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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