According to dynamic analyses of muscle contraction, jump rope is a typical stretch-shortening cycle (SSC) movement. It has been reported that the relationship with SSC is higher in double unders than in single unders (basic jumps); however, the relationship between jump rope and sprint performances has not been extensively studied. To clarify this relationship in elementary schoolchildren, we compared the sprint speed and SSC ability of children who were grouped according to gender and ability. The subjects were 143 elementary fifth and sixth graders (78 boys, 65 girls). The consecutive maximal number of double unders, reactivity index (index of SSC ability) by Myotest, and 20-m sprint time were measured. According to the mean of jump rope records, the children were divided into a superior ability group (more than average + 0.5 SD) and an inferior ability group (less than average - 0.5 SD) for each gender. In both genders, a significant difference was found in the 20-m sprint time between the inferior and superior ability groups. The times for the superior ability groups (boys, 3.75 ± 0.23 seconds; girls, 4.02 ± 0.24 seconds) were excellent compared with the inferior ability groups (boys, 4.17 ± 0.32 seconds; girls, 4.23 ± 0.21 seconds). This effect size was higher in boys (1.44) than in girls (0.93). The reactivity index in the superior ability group was excellent compared with that in the inferior ability group. In conclusion, children who perform better in double unders are also faster during a 20-m sprint run. This tendency may be higher in boys. Classic jump rope training, such as double unders, should be effective as elementary plyometrics for improving the sprint ability of children.

The strength required for motor activities should be exerted effectively according to the type of activity and the load size. This study examined the relationship between the accuracy of handgrip exertion for demanded forces of 20%-80% of the maximal voluntary contraction (MVC) and laterality. Subjects were 100 healthy young males (mean age, 22.4 ± 2.8 years). After the handgrip MVC was measured, subjects attempted to exert a handgrip at demanded forces of 20, 40, 60, and 80% of the MVC. All tests were performed twice and with dominant and nondominant hands, and mean values were used for statistical analysis. Differences between demanded forces and exerted forces were converted into relative values based on each subject's MVC. Two-way repeated measures ANOVA showed significant interaction between demanded forces and laterality. In multiple comparison tests, smaller demanded forces were associated with larger errors only in the non-dominant hand. For demanded forces of 20% and 40% MVC errors were smaller for the dominant hand than for the non-dominant hand. The non-dominant hand is used less than the dominant hand in daily life and in sport activities. It is therefore not unexpected that a laterality-based difference in the accuracy of exerted force for each demanded force is found, and the accuracy of exerted force at low demanded forces was inferior for the non-dominant hand. In conclusion, there was a difference in the accuracy of exerted force for each demanded force for the non-dominant hand with a larger error at lower demanded forces. In particular, the accuracy of exerted force in response to demanded forces of 40% MVC or less was inferior in the non-dominant hand compared with that in the dominant hand. Laterality is therefore a significant factor in force response to lower demanded force values.

This study aimed to examine the direction of errors between demand values and exerted forces using submaximal grip strength based on laterality. Subjects were 100 healthy young males (age: 22.4 ± 2.8 years). After maximum grip strength (MGS) was measured, each subject exerted handgrip for each demanded value (20%, 40%, 60%, and 80%), twice with a 2-min rest between each trial. The means of all subjects were used for statistical analysis. Evaluation parameters were errors between exertion and demand values, which are relative values based on MGS. The Shapiro-Wilk test was performed to confirm the normality of the frequency distribution. It was confirmed that errors in demand values, except for 80% in the dominant hand, show a normal distribution. The result of twoway repeated measures analysis of variance (ANOVA; demanded values × dominant/non-dominant) and multiple comparisons showed that significant differences were found among demand values, except between 60% and 80% in the dominant hand, and the error was greater as follows: 20% > 40% > 60% and 80%. The error in the nondominant hand was greater in the order of 20%, 40%, 60%, and 80%. A significant difference was found between errors of the dominant and non-dominant hands in demand values of 20%-60%, and the error was smaller in the dominant hand. Test of independence for the frequency difference of exertion direction showed significant difference in all demand values except for 80%; 20% and 40% did more in the positive direction (overexertion) in both hands, and 60% did more in the negative direction (underexertion) in the dominant hand and in the plus direction in the non-dominant hand. In conclusion, in the dominant hand, the error of grip force exertion toward demand values is greater in demand values under 60% and small grip vales are exerted. In contrast, in the nondominant hand，the error of grip force exertion toward demand values is greater in demand values under 80% and grip values are exerted largely in demand values under 60%. Accuracy of force exertion differs among demand values, and laterality is found in low demand values of 20%-60%.