Age-specific reliability of two grip-strength dynamometers when used by children

Background: The Jamar dynamometer and the Martin vigorimeter, two instruments often used to assess grip strength, have good validity and reliability when employed to evaluate adults. Grip strength measurements are often performed to assess children with hand disorders. However, the reliability of these measurements when used to assess children under the age of twelve years is unknown. In addition, it is not clear whether reliability differs between younger and older children. The purpose of this study was to establish test-retest reliability for different age groups and to determine which instrument is the most reliable. Methods: One hundred and four children from a primary school were included. Subjects were divided into three groups: four to six, seven to nine, and ten to twelve years of age. The grip strength of both hands was measured with the Lode dynamometer (equivalent to the Jamar dynamometer) and with the Martin vigorimeter. The mean of three maximum voluntary contractions was recorded for all measurements. A retest was performed after a mean interval of twenty-nine days. Results: In the total group, the intraclass correlation coefficient for the Lode dynamometer was 0.97 (95% confidence interval, 0.95 to 0.98) for the dominant hand and 0.95 (95% confidence interval, 0.92 to 0.96) for the nondominant hand and the intraclass correlation coefficient for the Martin vigorimeter was 0.84 (95% confidence interval, 0.77 to 0.89) for the dominant hand and 0.86 (95% confidence interval, 0.80 to 0.90) for the nondominant hand. The intraclass correlation coefficients in the different age groups were lower than those in the total group because of a lower between-subject variation. The normalized smallest detectable difference between the test and retest values was approximately 25% for the Lode dynamometer and approximately 31% for the Martin vigorimeter. Conclusions: Both the Lode dynamometer and the Martin vigorimeter are reliable instruments with which to measure the grip strength of children under twelve years of age; however, the Lode dynamometer has better test-retest reliability. Furthermore, comparison of the smallest detectable differences showed the Lode dynamometer to be a more accurate instrument.