing at a suboptimal level cannot, in principle, be answered in an absolutely affirmative or negative manner. Rather, a level of certainty must be attached to such a proclamation. Therefore, in questions belonging to the medicolegal arena, it is pertinent that proving or disproving a claim is based on tolerance interval limits (rather than confidence intervals). The former are obviously more stringent than the latter. For instance, it would be necessary for the CV to exceed 50.98% for the examiner to be 99% confident that a subject was feigning effort in a static test, whereas the corresponding value attached to 90% confidence would be 40.02. Clearly, a higher degree of confidence means a stronger identification power, but this comes at the expense of a higher rate of false-negatives because of the wider acceptance zone. In other words, more subjects can be classified as optimal performers when, indeed, they may not be. Consequently, the decision regarding the "cut-off" value should be linked to specific tolerance intervals and not be arbitrarily defined in terms of some fixed value of the particular identifier.
Selecting the CV as a verifier of the sincerity of muscular effort may have been erroneous from its very inception, as demonstrated in healthy subjects28 and patients alike.29, 30 There is a certain degree of inherent fluctuation in strength output, even in closely repeated contractions, which may vary as a function of the joint-muscle system, the protocol applied, the prevailing environmental conditions, and the health status and motivational level of the patient. Moreover, day-to-day fluctuations in the CV based on the same protocol have been observed,15 and these further confound a clear interpretation of the findings. Because hand musculature is specifically equipped to exert finely tuned force levels, it would have been reasonable to expect minimal variations on repeated contractions. However, based on the two well-researched and practiced methods of strength measurement, the findings of the present study strongly indicate that this is not the case. Combined with the above-mentioned studies, this inevitably leads to the conclusion that applying CV as an indicator of suboptimal effort, particularly in the medicolegal arena, should be seriously questioned.
We thank Chava Peretz for assistance with the statistical analysis.
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