Physical Fitness Promotion among Adolescents: Effects of a Jump Rope-Based Physical Activity Afterschool Program

The major purpose of this study was to examine the effects of a jump rope-based physical activity afterschool program on middle school students’ physical fitness. Sixty students (Mage = 13.37, SD = 0.58; 53.3% female) participated in a 12-week jump rope-based afterschool program (45 min/time, three times/week). Participants were randomly assigned to three groups: (a) freestyle rope skipping (N = 20), traditional jump rope (N = 20), and a control group (N = 20). Physical fitness tests, including muscular strength (standing long jump, right-hand grip, and left-hand grip), flexibility, body composition, and bone mineral density (BMD) were measured in pre- and post-tests. A 2 (time) × 3 (groups) repeated measure multivariate analysis of variance (MANOVA) was performed. The results found significant improvements in muscular strength (standing long jump, right-hand grip, and left-hand grip) in both intervention groups (p < 0.001; ds = 0.2–0.44). Only the freestyle rope skipping group had increased BMD (p < 0.05, d = 0.33). Compared to the traditional jump rope, the freestyle rope skipping group showed significantly higher improvement in flexibility (p < 0.05, d = 0.83). These findings suggest that the jump rope-based afterschool program with freestyle rope skipping would be more effective than traditional jump rope to promote physical fitness performance among adolescents.


Introduction
According to recent global physical activity (PA) guidelines, adolescents should engage in at least one hour of moderate to vigorous physical activity (MVPA) each day to gain sufficient health benefits [1][2][3][4]. However, more than 80% of adolescents failed to meet the daily MVPA recommendations worldwide [5], and only 29.9% of Chinese adolescents engage in the recommended daily MVPA. Adolescence (11)(12)(13)(14)(15) years old) is a crucial developmental stage for physiological maturation and physical fitness development [6]. Nevertheless, over one-third of adolescents are either overweight or obese [1,[7][8][9], and a substantial decline in cardiovascular and muscular fitness in adolescents has been reported across the globe [10][11][12][13]. Low levels of physical fitness during adolescence tend to track into adulthood and lead to both short-and long-terms health risks and chronic disease [2,[14][15][16]. It is an urgent need to implement effective interventions aiming to promote adolescents' PA participation and improve their physical fitness.

Participants and Study Design
Participants were 60 middle school students (M age = 13.37, SD = 0.58; 53.3% female) recruited from a public middle school in Shanghai, China. The inclusion criteria for participant recruitment were (a) students who returned the parental consent and assent forms, (b) students with no physical or psychological disabilities, and (c) no student athletes. The researchers received the university's ethical approval according to the Declaration of Helsinki (Project identification code: SU-1116) prior to undertaking the data collection. Among approximately 1800 students enrolled in 24 classrooms (i.e., about 75 students in each class; grades [6][7][8] in the middle school, 60 students were recruited according to the estimated sample size (N = 51; f2 = 0.35, π = 0.80, α = 0.05) from G*power 3.1 [37].

Procedure
The data collection and intervention were administrated during the afterschool program period (5:00 p.m.-6:00 p.m.). The Shanghai middle school system commonly provides middle school students with an afterschool program and unstructured free play (i.e., self-directed play without specific learning objectives) before dinner (6:00 p.m.-7:00 p.m.) and evening class sessions (7:00 p.m.-9:00 p.m.). Participants were recruited via fliers introducing the 12-week jump rope intervention project. Sixty students (N = 41 (6th grade), N = 16 (7th grade), and N = 3 (8th grade)) directly contacted the principal investigator and conveyed their interest and willingness to participate in the study. Parental consent and student assent forms were obtained before undertaking the project. Figure 1 presents the procedures of this intervention study. By applying random numbers generated from Microsoft Excel 2016, the 60 students were randomly assigned into three groups: freestyle rope skipping (N = 20; 12 girls), traditional jump rope (N = 20; 10 girls), and control (N = 20; 10 girls) groups. Pre-(week 1) and post-tests (week 14) were conducted in the indoor gym in the middle school. Prior to the measurements, trained graduate assistants instructed and demonstrated each physical fitness assessment to assure student comprehension. The intervention was administrated during the afterschool program for 12 weeks (weeks 2-13 in Figure 1). The participants from the control group engaged in free play without specific PA instructions.
Children 2020, 7, x FOR PEER REVIEW 3 of 13

Participants and Study Design
Participants were 60 middle school students (Mage = 13.37, SD = 0.58; 53.3% female) recruited from a public middle school in Shanghai, China. The inclusion criteria for participant recruitment were (a) students who returned the parental consent and assent forms, (b) students with no physical or psychological disabilities, and (c) no student athletes. The researchers received the university's ethical approval according to the Declaration of Helsinki (Project identification code: SU-1116) prior to undertaking the data collection. Among approximately 1800 students enrolled in 24 classrooms (i.e., about 75 students in each class; grades [6][7][8] in the middle school, 60 students were recruited according to the estimated sample size (N = 51; f2 = 0.35, π = 0.80, α = 0.05) from G*power 3.1 [37].

Procedure
The data collection and intervention were administrated during the afterschool program period (5:00 p.m.-6:00 p.m.). The Shanghai middle school system commonly provides middle school students with an afterschool program and unstructured free play (i.e., self-directed play without specific learning objectives) before dinner (6:00 p.m.-7:00 p.m.) and evening class sessions (7:00 p.m.-9:00 p.m.). Participants were recruited via fliers introducing the 12-week jump rope intervention project. Sixty students (N = 41 (6th grade), N = 16 (7th grade), and N = 3 (8th grade)) directly contacted the principal investigator and conveyed their interest and willingness to participate in the study. Parental consent and student assent forms were obtained before undertaking the project. Figure 1 presents the procedures of this intervention study. By applying random numbers generated from Microsoft Excel 2016, the 60 students were randomly assigned into three groups: freestyle rope skipping (N = 20; 12 girls), traditional jump rope (N = 20; 10 girls), and control (N = 20; 10 girls) groups. Pre-(week 1) and post-tests (week 14) were conducted in the indoor gym in the middle school. Prior to the measurements, trained graduate assistants instructed and demonstrated each physical fitness assessment to assure student comprehension. The intervention was administrated during the afterschool program for 12 weeks (weeks 2-13 in Figure 1). The participants from the control group engaged in free play without specific PA instructions.

Intervention Program
A summary of the intervention program is displayed in Table 1. Students from the freestyle rope skipping group and the traditional jump rope group participated in the 45-min (5:00 p.m.-5:45 p.m.) jump rope-based PA afterschool program guided by two jump rope instructors, respectively. This intervention was undertaken three times (Monday, Wednesday, and Friday) a week for 12 weeks (i.e., 36 sessions = three sessions × 12 weeks) in the indoor gym in the middle school. The 45-min jump rope-based PA afterschool program included: (a) 10 min of warm-up and instruction, (b) 30 min of jump rope-based exercise, and (c) 5 min of cooldown and stretching. During the warm-up and instruction, students were instructed to jog around the gym and stretch for warm-up. After that,

Intervention Program
A summary of the intervention program is displayed in Table 1. Students from the freestyle rope skipping group and the traditional jump rope group participated in the 45-min (5:00 p.m.-5:45 p.m.) jump rope-based PA afterschool program guided by two jump rope instructors, respectively. This intervention was undertaken three times (Monday, Wednesday, and Friday) a week for 12 weeks (i.e., 36 sessions = three sessions × 12 weeks) in the indoor gym in the middle school. The 45-min jump rope-based PA afterschool program included: (a) 10 min of warm-up and instruction, (b) 30 min of jump rope-based exercise, and (c) 5 min of cooldown and stretching. During the warm-up and instruction, students were instructed to jog around the gym and stretch for warm-up. After that, instructors delivered skill-based lessons (i.e., explanation and demonstration) and set up goals (e.g., frequency, time, intensity) to motivate students' engagement in the jump rope activity. For example, the participants set goals about how many jumps without stopping they could do (frequency) within a certain time (time) and achieving specified pedometer step counts (intensity). Bi-weekly lesson plans (i.e., repeating the same skills for two weeks) were applied to the 30 min of jump-rope activities to provide enough time for students to acquire essential skills in each lesson. During the 5 min of cooldown and stretching, students engaged in stretching exercises to allow the recovery of heart rate and breathing. In the freestyle rope skipping group, team performance and sharing fun with peers were emphasized. The primary instructor taught the formation of team performance and rhythm practice with music. Freestyle rope skipping allowed participants to engage in various rope skipping techniques both individually and as part of a team performance. In contrast, the traditional jump rope group focused on individual frequency and speed. The students performed the jump rope exercises based on one of the instructor's signals and recorded the number of jumps they did in the exercise log. The control group engaged in self-directed unsupervised free play. According to short interviews with students from the control group, they walked around on campus during the afterschool program.
As a fidelity check of the intervention, the instructors checked the students' afterschool program attendance; 97% were present for the intervention. Stopwatches were used to check the afterschool program duration. Digital pedometers (model: HS800822, B&K Electronic Technology Co., Ltd., Guangdong, China) were also employed to ensure the PA intensity of the intervention. Scruggs [38] suggested a cut point interval of 82-88 steps/min for middle school students' MVPA in PE. Therefore, we set up step count goals (2460-2640 steps/30 min) to maintain the intensity of intervention in the program.
The primary afterschool program instructor (40 years old) is an associate professor in PE and health promotion with a specialization in jump rope and dragon boat majors and has rich experience in teaching PA classes at a leading university in Shanghai, China. Another afterschool program instructor (24 years old) was a graduate teaching assistant enrolled in the M.S. program in PE with a concentration in leisure sport and jump rope. The program instructors' coaching experience of jump rope was 3 to 18 years at the K-12 level and 2 to 10 years at the undergraduate level. The instructors' major roles were to (1) teach essential rope skipping skills with explanation and demonstration; (2) organize the skill practice into a series of developmentally appropriate blocks; and (3) provide individualized suggestions and feedback about skills for students upon request.

Muscular Strength
Students' muscular strength was assessed using standing long jump and hand grip. In the standing long jump test, students stood on a line marked on the ground. They placed their feet slightly apart, swinging the arms and bending the knees, and then jumped as far as possible. The students were required to jump three times, and the longest jump distance was recorded and utilized in the data analysis. Students' handgrip strength was evaluated using the Camry digital handgrip dynamometer (model: EH101, Camry Industries Company Ltd., Kowloon, Hong Kong, China). Both left-and right-hands grip strength (kg) were measured three times in a standing position, and the highest number was used for the final data analyses in this study. Previous research showed that the standing long jump and handgrip tests were highly reliable and valid for measuring muscular strength [39].

Flexibility
The toe-touch test was used to measure students' lower body flexibility. The students stood on a box (5.75 × 2.5 × 4.4 ) bare-footed and maintained straight arms and knees during testing and bent over to try to touch the box for 10 s. The research assistants measured the distance (cm) between each students' fingers and the box using a measuring tape if participants could not reach it. If the students' fingertips did not contact the box, the scores were negative; if the student reached it, the scores were recorded as positive. The test's reliability and validity were shown in previous research [40].

Body Composition
Students' body composition was assessed using waist circumference. Research assistants measured students' bare waist circumference at a standing posture using a measuring tape. The research assistants started at the top of the participant's hip bone at the belly button level and wrapped the tape around the waist. The measurement was performed twice, and the mean score (cm) was recorded in this study.

Bone Mineral Density
Bone mineral density (BMD) from the students' right heel was assessed using the Sahara clinical bone sonometer (Hologic, MA, USA). This equipment estimated the students' BMD in g/cm 2 by measuring the transmission of high-frequency sound waves through the heel. Students were instructed to sit on the chair, and an oil-based coupling gel was applied to their heel. A previous study showed the validity and reliability of the equipment [41].

Data Analysis
Data were screened prior to the data analysis and no missing data, outliers, or normality issues were found in the present study. Three steps were used to analyze the data. First, one-way analysis of variance (ANOVA) was performed to test the differences in baseline physical fitness variables among groups (freestyle rope skipping vs. traditional jump rope vs. control group). Second, a 2 (time (pre-and post-test)) × 3 (groups) repeated measures multivariate analysis of variance (MANOVA) was conducted to examine the intervention effects and group effects on physical fitness variables. Finally, a further univariate ANOVA and post hoc tests using Bonferroni adjustment were performed to explore group differences. The effect size for the eta squared (η 2 ) was identified as >0.01 (small), ≥0.06 (medium), and ≥0.14 (large) [42]. To show statistical differences between the pre-test and post-test study variables by each group, we calculated the effect size using Cohen's d with the following criteria: ≥0.20 (small), ≥0.50 (medium), and ≥0.80 (large) [43]. An alpha level of 0.05 was used for all data analyses. Table 2 displays the descriptive results of the study variables in the pre-test by the three groups. The one-way ANOVA results showed no significant group differences in study variables (p > 0.05) in the pre-test (before the intervention). Table 2. Descriptive results of the study variables in the pre-test by the three groups (N = 60).

Variables
Freestyle RS (N = 20) Traditional JR (N = 20)   , with a small effect size (p < 0.001; ds = 0.2-0.44), in both intervention groups (freestyle rope skipping and traditional jump rope) compared to the control group. Notably, the freestyle rope skipping group significantly improved in flexibility, with a medium effect size (p < 0.05; d = 0.55) compared to the traditional jump rope and control groups, while body composition increased significantly in the traditional jump rope group compared with the control group (p < 0.05, d = 0.77). Although there was no group difference, the freestyle rope skipping group had increased BMD with a small effect size (p < 0.05; d = 0.33) across time.

Freestyle Rope Skipping vs. Traditional Jump Rope
The univariate ANOVA analysis testing group differences between freestyle rope skipping and traditional jump rope showed no significant differences (p > 0.05) in muscular strength (standing long jump, right-hand grip, and left-hand grip), while effect size (Cohen's d) analysis between the two groups indicated that freestyle rope skipping was more effective in improving students' standing long jump (d = 0.40) and left-hand grip (d = 0.23) than traditional jump rope. Furthermore, the flexibility score was statistically higher in the freestyle rope skipping group compared to the traditional jump rope group (p < 0.05, d = 0.83) (see Figure 2).
Children 2020, 7, x FOR PEER REVIEW 8 of 13 Figure 2. Gain in scores of physical fitness performances showing group differences between freestyle rope skipping and traditional jump rope. The asterisk represents a significant group difference between intervention groups, * p < 0.05.

Discussion
The major purpose of this study was to examine the effects of a 12-week jump rope-based PA afterschool intervention (i.e., freestyle rope skipping and traditional jump rope) on middle school students' physical fitness performances. The results of this study indicated significant improvements over time of physical fitness performances (standing long jump, right-hand grip, left-hand grip, and flexibility) in both intervention groups. Only the freestyle rope skipping group increased in BMD. However, the control group did not show any significant betterment in physical fitness performances in the post-test. The comparison between freestyle rope skipping and traditional jump rope indicated more improvements in muscular strength and flexibility in the freestyle rope skipping group. These results added empirical evidence to the effectiveness on physical fitness of a jump rope-based PA afterschool program, especially the effectiveness on muscular strength and flexibility of the freestyle rope skipping intervention.
After students participated in the 12-week jump rope-based PA afterschool program, statistically

Discussion
The major purpose of this study was to examine the effects of a 12-week jump rope-based PA afterschool intervention (i.e., freestyle rope skipping and traditional jump rope) on middle school students' physical fitness performances. The results of this study indicated significant improvements over time of physical fitness performances (standing long jump, right-hand grip, left-hand grip, and flexibility) in both intervention groups. Only the freestyle rope skipping group increased in BMD. However, the control group did not show any significant betterment in physical fitness performances in the post-test. The comparison between freestyle rope skipping and traditional jump rope indicated more improvements in muscular strength and flexibility in the freestyle rope skipping group. These results added empirical evidence to the effectiveness on physical fitness of a jump rope-based PA afterschool program, especially the effectiveness on muscular strength and flexibility of the freestyle rope skipping intervention.
After students participated in the 12-week jump rope-based PA afterschool program, statistically significant improvements in standing long jump were demonstrated in the group of freestyle rope skipping (5.29%, d = 0.40) and traditional jump rope (2.79%, d = 0.20). Yet, students in the control group showed deterioration in the standing long jump test (−2.14%, d = 0.11). This finding also supports previous literature that suggests that jump-based PA intervention can enhance adolescents' standing long jump performance [39]. Interestingly, Colakoglu et al. [39] applied heavy jump ropes (weight: 1.3-1.5 lb) to the weighted jump rope group to compare the effects of 15 min of jump rope training with other groups (standard jump rope group and control group), and found a significant increase in standing long jump among the female student athletes from the weighted jump rope group. Although our study used standard jump ropes (weight: 0.2-0.4 lb) for both intervention groups, the intervention groups' performance in the standing long jump was significantly improved. This may be due to the differences in duration and intensity from the previous study. We implemented 30 min of jump rope practice time and MVPA levels of intensity. In future studies, it would be interesting to examine differences in the effects of the jump rope-based PA intervention by implementing different weights of jump ropes for intervention groups in a school setting. Additionally, it is worth noting that the freestyle rope skipping group's standing long jump performance showed more improvement than that of the traditional jump rope group. This difference might be attributed to the fact that freestyle rope skipping has higher requirements for various jumps (i.e., short and long) in different directions (i.e., front, back, right, and left) [19,44]. Furthermore, the different skipping styles in freestyle rope skipping also require the integration of physical agility and explosive strength [45]. Thus, it will be an effective strategy to implement the freestyle rope skipping PA program for adolescents to enhance their muscular strength.
Consistent with previous study [39], jump rope-based intervention groups (freestyle rope skipping and traditional jump rope) demonstrated significant improvement in hand grip strength compared to the control group. Specifically, students in the freestyle rope skipping group considerably enhanced their hand grip strength (right hand: 12.28%, d = 0.41; left hand: 13.38%, d = 0.42), and the traditional jump rope group's students also notably increased their hand grip strength (right hand: 11.41%, d = 0.44; left hand: 9.79%, d = 0.31). However, participants in the control group had significantly decreased in their grip strength of both hands (right hand: −26.32%, d = 0.88; left hand: −37.82%, d = 1.53). Compared to traditional jump rope, the freestyle rope skipping group demonstrated slightly more increased left-hand grip strength. This may be due to the variation of rotating jump rope skills using body strength (i.e., wheel, double Dutch), which demands the force of both arms in freestyle rope skipping [19]. Hand grip strength is a significant indicator of multiple health outcomes (e.g., BMI, muscular strength) [46,47] and is a foundation of skill performance (i.e., martial arts) [48]. Adolescents with greater hand grip strength were found to have greater self-concept [48].
Body flexibility is one critical component of physical fitness, and being flexible is associated with a lower risk of injuries and better performance in physical activities and daily life activities [49,50]. The 12-week jump rope-based PA afterschool intervention in the present study revealed a significant improvement of flexibility among adolescents. Although the control group seemed to show significant changes in flexibility (633.30%, d = 0.14), their flexibility was obviously lower than that of the other two intervention groups. The finding is aligned with a few previous studies among different groups of adolescents (males, intellectual disabilities) [51,52]. Compared to previous studies, one unique aspect of this study was that freestyle rope skipping was demonstrated to be more effective on flexibility (163.97%, d = 0.55) than traditional jump rope (20.00%, d = 0.19). This might have been due to the fact that freestyle rope skipping integrates various movements, such as hand rolls, flips, flying feet, and assorted steps, with the use of more muscles [19].
Waist circumference is an important indicator to estimate individual body fatness [53]. Waist circumference is a biomarker for obesity and some diseases such as diabetes, heart disease, arthritis, and cancer [54,55]. Interestingly, the waist circumference of the traditional jump rope group increased significantly (7.88%; d = 0.84) while other two groups had no significant changes. The findings contradicted previous studies' results that jump rope-based interventions significantly reduced participants' body fatness [25,39]. This might be due to different measures, in which body mass index (BMI) was frequently used in the previous research [25,39]. Although it might be difficult to elucidate this contradictive result, our study recognized that the participants' waist circumferences were within the non-risky range of overweight or obese (≤88.9-101.6 cm) [56].
It was documented that various jump-based training (intensity, direction, and type) allowed for increasing body BMD [57]. To our surprise, there were no significant group differences or intervention effects for BMD. This might due to the limited intervention period in the present study. Kato et al. [58] demonstrated considerable changes in torso BMD among female college students after 6 months of jump training (i.e., two-legged maximum vertical jumps 10 times/three days/24 weeks). Given only a 12-week intervention in this study, freestyle rope skipping showed a small to medium effect size on BMD (7.14%, d = 0.33). This finding suggests that freestyle rope skipping would be more beneficial and effective in enhancing bone development among adolescents. A long-term jump rope-based intervention is warranted to test its effects on bone development among adolescents.
The findings of this study provided empirical evidence of a jump rope-based PA afterschool program in promoting adolescents' physical fitness performances, such as muscular strength and flexibility. Several limitations still existed in this study. First, this study did not objectively monitor the control group's PA during the afterschool program, while a series of short interviews were processed with participants from the control group regarding their physical activities every Friday during the 12-week intervention. Second, this study did not measure psychological factors concerning the afterschool program. Freestyle rope skipping is regarded as more fun and exciting than traditional jump rope [24]. Future studies may consider examining participants' changes in motivation or enjoyment with regard to different rope skipping interventions. Third, there were no follow-up tests (e.g., 3 months later) in the current study. At the end of the project, researchers provided jump ropes to students in all groups and encouraged them to maintain exercise using jump ropes. Thus, follow-up measures are suggested in the future to identify the lasting effects of the 12-week jump rope-based PA afterschool program on students' physical fitness performances or healthy behavioral changes.

Conclusions
To our knowledge, this study is the first investigation to examine the effects of a 12-week jump rope-based PA intervention in an afterschool programs on adolescents' physical fitness performances and, furthermore, it compared the effects between freestyle rope skipping and traditional rope jumping. Both intervention groups showed significant enhancements in muscular strength and flexibility after the 12-week jump rope-based PA intervention. The freestyle rope skipping group demonstrated better improvements in all study variables compared to the traditional rope jump group. PE teachers and afterschool program administrators are advised to adopt freestyle rope skipping to improve students' physical fitness performances. A jump rope-based PA program is recommended as a feasible curriculum element to be embedded in afterschool programs.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.