Jump rope is representative play carried out from a young age, and it has been considered a simple aerobic exercise because it improves respiratory and circulatory functions (10). To date, jump rope has been studied predominantly from the perspective of physiological responses, such as heart rate and oxygen uptake (9,22). However, its kinematic effect has not been sufficiently studied.
Jump rope is a consecutive jump exercise performed by both the lower limbs while the hands and arms rotate the rope over the head and under the feet. This motion is different from a press jump performed while largely flexing and extending the ankle, knee, and hip joints. Jump rope is a quick rebound jump performed almost without bending the joints by grounding the anterior part of the foot. Hence, in terms of muscle contraction, jumping rope is considered a stretch-shortening cycle (SSC) movement because anterior thigh muscles and calf muscles repeatedly contract and extend. Stretch-shortening cycle produces greater power output over a shorter period than a simple concentric contraction alone (13,21). However, the relationship between jump rope and SSC and the effect of jump rope on other movements in children have not been extensively examined.
The grounding time of rebound jumps, including jump rope, is approximately 160–190 milliseconds, which is extremely short when compared with simple vertical jumps (260–290 milliseconds) (4,14). However, the grounding time during sprints in adults is reported to be approximately 200 milliseconds just after the sprint start to 100 milliseconds at maximum speed (15–18,23). Moreover, grounding times for sprinting and rebound jumping are highly correlated (3,4,6). Furthermore, both movements may be similar in power exerted and exercise accomplished during the extremely short grounding time. Hence, this study adopted the following hypothesis: elementary school children superior in jump rope ability are also superior in sprint ability.
It has in fact been reported that repetitive jumping training (plyometrics) related to SSC is effective for improving sprint performance (3,4,6). However, conventional methods of plyometrics impose a heavy workload on the lower limbs, which may cause injury. Therefore, using repetition training to improve children's sprint performance is not advisable. Hence, we focused on ways of using the jump rope, an exercise that is familiar for children.
In addition, it was reported that, while performing jump rope exercises, a difference in jump frequency largely affects ground reaction force, contact time, and flight time (24). Miyaguchi et al. (20) reported that the relationship with SSC is higher in double unders (where the participant needs to jump up higher than usual while swinging the rope twice under his feet) than in the single unders (where both feet are slightly apart and jump at the same time over the rope). Therefore, we paid particular attention to double unders in this study. Clarifying the above will give us useful information for improving the sprint performance of children.
To clarify the effect of jump rope on sprint performance in elementary schoolchildren, this study aimed to examine the relationship between double unders and sprint performance by comparing sprint speed and SSC ability between the groups of differing jump rope ability.
Experimental Approach to the Problem
In the case of elementary school children, sprint performance is usually judged according to the results of a 50-m sprint. However, some children cannot maintain a maximum speed over this distance. From an analysis of a 50-m race, Ito (8) reported that fifth graders arrive at their maximum speed and best pitch in the 10- to 20-m segment after the starting point. Therefore, this study used a 20-m sprint time as an index of the sprint, which is measured in a gymnasium. Recently, it became possible to evaluate SSC ability using the reactivity index (jump height/ground contact time) when calculating ground contact time to jump height for rebound jumping. In addition, the reactivity index is automatically measured by simple equipment with a small acceleration sensor. Therefore, the reactivity index of elementary school children performing the rebound jump was measured with the Myotest (Myotest SA, Sion, Switzerland). Then, to clarify the effect of jump rope on sprint performance through comparison of groups with different jump rope achievements, sprint speed and reactivity index were examined.
The subjects were 143 elementary fifth and sixth graders (age range 10–12 years), 78 boys (mean height 142.1 ± 7.1 cm, mean weight 37.0 ± 8.7 kg) and 65 girls (mean height 144.4 ± 6.7 cm, mean weight 37.0 ± 6.6 kg). Their physical characteristics showed insignificant gender differences. They routinely jumped rope as a part of their physical education curriculum. Informed consent was obtained from all parents after a full explanation of the experimental project and its procedures. Oral explanation was given to the subjects on the measurement day. All subjects consented to the experimental measurement. All measurements were performed during physical education class time. The Ethics Committee on Human Experimentation of the Faculty of Human Science at Kanazawa University approved this study.
20-m Sprint Time Measurement
Wearing running shoes and starting from a standing posture, subjects were measured by a speed trap (Fitness Apollo Co., Ltd., Tokyo, Japan) with a phototube sensor. Sprint time was measured twice, with a sufficient rest (15 minutes) between sprints, and the higher value was selected for analysis.
The Measurement of Jump Rope Achievement
Double unders are representative tasks in physical education and are also frequently performed during play. To examine the achievement of double unders, the consecutive maximal number of movements performed were measured. The subjects performed double unders according to their individual styles (rhythm and timing). Before the experiment, they warmed up by performing exercises, which included stretch exercises and light jumping exercises. The same rope (Asics, Kobe City, Japan) was used for all attempts and was adjusted to a length suitable for each subject. The measurement was performed twice, and the higher value of consecutive maximal number was used for analysis. Intervals between trials were set at 3 minutes, taking into consideration the influence of muscle fatigue. Because the test time was limited, children were given appropriate supplementation and sufficient hydration before and after the test. The measurement was carried out in the morning (from 10:00 AM to 12:00 PM).
Measurement of Stretch-Shortening Cycle Ability
This study assessed SSC ability by using the reactivity index stated above during the rebound jump. However, because of this movement's relative difficulty, only sixth graders performed the measurement. The subjects attached the Myotest to their waistbands, placed both hands on their waists, and performed the continuation jump 5 times, as high and as quickly as possible. The best reactivity index result from the 5 rebound jumps was used for analysis. The subjects warmed up sufficiently before the experiment. They also practiced the rebound jump after seeing a demonstration by an experienced track-and-field athlete. The measurement was performed twice, and the higher value of reactivity index was used for analysis.
Reliability of the measurements was examined through intraclass correlation coefficient (ICC). According to the mean of jump rope records, a superior ability group (more than average + 0.5 SD) and an inferior ability group (less than average − 0.5 SD) were formed according to gender. An independent t-test revealed mean differences between groups with different jump rope abilities for 20-m sprint time and reactivity index. The criterion level for significance was set at p ≤ 0.05.
Table 1 shows the results for each measurement item by gender. Intraclass correlation coefficients of 2 trials for the 20-m sprint test were high (0.85). In addition, ICCs of the reactivity index evaluated by the Myotest were also high (0.81). Significant gender difference was found in 20-m sprint times. Figure 1 shows the 20-m sprint times of the superior and inferior ability groups in jump rope achievements. Significant differences were found between the groups in both genders. The times for the superior ability group (boys, 3.75 ± 0.23 seconds; girls, 4.02 ± 0.24 seconds) were excellent compared with inferior ability group (boys, 4.17 ± 0.32 seconds; girls, 4.23 ± 0.21 seconds). The effect size was larger in boys (1.44) than in girls (0.93). Figure 2 shows the reactivity index of the superior and inferior ability groups. Significant differences were found between the groups in both genders. The reactivity index of the superior group (boys, 3.88 ± 0.48 seconds; girls, 3.84 ± 0.74 seconds) was excellent compared with the inferior group (boys, 2.85 ± 0.91 seconds; girls, 3.15 ± 0.56 seconds). The effect size was larger in boys (1.46) than in girls (1.08).
Significant differences were found in the 20-m sprint times of both boys and girls between groups of elementary schoolchildren segregated according to their jump rope ability. This result may support the hypothesis in this study. Although the jump rope has been said to be a typical SSC movement, the present results suggest that including double unders in an exercise regimen may contribute to improve sprint performance in elementary schoolchildren.
In this study, double unders were adopted as a test movement. Kitagawa (12) examined jump frequency during various jump rope tasks, reporting that during 1 jump, grounding time decreases with increasing rope rotations. Although both jump rope tasks involve the same movement, when the number of rope rotations during 1 jump increases from single unders to double unders, the contribution degree of each muscle group changes, largely because of the changing contact time and jump height. In short, we may infer that increasing the number of rope rotations during 1 jump changes the jump motion, and consecutive jumps are performed without relaxation of lower limb muscle groups. As a result, the possibility of using SSC necessarily becomes higher.
Miyaguchi et al. (20) reported that, in the case of SSC beginners, the reactivity index of the rebound jump showed significant correlations with the double unders, but not the single unders. This indicates that the double under is effective for improving SSC ability. In fact, for SSC ability, the superior group showed a higher value than the inferior group. Therefore, we can infer that double unders contribute positively to elementary schoolchildren's 20-m sprint ability.
In addition, the mean difference between groups was smaller in girls than in boys. This suggests that there are gender differences in the effects of jump rope tasks on a 20-m sprint. Possibly, the jump rope contribution to the 20-m sprint is smaller in girls than in boys.
Aura and Komi (1) reported that women are superior to men in storage and recycling of elastic energy when an extension load is small, but inferior when it is large. This may depend on a gender difference in muscle stiffness (1) and inhibitory activities of the central nervous system (5). Because structural stiffness of muscle is lower in women than in men (2) and tendons are more compliant in women than in men (11), we believe tendons save elastic energy more in women than in men but cannot withstand the powerful counterattack force required in SSC action. In this regard, Miyaguchi et al. (19) examined gender differences of SSC ability in the upper extremities and reported that muscle power output in SSC may be lower in women than in men.
Similar functional gender differences may also exist in elementary school children. Hence, under a relatively light load, such as jump rope, girls can accomplish SSC well, but may not be able to use SSC as effectively when a large load is demanded, such as in the acceleration phase of a 20-m sprint. Consequently, we can infer that the relationships between jump rope tasks and the 20-m sprint were low in girls. Ito (8) analyzed a 50-m sprint by fifth graders and reported that boys are superior to girls in sprint ability from start to the acceleration phases. In this study, gender differences were not found in physical characteristics, jump rope accomplishment, or SSC ability, but boys (3.96 ± 0.31 seconds) were superior to girls (4.10 ± 0.25 seconds) in 20-m sprint times. This result may reflect the gender differences explained above.
Endo et al. (7) reported that rebound jumping ability develops during a growth spurt period between 9 and 13 years of age, and individual differences in skill enlarge at the same time. Thus, by effectively using the jump rope during this period, despite some small gender differences, the SSC ability of elementary children will develop and likely improve their sprint performance.
Rebound jumping to improve sprint ability is a representative plyometric exercise. Using it effectively as a training procedure requires reinforcing muscle stiffness by functionalizing the extension reflection mechanism of the gastrocnemius muscle–Achilles tendon complex (25). However, mastering this movement is very difficult. Because the growing and developing bodies of elementary school children differ from those of adults, using this training movement requires adaptation to children's physical abilities. Therefore, we focused on using the jump rope. Consequently, this study's results suggest that the classic double unders is effective for improving the sprint ability of elementary school children. The jump rope is a familiar training method, and it will provide sufficient SSC training by increasing the number of rotations during solo jump rope without supervision. Although until now, jump rope has been used to improve respiratory and circulatory functions, children's strength and conditioning coaches need to know that classic jump rope is also effective for improving sprint ability. The present results support the use of a combination of running exercises and jump rope tasks, such as double unders, to increase the sprint ability of elementary schoolchildren.
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