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Reliability of the grip strength coefficient of variation for detecting sincerity in normal and blocked median nerve in healthy adults
Available online at
ScienceDirect www.sciencedirect.com Hand Surgery and Rehabilitation 36 (2017) 90–96
Original article
Reliability of the grip strength coefficient of variation for detecting sincerity in normal and blocked median nerve in healthy adults Fiabilité du coefficient de variation de la force de poigne dans la détection de la sincérité chez des adultes sains avec des nerfs médians sains et bloqués N.J. Wachter a,*, M. Mentzel b, R. Hütz b, J. Gülke b b
a Clinic for Hand-, Plastic and Microsurgery, Klinikum Stuttgart, Kriegsbergstrasse 60, 70174 Stuttgart, Germany Clinic for Traumatology, Hand-, Plastic and Reconstructive Surgery, University of Ulm, Albert-Einstein-Allee 23, 89081 Ulm, Germany
Received 18 May 2016; received in revised form 27 September 2016; accepted 14 December 2016 Available online 7 February 2017
Abstract In the assessment of hand and upper limb function, detecting sincerity of effort (SOE) for grip strength is of major importance to identifying feigned loss of strength. Measuring maximal grip strength with a dynamometer is very common, often combined with calculating the coefficient of variation (CV), a measure of the variation over the three grip strength trials. Little data is available about the relevance of these measurements in patients with median nerve impairment due to the heterogeneity of patient groups. This study examined the reliability of grip strength tests as well as the CV to detect SOE in healthy subjects. The power distribution of the individual fingers and the thenar was taken into account. To assess reliability, the measurements were performed in subjects with a median nerve block to simulate a nerve injury. The ability of 21 healthy volunteers to exert maximal grip force and to deliberately exert half-maximal force to simulate reduced SOE in a power grip was examined using the Jamar1 dynamometer. The experiment was performed in a combined setting with and without median nerve block of the same subject. The force at the fingertips of digits 2–5 and at the thenar eminence was measured with a sensor glove with integrated pressure receptors. For each measurement, three trials were recorded subsequently and the mean and CV were calculated. When exerting submaximal force, the subjects reached 50–62% of maximal force, regardless of the median nerve block. The sensor glove revealed a significant reduction of force when exerting submaximal force (P1 sensor) with (P < 0.032) and without median nerve block (P < 0.017). An increase in CV at submaximal force was found, although it was not significant. SOE can be detected with the CV at the little finger at using a 10% cut-off (sensitivity 0.84 and 0.92 without and with median nerve block, respectively). These findings suggest low reliability of the power grip measurement with the Jamar1 dynamometer, as well as that of the CV for detecting SOE. However, the combination of finger forces including the thenar area and the CV at the little finger could lead to better reliability for detecting feigned reduction of grip strength. The methods were as reliable in subjects with a median nerve block as in healthy subjects. # 2017 SFCM. Published by Elsevier Masson SAS. All rights reserved. Keywords: Median nerve lesion; Sincerity of effort; Dynamometry; Coefficient of variation
Résumé Dans l’évaluation de la fonction de la main du membre supérieur, il est primordial de détecter la sincérité de l’effort (SDE) lors de la force de poigne pour identifier les pertes feintes de force. La mesure de la force maximale de poigne avec un dynamomètre est très répandue, souvent combinée au calcul du coefficient de variations (CV), une mesure de la variation lors de trois essais répétés successivement de la force de poigne. Peu de données sont disponibles concernant la pertinence de ces mesures chez les patients présentant une pathologie du nerf médian, en raison de l’hétérogénéité des groupes de patients. Le présent travail a évalué la fiabilité des tests de force de poigne et le CV pour détecter la SDE chez les sujets sains. La distribution de la puissance selon les différents doigts et l’éminence thénar a été prise en compte. Les mesures ont également été réalisées chez les mêmes sujets après un bloc du nerf médian simulant un groupe de patients présentant une lésion nerveuse pour en évaluer la
* Corresponding author. E-mail address: [email protected] (N.J. Wachter). http://dx.doi.org/10.1016/j.hansur.2016.12.003 2468-1229/# 2017 SFCM. Published by Elsevier Masson SAS. All rights reserved.
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fiabilité. Dans cette étude, 21 volontaires sains ont été examinés quant à leur capacité à développer une force de poigne maximale et une force de poigne délibérément diminuée de sa moitié pour simuler une SDE lors d’une prise de poigne évaluée avec le dynamomètre JAMAR. L’expérimentation a été réalisée avec et sans bloc du nerf médian chez le même sujet. De plus, la force développée aux pulpes des doigts 2 à 5 et au niveau de l’éminence thénar a été mesurée à l’aide d’un gant sensitif équipé de capteurs de pression. Pour chaque mesure, trois essais ont été enregistrés successivement, la moyenne et le CVont été calculés. Lors de la prise de force avec le dynamomètre JAMAR chez les sujets exerçant une force submaximale, les patients développaient 50 à 62 % de la force maximale, même après bloc du nerf médian. En mesurant de la force développée au niveau de l’éminence thénar par le gant sensitif, on constatait une réduction significative de la force lorsque que le sujet exerçait une force submaximale (capteur P1), avec ( p < 0,032) et sans bloc du nerf médian ( p < 0,017). On enregistrait également une augmentation du CV lors des prises à force submaximale, bien qu’elle ne soit pas significative. Il était possible de détecter la SDE et le CV au petit doigt avec un seuil de 10 % (sensibilité 0,84 et 0,92, respectivement sans et avec bloc du nerf médian). Ces expérimentations suggèrent une faible fiabilité des mesures de la force de poigne avec le dynamomètre JAMAR ainsi que celle du CV pour la détection de la SDE. Cependant, sa combinaison de la mesure des forces exercées par les doigts et l’éminence thénar et le CV au niveau du petit doigt pourrait améliorer la fiabilité lors du repérage d’une réduction feinte de force de poigne. La fiabilité de ces méthodes chez les sujets après bloc du nerf médian était comparable à celles des sujets sains. # 2017 SFCM. Publié par Elsevier Masson SAS. Tous droits réservés. Mots clés : Lésion du nerf médian ; Sincérité de l’effort ; Dynamométrie ; Coefficient de variation
1. Introduction Grip strength measurements are used to determine the extent of disability, loss in work capacity, readiness to return to work, and the amount of financial compensation for loss of function or work resulting from an injury [1]. Dynamometers such as the Jamar1 hydraulic dynamometer are used to measure grip strength. The Jamar1 dynamometer has been found to be a very accurate and reliable measurement tool of objective grip strength [2], but detecting sincerity of effort (SOE) remains difficult [2,3]. Several methods can be used by researchers and clinicians to determine SOE. The three SOE tests most commonly used in the clinic because of their affordability, simplicity and brevity are the five-rung grip test (5RGT) [4], the rapid exchange grip test (REGT) [5,6], and the coefficient of variation (CV). The CV has been used regularly for detecting SOE [3,7–9]. The CV measures the variability of three or more grip strength trials. It is calculated by dividing the standard deviation (SD) by the mean of these grip trials and multiplying the result by 100; thus, the CV is expressed as a percentage [8– 10]. According to the motor unit recruitment model [8,11], the variability during repeated grip trials with submaximal effort is higher than during a maximal effort [3]. Consequently, larger CV values indicate greater variability [3,7]. To consider a trial as inconsistent and hence insincere, a cut-off value must be established [8–10,12]. In the literature, the CV cut-off value ranges from 7.5% to 20% [7,12]. The measurements in most studies on this topic were performed in healthy subjects who simulate a feigned loss of grip strength by pressing the dynamometer with approximately 50% of maximal force [3,8]. Recent studies have questioned whether these measurements are also applicable to subjects with median nerve disorders such as carpal tunnel syndrome (CTS) or nerve injuries, where the sensory-motor deficits lead to a decrease in force accuracy and an increase in force variability [13,14]. The aim of this study was to evaluate the reliability of grip strength and the CV for detecting the SOE in healthy subjects with a distal median nerve block as a means to simulate a
patient group, in order to compare the measurements in the same subjects without a block as control. A median nerve block at the wrist is supposed to affect the thenar muscles (abductor pollicis brevis, superficial portion of the flexor pollicis brevis, opponens pollicis) and the lateral two lumbricals. The reduction of grip strength after median nerve damage ranges from 20% to 70%, depending on the measurement method [15]. For this reason, the power distribution of the fingers and the thenar prominence was taken into account. We also wanted to determine if using a cut-off value for the CV of single fingers could improve the detection of SOE.
2. Material and methods Twenty-one healthy subjects participated in this study who had an average age of 31.5 years (23–41 years). Only male subjects were included who were right-hand dominant. The hand length over the middle finger was 18.9 cm (1.1 cm) and the distance from the tip of the thumb and little finger was 21.8 cm (1.8 cm). A clinical examination performed by a hand surgeon revealed no pathology. The study examined the power grip using a hydraulic hand dynamometer set to the middle position (Baseline1 Hydraulic Hand Dynamometer, Fabrication Enterprises Inc., Irvingston, NY 10533, USA) (Fig. 1). A sensor glove was used for the finger measurements. The TUB-sensor glove (Technical University of Berlin) is equipped with 5 pressure sensors (Fig. 1) located on the palmar surface of the glove (FSR 151 NS, International Electronics and Engineering, Luxembourg). Data were displayed in force–time diagrams (Fig. 2). Each measurement was performed over a 15-s period, during which the grip was repeated three times according to the established protocol for measuring power grip strength [2]. For each of the three trials, the grip strength was determined at the beginning of the plateau of the force–time diagram [16,17] (Fig. 2). The mean value of the three trials (f1, f2, f3) was calculated. For all subjects, the mean and SD were determined and displayed in a diagram (Fig. 5).
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Fig. 1. Power grip using a Baseline1 dynamometer (middle position) and position of the five pressure sensors at the TUB Sensor-glove.
For the median nerve block, local anesthesia was used in all 21 subjects; 5 ml of Mepihexal sine 1% was injected under aseptic conditions using a sterile 25-gauge needle directly proximal to the transverse carpal ligament (Fig. 3). The sensitivity was evaluated before and after the block, using the two-point discrimination test with a threshold of 2 mm. Criteria for a successful block were loss of sensation in the median nerve area and a positive bottle sign [18]. Within approximately 6 h, all subjects regained their sensation and motor function. The application of local anesthesia was approved by the ethics board of Ulm University (No. 36/2003). Measurements were taken from the right hand of all participants in a standardized upright sitting position with the elbow in 90-degree flexion and the wrist joint in 15-degree extension. Before each measurement, the standardized position of the glove and the sensors was confirmed. The power grips were first carried out with the median nerve block and then without the median nerve block in another session with the dynamometer set to middle position (6 cm span) (Fig. 1). Both measurements were recorded on two different days within 1 week by random order to exclude the effects of fatigue and persistent effect of the block. For the submaximal effort measurements, the subjects were instructed to deliberately exert 50% of their maximal effort in the initial session and to exert 100% in a second session after a 1-h rest. These conditions were used to minimize the effect of fatigue. The maximal power f1
f2
f3
a1
a2
a3
grip strength was recorded using the dynamometer scale in kilograms and then converted into Newtons. The mean and SD were calculated for all measurements (Figs. 4–7). For statistical evaluation, the Mann Whitney-U test was used to compare the dynamometer and sensor data based on the factors of half-force or full force (P < 0.05). The CV was calculated by dividing the SD by the mean (AVE) of the three grip trials and multiplying by 100, using the formula: CV = (SD/AVE) 100 [7]. The sensitivity and specificity of the CV was calculated for the dynamometer and the P1 (thenar prominence), D2 (fingertip of index finger), D3 (fingertip of middle finger), D4 (fingertip of ring finger) and D5 (fingertip of little finger) sensors using the optimized cut-off level of 0.10 according to Robinson et al. [12]. In addition, ROC curves were used to determine an appropriate cut-off level from our own data (Fig. 8). All statistics were calculated with StatView software (SAS Institute Inc., Version 5.0).
3. Results 3.1. Grip strength The mean maximum grip strength was 249 N (66.9 N) without and 228 N (54.6 N) with median nerve block (Fig. 4). The difference was not significant. The submaximal force was
power [N]
8 6 4 2 0
0
4
8
12
time [s] Fig. 2. Assessing the force in a force–time diagram with 3 trials (a1–a3) within 15 s.
Fig. 3. Median nerve block at the wrist.
350
0.6
300
0.5
250 median nerve block no block
150 100 50
0.1
0
FF
power [N]
20 15
no block HF no block FF
10 5 0 D3
D4
D5
25
power [N]
20 15
median nerve block, HF median nerve block, FF
10 5
P1**
D2
D3
D4
D5
Fig. 5. Power distribution of P1 and D2–D5 (*P < 0.017, **P < 0.032). FF: full force; HF: half force.
120 100 80 [%]
no block median nerve block
60 40 20 0 D2
D3
D4
FF median nerve block FF no block
JAMAR
25
D2
HF no block
0
HF
Fig. 4. Measurement of grip strength using a dynamometer [N]. FF: full force; HF: half force.
P1*
HF median nerve block
0.3 0.2
0
93
0.4
200
CV
power [N]
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D5
Fig. 6. Submaximal force/maximal force 100 in [%] at the D2–D5 sensors.
106 N (28.6 N) without and 121 N (38.7 N) with median nerve block; the difference was not significant. A significant difference was found between the submaximal and maximal grip force in all subjects. This difference was not influenced by the median nerve block. 3.2. Power distribution The power distribution of the maximal grip strength measured at the fingertips showed a maximal force at the ring
P1
D2
D3
D4
D5
Fig. 7. Coefficient of variation (CV) during power grip with dynamometer and sensors. FF: full force; HF: half force.
finger, followed by the middle, index and little finger (Fig. 5, upper graph). Although there was a remarkable reduction when exerting submaximal force, the differences were not significant. The force measured at the thenar eminence (P1 sensor) was 13.0 N (0.9 N) during maximum effort and 5.8 N (0.32 N) during submaximal effort; this difference was significant (P < 0.017). In the trials with median nerve block, the power distribution at the finger tips was similar to the previous measurements, although the force levels were slightly lower (Fig. 5, lower graph). The force at the P1 sensor during maximal effort was 10.4 N (0.80 N); it was significantly reduced to 4.7 (2.2 N) during submaximal effort (P < 0.037). No significant difference was found between the grip strength of healthy subjects exerting submaximal force and subjects with a median nerve block exerting maximal force (Fig. 5). The fraction of the submaximal force to the maximal force was 50–62% for all fingers (Fig. 6). This fraction was not influenced significantly by the median nerve block. 3.3. Coefficient of variation In healthy subjects, the CV for the grip strength, thenar eminence (P1 sensor) and the fingertip sensors was higher during the submaximal effort than during the maximal effort; the difference however was not significant. With the median nerve block, the CV did not differ significantly between the submaximal and maximal efforts (P < 0.05, Fig. 7). Based on our data, an optimum cut-off level of 0.10 for dynamometer and all finger sensors was found using the ROC curve, which confirms published data (Fig. 8). The CV for the sensor at the little finger could distinguish between submaximal and maximal grip strength effort with an acceptable sensitivity (0.84) and specificity (0.50). This was also valid with the median nerve block (0.92 and 0.50). If the CV cut-off is set to 0.10, a relatively low sensitivity for the dynamometer and the P1 and D2–D4 sensors was found for detecting submaximal grip strength (Table 1). 4. Discussion In most studies in which grip strength is measured to detect injuries, healthy subjects are compared to patients with defined injuries. In these studies, there is a certain bias due to interindividual differences in a group of healthy subjects as well
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100%
D5 no block
100%
D5 block
0,05
80%
0,05
0,10
0,10
80% 0,15
60%
60%
0,15
20%
20%
sensitivity
40%
sensitivity
40%
0% 100% 80% 60% specificity
40%
20%
0%
0% 100% 80% 60% specificity
40%
20%
0%
Fig. 8. ROC curves for D5 displaying the cut-off labels in boxes. 95% confidence intervals are the dashed lines. Table 1 Sensitivity and specificity of the coefficient of variation at a cutoff level of 10%.
No block No block Median nerve block Median nerve block
Sensitivity Specificity Sensitivity Specificity
Jamar
P1
D2
D3
D4
D5
0.50 0.50 0.55 0.45
0.42 0.75 0.42 0.42
0.50 0.50 0.42 0.58
0.50 0.50 0.67 0.33
0.46 0.54 0.67 0.33
0.84 0.50 0.92 0.50
P1: thenar eminence; D2: fingertip of index finger; D3: fingertip of middle finger; D4: fingertip of ring finger; D5: fingertip of little finger.
as the heterogeneity of the injuries in the patient group [4,5,19]. To avoid these effects, we simulated a median nerve injury in healthy subjects by performing a median nerve block at the wrist [18]. Due to the standardized examination of the subjects in our study for clinical signs of median nerve palsy, the simulated injury was very homogenous in the median nerve block trials. In addition, the combined test setting eliminated interindividual differences in the force distribution of the fingers during a power grip. Our study found a reduction in grip strength when a median nerve block is applied; however this difference was not significant. In patients with neuropathy of the median nerve, reduced intrinsic muscle strength has been found with different dynamometers [20]; however, there was only a moderate decrease of total grip strength. For the thenar area (P1 sensor), a significant reduction of power after the median nerve block was found. This can be explained by the fact that the power vector of the sensor located at the thenar eminence differs from the power vector of the dynamometer, thus rather representing the force of the thenar muscles. During a power grip, palmar abduction and thumb flexion are most relevant. Boatright et al. reported an average 72.3% reduction in palmar abduction power with a median nerve block [21]. This was confirmed by other studies reporting a major loss of palmar abduction and thumb flexion strength with a median nerve block [18,22,23]. The dynamometer is an established grip strength measurement tool, indicating the maximal power of a trial [2,6,9,19,24– 26]. To evaluate the force distribution of single fingers in this study, a TUB-sensor glove equipped with five pressure sensors
located at the fingertips and thenar eminence was used [27]. A standardized experimental setting was defined to avoid measurement bias [27,28]. The absolute strength values are of less importance than the relationship between the grip strength of the various fingers [17]. By analyzing the force at the fingertips, we found reduced force values with the median nerve block, confirming the findings of previous studies [20]. The interindividual variation in the force at the fingers was higher with a median nerve block; the highest difference was found at the little finger, although it was not significant. Gulke et al. [16] reported that the little finger plays a decisive role in this as well. They found significant differences in strength distribution patterns, depending on whether the power grip trial was performed at maximal or submaximal effort. In maximal gripping, the total measured strength was distributed relatively evenly over all four fingers, with each finger contributing 23– 27% of the total strength. In submaximal gripping, the little finger was involved very little and only contributed 14–15% of the total strength, with the remainder of the force distributed relatively evenly between the index, middle and ring fingers, with each of them contributing 26–32% of the total [16]. Our data confirms these findings in the subjects with and without a median nerve block. However, these changes in the power distribution were not significant. The power distribution between the fingers could be one of the relevant criteria for detecting submaximal grip effort. Several studies have included healthy subjects who are asked to simulate a loss of grip force in order to detect a feigned loss of grip strength [7,8,12,25]. Most studies demonstrated that
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the subjects successfully exerted approximately 50% of maximal effort as instructed [8,12,29]. These findings were confirmed by our results with the subjects being able to exert half-force in the power grip position using the dynamometer (45%). With a median nerve block, there was only a moderate change of the median submaximal force to 56%. For the force at the fingertips, values between 41% and 63% were found, with higher variations during the median nerve block. The small effect of distal median nerve block on grip strength control can be explained by the fact that anticipatory force control highly influences grip strength, whereas sensory information from the grasping digits is less important and instead is needed to modify the grip strength and update sensory motor memories to support anticipatory grip strength control [22,30]. However, in our study we could not distinguish between subjects with and without a median nerve block in a combined setting that eliminated interindividual differences. Analyzing the force of the individual fingers also did not improve this distinction. This can be explained by the fact that there was a relatively large variation of grip force in the individual fingers. This variation can interfere with the reduction of total grip strength that was found when exerting submaximal effort in the power grip. A distinction could be made between submaximal and maximal effort by adding a sensor at the thenar eminence (P1), regardless of median nerve block. This could be a useful supplement for the evaluation of SOE. Since SOE detection with grip strength measurements is difficult, various statistical methods have been developed by clinicians and researchers in an attempt to detect SOE. Sindhu et al. examined the ability of three tests to detect SOE, namely the 5RGT, REGT, and CV. When examining the independent contribution of each tests to their combined predictive validity, they found that the CV contributed the most and the REGT contributed the least. The CV had the greatest independent contribution toward the combined predictive ability in both the logit and linear regression models. Although the predictive ability improved when combining the 5RGT, REGT and CV, it was still not sufficient to warrant their clinical use, either alone or in combination [3]. According to the motor unit recruitment model [10], a submaximal effort would have more variability and less consistency during repeated grip trials than a maximal effort. Consequently, larger CV values indicate greater variability [3]. In our study of the power grip, the CV was higher when exerting submaximal force versus maximal force for almost all sensors; this difference however was not significant. When exerting maximum force, the CV was higher with a median nerve block than without the block; however this was not significant. Robinson et al. [12] found that the CV was significantly greater for submaximal than for maximal force when the subjects were performing three trials; however, they reported that using the CV to determine SOE is questionable. In other studies, no significant differences were reported between the CV at maximal and submaximal effort [2,27,31,32], which is consistent with our data. Still the use of CV for detecting SOE is controversial [7,12]. Ashford et al. found no significant differences in variability between maximal and submaximal
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efforts in 22 healthy subjects. They concluded that the current protocol for testing grip strength with a dynamometer allows for consistent submaximal effort [2]. Shechtman examined a group of patients with CTS and concluded that the CV as a measure of SOE, especially in injured individuals with compromised hand strength, can produce contradictory results [9,12,31]. Some studies indicate that the CV is not a valid measure of SOE because it increases during submaximal effort due to a decrease in the mean, not to an increase in absolute variability, concluding that determinations of SOE should not be made based solely on the CV [8,32]. The CV has been reported to possess low sensitivity and/or specificity as a screening tool [7,12]. For grip strength, the percentage of submaximal effort misclassified as maximal is 22–69%, depending on the CV cutoff value, a threshold above which an effort is considered to be inconsistent and hence insincere [7,12]. The CV cut-off value ranges from 7.5% to 20% in the literature; thus, the disagreement regarding the validity of the CV is not surprising [2,7,33]. In our study, a cutoff value of 10% was determined to be optimum, which lies in the range of published values and corresponds to the optimized cutoff level reported by Robinson et al. [12]. A high sensitivity for CV at the 10% cutoff level was only found for the D5 sensor. This could imply that the differences between the fingers should be taken into account; force measurement and CV calculation for the little finger can help to determine the SOE. Recent studies have reported that sensorimotor deficits associated with CTS reduce the accuracy and stability in finger force control. The fact that the CV of the little finger was a good predictor for SOE could therefore be a consequence of the sensorimotor deficits caused by the median nerve block [22]. The limitations of this study include the use of healthy subjects and the nonclinical setting. These limitations reduce our ability to generalize the results to subjects with upper extremity injuries in a clinical setting. 5. Conclusion In this study, the reliability of grip strength, finger power and the CV for detecting SOE was examined. Twenty-one healthy subjects performed a power grip maneuver, exerting maximal force and submaximal force in a combined setting; the measurements were repeated with a median nerve block on a different day. The subjects could deliberately exert halfmaximal grip force and the SOE could not be detected by measuring only grip strength. The difference between feigned submaximal grip force of healthy subjects and maximal force with median nerve block was significant. By adding a sensor at the thenar area, a distinction between maximal power and feigned loss of grip force in healthy subjects and in subjects with median nerve block could be made. When stimulating a feigned effort, the CV was reduced when compared to maximal force; however it was not significant. No significant difference in CV was found between the subjects with median nerve block exerting maximal force and the healthy subjects feigning reduced grip strength. At an optimal CV cut-off of 10%, a
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sensitivity of 0.84 for the grip force at the fifth finger was found (0.92 with nerve block). These experiments suggest a low reliability of the grip strength measurement and the CV of the power grip for the detecting SOE in healthy subjects. However, the combination of measuring the finger force including the thenar area and the decisive role of the little finger in the power distribution and CV could lead to better reliability for detecting a feigned reduction of grip strength. The reliability of these methods in subjects with a median nerve block was comparable to that in healthy subjects. Future studies should evaluate whether the method can also be transferred to patient groups like the median nerve block group in this study. Disclosure of interest The authors declare that they have no competing interest. References [1] Shechtman O, Sindhu BS, Davenport PW. Using the ‘‘visual target grip test’’ to identify sincerity of effort during grip strength testing. J Hand Ther 2012;25:320–8. [2] Ashford RF, Nagelburg S, Adkins R. Sensitivity of the Jamar dynamometer in detecting submaximal grip effort. J Hand Surg Br 1996;21:402–5. [3] Sindhu BS, Shechtman O, Veazie PJ. Identifying sincerity of effort based on the combined predictive ability of multiple grip strength tests. J Hand Ther 2012;25:308–18. [4] Goldman S, Cahalan TD, An KN. The injured upper extremity and the JAMAR five-handle position grip test. Am J Phys Med Rehabil 1991;70:306–8. [5] 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. [6] Westbrook AP, Tredgett MW, Davis TR, Oni JA. The rapid exchange grip strength test and the detection of submaximal grip effort. J Hand Surg Am 2002;27:329–33. [7] Shechtman O, Anton SD, Kanasky Jr WF, Robinson ME. The use of the coefficient of variation in detecting sincerity of effort: a meta-analysis. Work 2006;26:335–41. [8] 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. [9] Shechtman O. Is the coefficient of variation a valid measure for detecting sincerity of effort of grip strength? Work 1999;13:163–9. [10] 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. [11] Kroemer KH, Marras WS. Towards an objective assessment of the ‘‘maximal voluntary contraction’’ component in routine muscle strength measurements. Eur J Appl Physiol Occup Physiol 1980;45:1–9. [12] 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.
[13] Geere J, Chester R, Kale S, Jerosch-Herold C. Power grip, pinch grip, manual muscle testing or thenar atrophy – which should be assessed as a motor outcome after carpal tunnel decompression? A systematic review. BMC Musculoskelet Disord 2007;8:114. [14] Li K, Evans PJ, Seitz Jr WH, Li ZM. Carpal tunnel syndrome impairs sustained precision pinch performance. Clin Neurophysiol 2015;126:194– 201. [15] Rajan P, Premkumar R, Rajkumar P, Richard J. The impact of hand dominance and ulnar and median nerve impairment on strength and basic daily activities. J Hand Ther 2005;18:40–5. [16] Gulke J, Wachter NJ, Katzmaier P, Ebinger T, Mentzel M. Detecting submaximal effort in power grip by observation of the strength distribution pattern. J Hand Surg Br 2007;32:677–83. [17] Mentzel M, Apic G, Wachter NJ, Bauknecht S, Gulkin D, Gulke J. Assessment of force patterns of different primary grips through dynamic force measurement using a sensor glove. Handchir Mikrochir Plast Chir 2011;43:155–61. [18] Kozin SH, Porter S, Clark P, Thoder JJ. The contribution of the intrinsic muscles to grip and pinch strength. J Hand Surg Br 1999;24:64–72. [19] Stokes HM, Landrieu KW, Domangue B, Kunen S. Identification of loweffort patients through dynamometry. J Hand Surg Am 1995;20:1047–56. [20] Schreuders TA, Roebroeck ME, Jaquet JB, Hovius SE, Stam HJ. Longterm outcome of muscle strength in ulnar and median nerve injury: comparing manual muscle strength testing, grip and pinch strength dynamometers and a new intrinsic muscle strength dynamometer. J Rehabil Med 2004;36:273–8. [21] Boatright JR, Kiebzak GM. The effects of low median nerve block on thumb abduction strength. J Hand Surg Br 1997;22:849–52. [22] Li ZM, Harkness DA, Goitz RJ. Thumb force deficit after lower median nerve block. J Neuroeng Rehabil 2004;1:3. [23] Kaufman KR, An KN, Litchy WJ, Cooney III WP, Chao EY. In-vivo function of the thumb muscles. Clin Biomech 1999;14:141–50. [24] Fairfax AH, Balnave R, Adams RD. Variability of grip strength during isometric contraction. Ergonomics 1995;38:1819–30. [25] Niebuhr BR. Detecting submaximal grip exertions of variable effort by electromyography. Electromyogr Clin Neurophysiol 1996;36:113–20. [26] Smith GA, Nelson RC, Sadoff SJ, Sadoff AM. Assessing sincerity of effort in maximal grip strength tests. Am J Phys Med Rehabil 1989;68:73–80. [27] Mentzel M, Hofmann F, Ebinger T, Gülke J, Kinzl L, Wachter NJ. Measuring grip strength of the hand with a sensor glove by gripping with submaximal and maximal strength. Handchir Mikrochir Plast Chir 2001;33:52–7. [28] Mathiowetz V, Rennells C, Donahoe L. Effect of elbow position on grip and key pinch strength. J Hand Surg Am 1985;10:694–7. [29] Krombholz H. On the association of effort and force of handgrip. Percept Mot Skills 1985;60:161–2. [30] Nowak DA, Hermsdörfer J. Selective deficits of grip force control during object manipulation in patients with reduced sensibility of the grasping digits. Neurosci Res 2003;47:65–72. [31] Lechner DE, Bradbury SF, Bradley LA. Detecting sincerity of effort: a summary of methods and approaches. Phys Ther 1998;78:867–88. [32] Simonsen JC. Coefficient of variation as a measure of subject effort. Arch Phys Med Rehabil 1995;76:516–20. [33] Mitterhauser MD, Muse VL, Dellon AL, Jetzer TC. Detection of submaximal effort with computer-assisted grip strength measurements. J Occup Environ Med 1997;39:1220–7.
ScienceDirect www.sciencedirect.com Hand Surgery and Rehabilitation 36 (2017) 90–96
Original article
Reliability of the grip strength coefficient of variation for detecting sincerity in normal and blocked median nerve in healthy adults Fiabilité du coefficient de variation de la force de poigne dans la détection de la sincérité chez des adultes sains avec des nerfs médians sains et bloqués N.J. Wachter a,*, M. Mentzel b, R. Hütz b, J. Gülke b b
a Clinic for Hand-, Plastic and Microsurgery, Klinikum Stuttgart, Kriegsbergstrasse 60, 70174 Stuttgart, Germany Clinic for Traumatology, Hand-, Plastic and Reconstructive Surgery, University of Ulm, Albert-Einstein-Allee 23, 89081 Ulm, Germany
Received 18 May 2016; received in revised form 27 September 2016; accepted 14 December 2016 Available online 7 February 2017
Abstract In the assessment of hand and upper limb function, detecting sincerity of effort (SOE) for grip strength is of major importance to identifying feigned loss of strength. Measuring maximal grip strength with a dynamometer is very common, often combined with calculating the coefficient of variation (CV), a measure of the variation over the three grip strength trials. Little data is available about the relevance of these measurements in patients with median nerve impairment due to the heterogeneity of patient groups. This study examined the reliability of grip strength tests as well as the CV to detect SOE in healthy subjects. The power distribution of the individual fingers and the thenar was taken into account. To assess reliability, the measurements were performed in subjects with a median nerve block to simulate a nerve injury. The ability of 21 healthy volunteers to exert maximal grip force and to deliberately exert half-maximal force to simulate reduced SOE in a power grip was examined using the Jamar1 dynamometer. The experiment was performed in a combined setting with and without median nerve block of the same subject. The force at the fingertips of digits 2–5 and at the thenar eminence was measured with a sensor glove with integrated pressure receptors. For each measurement, three trials were recorded subsequently and the mean and CV were calculated. When exerting submaximal force, the subjects reached 50–62% of maximal force, regardless of the median nerve block. The sensor glove revealed a significant reduction of force when exerting submaximal force (P1 sensor) with (P < 0.032) and without median nerve block (P < 0.017). An increase in CV at submaximal force was found, although it was not significant. SOE can be detected with the CV at the little finger at using a 10% cut-off (sensitivity 0.84 and 0.92 without and with median nerve block, respectively). These findings suggest low reliability of the power grip measurement with the Jamar1 dynamometer, as well as that of the CV for detecting SOE. However, the combination of finger forces including the thenar area and the CV at the little finger could lead to better reliability for detecting feigned reduction of grip strength. The methods were as reliable in subjects with a median nerve block as in healthy subjects. # 2017 SFCM. Published by Elsevier Masson SAS. All rights reserved. Keywords: Median nerve lesion; Sincerity of effort; Dynamometry; Coefficient of variation
Résumé Dans l’évaluation de la fonction de la main du membre supérieur, il est primordial de détecter la sincérité de l’effort (SDE) lors de la force de poigne pour identifier les pertes feintes de force. La mesure de la force maximale de poigne avec un dynamomètre est très répandue, souvent combinée au calcul du coefficient de variations (CV), une mesure de la variation lors de trois essais répétés successivement de la force de poigne. Peu de données sont disponibles concernant la pertinence de ces mesures chez les patients présentant une pathologie du nerf médian, en raison de l’hétérogénéité des groupes de patients. Le présent travail a évalué la fiabilité des tests de force de poigne et le CV pour détecter la SDE chez les sujets sains. La distribution de la puissance selon les différents doigts et l’éminence thénar a été prise en compte. Les mesures ont également été réalisées chez les mêmes sujets après un bloc du nerf médian simulant un groupe de patients présentant une lésion nerveuse pour en évaluer la
* Corresponding author. E-mail address: [email protected] (N.J. Wachter). http://dx.doi.org/10.1016/j.hansur.2016.12.003 2468-1229/# 2017 SFCM. Published by Elsevier Masson SAS. All rights reserved.
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fiabilité. Dans cette étude, 21 volontaires sains ont été examinés quant à leur capacité à développer une force de poigne maximale et une force de poigne délibérément diminuée de sa moitié pour simuler une SDE lors d’une prise de poigne évaluée avec le dynamomètre JAMAR. L’expérimentation a été réalisée avec et sans bloc du nerf médian chez le même sujet. De plus, la force développée aux pulpes des doigts 2 à 5 et au niveau de l’éminence thénar a été mesurée à l’aide d’un gant sensitif équipé de capteurs de pression. Pour chaque mesure, trois essais ont été enregistrés successivement, la moyenne et le CVont été calculés. Lors de la prise de force avec le dynamomètre JAMAR chez les sujets exerçant une force submaximale, les patients développaient 50 à 62 % de la force maximale, même après bloc du nerf médian. En mesurant de la force développée au niveau de l’éminence thénar par le gant sensitif, on constatait une réduction significative de la force lorsque que le sujet exerçait une force submaximale (capteur P1), avec ( p < 0,032) et sans bloc du nerf médian ( p < 0,017). On enregistrait également une augmentation du CV lors des prises à force submaximale, bien qu’elle ne soit pas significative. Il était possible de détecter la SDE et le CV au petit doigt avec un seuil de 10 % (sensibilité 0,84 et 0,92, respectivement sans et avec bloc du nerf médian). Ces expérimentations suggèrent une faible fiabilité des mesures de la force de poigne avec le dynamomètre JAMAR ainsi que celle du CV pour la détection de la SDE. Cependant, sa combinaison de la mesure des forces exercées par les doigts et l’éminence thénar et le CV au niveau du petit doigt pourrait améliorer la fiabilité lors du repérage d’une réduction feinte de force de poigne. La fiabilité de ces méthodes chez les sujets après bloc du nerf médian était comparable à celles des sujets sains. # 2017 SFCM. Publié par Elsevier Masson SAS. Tous droits réservés. Mots clés : Lésion du nerf médian ; Sincérité de l’effort ; Dynamométrie ; Coefficient de variation
1. Introduction Grip strength measurements are used to determine the extent of disability, loss in work capacity, readiness to return to work, and the amount of financial compensation for loss of function or work resulting from an injury [1]. Dynamometers such as the Jamar1 hydraulic dynamometer are used to measure grip strength. The Jamar1 dynamometer has been found to be a very accurate and reliable measurement tool of objective grip strength [2], but detecting sincerity of effort (SOE) remains difficult [2,3]. Several methods can be used by researchers and clinicians to determine SOE. The three SOE tests most commonly used in the clinic because of their affordability, simplicity and brevity are the five-rung grip test (5RGT) [4], the rapid exchange grip test (REGT) [5,6], and the coefficient of variation (CV). The CV has been used regularly for detecting SOE [3,7–9]. The CV measures the variability of three or more grip strength trials. It is calculated by dividing the standard deviation (SD) by the mean of these grip trials and multiplying the result by 100; thus, the CV is expressed as a percentage [8– 10]. According to the motor unit recruitment model [8,11], the variability during repeated grip trials with submaximal effort is higher than during a maximal effort [3]. Consequently, larger CV values indicate greater variability [3,7]. To consider a trial as inconsistent and hence insincere, a cut-off value must be established [8–10,12]. In the literature, the CV cut-off value ranges from 7.5% to 20% [7,12]. The measurements in most studies on this topic were performed in healthy subjects who simulate a feigned loss of grip strength by pressing the dynamometer with approximately 50% of maximal force [3,8]. Recent studies have questioned whether these measurements are also applicable to subjects with median nerve disorders such as carpal tunnel syndrome (CTS) or nerve injuries, where the sensory-motor deficits lead to a decrease in force accuracy and an increase in force variability [13,14]. The aim of this study was to evaluate the reliability of grip strength and the CV for detecting the SOE in healthy subjects with a distal median nerve block as a means to simulate a
patient group, in order to compare the measurements in the same subjects without a block as control. A median nerve block at the wrist is supposed to affect the thenar muscles (abductor pollicis brevis, superficial portion of the flexor pollicis brevis, opponens pollicis) and the lateral two lumbricals. The reduction of grip strength after median nerve damage ranges from 20% to 70%, depending on the measurement method [15]. For this reason, the power distribution of the fingers and the thenar prominence was taken into account. We also wanted to determine if using a cut-off value for the CV of single fingers could improve the detection of SOE.
2. Material and methods Twenty-one healthy subjects participated in this study who had an average age of 31.5 years (23–41 years). Only male subjects were included who were right-hand dominant. The hand length over the middle finger was 18.9 cm (1.1 cm) and the distance from the tip of the thumb and little finger was 21.8 cm (1.8 cm). A clinical examination performed by a hand surgeon revealed no pathology. The study examined the power grip using a hydraulic hand dynamometer set to the middle position (Baseline1 Hydraulic Hand Dynamometer, Fabrication Enterprises Inc., Irvingston, NY 10533, USA) (Fig. 1). A sensor glove was used for the finger measurements. The TUB-sensor glove (Technical University of Berlin) is equipped with 5 pressure sensors (Fig. 1) located on the palmar surface of the glove (FSR 151 NS, International Electronics and Engineering, Luxembourg). Data were displayed in force–time diagrams (Fig. 2). Each measurement was performed over a 15-s period, during which the grip was repeated three times according to the established protocol for measuring power grip strength [2]. For each of the three trials, the grip strength was determined at the beginning of the plateau of the force–time diagram [16,17] (Fig. 2). The mean value of the three trials (f1, f2, f3) was calculated. For all subjects, the mean and SD were determined and displayed in a diagram (Fig. 5).
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Fig. 1. Power grip using a Baseline1 dynamometer (middle position) and position of the five pressure sensors at the TUB Sensor-glove.
For the median nerve block, local anesthesia was used in all 21 subjects; 5 ml of Mepihexal sine 1% was injected under aseptic conditions using a sterile 25-gauge needle directly proximal to the transverse carpal ligament (Fig. 3). The sensitivity was evaluated before and after the block, using the two-point discrimination test with a threshold of 2 mm. Criteria for a successful block were loss of sensation in the median nerve area and a positive bottle sign [18]. Within approximately 6 h, all subjects regained their sensation and motor function. The application of local anesthesia was approved by the ethics board of Ulm University (No. 36/2003). Measurements were taken from the right hand of all participants in a standardized upright sitting position with the elbow in 90-degree flexion and the wrist joint in 15-degree extension. Before each measurement, the standardized position of the glove and the sensors was confirmed. The power grips were first carried out with the median nerve block and then without the median nerve block in another session with the dynamometer set to middle position (6 cm span) (Fig. 1). Both measurements were recorded on two different days within 1 week by random order to exclude the effects of fatigue and persistent effect of the block. For the submaximal effort measurements, the subjects were instructed to deliberately exert 50% of their maximal effort in the initial session and to exert 100% in a second session after a 1-h rest. These conditions were used to minimize the effect of fatigue. The maximal power f1
f2
f3
a1
a2
a3
grip strength was recorded using the dynamometer scale in kilograms and then converted into Newtons. The mean and SD were calculated for all measurements (Figs. 4–7). For statistical evaluation, the Mann Whitney-U test was used to compare the dynamometer and sensor data based on the factors of half-force or full force (P < 0.05). The CV was calculated by dividing the SD by the mean (AVE) of the three grip trials and multiplying by 100, using the formula: CV = (SD/AVE) 100 [7]. The sensitivity and specificity of the CV was calculated for the dynamometer and the P1 (thenar prominence), D2 (fingertip of index finger), D3 (fingertip of middle finger), D4 (fingertip of ring finger) and D5 (fingertip of little finger) sensors using the optimized cut-off level of 0.10 according to Robinson et al. [12]. In addition, ROC curves were used to determine an appropriate cut-off level from our own data (Fig. 8). All statistics were calculated with StatView software (SAS Institute Inc., Version 5.0).
3. Results 3.1. Grip strength The mean maximum grip strength was 249 N (66.9 N) without and 228 N (54.6 N) with median nerve block (Fig. 4). The difference was not significant. The submaximal force was
power [N]
8 6 4 2 0
0
4
8
12
time [s] Fig. 2. Assessing the force in a force–time diagram with 3 trials (a1–a3) within 15 s.
Fig. 3. Median nerve block at the wrist.
350
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0.5
250 median nerve block no block
150 100 50
0.1
0
FF
power [N]
20 15
no block HF no block FF
10 5 0 D3
D4
D5
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power [N]
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median nerve block, HF median nerve block, FF
10 5
P1**
D2
D3
D4
D5
Fig. 5. Power distribution of P1 and D2–D5 (*P < 0.017, **P < 0.032). FF: full force; HF: half force.
120 100 80 [%]
no block median nerve block
60 40 20 0 D2
D3
D4
FF median nerve block FF no block
JAMAR
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0
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Fig. 4. Measurement of grip strength using a dynamometer [N]. FF: full force; HF: half force.
P1*
HF median nerve block
0.3 0.2
0
93
0.4
200
CV
power [N]
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D5
Fig. 6. Submaximal force/maximal force 100 in [%] at the D2–D5 sensors.
106 N (28.6 N) without and 121 N (38.7 N) with median nerve block; the difference was not significant. A significant difference was found between the submaximal and maximal grip force in all subjects. This difference was not influenced by the median nerve block. 3.2. Power distribution The power distribution of the maximal grip strength measured at the fingertips showed a maximal force at the ring
P1
D2
D3
D4
D5
Fig. 7. Coefficient of variation (CV) during power grip with dynamometer and sensors. FF: full force; HF: half force.
finger, followed by the middle, index and little finger (Fig. 5, upper graph). Although there was a remarkable reduction when exerting submaximal force, the differences were not significant. The force measured at the thenar eminence (P1 sensor) was 13.0 N (0.9 N) during maximum effort and 5.8 N (0.32 N) during submaximal effort; this difference was significant (P < 0.017). In the trials with median nerve block, the power distribution at the finger tips was similar to the previous measurements, although the force levels were slightly lower (Fig. 5, lower graph). The force at the P1 sensor during maximal effort was 10.4 N (0.80 N); it was significantly reduced to 4.7 (2.2 N) during submaximal effort (P < 0.037). No significant difference was found between the grip strength of healthy subjects exerting submaximal force and subjects with a median nerve block exerting maximal force (Fig. 5). The fraction of the submaximal force to the maximal force was 50–62% for all fingers (Fig. 6). This fraction was not influenced significantly by the median nerve block. 3.3. Coefficient of variation In healthy subjects, the CV for the grip strength, thenar eminence (P1 sensor) and the fingertip sensors was higher during the submaximal effort than during the maximal effort; the difference however was not significant. With the median nerve block, the CV did not differ significantly between the submaximal and maximal efforts (P < 0.05, Fig. 7). Based on our data, an optimum cut-off level of 0.10 for dynamometer and all finger sensors was found using the ROC curve, which confirms published data (Fig. 8). The CV for the sensor at the little finger could distinguish between submaximal and maximal grip strength effort with an acceptable sensitivity (0.84) and specificity (0.50). This was also valid with the median nerve block (0.92 and 0.50). If the CV cut-off is set to 0.10, a relatively low sensitivity for the dynamometer and the P1 and D2–D4 sensors was found for detecting submaximal grip strength (Table 1). 4. Discussion In most studies in which grip strength is measured to detect injuries, healthy subjects are compared to patients with defined injuries. In these studies, there is a certain bias due to interindividual differences in a group of healthy subjects as well
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100%
D5 no block
100%
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80%
0,05
0,10
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80% 0,15
60%
60%
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20%
20%
sensitivity
40%
sensitivity
40%
0% 100% 80% 60% specificity
40%
20%
0%
0% 100% 80% 60% specificity
40%
20%
0%
Fig. 8. ROC curves for D5 displaying the cut-off labels in boxes. 95% confidence intervals are the dashed lines. Table 1 Sensitivity and specificity of the coefficient of variation at a cutoff level of 10%.
No block No block Median nerve block Median nerve block
Sensitivity Specificity Sensitivity Specificity
Jamar
P1
D2
D3
D4
D5
0.50 0.50 0.55 0.45
0.42 0.75 0.42 0.42
0.50 0.50 0.42 0.58
0.50 0.50 0.67 0.33
0.46 0.54 0.67 0.33
0.84 0.50 0.92 0.50
P1: thenar eminence; D2: fingertip of index finger; D3: fingertip of middle finger; D4: fingertip of ring finger; D5: fingertip of little finger.
as the heterogeneity of the injuries in the patient group [4,5,19]. To avoid these effects, we simulated a median nerve injury in healthy subjects by performing a median nerve block at the wrist [18]. Due to the standardized examination of the subjects in our study for clinical signs of median nerve palsy, the simulated injury was very homogenous in the median nerve block trials. In addition, the combined test setting eliminated interindividual differences in the force distribution of the fingers during a power grip. Our study found a reduction in grip strength when a median nerve block is applied; however this difference was not significant. In patients with neuropathy of the median nerve, reduced intrinsic muscle strength has been found with different dynamometers [20]; however, there was only a moderate decrease of total grip strength. For the thenar area (P1 sensor), a significant reduction of power after the median nerve block was found. This can be explained by the fact that the power vector of the sensor located at the thenar eminence differs from the power vector of the dynamometer, thus rather representing the force of the thenar muscles. During a power grip, palmar abduction and thumb flexion are most relevant. Boatright et al. reported an average 72.3% reduction in palmar abduction power with a median nerve block [21]. This was confirmed by other studies reporting a major loss of palmar abduction and thumb flexion strength with a median nerve block [18,22,23]. The dynamometer is an established grip strength measurement tool, indicating the maximal power of a trial [2,6,9,19,24– 26]. To evaluate the force distribution of single fingers in this study, a TUB-sensor glove equipped with five pressure sensors
located at the fingertips and thenar eminence was used [27]. A standardized experimental setting was defined to avoid measurement bias [27,28]. The absolute strength values are of less importance than the relationship between the grip strength of the various fingers [17]. By analyzing the force at the fingertips, we found reduced force values with the median nerve block, confirming the findings of previous studies [20]. The interindividual variation in the force at the fingers was higher with a median nerve block; the highest difference was found at the little finger, although it was not significant. Gulke et al. [16] reported that the little finger plays a decisive role in this as well. They found significant differences in strength distribution patterns, depending on whether the power grip trial was performed at maximal or submaximal effort. In maximal gripping, the total measured strength was distributed relatively evenly over all four fingers, with each finger contributing 23– 27% of the total strength. In submaximal gripping, the little finger was involved very little and only contributed 14–15% of the total strength, with the remainder of the force distributed relatively evenly between the index, middle and ring fingers, with each of them contributing 26–32% of the total [16]. Our data confirms these findings in the subjects with and without a median nerve block. However, these changes in the power distribution were not significant. The power distribution between the fingers could be one of the relevant criteria for detecting submaximal grip effort. Several studies have included healthy subjects who are asked to simulate a loss of grip force in order to detect a feigned loss of grip strength [7,8,12,25]. Most studies demonstrated that
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the subjects successfully exerted approximately 50% of maximal effort as instructed [8,12,29]. These findings were confirmed by our results with the subjects being able to exert half-force in the power grip position using the dynamometer (45%). With a median nerve block, there was only a moderate change of the median submaximal force to 56%. For the force at the fingertips, values between 41% and 63% were found, with higher variations during the median nerve block. The small effect of distal median nerve block on grip strength control can be explained by the fact that anticipatory force control highly influences grip strength, whereas sensory information from the grasping digits is less important and instead is needed to modify the grip strength and update sensory motor memories to support anticipatory grip strength control [22,30]. However, in our study we could not distinguish between subjects with and without a median nerve block in a combined setting that eliminated interindividual differences. Analyzing the force of the individual fingers also did not improve this distinction. This can be explained by the fact that there was a relatively large variation of grip force in the individual fingers. This variation can interfere with the reduction of total grip strength that was found when exerting submaximal effort in the power grip. A distinction could be made between submaximal and maximal effort by adding a sensor at the thenar eminence (P1), regardless of median nerve block. This could be a useful supplement for the evaluation of SOE. Since SOE detection with grip strength measurements is difficult, various statistical methods have been developed by clinicians and researchers in an attempt to detect SOE. Sindhu et al. examined the ability of three tests to detect SOE, namely the 5RGT, REGT, and CV. When examining the independent contribution of each tests to their combined predictive validity, they found that the CV contributed the most and the REGT contributed the least. The CV had the greatest independent contribution toward the combined predictive ability in both the logit and linear regression models. Although the predictive ability improved when combining the 5RGT, REGT and CV, it was still not sufficient to warrant their clinical use, either alone or in combination [3]. According to the motor unit recruitment model [10], a submaximal effort would have more variability and less consistency during repeated grip trials than a maximal effort. Consequently, larger CV values indicate greater variability [3]. In our study of the power grip, the CV was higher when exerting submaximal force versus maximal force for almost all sensors; this difference however was not significant. When exerting maximum force, the CV was higher with a median nerve block than without the block; however this was not significant. Robinson et al. [12] found that the CV was significantly greater for submaximal than for maximal force when the subjects were performing three trials; however, they reported that using the CV to determine SOE is questionable. In other studies, no significant differences were reported between the CV at maximal and submaximal effort [2,27,31,32], which is consistent with our data. Still the use of CV for detecting SOE is controversial [7,12]. Ashford et al. found no significant differences in variability between maximal and submaximal
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efforts in 22 healthy subjects. They concluded that the current protocol for testing grip strength with a dynamometer allows for consistent submaximal effort [2]. Shechtman examined a group of patients with CTS and concluded that the CV as a measure of SOE, especially in injured individuals with compromised hand strength, can produce contradictory results [9,12,31]. Some studies indicate that the CV is not a valid measure of SOE because it increases during submaximal effort due to a decrease in the mean, not to an increase in absolute variability, concluding that determinations of SOE should not be made based solely on the CV [8,32]. The CV has been reported to possess low sensitivity and/or specificity as a screening tool [7,12]. For grip strength, the percentage of submaximal effort misclassified as maximal is 22–69%, depending on the CV cutoff value, a threshold above which an effort is considered to be inconsistent and hence insincere [7,12]. The CV cut-off value ranges from 7.5% to 20% in the literature; thus, the disagreement regarding the validity of the CV is not surprising [2,7,33]. In our study, a cutoff value of 10% was determined to be optimum, which lies in the range of published values and corresponds to the optimized cutoff level reported by Robinson et al. [12]. A high sensitivity for CV at the 10% cutoff level was only found for the D5 sensor. This could imply that the differences between the fingers should be taken into account; force measurement and CV calculation for the little finger can help to determine the SOE. Recent studies have reported that sensorimotor deficits associated with CTS reduce the accuracy and stability in finger force control. The fact that the CV of the little finger was a good predictor for SOE could therefore be a consequence of the sensorimotor deficits caused by the median nerve block [22]. The limitations of this study include the use of healthy subjects and the nonclinical setting. These limitations reduce our ability to generalize the results to subjects with upper extremity injuries in a clinical setting. 5. Conclusion In this study, the reliability of grip strength, finger power and the CV for detecting SOE was examined. Twenty-one healthy subjects performed a power grip maneuver, exerting maximal force and submaximal force in a combined setting; the measurements were repeated with a median nerve block on a different day. The subjects could deliberately exert halfmaximal grip force and the SOE could not be detected by measuring only grip strength. The difference between feigned submaximal grip force of healthy subjects and maximal force with median nerve block was significant. By adding a sensor at the thenar area, a distinction between maximal power and feigned loss of grip force in healthy subjects and in subjects with median nerve block could be made. When stimulating a feigned effort, the CV was reduced when compared to maximal force; however it was not significant. No significant difference in CV was found between the subjects with median nerve block exerting maximal force and the healthy subjects feigning reduced grip strength. At an optimal CV cut-off of 10%, a
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sensitivity of 0.84 for the grip force at the fifth finger was found (0.92 with nerve block). These experiments suggest a low reliability of the grip strength measurement and the CV of the power grip for the detecting SOE in healthy subjects. However, the combination of measuring the finger force including the thenar area and the decisive role of the little finger in the power distribution and CV could lead to better reliability for detecting a feigned reduction of grip strength. The reliability of these methods in subjects with a median nerve block was comparable to that in healthy subjects. Future studies should evaluate whether the method can also be transferred to patient groups like the median nerve block group in this study. Disclosure of interest The authors declare that they have no competing interest. References [1] Shechtman O, Sindhu BS, Davenport PW. Using the ‘‘visual target grip test’’ to identify sincerity of effort during grip strength testing. J Hand Ther 2012;25:320–8. [2] Ashford RF, Nagelburg S, Adkins R. Sensitivity of the Jamar dynamometer in detecting submaximal grip effort. J Hand Surg Br 1996;21:402–5. [3] Sindhu BS, Shechtman O, Veazie PJ. Identifying sincerity of effort based on the combined predictive ability of multiple grip strength tests. J Hand Ther 2012;25:308–18. [4] Goldman S, Cahalan TD, An KN. The injured upper extremity and the JAMAR five-handle position grip test. Am J Phys Med Rehabil 1991;70:306–8. [5] 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. [6] Westbrook AP, Tredgett MW, Davis TR, Oni JA. The rapid exchange grip strength test and the detection of submaximal grip effort. J Hand Surg Am 2002;27:329–33. [7] Shechtman O, Anton SD, Kanasky Jr WF, Robinson ME. The use of the coefficient of variation in detecting sincerity of effort: a meta-analysis. Work 2006;26:335–41. [8] 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. [9] Shechtman O. Is the coefficient of variation a valid measure for detecting sincerity of effort of grip strength? Work 1999;13:163–9. [10] 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. [11] Kroemer KH, Marras WS. Towards an objective assessment of the ‘‘maximal voluntary contraction’’ component in routine muscle strength measurements. Eur J Appl Physiol Occup Physiol 1980;45:1–9. [12] 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.
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