The measure of handgrip strength is influenced by several factors including age, sex, hand size and grip span, posture, and position of the shoulder, forearm, and wrist (12,15,20,24,31-33,36,39). The angle of the elbow is another important factor when measuring handgrip strength (15,18). The most appropriate elbow position for achieving the maximal handgrip strength remains unclear, and the findings reported are inconsistent (3,11,13,22,24,30). The American Society of Hand Therapists (ASHT) recommends that the handgrip strength test should be performed with the elbow flexed at 90° (14), whereas others suggest that the test can be performed with the elbow in full extension or with the elbow flexed at 30°, 60°, 120°, and 135° (21,28,38). The influence of elbow position in the assessment of handgrip strength test has mainly been studied in adults (22,35) and elderly people (11), yet studies examining the effect of the elbow position on the assessment of handgrip strength in young people are scarce. Therefore, the first purpose of this study was to examine whether elbow position affects the handgrip strength in adolescents.
The handgrip strength test has traditionally been assessed by using the Jamar dynamometer in both clinical and epidemiologic settings. The validity and reliability of the Jamar has been reported in several studies (4,17,38). Subsequent studies have used this dynamometer as the gold standard to examine the criterion-related validity of other dynamometers (23,25,34,35). These studies have been mainly confined to correlation analyses, yet this method is not considered appropriate for that purpose (2,8). Therefore, we analyzed the criterion-related validity of the Jamar dynamometer and other widely used dynamometers (i.e., DynEx and TKK) by using more appropriate statistical methods. Moreover, the reliability of these dynamometers was examined.
Experimental Approach to the Problem
To examine whether elbow position affects the handgrip strength, we compared the handgrip strength measurements at 2 different elbow positions: in full extension and at 90° using the Jamar, DynEx, and TKK dynamometers. To analyze the criterion-related validity and the reliability of the Jamar, DynEx, and TKK dynamometers, we used the Bland-Altman approach, which is a more appropriate method to study the agreement between 2 measurements (5). In addition, the potential systematic bias, 95% confidence intervals of the bias, and the 95% limits of agreement (bias ± 1.96 SD of the differences) were also calculated. Finally, the association between the difference and the magnitude of the measurement (i.e., heteroscedasticity) was examined.
A total of 66 adolescents (31 males and 35 females) aged 12 to 16 years from the HELENA (Healthy Lifestyle in Europe by Nutrition in Adolescence) cross-sectional study sample recruited in Zaragoza (Aragón, Spain) voluntarily participated in this study, which was carried out in the Spring of 2007 (27). All the adolescents were free of any lesion or impairment in the upper limbs. Participants were asked to refrain from exercise the day before testing and to avoid eating within the preceding 2 hours of the tests.
A comprehensive verbal description of the nature and purpose of the study, as well as on the experimental risks, was given to the adolescents, their parents/guardians, and their teachers. This information was also sent to parents/guardians by regular mail, and written informed consent was obtained from the parents and adolescents before participation. The study protocol was performed in accordance with the ethical standards established in the 1961 Declaration of Helsinki (as revised in Hong Kong in 1989 and in Edinburgh, Scotland, in 2000) and was approved by the Research Ethics Committee of the Government of Aragón.
Handgrip strength was measured with 3 different hand dynamometers:
- Jamar hydraulic hand dynamometer (J. A. Preston Corporation, Clifton, NJ, USA). The Jamar dynamometer is a hydraulic tool with 5 fixed grip positions. The precision of the dynamometer is 2 kg. The grip span equivalences for the different positions in the dynamometer are I, 3.5 cm; II, 4.8 cm; III, 6.0; IV, 7.3 cm; and V, 8.6 cm.
- DynEx electronic hand dynamometer (MD System, Inc., Westerville, OH, USA). The DynEx dynamometer is an electronic device with 3 fixed grip positions that are similar to positions II, III, and IV of the Jamar dynamometer. The precision of the dynamometer is 0.1 kg.
- TKK digital hand dynamometer (TKK 5101 Grip-D, Takey, Tokyo, Japan). The TKK dynamometer is a digital tool with an adjustable grip span, ranging from 3.5 to 7 cm. The precision of the dynamometer is 0.1 kg.
Handgrip Strength Protocols
Two tests were performed with each dynamometer, 1 test with the elbow in full extension and the other with the elbow flexed at 90°. In the protocol with the elbow extended, the adolescents were standing during the entire test with the arm straight down at the side, with the shoulder slightly abducted (approximately 10°), the elbow in full extension, the forearm in neutral position, and the wrist also extended.
When performing the protocol with the elbow flexed at 90°, the standard procedures recommended by the ASHT were followed (14), but the adolescents were standing. Handgrip strength levels are higher when the test is performed in a standing position compared with sitting (19). The participants were instructed to maintain the shoulder slightly abducted (approximately 10°), elbow flexed at 90° (fixed with universal goniometer), forearm in neutral position, and wrist flexed between 0° and 30° and between 0° and 15° of ulnar deviation.
In both protocols, the adolescents looked forward, with feet shoulder-width apart, and were instructed not to touch any part of the body with the dynamometer except the hand being measured. The display of dynamometers was aligned to face the examiner, providing blind measurements to the adolescent.
Each adolescent performed the test twice with each hand (right and left alternatively), with 1-minute rest between trials. Participants were instructed to squeeze gradually and continuously for at least 2 seconds and were encouraged to do their best when performing the tests. For each measure, the order of the dynamometer to be used, the hand to be tested first, and the elbow position (in full extension or flexed at 90°) were chosen randomly.
The third grip position was used for the Jamar dynamometer, and the second position for the DynEx dynamometer (6,9,23,34,35). The grip position of the TKK dynamometer was adjusted to the individual's hand size according to the equation developed by Ruiz et al. (32). All measurements were performed the same day, and 2-hours rest between dynamometers was allowed.
Criterion-Related Validity and Reliability Procedures
The criterion-related validity (dynamometers vs. known weights) and reliability (repeated measurements) of the 3 dynamometers were analyzed by using known weights (ranging from 20-70 kg, every 5 kg) held with a rope from the center of the dynamometer's handle. The weights were added in a randomized order. Each weight measurement was repeated twice. The time between trials was approximately 40 to 60 seconds. The dynamometer's handle was marked for consistent placement of the rope with the known weight. The verification of all the weights was performed by using a high-precision SECA (861 scale, Hamburg, Germany) digital beam balance, which was calibrated by the manufacturer.
Distributions of handgrip strength values were tested for normality using the Kolmogorov-Smirnov test. All the variables showed a satisfactory pattern. Differences between elbow positions (full extension vs. 90°) were analyzed by one-way analysis of variance (ANOVA) for repeated measures for the right and left hands separately. Additional analyses were performed after summing up the scores obtained with the right and left hands. Difference among dynamometers (Jamar vs. DynEx vs. TKK) was analysed by one-way ANOVA for repeated measures for the elbow in full extension and flexed at 90° separately. Because no interaction between sex × elbow position and sex × dynamometer was found, all the analyses were performed for both males and females together. All the analyses were adjusted for sex.
Agreement between each dynamometer and the known weights (criterion-related validity), as well as between different trials with the same weight (reliability), was assessed following the Bland and Altman method (5). The bias (mean difference), 95% confidence intervals of the bias, and the 95% limits of agreement (bias ± 1.96 SD of the differences) were calculated. Validity and reliability variables were also graphically examined by plotting the differences against their mean, according to the Bland and Altman approach (5). Potential systematic bias was analysed by 1-sample t-test (H0: mean intertrial differences = 0; H1: mean intertrial difference ≠ 0).
In both validity and reliability analysis, heteroscedasticity was examined by using one-way ANOVA (quintiles of this variable as fixed factor) to determine whether the absolute interscore differences were associated with the magnitude of the measurements (i.e., interscore mean). A significant association (p ≤ 0.05) between these 2 measures would confirm heteroscedasticity. The significance level was set at 5% for all the analyses.
Handgrip Strength Protocols
Handgrip strength levels obtained with each dynamometer and each hand and each elbow position are shown in Table 1. Levels of handgrip strength were significantly higher when the test was performed with the elbow extended compared with those obtained with the elbow flexed at 90°. This was true when using the TKK dynamometer with the right or left hand. The results did not materially change after summing up the results obtained with both hands (Figure 1). No significant differences between elbow positions were observed when the test was performed with the Jamar or the DynEx dynamometer. A significantly higher score was obtained with the TKK compared with the Jamar and DynEx dynamometer when performing the tests with the elbow in full extension (49.62 ± 1.44, 45.03 ± 1.37, 44.05 ± 1.54 kg, for TKK, Jamar, and DynEx, respectively) (Figure 1). The observed statistical power for the n size used was between 0.8 and 1.
A negative systematic bias (underestimation) was observed for the Jamar and DynEx dynamometer (−1.92 and −1.43, respectively, p < 0.05), with a 95% limits of agreements of 1.92 and 3.56, respectively (Figure 2). Bias and limits of agreement for the TKK dynamometer were 0.49 (p < 0.05) and 1.32, respectively (Figure 2). A positive significant association was found between intertrials difference and intertrials mean (i.e, heteroscedasticity) in the Jamar (p = 0.006), DynEx (p < 0.001), and TKK (p = 0.038) dynamometers.
The observed systematic bias was 0.23, 0.26, and 0.02 for the Jamar, DynEx, and TKK dynamometers, respectively (all p > 0.05), whereas 95% limits of agreements were 1.20, 1.42, and 1.57, respectively (Figure 3). No patterns of heteroscedasticity were observed with any of the dynamometers.
The main results indicate that handgrip strength levels are significantly higher when the test is performed with the elbow extended compared with those obtained with the elbow flexed at 90° when using the TKK dynamometer. These findings are in accordance with other studies (3,16). Handgrip strength levels did not significantly differ between elbow positions when the test was performed with the Jamar or the DynEx dynamometer, which is in agreement with studies in young adults (13,21,28). These findings might be partially explained by the fact that the grip span of the TKK dynamometer was accurately adjusted to the adolescent's hand size (32), whereas an accurate grip adaptation to the adolescent's hand size was not practically possible for the Jamar and the DynEx dynamometers. The third grip position used for the Jamar and the second for the DynEx dynamometer were as recommended in the literature (6,9,23,34,35). Several studies using the TKK dynamometer have shown that there is an optimal grip span for achieving the maximum handgrip strength and that optimal grip span is related to the individual's hand size (12,32,33).
The results of the present study provide useful and relevant information that may help to elucidate which elbow position yields maximal handgrip strength in adolescents. Our findings suggest that, for the assessment of handgrip strength in adolescents, the elbow should be in full extension when using the TKK dynamometer, whereas, when using either Jamar or DynEx dynamometers, the elbow can be either in full extension or flexed at 90°. There are no data available regarding the influence of the elbow position on the assessment of handgrip strength in adolescents when using the TKK or the DynEx dynamometer. Handgrip strength is part of several health-related fitness test batteries (7,10,37), and it has been widely used in experimental and epidemiologic studies (1,26,29). Therefore, from a public health perspective, it is important to standardize the procedure because otherwise the measurement error may be too large to detect actual changes in strength.
The criterion-related validity analyses suggest that the Jamar and DynEx dynamometers underestimate the handgrip strength levels (−192 and −1.43 kg), whereas the TKK dynamometer provides the lowest systematic error (0.49 kg). The heteroscedasticity analysis showed a significant association in all the studied dynamometers, which indicates that the error of the measurement is associated with the magnitude of the measured weight. A higher error of measure in those individuals with high levels of handgrip strength can therefore be assumed. The Bland-Altman plot for the DynEx and Jamar dynamometers showed a trend to underestimate the measurement with higher weights, whereas the TKK dynamometer tended to overestimate the measurement as the weight increased. Bearing in mind that adolescents do not achieve high grip levels, we conclude that the TKK appears to be the most appropriate dynamometer to assess handgrip strength in this particular population.
The TKK dynamometer showed the lowest systematic error (0.02), whereas the systematic errors showed by the Jamar and DynEx dynamometers were similar (0.23 and 0.26, respectively). The heteroscedasticity analysis showed no significant association between known weights and different trials, which indicates that the intertrials difference was not associated with the magnitude of the measured weight.
In conclusion, the results of the present study suggest that performing the handgrip strength test with the elbow in full extension is the most appropriate protocol to assess maximal handgrip strength in adolescents when using the TKK dynamometer. In addition, the results indicate that, among the 3 dynamometers studied, the TKK dynamometer appears to provide the highest criterion-related validity and reliability for measuring maximal handgrip strength in this particular population.
The present study provides useful and relevant information indicating which elbow position, and which type of dynamometer, yields maximal handgrip strength in adolescents. The elbow should be in full extension when performing the test, and the TKK dynamometer is the most appropriate dynamometer to assess handgrip strength at the ages studied.
The authors thank Prof. Manuel J Castillo (Director of EFFECTS-262 Research Group, Department of Physiology, University of Granada, Spain) for his highly valuable comments on the article, his key role in the study concept, design, and supervision, as well as in the funding. This study was supported by the HELENA study, which takes place with the financial support of the European Community Sixth RTD Framework Programme (Contract FOOD-CT-2005-007034); the ALPHA study, which takes place with the financial support of the Public Health Executive Agency, DG Sanco, Health Information Strand (Ref: 2006120); the Ministerio de Educación y Ciencia, Spain (EX-2007-1124, EX-2008-0641, and AP2005-4358), Consejo Superior de Deportes, Spain (109/UPB31/03 and 13/UPB20/04), Fundación Mapfre (Spain), and Fundación Cuenca Villoro. The content of this article reflects only the authors' views, and the European Community is not liable for any use that may be made of the information contained therein.
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