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Using the “Visual Target Grip Test” to Identify Sincerity of Effort during Grip Strength Testing
SCIENTIFIC/CLINICAL ARTICLE JHT READ
FOR
CREDIT ARTICLE #233.
Using the ‘‘Visual Target Grip Test’’ to Identify Sincerity of Effort during Grip Strength Testing Orit Shechtman, PhD, OTR/L Department of Occupational Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida
Bhagwant S. Sindhu, PhD, OTR Department of Occupational Science and Technology, College of Health Sciences, University of WisconsinMilwaukee, Milwaukee, Wisconsin
Paul W. Davenport, PhD Department of Physiological Sciences, University of Florida, Gainesville, Florida
Individuals who have musculoskeletal and connective tissue disorders or orthopedic impairments constitute a predominant portion of people with disabilities.1 Musculoskeletal disorders (MSD) is a term for a variety of conditions, which involve gradually developing pain and discomfort in soft-tissue structures including nerves, muscles, tendons, blood vessels, and their related connective tissues. MSD are usually associated with the upper extremities and with work activities. Work-related musculoskeletal disorders (WMSD) commonly consist of tendonitis/tendinosis and nerve compression, occurring mostly in specific regions of the upper extremities.2 The incidence of WMSD has been on the rise, impacting the individual worker, industry, and even society,3 with approximately 20 billion dollars being spent annually.4 Some people with WMSD exert less than a maximal voluntary contraction during evaluation and treatment. Exerting low effort may be either Correspondence and reprint requests to Orit Shechtman, PhD, OTR/L, Department of Occupational Therapy, College of Public Health and Health Professions, University of Florida, PO Box 100164, Gainesville, FL 32610-0164; e-mail:. 0894-1130/$ - see front matter Ó 2012 Hanley & Belfus, an imprint of Elsevier Inc. All rights reserved. doi:10.1016/j.jht.2011.12.007
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ABSTRACT: We devised a sincerity of effort assessment based on ‘‘tricking’’ a person into exerting maximal effort by providing incorrect visual feedback. The assessment involves deriving a target line from nonvisual peak gripping force, instructing participants to reach it with each grip repetition, and then secretly changing its position, which requires doubling the force necessary to reach it. Accordingly, participants are tricked into exerting more force than intended to reach the deceptive target line. We examined the validity of this test by comparing force values between ‘‘trick’’ and ‘‘non-trick’’ trials in 30 healthy participants. The study design used was a prospective cohort. Providing incorrect visual feedback caused significantly greater increases in force during submaximal effort (69%) than during maximal effort (28%). This test effectively detected submaximal effort (sensitivity ¼ 0.83 and specificity ¼ 0.93). Although this test is not safe for patients during initial therapy, it may be appropriate for patients who can safely exert maximal grip force. Level of Evidence: Not applicable. J HAND THER. 2012;25:320–9.
intentional or unintentional. Unintentional low effort may be exerted by people with upper extremity injuries because of pain, fear of pain, or fear of reinjury.5e7 Intentional low effort may be exerted for a secondary gain of money, benefits, and/or attention.8e11 Exerting low effort for secondary gain is termed malingering or disability exaggeration, a term used to describe a person who exaggerates his or her disability either consciously or unconsciously for a secondary gain such as money, benefits, or attention. Other indicators of symptom magnification include exaggerated pain symptoms and overstated perception of disability. Malingering is a legal term implying intention.12e14 It is suspected to occur in one of four cases of worker’s compensation, disability claims, or personal injury litigation.9 Health care professionals have attempted to identify if a client is malingering or displaying symptom magnification. Detecting the level of effort is essential for effective rehabilitation because without putting forth a sincere effort, a person cannot be effectively rehabilitated despite advanced technology, equipment, or treatment methods. One of the most common ways to evaluate upper extremity disability is grip strength.15 Currently, however, there are no valid, reliable, or widely accepted assessment methods for detecting sincerity of effort of grip strength testing.16e23 Existing
sincerity of effort tests that are commonly used in the clinic include the coefficient of variation,16,21e26 the five-handle position test,27e33 and the rapid exchange grip test.17,18,27,34,35 In general, these assessments lack standardized administration protocols and empirical support.16,17,21e23,25,26,28,32,33 Specifically, some of these assessments are not reliable and/or valid16,17,21,26,32 and all of them possess insufficient specificity and sensitivity values.17,22,33 There are grave clinical implications to sincerity of effort tests that possess insufficient sensitivity and specificity. As a result of low sensitivity, a submaximal effort may be misclassified as maximal and accordingly a feigning individual would be mistakenly labeled as sincere, an error that leads to elevated disability and health care costs.15 In addition, the consequence of low specificity is the misclassification of a maximal effort as submaximal and thus mistakenly labeling sincere individuals as insincere. This mistake could be very damaging to the patient as it may possibly lead to diminished medicolegal compensation and even a job loss.16,17 Other sincerity of effort assessments, such as electromyography28,36e38 and the forceetime curve39,40 have been shown to be more effective but are clinically irrelevant because of their complexity and lack of commercial availability. The American Society of Hand Therapists (ASHTs) recommends that grip testing be performed with the dynamometer facing away from the client.41 In the absence of visual feedback, the person has to rely on proprioceptive feedback to replicate grip force. Visual feedback allows for better grip force control than proprioceptive feedback alone.42 Many sincerity of effort testing methods are founded on the premise that it is difficult to replicate a submaximal grip force during repeated trials without visual feedback, as the client is forced to rely on proprioceptive feedback alone. In the present study, we devised an assessment and named it the ‘‘Visual Target Grip Test.’’ This assessment is based on ‘‘tricking’’ a person to exert maximal effort by providing erroneous visual feedback. This method is based on allowing a person to depend on visual instead of proprioceptive feedback for replicating grip force and then providing him or her with an incorrect visual feedback to trick them into exerting greater grip force than they intended. More specifically, a ‘‘target line’’ derived from peak force of three ‘‘nonvisual’’ grip strength trials (exerted without visual feedback) is set on an oscilloscope (Figure 1). Then, the person is permitted to look at the oscilloscope while performing repeated grip strength trials and is instructed to reach that target line with each grip force exertion. Finally, the gain (sensitivity) of the oscilloscope is secretly cut in half requiring the person to exert twice as much force to reach the target line, which appears to remain in the same position on the oscilloscope (Figure 2). We
FIGURE 1. An example of setting the target line on the oscilloscope.
named the grip trials performed with the deceptive target line (incorrect visual feedback) ‘‘trick’’ trials and those performed with the correct visual feedback ‘‘non-trick’’ trials. Our first hypothesis was that the deceptive target line would cause participants to exert greater force during both maximal and submaximal efforts, as a pervious study found that visual feedback increased static grip by 7.7%.43 Our second hypothesis was that for the submaximal trials, the increase in force would be significantly greater than for the maximal trials. We predicted that when attempting to reach the deceptive target line, participants who were instructed to exert a submaximal effort would be ‘‘tricked’’ into exerting a lot more force than they intended. In addition, we expected that when participants exert a maximal effort, they would not be able to exert enough force to reach the deceptive target line, although they would exhibit an increase in grip strength. As true maximal voluntary contraction is almost never achieved during muscle strength testing, due to protective mechanisms,44 we expect that the deceptive visual feedback would result in increased grip force even during maximal effort exertion.43 However, we expected the grip force to be significantly more elevated for submaximal efforts than for maximal efforts. The validity of this sincerity of effort test is based on the second hypothesis, which states that submaximal effort is expected to yield a significantly greater increase in force between trick and non-trick trials than maximal effort and thus differentiate between maximal and submaximal efforts. The purpose of the study was threefold: 1) to examine the hypothesis that deceptive visual feedback will cause participants to exert greater force during both maximal and submaximal efforts, 2) to JulyeSeptember 2012 321
* An example taken from one subject. Because the gain of the “Handgrip Force” channel was changed from 1.0 to 0.5 in the “Trick Effort” column, the force has to be multiplied by 2. Thus, the curve is actually twice as high as it seems in the picture.
FIGURE 2. An example of grip force curves during gripping. examine the validity of the ‘‘Visual Target Grip Test’’ by a) calculating the differences in force between the ‘‘trick’’ and ‘‘non-trick’’ grip strength trials of maximal vs. submaximal efforts, b) calculating the sensitivity and specificity of this test, and 3) to examine the testeretest reliability of the repeated grip strength trials.
METHODS Participants Thirty healthy participants (15 males and 15 females) aged 21.3 þ 1.8 years participated in the study. Inclusion criteria included healthy adults aged 18e65 years, without upper extremity injury. Before testing, all participants read and signed the informed consent form approved by the Institutional Review Board at the University of Florida.
Equipment Force measurements were collected using a specialized dynamometer with a force transducer (Biopac Instruments, Goleta, CA). When gripping the dynamometer, the transducer converted the force into electrical impulses, which were measured as a change in voltage of direct current and transmitted to a digital oscilloscope (Gould Instruments, Hainault, Essex, England). From the oscilloscope, the impulses were sent to a signal processor unit (Grass Polygraph, Model 79, Grass Instruments, Inc., Braintree, MA) and then to an analog-to-digital converter (PowerLab, ADInstruments, Colorado Springs, CO). These digital signals were then transmitted to a computer with polygraph software (Chart, ADInstruments), which used a sampling frequency of 2000 Hz to store them and to calculate peak force values. The dynamometer was calibrated using the Grass polygraph so that each mVof current resulted in a 1-cm movement of the pen on the graph paper. Conversion values were calculated as 24.14 lb per Volt of current. In addition, we used the oscilloscope (Gould Instruments) for setting the target line, which the participants aimed to reach during the visual feedback grip strength trials. 322
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Procedure Each subject performed two sessions of grip testing, one maximal and one submaximal, in a randomized order after performing four practice trials to familiarize them with the task. The test administrator was blinded to the level of effort. A different investigator, uninvolved with the test administration, assigned the subject their level of effort using standardized instructions (for exact instructions, see Appendix A). The subject first performed three grip trials without visual feedback and a target line was set on the oscilloscope corresponding to the obtained maximal grip force. After the target line was established, the subject was permitted to look at the oscilloscope and was instructed to watch the target line and attempt to reach it during the eight subsequent grip trials. The first four grip trials were performed with the gain maintained at 1.0. The next four trials were randomized such that the gain remained at 1.0 for two trials (we called these ‘‘non-trick trials’’) and was cut in half (at a gain of 0.5) for the other two trials (we called these ‘‘trick trials’’). Cutting the gain in half meant that the subject had to exert twice as much grip force to reach the target line, although the appearance of the target line remained the same as that of a gain of 1.0. For all grip strength trials, the subject was seated in an adjustable chair without arm rests. The subject assumed the testing position recommended by the ASHTs.41 The subject’s feet were fully resting on the floor and the hips were as far back in the chair as possible, with the hips and knees positioned at approximately 90 degrees. The shoulder of the tested extremity was adducted and neutrally rotated, the elbow flexed at 90 degrees, with the forearm and wrist in a neutral position. The subject was instructed to maintain this position during all tests and was reminded and corrected by the investigators as needed. All grip trials consisted of a 5-second contraction, with a 30-second rest interval between trials. After a 10-minute break, the session was repeated with a different level of effort (either maximal or submaximal). The order of maximal and submaximal sessions was randomly assigned. The peak force generated
during each trial was recorded and used in the statistical analysis.
Data Analysis First, we wanted to examine our hypothesis that providing incorrect visual feedback will cause a person to exert greater grip force during both maximal and submaximal efforts. To do so, we compared the peak force between the trick and non-trick trials for both maximal and submaximal efforts. We then wanted to examine the validity of this test; in other words, our hypothesis that the increase in force brought about by the incorrect visual feedback will be significantly greater for the submaximal than the maximal grip strength trials. To accomplish this, we first determined the increase in force between trick and non-trick trials by calculating the percent difference using the formula: [(trick non-trick)/target line value 3 100]. We then compared the percent difference in peak force between maximal and submaximal efforts. Because significant gender differences in grip strength are well documented,45e47 we compared all force measurements separately for men and women. However, gender differences in percent difference in peak force between maximal and submaximal efforts are uncertain. Thus, we also calculated force measurements for all participants, that is, men and women combined. We performed three paired t-tests per measurement to compare 1) maximal trick vs. maximal non-trick trials, 2) submaximal trick vs. submaximal non-trick trials, and 3) the percent difference between maximal and submaximal efforts. The first two comparisons were conducted to establish whether or not there are differences between the trick and non-trick trials. These comparisons tested our first hypothesis that the deceptive target line will cause greater force exertion during both maximal and submaximal efforts. The third comparison was conducted to test our second hypothesis that there are greater differences between trick and non-trick trials for submaximal effort than for maximal effort. Because three t-tests were conducted on each
measurement for three separate samples (men, women, and all participants), we used the Bonferroni correction to adjust for nine comparisons, which set the alpha level at 0.05/9 ¼ 0.0056. In addition, sensitivity, specificity, and overall error rate values39,48 were calculated for percent difference of grip force for men, women, and all participants. Sensitivity was calculated by dividing the number of true positives by the total number of positives and specificity was calculated by dividing the number of true negatives by the total number of negatives.39,48 Overall error rate was calculated by the formula (1 sensitivity) þ (1 specificity). To find the optimal cutoff value, with the best combination of sensitivity and specificity, we generated three receiver operator characteristic (ROC) curves48,49 for men, women, and all participants, using multiple cutoff values of percent difference. The ROC curve was generated by plotting the true-positive rate against the false-positive rate for each of the above cutoff values. The true-positive rate is the sensitivity, whereas the false-positive rate is calculated by subtracting the specificity value from 1.00 (1.00 specificity).48,49 The proportional area under the curve (AUC) was calculated by counting the number of squares beneath the curve and dividing it by the total number of squares to obtain an index of discriminability.49 Finally, we examined testeretest reliability of force between two repeated trials of the various grip strength efforts by calculating Pearson’s Product Moment correlation coefficients.
RESULTS Values of peak force for men, women, and all participants are presented in Table 1. Our first hypothesis was accepted, as peak force of the trick trials was significantly greater than that of the nontrick trials, indicating that deceptive visual feedback caused participants to exert greater grip force during both maximal (a 28% increase for all participants) and submaximal (a 69% increase for all participants) efforts (the percent increase values for men and
TABLE 1. Comparisons of Peak Force between Trick and Non-trick Trials of Maximal and Submaximal Efforts for Men, Women, and All Participants Measurement Maximal effort (lb) Men Women All Submaximal effort (lb) Men Women All
Trick Trial (Average 6 SD)
Non-trick (Average 6 SD)
t-Test Value
Probability (p)
104.02 6 18.53 76.42 6 11.39 90.04 6 20.52
82.15 6 19.64 59.04 6 12.47 70.49 6 19.79
10.23 13.15 15.06
0.001* 0.001* 0.001*
61.98 6 14.90 52.63 6 14.22 57.94 6 15.45
38.07 6 8.32 29.63 6 8.26 34.04 6 9.41
12.16 12.56 17.73
0.001* 0.001* 0.001*
*Indicates significance at the 0.0056 alpha level.
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TABLE 2. Comparisons of Percent Difference of Peak Force between Maximal and Submaximal Efforts for Men, Women, and All Participants Percent Difference of Peak Force Men Women All
Maximal Effort (Average 6 SD)
Submaximal Effort (Average 6 SD)
t-Test Value
Probability (p)
25% 6 12 31% 6 15 28% 6 13
66% 6 17 73% 6 24 69% 6 20
7.94 6.58 10.28
0.001* 0.001* 0.001*
Percent difference was calculated using the formula: [(trick non-trick)/target line value 3 100]. *Indicates significance at the 0.0056 alpha level.
women are presented in Table 2). Our second hypothesis was accepted, as the change in force (percent difference) was significantly greater during submaximal effort than during maximal effort for men, women, and all participants (Table 2). Sensitivity and specificity values were calculated for percent difference cutoff values ranging from 0% to 115% (Table 3). The ROC curve revealed that for all participants the optimal cutoff value of the percent difference in force was 50%, yielding a sensitivity value of 0.83 (17% error) and a specificity value of 0.93 (7% error) for an overall error rate of 23% (Figure 3). The area under the ROC curve was 96.3% indicating excellent discriminability. When analyzing men and women separately, the optimal cutoff point remained 50% but the sensitivity and specificity values changed. For women, the optimal cutoff value yielded sensitivity of 0.87 and specificity of 0.87, an overall error rate of 26%, and an AUC of 94.6%. For men, the optimal cutoff value yielded sensitivity of 0.80 and specificity of 1.0, an overall error rate of 20%, and an AUC of 97.0%. Finally, teste retest reliability between repeated grip trials of maximal efforts ranged from r ¼ 0.87 to r ¼ 0.98 and for submaximal efforts ranged from r ¼ 0.67 to r ¼ 0.96 (Table 4).
DISCUSSION Deceptive Visual Feedback The findings of the present study supported the hypothesis that deceptive visual feedback caused participants to unknowingly exert greater force during both maximal and submaximal efforts. Reaching the deceptive target line required twice as much force as was needed to reach the original target line. Thus, attempting to reach the deceptive target line caused participants to exert a significantly greater force during both maximal and submaximal efforts. These increases were calculated in relationship to the force that was generated by the nonvisual grip strength trials, which was used to establish the target line. The formula used to calculate the percent increase was [(trick non-trick)/target line value 3 100]. A previous study found that providing a correct
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visual feedback increased static grip by 7.7% when compared with grip trials performed without visual feedback.43 In our study, the deceptive target line, which provided erroneously low visual feedback, caused grip force to increase by 28% during maximal effort (ranging from 11% to 58%) and by 69% during submaximal effort (ranging from 30% to 113%) (Table 2). These increases in force may be explained by two factors. First, humans rely on visual feedback for force control more readily than on any other sensory feedback.50 Secondly, healthy persons do not voluntarily exert a true maximal muscular contraction under most circumstances. Because exerting true maximal contraction requires the recruitment of all motor units available in a muscle, the inability of an untrained person to perform an ‘‘all out’’ muscle contraction serves as a defense mechanism against muscle injury.51 The twitch interpolation technique is used to study if humans can voluntarily elicit full muscle force.44,51,52 In this technique, electrical stimulation is superimposed during a voluntary contraction. When this electrical stimulation causes an increase in twitch force, the muscular contraction is considered to be submaximal and the voluntary activation is less than 100%.53 Without training, most humans cannot willingly exert true maximal contraction.44,51,52 For example, healthy participants were able to increase force produced during maximal grips when presented with verbal motivators,54 monetary incentives,55 and emotionally arousing photographs.56 In contrast, the ability to produce grip force is reduced when injury-related factors such as pain and fear of pain are present.57e60 In addition, the perception of the exerted force is increased in the presence of muscular fatigue.61
Validity of the ‘‘Visual Target Grip Test’’ To examine the validity of this test, we compared the percent difference between maximal and submaximal grip efforts. As expected, the increase in grip force between non-trick (nonvisual) and trick (the deceptive target line) grip strength trials was significantly greater for submaximal efforts than for maximal efforts (Table 2). This significantly greater percent difference [(trick non-trick)/target line value 3 100]
0.00 1.00 1.00 0.03 1.00 0.97 0.07 1.00 0.93 0.07 1.00 0.93 0.10 1.00 0.90 0.13 1.00 0.87 Bolded and italic numbers indicate the optimal cutoff value (50%) and its associated sensitivity, specificity, and overall error rate values.
0.27 1.00 0.73 0.33 1.00 0.67 0.37 1.00 0.63 0.47 1.00 0.53 0.53 1.00 0.47 0.63 1.00 0.37 0.67 0.97 0.37 0.83 0.93 0.23 0.93 0.80 0.27 0.97 0.80 0.23 0.97 0.73 0.30 0.97 0.67 0.37 1.00 0.50 0.50 1.00 0.27 0.73 1.00 0.20 0.80 1.00 0.00 1.00 1.00 0.00 1.00 All Sensitivity Specificity Overall error rate
1.00 0.00 1.00
0.00 1.00 1.00 0.07 1.00 0.93 0.13 1.00 0.87 0.13 1.00 0.87 0.20 1.00 0.80 0.27 1.00 0.73 0.40 1.00 0.60 0.40 1.00 0.60 0.47 1.00 0.53 0.47 1.00 0.53 0.47 1.00 0.53 0.60 1.00 0.40 0.60 0.93 0.47 0.87 0.87 0.26 0.93 0.73 0.33 1.00 0.73 0.27 1.00 0.67 0.33 1.00 0.67 0.33 1.00 0.33 0.67 1.00 0.20 0.80 1.00 0.13 0.87 1.00 0.00 1.00 1.00 0.00 1.00 Women Sensitivity Specificity Overall error rate
1.00 0.00 1.00
0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.13 1.00 0.87 0.27 1.00 0.73 0.27 1.00 0.73 0.47 1.00 0.53 0.60 1.00 0.40 0.67 1.00 0.33 0.73 1.00 0.27 0.80 1.00 0.20 0.93 0.87 0.20 0.93 0.87 0.20 0.93 0.80 0.27 0.93 0.67 0.40 1.00 0.67 0.33 1.00 0.33 0.67 1.00 0.27 0.73 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 Men Sensitivity Specificity Overall error rate
115 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 Cutoff Value (%)
TABLE 3. Sensitivity, Specificity, and Overall Error Rate of Percent Difference in Peak Force for Men, Women, and All Participants
indicates that the ‘‘Visual Target Grip Test’’ can indeed differentiate between maximal and submaximal efforts. To further assess the validity of this sincerity of effort test, we explored the test’s effectiveness by examining the sensitivity and specificity of the percent difference of force. A test’s effectiveness is defined as its ability to accurately assess the presence and absence of a submaximal (insincere) effort.48 Sensitivity corresponds to true positives, reflecting the percentage of people who were considered to exert a submaximal effort and really exerted a submaximal effort, whereas specificity corresponds to true negatives reflecting the percentage of people who were considered to exert a maximal effort and really exerted a maximal effort. There are trade-offs between specificity and sensitivity: as one increases the other decreases.48 Thus, it is imperative to establish an optimal cutoff value, which yields the best combination between sensitivity and specificity.49 A test that possesses low specificity will mistakenly classify people who exert maximal effort as having exerted submaximal effort. This error could lead to inappropriate diagnosis and treatment, reduced compensation settlement, withheld payments, and job loss.20,22 Conversely, a test that possesses low sensitivity would misclassify people who exert submaximal effort as having exerted maximal effort. This error could lead to unnecessary procedures, ineffective therapy, and increased disability and health care costs.22,25 We used two methods to find the optimal cutoff value: finding the overall error rate and constructing an ROC curve. The overall error rate is the percent of combined errors (false positive plus false negatives) at a specific cutoff value. The lowest overall error rate reflects the optimal cutoff value, which provides the best combination of sensitivity and specificity.48 The common sincerity of effort assessments, such as the coefficient of variation, rapid exchange grip test, and the fiverung test, possess overall error rates ranging from 47% to 69%.39 When compared, the ‘‘Visual Target Grip Test,’’ at its optimal cutoff value (percent change ¼ 50%), misclassified only 17% of submaximal efforts and 7% of maximal efforts of all participants for an overall error rate of 23% (Table 3). This overall error rate is exceptional, considering the overall error rates of most other sincerity of effort tests. This test was somewhat more effective in men as indicated by a lower overall error rate (20%) compared with women (26%). Similar to the overall error rate, the ROC curve illustrates the accuracy of detecting sincerity of effort over a range of percent difference cutoff values and reveals the best combination of sensitivity and specificity. In the present study, both the overall error rate and the ROC curve revealed that when the percent JulyeSeptember 2012 325
1.00
40%
0.90
50%*
0.80
0.70
55% Sensitivity
0.60
0.50
0.40
0.30
0.20
0.10
0.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1-Specificity
* The optimal cutoff point for peak force percent difference for all participants is 50% (sensitivity = 0.83; specificity = 0.93)
FIGURE 3. Receiver operator characteristic curve for percent difference in force for all participants.
difference (the increase in force) between trick and non-trick trials is 50% or larger (the optimal cutoff value), one can diagnose the effort as submaximal and misclassify only 7% of all participants who exerted maximal effort as having exerted a submaximal effort. Conversely, when the percent difference is smaller than 50%, one can diagnose the effort as maximal and misclassify only 17% of all participants who exerted submaximal effort as having exerted maximal effort. At the 50% cutoff value, men had better specificity but poorer sensitivity than women. None of the men were misclassified as exerting submaximal effort when really exerting maximal effort and 20% of them were misclassified as exerting maximal effort when exerting submaximal effort. For women, 13% were misclassified as exerting submaximal effort and 13% as exerting maximal effort. These error rates suggest that the ‘‘Visual Target Grip Test’’ possesses acceptable levels of sensitivity and specificity and can effectively detect sincerity of effort.
TABLE 4. Testeretest Reliability of Peak Force for Trick and Non-trick Trials of Maximal and Submaximal Efforts for Men, Women, and All Participants Men Peak Force
Women
All
r
p
r
p
r
p
0.869 0.936
0.001* 0.001*
0.905 0.976
0.001* 0.001*
0.931 0.959
0.001* 0.001*
Submaximal effort Trick trials 0.964 Non-trick trials 0.666
0.001* 0.007*
0.906 0.955
0.001* 0.001*
0.937 0.83
0.001* 0.001*
Maximal effort Trick trials Non-trick trials
Testeretest Reliability We wanted to assess testeretest reliability because a test that is not reliable cannot be valid.48 However, we did not administer the test twice, so we could not assess the reliability of the actual ‘‘Visual Target Grip Test.’’ Rather, we assessed the reliability of its components; namely, the force of repeated grip trials. The testeretest reliability values for repeated grip strength trials (Table 4) were somewhat higher for maximal effort (ranging from r ¼ 0.87 to r ¼ 0.98) than for submaximal efforts (r ¼ 0.67 to r ¼ 0.96).
Limitations
*Indicates significance at 0.05 alpha level.
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The proportional area under the ROC curve can be used as an index for the amount of overlap between the distributions of true positives and false positives, which provides a measure of discriminability. A perfect test will have an area of 100% under the curve.49 When considering sincerity of effort testing, discriminability is the ability to discriminate between maximal and submaximal efforts. A larger AUC indicates a better ability of a test to discriminate between maximal and submaximal efforts. In the present study, the AUC was 96.3%, indicating a good ability to discriminate between maximal and submaximal efforts. The AUC indicated somewhat better discriminability for men (97.0%) than for women (94.6%).
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The main limitation of this study include a small sample size and the use of a noninjured population. The present study was conducted on healthy participants, as it is only the first step in establishing the ‘‘Visual Target Grip Test’’ as a valid sincerity of effort
assessment. Difference may exist between healthy participants instructed to fake weakness in a laboratory experiment and patients who exert insincere effort for a secondary gain. To validate this test for the proper patient population and deem it clinically appropriate, future studies must be conducted on persons with upper extremity injury. Most importantly, this test may not be safe for clinical use, especially during initial therapy, because it leads to an unconscious and unintentional increased exertion of grip force, which could raise the potential for reinjury. However, the test may be appropriate for patients who can safely exert maximal grip force. An additional limitation is that the testeretest reliability was assessed only for the components of the test but not for the test as a whole. Future studies must examine testeretest reliability of the ‘‘Visual Target Grip Test’’ as a whole. Lastly, the testing device is not available to clinicians as it was assembled in our laboratory. Future work would include developing and testing a clinically friendly version of our device.
CONCLUSIONS The present study comprises the first step in establishing the feasibility and validity of a new method for testing sincerity of effort by using a deceptive visual feedback to differentiate between maximal and submaximal efforts. We found that the increase in grip force between trick and non-trick grip trials was significantly greater for submaximal effort than for maximal effort, indicating that the ‘‘Visual Target Grip Test’’ can detect submaximal grip strength efforts. Further, the test possesses good effectiveness as indicated by a relatively small overall error rate of sensitivity and specificity and a large area under the ROC curve. Incorporating this assessment into a functional capacity evaluation would be relatively simple and clinically beneficial. However, this test may not be safe for clinical use during the initial therapy period because it causes the individual to unknowingly exert greater grip force than intended, and thus raises the potential for reinjury. Yet, this test is appropriate for patients who can safely exert maximal grip force. Acknowledgment This work was supported in part by the Research Opportunity Fund of The University of Florida.
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2. Sanders MJ. The medical context. In: Sanders MJ (ed). Management of Cumulative Trauma Disorders. Boston, MA: Butterworth-Heinemann, 1997. pp. 21–6. 3. Sanders MJ. The industrial perspective. In: Sanders MJ (ed). Management of Cumulative Trauma Disorders. Boston, MA: Butterworth-Heinemann, 1997. pp. 27–38. 4. Bernard BP. Musculoskeletal Disorders and Workplace Factors. Cincinnati, OH: NIOSH Publication No. 97, 1997. pp. 97e141. 5. Czitrom AA, Lister GD. Measurement of grip strength in the diagnosis of wrist pain. J Hand Surg [Am]. 1988;13:16–9. 6. van Wilgen CP, Akkerman L, Wieringa J, Dijkstra PU. Muscle strength in patients with chronic pain. Clin Rehabil. 2003;17: 885–9. 7. Pienimaki T, Tarvainen T, Siira P, Malmivaara A, Vanharanta H. Associations between pain, grip strength, and manual tests in the treatment evaluation of chronic tennis elbow. Clin J Pain. 2002;18:164–70. 8. Fishbain DA, Cutler RB, Rosomoff HL, Rosomoff RS. Chronic pain disability exaggeration/malingering and submaximal effort research. Clin J Pain. 1999;15:244–74. 9. Mittenberg W, Patton C, Canyock EM, Condit DC. Base rates of malingering and symptom exaggeration. J Clin Exp Neuropsychol. 2002;24:1094–102. 10. Green P, Rohling ML, Lees-Haley PR, Allen LM. Effort has a greater effect on test scores than severe brain injury in compensation claimants. Brain Inj. 2001;15:1045–60. 11. Lees-Haley PR. MMPI-2 base rates for 492 personal injury plaintiffs: implications and challenges for forensic assessment. J Clin Psychol. 1997;53:745–55. 12. Matheson LN, Isenhagen SJ, Hart DL. Functional capacity evaluation as a facilitator of social security disability program reform. Work. 1998;10:77–84. 13. Niemeyer LO. The issue of abnormal illness behavior in work hardening. In: Niemeyer LO, Jacobs K (eds). Work Hardening: State of the Art. Thorofare, NJ: Slack, 1989. pp. 67–109. 14. Velozo CA. Work evaluations: critique of the state of the art of functional assessment of work. Am J Occup Ther. 1993;47: 203–9. 15. King JW, Berryhill BH. Assessing maximum effort in upperextremity functional testing. Work. 1991;1:65–76. 16. Shechtman O. Using the coefficient of variation to detect sincerity of effort of grip strength: a literature review. J Hand Ther. 2000;13:25–32. 17. Shechtman O, Taylor C. The use of the rapid exchange grip test in detecting sincerity of effort, part II: validity of the test. J Hand Ther. 2000;13:203–10. 18. Taylor C, Shechtman O. The use of the rapid exchange grip test in detecting sincerity of effort, part I: administration of the test. J Hand Ther. 2000;13:195–202. 19. King PM. Analysis of approaches to detection of sincerity of effort through grip strength measurement. Work. 1998;10:9–13. 20. Lechner DE, Bradbury SF, Bradley LA. Detecting sincerity of effort: a summary of methods and approaches. Phys Ther. 1998;78:867–88. 21. Shechtman O. The coefficient of variation as a measure of sincerity of effort of grip strength, part I: the statistical principle. J Hand Ther. 2001;14:180–7. 22. Shechtman O. The coefficient of variation as a measure of sincerity of effort of grip strength, part II: sensitivity and specificity. J Hand Ther. 2001;14:188–94. 23. Hamilton Fairfax A, Balnave R, Adams RD. Variability of grip strength during isometric contraction. Ergonomics. 1995;38: 1819–30. 24. Dvir Z. Coefficient of variation in maximal and feigned static and dynamic grip efforts. Am J Phys Med Rehabil. 1999;78: 216–21. 25. Robinson ME, Geisser ME, Hanson CS, O’Connor PD. Detecting submaximal efforts in grip strength testing with the coefficient of variation. J Occup Rehabil. 1993;3:45–50. 26. Shechtman O. Is the coefficient of variation a valid measure for detecting sincerity of effort of grip strength? Work. 1999;13: 163–9.
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27. Stokes HM, Landrieu KW, Domangue B, Kunen S. Identification of low-effort patients through dynamometry. J Hand Surg. 1995;20A:1047–56. 28. Hoffmaster E, Lech R, Niebuhr BR. Consistency of sincere and feigned grip exertions with repeated testing. J Occup Med. 1993;35:788–94. 29. Niebuhr BR, Marion R. Detecting sincerity of effort when measuring grip strength. Am J Phys Med. 1987;66:16–24. 30. Niebuhr BR, Marion R. Voluntary control of submaximal grip strength. Am J Phys Med Rehabil. 1990;69:96–101. 31. Stokes HM. The seriously uninjured hand—weakness of grip. J Occup Med. 1983;25:683–4. 32. Shechtman O, Gutierrez Z, Kokendofer E. Analysis of methods used to detect submaximal effort with the five-rung grip strength test. J Hand Ther. 2005;18:10–8. 33. Gutierrez Z, Shechtman O. Effectiveness of the five-handle position grip strength test in detecting sincerity of effort in men and women. Am J Phys Med Rehabil. 2003;82:847–55. 34. Hildreth DH, Breidenbach WC, Lister GD, Hodges AD. Detection of submaximal effort by use of the rapid exchange grip. J Hand Surg [Am]. 1989;14:742–5. 35. Joughin K, Gulati P, Mackinnon SE, et al. An evaluation of rapid exchange and simultaneous grip tests. J Hand Surg [Am]. 1993;18:245–52. 36. Janda DH, Geiringer SR, Hankin FM, Barry DT. Objective evaluation of grip strength. J Occup Med. 1987;29:569–71. 37. Niebuhr BR, Marion R, Hasson SM. Electromyographic analysis of effort in grip strength assessment. Electromyogr Clin Neurophysiol. 1993;33:149–56. 38. Niebuhr BR. Detecting submaximal grip exertions of variable effort by electromyography. Electromyogr Clin Neurophysiol. 1996;36:113–20. 39. Shechtman O, Sindhu BS, Davenport PW. Using the force-time curve to detect maximal grip strength effort. J Hand Ther. 2007; 20:37–47. 40. Sindhu BS, Shechtman O. Using the force-time curve to determine sincerity of effort in people with upper extremity injuries. J Hand Ther. 2011;24:22–9. 41. Fess EE. Grip strength. In: Casanova JS (ed). Clinical Assessment Recommendations. 2nd ed. Chicago, IL: The American Society of Hand Therapists, 1992. pp. 41–5. 42. Blank R, Heizer W, von Voss H. Externally guided control of static grip forces by visual feedback-age and task effects in 3-6-year old children and in adults. Neurosci Lett. 1999;271:41–4. 43. Jung MC, Hallbeck MS. Quantification of the effects of instruction type, verbal encouragement, and visual feedback on static and peak handgrip strength. Int J Ind Ergon. 2004; 34:367–74. 44. Yue GH, Ranganathan VK, Siemionow V, Liu JZ, Sahgal V. Evidence of inability to fully activate human limb muscle. Muscle Nerve. 2000;23:376–84. 45. Mathiowetz V, Kashman N, Volland G, Weber K, Dowe M, Rogers S. Grip and pinch strength—normative data for adults. Arch Phys Med Rehabil. 1985;66:69–74. 46. Shechtman O, Davenport R, Malcolm M, Nabavi D. Reliability and validity of the BTE-Primus grip tool. J Hand Ther. 2003;16: 36–42. 47. Shechtman O, Gestewitz L, Kimble C. Reliability and validity of the DynEx dynamometer. J Hand Ther. 2005;18:339–47. 48. Portney LG, Watkins MP. Foundations of Clinical Research: Applications to Practice. 2nd ed. Norwalk, CT: Appleton & Lange, 2000. 49. McNicol DA. Primer of Signal Detection Theory. London, UK: Lawrence Erlbaum Associates, Publishers, 2005. 50. De Serres SJ, Fang NZ. The accuracy of perception of a pinch grip force in older adults. Can J Physiol Pharmacol. 2004;82:693–701. 51. Belanger AY, McComas AJ. Extent of motor unit activation during effort. J Appl Physiol. 1981;51:1131–5. 52. Allen GM, Gandevia SC, McKenzie DK. Reliability of measurements of muscle strength and voluntary activation using twitch interpolation. Muscle Nerve. 1995;18:593–600.
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53. Awiszus F, Wahl B, Meinecke I. Influence of stimulus cross talk on results of the twitch-interpolation technique at the biceps brachii muscle. Muscle Nerve. 1997;20:1187–90. 54. Aarts H, Custers R, Marien H. Preparing and motivating behavior outside of awareness. Science. 2008;319:1639. 55. Schmidt L, Palminteri S, Lafargue G, Pessiglione M. Splitting motivation: unilateral effects of subliminal incentives. Psychol Sci. 2010;21:977–83. 56. Pessiglione M, Schmidt L, Clery-Melin ML, et al. Get aroused and be stronger: emotional facilitation of physical effort in the human brain. J Neurosci. 2009;29:9450–7. 57. Farina D, Arendt-Nielsen L, Graven-Nielsen T. Experimental muscle pain decreases voluntary EMG activity but does not affect the muscle potential evoked by transcutaneous electrical stimulation. Clin Neurophysiol. 2005;116:1558–65. 58. Farina D, Arendt-Nielsen L, Merletti R, Graven-Nielsen T. Effect of experimental muscle pain on motor unit firing rate and conduction velocity. J Neurophysiol. 2004;91:1250–9. 59. Graven-Nielsen T, Svensson P, Arendt-Nielsen L. Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function. Electroencephalogr Clin Neurophysiol. 1997;105:156–64. 60. Mense S, Skeppar P. Discharge behaviour of feline gammamotoneurones following induction of an artificial myositis. Pain. 1991;46:201–10. 61. Jones LA. The senses of effort and force during fatiguing contractions. Adv Exp Med Biol. 1995;384:305–13.
APPENDIX A STANDARDIZED INSTRUCTIONS FOR MAXIMAL AND SUBMAXIMAL TRIALS, AND FOR REACHING THE TARGET LINE Instructions for maximal effort were as follows: ‘‘In this session, I want you to give maximal effort with your dominant hand while performing all tests. Follow the directions of the test administrator to exert full effort. Do you have any questions?’’
Instructions for submaximal effort were as follows: ‘‘In this session, I want you to imagine yourself in the following scenario: Imagine that 2 years ago you were involved in an accident at work in which your right hand was injured. Although now completely recovered, you are suing the insurance company for weakness of grip. In other words, you will be faking weakness during the grip strength tests because you are trying to gain money unlawfully. Please do your best to convince the test administrator that your claim is sincere, in other words, try to be consistent in repeating the force of your grip. The test administrator will be asking you, throughout the testing session, to give maximal effort, but you need to remember to feign weakness of grip with your ‘injured’ hand. Do you have any questions?’’
Instructions for reaching the target line during the visual grip strength trials were as follows: ‘‘Make sure that the grip instrument is not resting on your lap. Remember to sit in the right position. Try to reach the line in front of you at GO and keep giving consistent effort till I say STOP.’’ ‘‘READY.SET.GO.’’ Five seconds later, ‘‘STOP.’’
JHT Read for Credit Quiz: Article #233
Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to JHTReadforCredit.com. There is only one best answer for each question. #1. The method described showed ______________in detecting sub-maximal effort a. moderate validity b. high specificity, but low sensitivity c. high sensitivity, but low specificity d. both high sensitivity and specificity #2. The method a. was shown to be safe for initial evaluation b. has been approved by the ASHT c. tricked subjects to exert maximal effort d. tricked subjects to exert sub-maximal effort #3. The key to this method is to provide
a. incorrect visual feedback b. forceful verbal cues c. deceptive voice modulation in cuing d. blindfold the subject during testing #4. Clinically, sub-maximal effort a. is almost always intentional b. may be either intentional or unintentional c. is almost always unintentional d. is usually associated with mild psycho pathology #5. The study established the validity of this method for orthopedic patients a. true b. false When submitting to the HTCC for re-certification, please batch your JHT RFC certificates in groups of 3 or more to get full credit.
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FOR
CREDIT ARTICLE #233.
Using the ‘‘Visual Target Grip Test’’ to Identify Sincerity of Effort during Grip Strength Testing Orit Shechtman, PhD, OTR/L Department of Occupational Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida
Bhagwant S. Sindhu, PhD, OTR Department of Occupational Science and Technology, College of Health Sciences, University of WisconsinMilwaukee, Milwaukee, Wisconsin
Paul W. Davenport, PhD Department of Physiological Sciences, University of Florida, Gainesville, Florida
Individuals who have musculoskeletal and connective tissue disorders or orthopedic impairments constitute a predominant portion of people with disabilities.1 Musculoskeletal disorders (MSD) is a term for a variety of conditions, which involve gradually developing pain and discomfort in soft-tissue structures including nerves, muscles, tendons, blood vessels, and their related connective tissues. MSD are usually associated with the upper extremities and with work activities. Work-related musculoskeletal disorders (WMSD) commonly consist of tendonitis/tendinosis and nerve compression, occurring mostly in specific regions of the upper extremities.2 The incidence of WMSD has been on the rise, impacting the individual worker, industry, and even society,3 with approximately 20 billion dollars being spent annually.4 Some people with WMSD exert less than a maximal voluntary contraction during evaluation and treatment. Exerting low effort may be either Correspondence and reprint requests to Orit Shechtman, PhD, OTR/L, Department of Occupational Therapy, College of Public Health and Health Professions, University of Florida, PO Box 100164, Gainesville, FL 32610-0164; e-mail:
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ABSTRACT: We devised a sincerity of effort assessment based on ‘‘tricking’’ a person into exerting maximal effort by providing incorrect visual feedback. The assessment involves deriving a target line from nonvisual peak gripping force, instructing participants to reach it with each grip repetition, and then secretly changing its position, which requires doubling the force necessary to reach it. Accordingly, participants are tricked into exerting more force than intended to reach the deceptive target line. We examined the validity of this test by comparing force values between ‘‘trick’’ and ‘‘non-trick’’ trials in 30 healthy participants. The study design used was a prospective cohort. Providing incorrect visual feedback caused significantly greater increases in force during submaximal effort (69%) than during maximal effort (28%). This test effectively detected submaximal effort (sensitivity ¼ 0.83 and specificity ¼ 0.93). Although this test is not safe for patients during initial therapy, it may be appropriate for patients who can safely exert maximal grip force. Level of Evidence: Not applicable. J HAND THER. 2012;25:320–9.
intentional or unintentional. Unintentional low effort may be exerted by people with upper extremity injuries because of pain, fear of pain, or fear of reinjury.5e7 Intentional low effort may be exerted for a secondary gain of money, benefits, and/or attention.8e11 Exerting low effort for secondary gain is termed malingering or disability exaggeration, a term used to describe a person who exaggerates his or her disability either consciously or unconsciously for a secondary gain such as money, benefits, or attention. Other indicators of symptom magnification include exaggerated pain symptoms and overstated perception of disability. Malingering is a legal term implying intention.12e14 It is suspected to occur in one of four cases of worker’s compensation, disability claims, or personal injury litigation.9 Health care professionals have attempted to identify if a client is malingering or displaying symptom magnification. Detecting the level of effort is essential for effective rehabilitation because without putting forth a sincere effort, a person cannot be effectively rehabilitated despite advanced technology, equipment, or treatment methods. One of the most common ways to evaluate upper extremity disability is grip strength.15 Currently, however, there are no valid, reliable, or widely accepted assessment methods for detecting sincerity of effort of grip strength testing.16e23 Existing
sincerity of effort tests that are commonly used in the clinic include the coefficient of variation,16,21e26 the five-handle position test,27e33 and the rapid exchange grip test.17,18,27,34,35 In general, these assessments lack standardized administration protocols and empirical support.16,17,21e23,25,26,28,32,33 Specifically, some of these assessments are not reliable and/or valid16,17,21,26,32 and all of them possess insufficient specificity and sensitivity values.17,22,33 There are grave clinical implications to sincerity of effort tests that possess insufficient sensitivity and specificity. As a result of low sensitivity, a submaximal effort may be misclassified as maximal and accordingly a feigning individual would be mistakenly labeled as sincere, an error that leads to elevated disability and health care costs.15 In addition, the consequence of low specificity is the misclassification of a maximal effort as submaximal and thus mistakenly labeling sincere individuals as insincere. This mistake could be very damaging to the patient as it may possibly lead to diminished medicolegal compensation and even a job loss.16,17 Other sincerity of effort assessments, such as electromyography28,36e38 and the forceetime curve39,40 have been shown to be more effective but are clinically irrelevant because of their complexity and lack of commercial availability. The American Society of Hand Therapists (ASHTs) recommends that grip testing be performed with the dynamometer facing away from the client.41 In the absence of visual feedback, the person has to rely on proprioceptive feedback to replicate grip force. Visual feedback allows for better grip force control than proprioceptive feedback alone.42 Many sincerity of effort testing methods are founded on the premise that it is difficult to replicate a submaximal grip force during repeated trials without visual feedback, as the client is forced to rely on proprioceptive feedback alone. In the present study, we devised an assessment and named it the ‘‘Visual Target Grip Test.’’ This assessment is based on ‘‘tricking’’ a person to exert maximal effort by providing erroneous visual feedback. This method is based on allowing a person to depend on visual instead of proprioceptive feedback for replicating grip force and then providing him or her with an incorrect visual feedback to trick them into exerting greater grip force than they intended. More specifically, a ‘‘target line’’ derived from peak force of three ‘‘nonvisual’’ grip strength trials (exerted without visual feedback) is set on an oscilloscope (Figure 1). Then, the person is permitted to look at the oscilloscope while performing repeated grip strength trials and is instructed to reach that target line with each grip force exertion. Finally, the gain (sensitivity) of the oscilloscope is secretly cut in half requiring the person to exert twice as much force to reach the target line, which appears to remain in the same position on the oscilloscope (Figure 2). We
FIGURE 1. An example of setting the target line on the oscilloscope.
named the grip trials performed with the deceptive target line (incorrect visual feedback) ‘‘trick’’ trials and those performed with the correct visual feedback ‘‘non-trick’’ trials. Our first hypothesis was that the deceptive target line would cause participants to exert greater force during both maximal and submaximal efforts, as a pervious study found that visual feedback increased static grip by 7.7%.43 Our second hypothesis was that for the submaximal trials, the increase in force would be significantly greater than for the maximal trials. We predicted that when attempting to reach the deceptive target line, participants who were instructed to exert a submaximal effort would be ‘‘tricked’’ into exerting a lot more force than they intended. In addition, we expected that when participants exert a maximal effort, they would not be able to exert enough force to reach the deceptive target line, although they would exhibit an increase in grip strength. As true maximal voluntary contraction is almost never achieved during muscle strength testing, due to protective mechanisms,44 we expect that the deceptive visual feedback would result in increased grip force even during maximal effort exertion.43 However, we expected the grip force to be significantly more elevated for submaximal efforts than for maximal efforts. The validity of this sincerity of effort test is based on the second hypothesis, which states that submaximal effort is expected to yield a significantly greater increase in force between trick and non-trick trials than maximal effort and thus differentiate between maximal and submaximal efforts. The purpose of the study was threefold: 1) to examine the hypothesis that deceptive visual feedback will cause participants to exert greater force during both maximal and submaximal efforts, 2) to JulyeSeptember 2012 321
* An example taken from one subject. Because the gain of the “Handgrip Force” channel was changed from 1.0 to 0.5 in the “Trick Effort” column, the force has to be multiplied by 2. Thus, the curve is actually twice as high as it seems in the picture.
FIGURE 2. An example of grip force curves during gripping. examine the validity of the ‘‘Visual Target Grip Test’’ by a) calculating the differences in force between the ‘‘trick’’ and ‘‘non-trick’’ grip strength trials of maximal vs. submaximal efforts, b) calculating the sensitivity and specificity of this test, and 3) to examine the testeretest reliability of the repeated grip strength trials.
METHODS Participants Thirty healthy participants (15 males and 15 females) aged 21.3 þ 1.8 years participated in the study. Inclusion criteria included healthy adults aged 18e65 years, without upper extremity injury. Before testing, all participants read and signed the informed consent form approved by the Institutional Review Board at the University of Florida.
Equipment Force measurements were collected using a specialized dynamometer with a force transducer (Biopac Instruments, Goleta, CA). When gripping the dynamometer, the transducer converted the force into electrical impulses, which were measured as a change in voltage of direct current and transmitted to a digital oscilloscope (Gould Instruments, Hainault, Essex, England). From the oscilloscope, the impulses were sent to a signal processor unit (Grass Polygraph, Model 79, Grass Instruments, Inc., Braintree, MA) and then to an analog-to-digital converter (PowerLab, ADInstruments, Colorado Springs, CO). These digital signals were then transmitted to a computer with polygraph software (Chart, ADInstruments), which used a sampling frequency of 2000 Hz to store them and to calculate peak force values. The dynamometer was calibrated using the Grass polygraph so that each mVof current resulted in a 1-cm movement of the pen on the graph paper. Conversion values were calculated as 24.14 lb per Volt of current. In addition, we used the oscilloscope (Gould Instruments) for setting the target line, which the participants aimed to reach during the visual feedback grip strength trials. 322
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Procedure Each subject performed two sessions of grip testing, one maximal and one submaximal, in a randomized order after performing four practice trials to familiarize them with the task. The test administrator was blinded to the level of effort. A different investigator, uninvolved with the test administration, assigned the subject their level of effort using standardized instructions (for exact instructions, see Appendix A). The subject first performed three grip trials without visual feedback and a target line was set on the oscilloscope corresponding to the obtained maximal grip force. After the target line was established, the subject was permitted to look at the oscilloscope and was instructed to watch the target line and attempt to reach it during the eight subsequent grip trials. The first four grip trials were performed with the gain maintained at 1.0. The next four trials were randomized such that the gain remained at 1.0 for two trials (we called these ‘‘non-trick trials’’) and was cut in half (at a gain of 0.5) for the other two trials (we called these ‘‘trick trials’’). Cutting the gain in half meant that the subject had to exert twice as much grip force to reach the target line, although the appearance of the target line remained the same as that of a gain of 1.0. For all grip strength trials, the subject was seated in an adjustable chair without arm rests. The subject assumed the testing position recommended by the ASHTs.41 The subject’s feet were fully resting on the floor and the hips were as far back in the chair as possible, with the hips and knees positioned at approximately 90 degrees. The shoulder of the tested extremity was adducted and neutrally rotated, the elbow flexed at 90 degrees, with the forearm and wrist in a neutral position. The subject was instructed to maintain this position during all tests and was reminded and corrected by the investigators as needed. All grip trials consisted of a 5-second contraction, with a 30-second rest interval between trials. After a 10-minute break, the session was repeated with a different level of effort (either maximal or submaximal). The order of maximal and submaximal sessions was randomly assigned. The peak force generated
during each trial was recorded and used in the statistical analysis.
Data Analysis First, we wanted to examine our hypothesis that providing incorrect visual feedback will cause a person to exert greater grip force during both maximal and submaximal efforts. To do so, we compared the peak force between the trick and non-trick trials for both maximal and submaximal efforts. We then wanted to examine the validity of this test; in other words, our hypothesis that the increase in force brought about by the incorrect visual feedback will be significantly greater for the submaximal than the maximal grip strength trials. To accomplish this, we first determined the increase in force between trick and non-trick trials by calculating the percent difference using the formula: [(trick non-trick)/target line value 3 100]. We then compared the percent difference in peak force between maximal and submaximal efforts. Because significant gender differences in grip strength are well documented,45e47 we compared all force measurements separately for men and women. However, gender differences in percent difference in peak force between maximal and submaximal efforts are uncertain. Thus, we also calculated force measurements for all participants, that is, men and women combined. We performed three paired t-tests per measurement to compare 1) maximal trick vs. maximal non-trick trials, 2) submaximal trick vs. submaximal non-trick trials, and 3) the percent difference between maximal and submaximal efforts. The first two comparisons were conducted to establish whether or not there are differences between the trick and non-trick trials. These comparisons tested our first hypothesis that the deceptive target line will cause greater force exertion during both maximal and submaximal efforts. The third comparison was conducted to test our second hypothesis that there are greater differences between trick and non-trick trials for submaximal effort than for maximal effort. Because three t-tests were conducted on each
measurement for three separate samples (men, women, and all participants), we used the Bonferroni correction to adjust for nine comparisons, which set the alpha level at 0.05/9 ¼ 0.0056. In addition, sensitivity, specificity, and overall error rate values39,48 were calculated for percent difference of grip force for men, women, and all participants. Sensitivity was calculated by dividing the number of true positives by the total number of positives and specificity was calculated by dividing the number of true negatives by the total number of negatives.39,48 Overall error rate was calculated by the formula (1 sensitivity) þ (1 specificity). To find the optimal cutoff value, with the best combination of sensitivity and specificity, we generated three receiver operator characteristic (ROC) curves48,49 for men, women, and all participants, using multiple cutoff values of percent difference. The ROC curve was generated by plotting the true-positive rate against the false-positive rate for each of the above cutoff values. The true-positive rate is the sensitivity, whereas the false-positive rate is calculated by subtracting the specificity value from 1.00 (1.00 specificity).48,49 The proportional area under the curve (AUC) was calculated by counting the number of squares beneath the curve and dividing it by the total number of squares to obtain an index of discriminability.49 Finally, we examined testeretest reliability of force between two repeated trials of the various grip strength efforts by calculating Pearson’s Product Moment correlation coefficients.
RESULTS Values of peak force for men, women, and all participants are presented in Table 1. Our first hypothesis was accepted, as peak force of the trick trials was significantly greater than that of the nontrick trials, indicating that deceptive visual feedback caused participants to exert greater grip force during both maximal (a 28% increase for all participants) and submaximal (a 69% increase for all participants) efforts (the percent increase values for men and
TABLE 1. Comparisons of Peak Force between Trick and Non-trick Trials of Maximal and Submaximal Efforts for Men, Women, and All Participants Measurement Maximal effort (lb) Men Women All Submaximal effort (lb) Men Women All
Trick Trial (Average 6 SD)
Non-trick (Average 6 SD)
t-Test Value
Probability (p)
104.02 6 18.53 76.42 6 11.39 90.04 6 20.52
82.15 6 19.64 59.04 6 12.47 70.49 6 19.79
10.23 13.15 15.06
0.001* 0.001* 0.001*
61.98 6 14.90 52.63 6 14.22 57.94 6 15.45
38.07 6 8.32 29.63 6 8.26 34.04 6 9.41
12.16 12.56 17.73
0.001* 0.001* 0.001*
*Indicates significance at the 0.0056 alpha level.
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TABLE 2. Comparisons of Percent Difference of Peak Force between Maximal and Submaximal Efforts for Men, Women, and All Participants Percent Difference of Peak Force Men Women All
Maximal Effort (Average 6 SD)
Submaximal Effort (Average 6 SD)
t-Test Value
Probability (p)
25% 6 12 31% 6 15 28% 6 13
66% 6 17 73% 6 24 69% 6 20
7.94 6.58 10.28
0.001* 0.001* 0.001*
Percent difference was calculated using the formula: [(trick non-trick)/target line value 3 100]. *Indicates significance at the 0.0056 alpha level.
women are presented in Table 2). Our second hypothesis was accepted, as the change in force (percent difference) was significantly greater during submaximal effort than during maximal effort for men, women, and all participants (Table 2). Sensitivity and specificity values were calculated for percent difference cutoff values ranging from 0% to 115% (Table 3). The ROC curve revealed that for all participants the optimal cutoff value of the percent difference in force was 50%, yielding a sensitivity value of 0.83 (17% error) and a specificity value of 0.93 (7% error) for an overall error rate of 23% (Figure 3). The area under the ROC curve was 96.3% indicating excellent discriminability. When analyzing men and women separately, the optimal cutoff point remained 50% but the sensitivity and specificity values changed. For women, the optimal cutoff value yielded sensitivity of 0.87 and specificity of 0.87, an overall error rate of 26%, and an AUC of 94.6%. For men, the optimal cutoff value yielded sensitivity of 0.80 and specificity of 1.0, an overall error rate of 20%, and an AUC of 97.0%. Finally, teste retest reliability between repeated grip trials of maximal efforts ranged from r ¼ 0.87 to r ¼ 0.98 and for submaximal efforts ranged from r ¼ 0.67 to r ¼ 0.96 (Table 4).
DISCUSSION Deceptive Visual Feedback The findings of the present study supported the hypothesis that deceptive visual feedback caused participants to unknowingly exert greater force during both maximal and submaximal efforts. Reaching the deceptive target line required twice as much force as was needed to reach the original target line. Thus, attempting to reach the deceptive target line caused participants to exert a significantly greater force during both maximal and submaximal efforts. These increases were calculated in relationship to the force that was generated by the nonvisual grip strength trials, which was used to establish the target line. The formula used to calculate the percent increase was [(trick non-trick)/target line value 3 100]. A previous study found that providing a correct
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visual feedback increased static grip by 7.7% when compared with grip trials performed without visual feedback.43 In our study, the deceptive target line, which provided erroneously low visual feedback, caused grip force to increase by 28% during maximal effort (ranging from 11% to 58%) and by 69% during submaximal effort (ranging from 30% to 113%) (Table 2). These increases in force may be explained by two factors. First, humans rely on visual feedback for force control more readily than on any other sensory feedback.50 Secondly, healthy persons do not voluntarily exert a true maximal muscular contraction under most circumstances. Because exerting true maximal contraction requires the recruitment of all motor units available in a muscle, the inability of an untrained person to perform an ‘‘all out’’ muscle contraction serves as a defense mechanism against muscle injury.51 The twitch interpolation technique is used to study if humans can voluntarily elicit full muscle force.44,51,52 In this technique, electrical stimulation is superimposed during a voluntary contraction. When this electrical stimulation causes an increase in twitch force, the muscular contraction is considered to be submaximal and the voluntary activation is less than 100%.53 Without training, most humans cannot willingly exert true maximal contraction.44,51,52 For example, healthy participants were able to increase force produced during maximal grips when presented with verbal motivators,54 monetary incentives,55 and emotionally arousing photographs.56 In contrast, the ability to produce grip force is reduced when injury-related factors such as pain and fear of pain are present.57e60 In addition, the perception of the exerted force is increased in the presence of muscular fatigue.61
Validity of the ‘‘Visual Target Grip Test’’ To examine the validity of this test, we compared the percent difference between maximal and submaximal grip efforts. As expected, the increase in grip force between non-trick (nonvisual) and trick (the deceptive target line) grip strength trials was significantly greater for submaximal efforts than for maximal efforts (Table 2). This significantly greater percent difference [(trick non-trick)/target line value 3 100]
0.00 1.00 1.00 0.03 1.00 0.97 0.07 1.00 0.93 0.07 1.00 0.93 0.10 1.00 0.90 0.13 1.00 0.87 Bolded and italic numbers indicate the optimal cutoff value (50%) and its associated sensitivity, specificity, and overall error rate values.
0.27 1.00 0.73 0.33 1.00 0.67 0.37 1.00 0.63 0.47 1.00 0.53 0.53 1.00 0.47 0.63 1.00 0.37 0.67 0.97 0.37 0.83 0.93 0.23 0.93 0.80 0.27 0.97 0.80 0.23 0.97 0.73 0.30 0.97 0.67 0.37 1.00 0.50 0.50 1.00 0.27 0.73 1.00 0.20 0.80 1.00 0.00 1.00 1.00 0.00 1.00 All Sensitivity Specificity Overall error rate
1.00 0.00 1.00
0.00 1.00 1.00 0.07 1.00 0.93 0.13 1.00 0.87 0.13 1.00 0.87 0.20 1.00 0.80 0.27 1.00 0.73 0.40 1.00 0.60 0.40 1.00 0.60 0.47 1.00 0.53 0.47 1.00 0.53 0.47 1.00 0.53 0.60 1.00 0.40 0.60 0.93 0.47 0.87 0.87 0.26 0.93 0.73 0.33 1.00 0.73 0.27 1.00 0.67 0.33 1.00 0.67 0.33 1.00 0.33 0.67 1.00 0.20 0.80 1.00 0.13 0.87 1.00 0.00 1.00 1.00 0.00 1.00 Women Sensitivity Specificity Overall error rate
1.00 0.00 1.00
0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.13 1.00 0.87 0.27 1.00 0.73 0.27 1.00 0.73 0.47 1.00 0.53 0.60 1.00 0.40 0.67 1.00 0.33 0.73 1.00 0.27 0.80 1.00 0.20 0.93 0.87 0.20 0.93 0.87 0.20 0.93 0.80 0.27 0.93 0.67 0.40 1.00 0.67 0.33 1.00 0.33 0.67 1.00 0.27 0.73 1.00 0.00 1.00 1.00 0.00 1.00 1.00 0.00 1.00 Men Sensitivity Specificity Overall error rate
115 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 Cutoff Value (%)
TABLE 3. Sensitivity, Specificity, and Overall Error Rate of Percent Difference in Peak Force for Men, Women, and All Participants
indicates that the ‘‘Visual Target Grip Test’’ can indeed differentiate between maximal and submaximal efforts. To further assess the validity of this sincerity of effort test, we explored the test’s effectiveness by examining the sensitivity and specificity of the percent difference of force. A test’s effectiveness is defined as its ability to accurately assess the presence and absence of a submaximal (insincere) effort.48 Sensitivity corresponds to true positives, reflecting the percentage of people who were considered to exert a submaximal effort and really exerted a submaximal effort, whereas specificity corresponds to true negatives reflecting the percentage of people who were considered to exert a maximal effort and really exerted a maximal effort. There are trade-offs between specificity and sensitivity: as one increases the other decreases.48 Thus, it is imperative to establish an optimal cutoff value, which yields the best combination between sensitivity and specificity.49 A test that possesses low specificity will mistakenly classify people who exert maximal effort as having exerted submaximal effort. This error could lead to inappropriate diagnosis and treatment, reduced compensation settlement, withheld payments, and job loss.20,22 Conversely, a test that possesses low sensitivity would misclassify people who exert submaximal effort as having exerted maximal effort. This error could lead to unnecessary procedures, ineffective therapy, and increased disability and health care costs.22,25 We used two methods to find the optimal cutoff value: finding the overall error rate and constructing an ROC curve. The overall error rate is the percent of combined errors (false positive plus false negatives) at a specific cutoff value. The lowest overall error rate reflects the optimal cutoff value, which provides the best combination of sensitivity and specificity.48 The common sincerity of effort assessments, such as the coefficient of variation, rapid exchange grip test, and the fiverung test, possess overall error rates ranging from 47% to 69%.39 When compared, the ‘‘Visual Target Grip Test,’’ at its optimal cutoff value (percent change ¼ 50%), misclassified only 17% of submaximal efforts and 7% of maximal efforts of all participants for an overall error rate of 23% (Table 3). This overall error rate is exceptional, considering the overall error rates of most other sincerity of effort tests. This test was somewhat more effective in men as indicated by a lower overall error rate (20%) compared with women (26%). Similar to the overall error rate, the ROC curve illustrates the accuracy of detecting sincerity of effort over a range of percent difference cutoff values and reveals the best combination of sensitivity and specificity. In the present study, both the overall error rate and the ROC curve revealed that when the percent JulyeSeptember 2012 325
1.00
40%
0.90
50%*
0.80
0.70
55% Sensitivity
0.60
0.50
0.40
0.30
0.20
0.10
0.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1-Specificity
* The optimal cutoff point for peak force percent difference for all participants is 50% (sensitivity = 0.83; specificity = 0.93)
FIGURE 3. Receiver operator characteristic curve for percent difference in force for all participants.
difference (the increase in force) between trick and non-trick trials is 50% or larger (the optimal cutoff value), one can diagnose the effort as submaximal and misclassify only 7% of all participants who exerted maximal effort as having exerted a submaximal effort. Conversely, when the percent difference is smaller than 50%, one can diagnose the effort as maximal and misclassify only 17% of all participants who exerted submaximal effort as having exerted maximal effort. At the 50% cutoff value, men had better specificity but poorer sensitivity than women. None of the men were misclassified as exerting submaximal effort when really exerting maximal effort and 20% of them were misclassified as exerting maximal effort when exerting submaximal effort. For women, 13% were misclassified as exerting submaximal effort and 13% as exerting maximal effort. These error rates suggest that the ‘‘Visual Target Grip Test’’ possesses acceptable levels of sensitivity and specificity and can effectively detect sincerity of effort.
TABLE 4. Testeretest Reliability of Peak Force for Trick and Non-trick Trials of Maximal and Submaximal Efforts for Men, Women, and All Participants Men Peak Force
Women
All
r
p
r
p
r
p
0.869 0.936
0.001* 0.001*
0.905 0.976
0.001* 0.001*
0.931 0.959
0.001* 0.001*
Submaximal effort Trick trials 0.964 Non-trick trials 0.666
0.001* 0.007*
0.906 0.955
0.001* 0.001*
0.937 0.83
0.001* 0.001*
Maximal effort Trick trials Non-trick trials
Testeretest Reliability We wanted to assess testeretest reliability because a test that is not reliable cannot be valid.48 However, we did not administer the test twice, so we could not assess the reliability of the actual ‘‘Visual Target Grip Test.’’ Rather, we assessed the reliability of its components; namely, the force of repeated grip trials. The testeretest reliability values for repeated grip strength trials (Table 4) were somewhat higher for maximal effort (ranging from r ¼ 0.87 to r ¼ 0.98) than for submaximal efforts (r ¼ 0.67 to r ¼ 0.96).
Limitations
*Indicates significance at 0.05 alpha level.
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The proportional area under the ROC curve can be used as an index for the amount of overlap between the distributions of true positives and false positives, which provides a measure of discriminability. A perfect test will have an area of 100% under the curve.49 When considering sincerity of effort testing, discriminability is the ability to discriminate between maximal and submaximal efforts. A larger AUC indicates a better ability of a test to discriminate between maximal and submaximal efforts. In the present study, the AUC was 96.3%, indicating a good ability to discriminate between maximal and submaximal efforts. The AUC indicated somewhat better discriminability for men (97.0%) than for women (94.6%).
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The main limitation of this study include a small sample size and the use of a noninjured population. The present study was conducted on healthy participants, as it is only the first step in establishing the ‘‘Visual Target Grip Test’’ as a valid sincerity of effort
assessment. Difference may exist between healthy participants instructed to fake weakness in a laboratory experiment and patients who exert insincere effort for a secondary gain. To validate this test for the proper patient population and deem it clinically appropriate, future studies must be conducted on persons with upper extremity injury. Most importantly, this test may not be safe for clinical use, especially during initial therapy, because it leads to an unconscious and unintentional increased exertion of grip force, which could raise the potential for reinjury. However, the test may be appropriate for patients who can safely exert maximal grip force. An additional limitation is that the testeretest reliability was assessed only for the components of the test but not for the test as a whole. Future studies must examine testeretest reliability of the ‘‘Visual Target Grip Test’’ as a whole. Lastly, the testing device is not available to clinicians as it was assembled in our laboratory. Future work would include developing and testing a clinically friendly version of our device.
CONCLUSIONS The present study comprises the first step in establishing the feasibility and validity of a new method for testing sincerity of effort by using a deceptive visual feedback to differentiate between maximal and submaximal efforts. We found that the increase in grip force between trick and non-trick grip trials was significantly greater for submaximal effort than for maximal effort, indicating that the ‘‘Visual Target Grip Test’’ can detect submaximal grip strength efforts. Further, the test possesses good effectiveness as indicated by a relatively small overall error rate of sensitivity and specificity and a large area under the ROC curve. Incorporating this assessment into a functional capacity evaluation would be relatively simple and clinically beneficial. However, this test may not be safe for clinical use during the initial therapy period because it causes the individual to unknowingly exert greater grip force than intended, and thus raises the potential for reinjury. Yet, this test is appropriate for patients who can safely exert maximal grip force. Acknowledgment This work was supported in part by the Research Opportunity Fund of The University of Florida.
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53. Awiszus F, Wahl B, Meinecke I. Influence of stimulus cross talk on results of the twitch-interpolation technique at the biceps brachii muscle. Muscle Nerve. 1997;20:1187–90. 54. Aarts H, Custers R, Marien H. Preparing and motivating behavior outside of awareness. Science. 2008;319:1639. 55. Schmidt L, Palminteri S, Lafargue G, Pessiglione M. Splitting motivation: unilateral effects of subliminal incentives. Psychol Sci. 2010;21:977–83. 56. Pessiglione M, Schmidt L, Clery-Melin ML, et al. Get aroused and be stronger: emotional facilitation of physical effort in the human brain. J Neurosci. 2009;29:9450–7. 57. Farina D, Arendt-Nielsen L, Graven-Nielsen T. Experimental muscle pain decreases voluntary EMG activity but does not affect the muscle potential evoked by transcutaneous electrical stimulation. Clin Neurophysiol. 2005;116:1558–65. 58. Farina D, Arendt-Nielsen L, Merletti R, Graven-Nielsen T. Effect of experimental muscle pain on motor unit firing rate and conduction velocity. J Neurophysiol. 2004;91:1250–9. 59. Graven-Nielsen T, Svensson P, Arendt-Nielsen L. Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function. Electroencephalogr Clin Neurophysiol. 1997;105:156–64. 60. Mense S, Skeppar P. Discharge behaviour of feline gammamotoneurones following induction of an artificial myositis. Pain. 1991;46:201–10. 61. Jones LA. The senses of effort and force during fatiguing contractions. Adv Exp Med Biol. 1995;384:305–13.
APPENDIX A STANDARDIZED INSTRUCTIONS FOR MAXIMAL AND SUBMAXIMAL TRIALS, AND FOR REACHING THE TARGET LINE Instructions for maximal effort were as follows: ‘‘In this session, I want you to give maximal effort with your dominant hand while performing all tests. Follow the directions of the test administrator to exert full effort. Do you have any questions?’’
Instructions for submaximal effort were as follows: ‘‘In this session, I want you to imagine yourself in the following scenario: Imagine that 2 years ago you were involved in an accident at work in which your right hand was injured. Although now completely recovered, you are suing the insurance company for weakness of grip. In other words, you will be faking weakness during the grip strength tests because you are trying to gain money unlawfully. Please do your best to convince the test administrator that your claim is sincere, in other words, try to be consistent in repeating the force of your grip. The test administrator will be asking you, throughout the testing session, to give maximal effort, but you need to remember to feign weakness of grip with your ‘injured’ hand. Do you have any questions?’’
Instructions for reaching the target line during the visual grip strength trials were as follows: ‘‘Make sure that the grip instrument is not resting on your lap. Remember to sit in the right position. Try to reach the line in front of you at GO and keep giving consistent effort till I say STOP.’’ ‘‘READY.SET.GO.’’ Five seconds later, ‘‘STOP.’’
JHT Read for Credit Quiz: Article #233
Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to JHTReadforCredit.com. There is only one best answer for each question. #1. The method described showed ______________in detecting sub-maximal effort a. moderate validity b. high specificity, but low sensitivity c. high sensitivity, but low specificity d. both high sensitivity and specificity #2. The method a. was shown to be safe for initial evaluation b. has been approved by the ASHT c. tricked subjects to exert maximal effort d. tricked subjects to exert sub-maximal effort #3. The key to this method is to provide
a. incorrect visual feedback b. forceful verbal cues c. deceptive voice modulation in cuing d. blindfold the subject during testing #4. Clinically, sub-maximal effort a. is almost always intentional b. may be either intentional or unintentional c. is almost always unintentional d. is usually associated with mild psycho pathology #5. The study established the validity of this method for orthopedic patients a. true b. false When submitting to the HTCC for re-certification, please batch your JHT RFC certificates in groups of 3 or more to get full credit.
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