The jump rope has existed all over the world from ancient times as a means of promoting health and fitness, and it has been considered a simple aerobic exercise because of having an improvement effect on respiratory and circulatory functions (9). In the 1970s, interest heightened in the jump rope as a way to improve physical fitness and health. Since then, events and programs with the jump rope have emerged and jumping rope has become part of exercise training regimens. Jump rope was introduced as a teaching tool in Japan's school education system in 1902 and has been integrated in exercise domains for improving physical fitness in elementary schools.
Until now, studies on the rope jump have been mainly performed from the perspective of physiological responses, such as jump frequency and energy expenditure (8,15,16), the relationship between jump frequency and exercise intensity (13), and training effect (2,3); however, there are few studies from a kinematic perspective, such as the effect of jumping rope on acquisition of movement.
Jumping rope is a consecutive jump exercise with turning of the rope. This motion is different from a press jump performed while largely flexing and extending ankle, knee, and hip joints. Rather, jumping rope involves a quick rebound jump wherein there is very little bending of the foot, knee, and hip joints. Hence, when considering muscle contraction, the rope jump is considered to be a kind of stretch-shortening cycle movement (SSC) because the muscle groups of the anterior thighs and the posterior crural regions shorten and extend. Use of the SSC produces greater power output over a shorter period of time than a simple concentric contraction alone (10,14). However, there are few research reports that focus on this point.
Plyometrics are typically employed to improve jumping power. Also, a box jump and a hurdle jump are often used. To perform these exercises effectively, it is important to reinforce the stiffness of the muscle by functionalizing the extension-reflection mechanism of the MTC (Gastrocnemius Muscle-Achilles Tendon Complex) (18). However, it is very difficult to master the above movement. This is especially true in the case of junior athletes who are still physically developing and are at risk of injury during training because of large physical strain (11). Hence, the familiar jump rope exercise for athletes without prior experience in specialized plyometrics was examined in this study.
Yamaguchi et al. (17) reported that differences in jump frequency while jumping rope largely affected the ground reaction force, contact time, and flight time. Hence, the following hypothesis was set in this study: When the rotation frequency increases during solo rope jumping, the jump motion shifts to consecutive jumping without the unweighting movement and the SSC is enhanced as a result.
Recently, it was possible to evaluate SSC ability using the rebound jump index (RJ-index) when it was calculated with respect to ground contact time and jump height during the rebound jump. It is also considered that the use ratio of SSC during rope jumping can be evaluated by applying this index. We paid attention to the representative basic jump (where both feet are slightly apart and are used to jump at the same time over the rope) and the double-under jump (where the participant needs to jump higher than usual while swinging the rope twice under his feet) in this study. It will be necessary to examine how the SSC value changes with respect to increasing rotation frequency during solo rope jumping as it will be useful in the design of training programs that reinforce jumping power.
This study aimed to examine the possibility of using rope jumping in SSC training by comparing the RJ-index of the rebound jump (standard value) and the 2 different methods of rope jumping.
Experimental Approach to the Problem
Jumping rope involves a quick rebound jump in which the foot, knee, and hip joints bend very little. Hence, when focusing on muscle contraction, it can be considered to be a kind of SSC. Even if jump height is the same in the case of the rebound jump, takeoff time is considerably different with respect to the subject's skill level from the eccentric muscle action to the concentric one of the leg muscle groups at takeoff. Hence, in this study, we defined the RJ-index based on a formula of jump height divided by ground contact time as the index of the SSC ability and used it for analysis (19).
To begin this study, the SSC ability of each subject was evaluated by calculating the RJ-index based on 5 consecutive vertical jumps (rebound jumps). The RJ-index as to rebound jump was assumed standard value of SSC ability. Next, participants performed basic and double-under jumps, according to the style of each individual, and the RJ-index during rope jumping was calculated by the same method as above. To examine the possibility of using a jump rope in SSC training, after comparing each RJ-index, the contribution-degree of the SSC was examined based on the ratio of the RJ-index of the jump rope tasks to the standard value. In addition, we focused particularly on subjects with inferior SSC ability and examined the relationship between the standard value and the RJ-index of the jump rope in considering the improvement of the SSC.
The subjects were 76 healthy young men (mean age, 19.5 ± 1.2 years; mean height, 172.2 ± 5.3 cm; mean weight, 62.3 ± 7.6 kg). Most subjects performed sports training 2–3 times per week routinely. All subjects could perform the basic and the double-under jump with the jump rope. They were selected from a variety of popular sports backgrounds, such as baseball, soccer, tennis, and basketball. A mean of the RJ-index of the Japanese top-level male athletes was about 2.3, and 2.5 corresponding to percentile 70 of the top-level is often used as the target value of athletes (6). In this study, participants with an RJ-index less than 2.5 were judged as SSC beginners. Informed consent was obtained from all subjects after a full explanation of the experimental project and its procedures was provided. This study was approved by The Ethics Committee on Human Experimentation of Faculty of Human Science at Kanazawa University.
Measurement of Rebound Jump Index
The RJ-index was evaluated using the OptoJump system (Microgate, Bolzano, Italy). This instrument was used to measure all flight and ground contact times while the subjects performed a sequence of jumps with 1/1,000th second precision (Figure 1).
The transmitting and receiving bars of OptoJump system (single meter) were placed face to face at distance of 1.5 m, and the subjects performed 5 consecutive vertical jumps (rebound jump) between the bars. Flight time (time in air: TA) and ground contact time (time in contact: TC) were measured. They will provide information on subject's bounce ability. The measurement was performed twice and the better value (maximum flight time and minimum ground contact time) was used for analysis. The rebound jump is generally performed with the hands placed on the waist to remove the effect of the arms' counter movement. However, to compare the rebound jump with rope jumping tasks, the counter movements of the arms were deemed acceptable in this study. The rebound jump was performed by each subject under the following instructions: “Jump as high as you can over as short a period of time as possible.”
From these measurements, the jump height (h) was calculated from the following formula (1):
where g is the acceleration of gravity.
In addition, as an index of SSC ability in the rebound jump, the RJ-index was calculated based on the method of Zushi et al. (19) from the following formula:
Next, subjects performed the basic and double-under jumps with a jump rope 10 times, respectively, according to their individual style (rhythm and timing), and the RJ-indexes were calculated using the same method described above. The measurement was performed twice and the larger value was used in the analysis. The same rope (Asics jumping rope INF, Kobe City, Japan) was used for all attempts, and the rope was regulated to a length that made it easy for each subject to jump. The intervals between trials and conditions were set for 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).
The reliability of the RJ-index evaluated by each jump test was examined using the intraclass correlation coefficient (ICC). One-way analysis of variance (ANOVA) was used to identify the mean differences between the RJ-indexes of each jump test. Post hoc comparisons were made using Tukey's Honestly Significant Difference (HSD) tests. The ratio of the RJ-index of each jump rope was calculated based on the RJ-index of the rebound jump, which is a standard value (100%).
In addition, relationships between each RJ-index were examined using Pearson's correlation coefficient. The criterion level for significance was set at p ≤ 0.05.
Table 1 shows ICCs of the RJ-index in each jump. There were insignificant differences between the 2 rebound jump trials, and ICC was high at 0.77. In addition, ICCs of the basic jump (ICC: 0.84) and double-under jump (0.92) were high. Table 2 shows the descriptive statistics of the RJ-index in each jump. Rebound jump index of the rebound jump was 1.97 ± 0.38, where the maximum was 2.94 and the minimum was 1.11. As for the jump rope, RJ-index of the basic jump was 0.60 ± 0.21 and the double-under was 1.34 ± 0.24. The coefficient of variance of the latter was at the same level as the rebound jump but that of the former was larger.
Figure 2 shows the results of ANOVA for the mean differences between the RJ-indexes for each jump. The RJ-index increased in value in the following order: rebound jump (1.97 ± 0.38), double-under jump (1.34 ± 0.24), and basic jump (0.60 ± 0.21) (F value: 628.7, p < 0.01, partial η2: 0.94). When using the RJ-index of the rebound jump as a standard (100%), the rate of basic jumping was 30.4% and double-under jumping was 68.0%.
Figures 3 and 4 show the typical patterns of the change in contact time, jump height, and RJ-index during the rebound jump and the 2 different kinds of jump rope tasks. Figures 5 and 6 show the relationships of the RJ-indexes between the rebound jump and the jump rope tasks in the SSC beginners with an RJ-index less than 2.5. The double-under showed significant and low correlations (r = 0.28) with the rebound jump.
The rebound jump is one of the representative plyometric exercises (18). To use this as an effective means of training requires considerable skill. Therefore, this study examined the possibility of SSC training that used the familiar jump rope.
Stretch-shortening cycle movement ability is evaluated by the RJ-index, calculated with contact time and jump height during the rebound jump (19). The RJ-indexes of the 2 jump rope tasks were evaluated by the same method and compared with that of the rebound jump. The RJ-index of the rebound jump in this study was 1.97 ± 0.38. Iwatake et al. (7) calculated the RJ-index of 145 technical college students and reported that it was 1.96 ± 0.45. In addition, Endo et al. (4) reported that the RJ-index of 82 boys of age 17–18 years was 1.91 ± 0.46. These are indexes at the time of the rebound jump without the counter movement of the arms. The SSC ability of the present participants may be slightly low in comparison with that in previous studies when considering that the squat jump becomes higher with counter movements of the arms (5,12).
As for RJ-index, the basic jump showed only moderate correlations (r = 0.62) with the double-under jump. Although both use the same jump rope, when increasing the number of rotations during one jump, it is inferred that the contribution degree of each muscle group changed because of changing the contact time and jump height. In this study, subjects performed the jump rope tasks according to the individual style of each participant. Therefore, it is considered that they changed jump strategy instinctively in response to a change in rotation-number. When performing the rebound jump, instructions such as “please jump as quickly as possible” and “jump as high as possible” are usually used. In the case of jump rope, the movement meeting the above condition may be achieved by increasing the number of rotations (e.g., double-under and triple-under).
There were significant differences among each RJ-index. However, RJ-index of the double-under was similar to that of the rebound jump, which was the standard value. It is inferred that the double-under uses about 70% of SSC ability of the rebound jump, judging from the RJ-index.
Yamaguchi et al. (17) analyzed the effects of the differences in jump frequency during rope jumping (basic jump) on the ground reaction force and reported that peak force changed depending on jump frequency. A jumping frequency of less than 92 times per minute showed a biphasic waveform that included unweighting movement; in contrast, a jumping frequency greater than 112 times per minute produced a monophasic waveform.
In this study, each subject performed the jump rope tasks according to their own rhythm. However, when the rotation frequency was increased as shown in Figure 4, it was found that the jump height tended to increase and contact time shortened. As a result, it is inferred that the jumping motion shifted to consecutive jumps without unweighting motions and, furthermore, that biodynamic responses such as the contribution of each muscle group or the articular angle control of the lower legs changed necessarily.
In this study, we paid particular attention to SSC beginners and examined the relationships between the standard value and the RJ-indexes of the jump rope tasks, considering improvement of the SSC ability. As a result, in the case of SSC beginners, the standard RJ-index of the rebound jump only showed significant correlations (r = 0.28) with the double-under but not the basic jump. This indicates that the double-under of the jump rope is effective for improving SSC ability in the SSC beginners. In addition, for SSC experts, a marked difference was not found between the basic jump and the double-under. It will also be necessary to introduce an experimental setting in the future that increases the number of rotations, such as a triple under. From the above results, when using the RJ-index of the rebound jump, it is judged that the double-under using about 70% of the SSC ability is effective for reinforcement of SSC.
Because plyometrics using the rebound jump tend to produce a large ground reaction over a short period of time, a large weight burden is placed on the lower limbs. In short, a rebound jump carried out using inappropriate may result in injury, in addition to having little effect on the SSC value. Particularly, in the case of the junior athletes, it will be necessary to perform exercise according to each person's individual physical ability.
The jump rope is a familiar training method, and it is believed that it can provide sufficient SSC training if the number of rotations is increased during solo rope jumping without supervision. Hence, strength and conditioning coaches must know that the classic jump rope is effective not only for the improvement of respiratory and circulatory functions but also for the improvement of SSC ability in athletes who do not have specialized experience in plyometrics.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
rebound jump; OptoJump; plyometrics; double-under