Effect of Plyometric Training on Jumping, Sprinting and Change of Direction Speed in Child Female Athletes

Background: This study examined the e ects of 8 weeks of plyometric training on jumping, sprinting, and change of direction (COD) performance. Methods: Fifty female 7–9-year-old gymnasts were randomly assigned to a plyometric training group (PG;n=33), that performed supplementary plyometric training twice per week, and a control group (CG;n=17) that continued regular training. The following tests were performed before and after the intervention: 10 and 20 m sprints,5+5 m and 10+10 m COD tests, one-leg and two-leg countermovement jump (CMJ), drop jump (DJ), squat jump (SJ), and standing long jump (SLJ). Results: Only a main e ect for time was found for all jumping performance parameters (p=0.001). However, the improvement of one- and two-leg CMJ in PG had a greater e ect size than CG (0.72 and 0.67 vs. 0.34 and 0.18, respectively). Group time interactions were found for 10 and 20 m sprint tests (p=0.018 andp=0.011, respectively) and for 10+10 m COD (p=0.008) with the post hoc test showing improvement only for the PG (p=0.001, 0.001, and 0.003 and d=1.1, 1.14, and 0.6, respectively). Conclusions: Supplementary plyometric training increased sprint and COD performance more than regular gymnastics training, while jumping performance was equally improved in both groups. Keywords:muscle power; stretch–shortening cycle; children; gymnastics 1. Introduction Leg muscle power and sprinting ability are important performance parameters in several youth sports [1,2]. The e ects of training on muscular power and jumping ability are more pronounced in adolescence than childhood, with boys improving more than

more than regular gymnastics training, while jumping performance was equally improved in both groups. Keywords:muscle power; stretch–shortening cycle; children; gymnastics 1. Introduction Leg muscle power and sprinting ability are important performance parameters in several youth sports [1,2]. The e ects of training on muscular power and jumping ability are more pronounced in adolescence than childhood, with boys improving more than girls [3]. Adaptations to sprint training are more variable, showing an initial spurt during the ages of 5–9 years, which is followed by a second period of rapid improvement during puberty and beyond [2,4]. A more complex tness component associated with straight line sprinting, muscular power, and motor coordination, is the change of direction (COD) speed [5]. In most youth development models, drills including short sprints, accelerations, decelerations, and COD may be used as training tools, from a very young age, to develop general athleticism as well as physical qualities, such as speed and muscle power [1,2,6]. Plyometric training is e ective in increasing sprinting and jumping ability, reactive strength, and CODin prepubertal athletes [7,8]. Age, maturation, and task complexity in uence plyometric training e ectiveness [9,10]. For example, Moran et al. [11] reported a greater increase in vertical jumping following plyometric training in males before and after peak height velocity (PHV) compared Sports2019,7, 116; doi:10.3390 /sports7050116 /journal/sports

Sports2019,7, 116 2 of 10 to mid-PHV (d>0.90, and d=0.47, respectively), while Asadi et al. [12] found lower adaptive responses in COD abilities, in male participants before PHV (d=0.68), compared with mid-and post-PHV (d=0.95and 0.99, respectively). However, most studies have examined male youth athletes, while females may respond di erently due to the gender-speci c e ects of maturation on body composition and adaptability to training [3,13]. Recently, Moran et al. [14] reported that plyometric training induced small e ects (d=0.57) on jumping performance in female youth (8–18 years), while there is limited evidence on the degree to which prepubescent girls adapt to plyometric training. The fundamental locomotion skills of jumping, leaping, hopping, and running may be signi cantly improved during middle childhood (7–10 years), while it has been recommended that children may start resistance training with their own body weight as early as they are able to follow instructions and safety rules (i.e., 7–8 years of age) [1,15,16]. However, age- and sport-speci c information is limited regarding the e ects of plyometric training on speed and power indices in very young athletes, and especially females, although in some sports like gymnastics, children are training systematically from a very early age [3,6]. Young gymnasts are required to execute complex technical skills that require high power output acquired from a young age [6]. Thus, the purpose of this study was to examine the e ects of 8 weeks of plyometric training on jumping performance, sprinting, and COD in 7–9 years old female gymnasts. It was hypothesized that supplementary plyometric training would improve these abilities more than gymnastics training alone. 2. Materials and Methods 2.1. Participants Power calculations indicated a sample of 12 participants would be needed to detect an e ect size (ES) of 0.5, obtained from the minimum ES reported in the meta-analysis of Moran et al. [11] for plyometric training in pre-PHV boys (within–between ANOVA power=0.80, alpha=0.05, correlation between repeated measures r=0.5; G-Power 3.1.9.2). Gymnast ages ranged from 7 to 9 years,and they were recruited from the same gymnastics club and trained with the same coach in three groups

size (ES) of 0.5, obtained from the minimum ES reported in the meta-analysis of Moran et al. [11] for plyometric training in pre-PHV boys (within–between ANOVA power=0.80, alpha=0.05, correlation between repeated measures r=0.5; G-Power 3.1.9.2). Gymnast ages ranged from 7 to 9 years,and they were recruited from the same gymnastics club and trained with the same coach in three groups of 16–19 athletes each. None of the participants had performed systematic plyometric training in the past. All participants were injury-free six months prior to the study start and no gymnast was injured during the study. Since the minimum sample size was 12, it was decided that two of the groups would be randomly allocated to the plyometric intervention (n=16 andn=19) while the other group acted as a control (n=18). Of these, two gymnasts from the training group and one athlete from the control group did not perform all preliminary tests and visits, and were excluded from the study. In total, fty female `Gymnastics for All' gymnasts participated in this study forming the plyometric group (PG;n=33) and the control group (CG;n=17). No athlete dropped out during the course of the intervention. Anthropometric and maturity characteristics of the participants are shown in Table. All gymnasts trained 3 days per week, (Monday, Wednesday, Friday) for 90 min each day, and competed in the national championships. Maturity o set was calculated at the study beginning and end, according to the prediction equation of Mirwald et al. [17]. Based on the chronological age and maturity o set ( 4.9 0.4 years from peak height velocity—PHV) the participants of the PG and the CG were characterized as prepubertal. Prior to the study, the athletes and their parents were fully informed about the purpose and risks of this study. Written parental consent was obtained for each participant. Procedures were approved by the Institutional Ethics Review Committee (reference number: 1000/30-09-2016) and complied with the Code of Ethics of the World Medical Association (Helsinki declaration of 1964, as revised in 2013).

this study. Written parental consent was obtained for each participant. Procedures were approved by the Institutional Ethics Review Committee (reference number: 1000/30-09-2016) and complied with the Code of Ethics of the World Medical Association (Helsinki declaration of 1964, as revised in 2013).

Sports2019,7, 116 3 of 10 Table 1. Age, training experience, maturity o set, and anthropometric characteristics of the participants in the plyometric training group (PG) and the control group (CG) (mean SD). Variables PG (n=33) CG (n=17) p Age (year) 8.1 0.7 7.9 0.8 0.333 Training experience (year) 2.5 0.6 2.3 0.5 0.140 Height (cm) 129.3 6.1 129.8 7.6 0.829 Body mass (kg) 28.7 5.8 27.5 6.0 0.475 BMI (kg/m 2 ) 17.1 2.5 16.3 2.0 0.194 Leg length (cm) 60.1 4.1 61.2 4.7 0.420 2.2. Study Design The e ects of 8 weeks of plyometric training, in addition to regular gymnastics training,on jumping, sprinting, and COD were examined using a repeated measures design. This 8-week period was chosen based on the duration of a typical mesocycle. The study took place during the gymnastics preseason (October to December). All gymnasts underwent the following tests at baseline, and after 8 weeks of training: 10 and 20 m linear sprint test, 5+5 m and 10+10 m COD sprint tests with a 180 turn, one-leg and two-leg countermovement jump (CMJ), drop jump (DJ), squat jump (SJ), and standing long jump (SLJ). The additional plyometric program was performed during non-consecutive days (Monday, Wednesday) before the main training and lasted ~27 min (total of 16 sessions). In the two weeks preceding baseline testing, four familiarization sessions were held to get participants familiarized with the plyometric training and testing procedures and to calculate intra-class correlation coe cients (ICC) using a two-way mixed model. 2.3. Methodology Body mass, (0.1 kg, Seca 700, Seca Ltd., Birmingham, UK) standing height and sitting height (0.1 cm, Charder HM-200P, Charder Electronic Co., Ltd., Taichung, Taiwan) were measured before and after training. 2.3.1. Jumping Performance Jump height was determined from ight time, using an Optojump system (Microgate, SRL, Bolzano, Italy). The validity and reliability of this system for measuring jump height have been previously reported [18]. Participants were instructed to perform maximum e ort jumps. For the one- and two-leg CMJ, DJ, and SJ, participants were instructed to maintain their hands akimbo, to take o with the ankles and knees fully extended,

using an Optojump system (Microgate, SRL, Bolzano, Italy). The validity and reliability of this system for measuring jump height have been previously reported [18]. Participants were instructed to perform maximum e ort jumps. For the one- and two-leg CMJ, DJ, and SJ, participants were instructed to maintain their hands akimbo, to take o with the ankles and knees fully extended, and to land balanced on the same spot. During all jumping tests, gymnasts wore gymnastics shoes. For the one- and the two-leg CMJ, gymnasts were instructed to perform a countermovement until the knees bent to approximately 90 , and then immediately jump up. ICC for the two-leg CMJ was 0.94 (95% CI: 0.90–0.97) (SEM=4.3%, MDC90=1.8 cm) and for the sum of the right and the left-leg CMJ it was 0.97 (95% CI: 0.94–0.98) (SEM=5.1%, MDC90=1.7 cm). For the DJ, gymnasts stepped horizontally o a 20 cm box on the gymnastics carpet and then immediately performed a maximal rebound vertical jump with minimal ground contact time (CT). The ICC for the DJ height was 0.93 (95% CI: 0.87–0.96) (SEM=7.4%, MDC90=2.6 cm), while the ICC for CT was 0.85 (95% CI: 0.73–0.91) (SEM=10.3%, MDC90=100 ms). Reactive strength index (RSI) was determined during drop jump as the ratio between jump height and time spent in contact with the ground as follows [19]: RSI=jump height (millimeters)/ground contact time (milliseconds). The ICC for RSI was 0.86 (95% CI: 0.75–0.92) (SEM=16.6%, MDC90=0.20 mm/ms). For the SJ, participants started from a stationary semi-squat position (~90 knee exion determined by a manual goniometer) and were instructed to jump as high as possible, without a countermovement. For each trial of the SLJ, the participants started behind a line on the ground, feet at shoulder width

Sports2019,7, 116 4 of 10 and hands neutral. On the command, participants executed a countermovement and then jumped maximally in the horizontal direction. Participants landed with both feet simultaneously and were not allowed to fall. Horizontal distance from the toes at the start to the landing at heel contact was used for statistical analysis. All trials were measured to the nearest 0.01 m. The ICC for the SJ was 0.89 (95% CI: 0.80–0.94) (SEM=7.2%, MDC90=2.8 cm) and for the SLJ, 0.85 (95% CI: 0.74–0.92) (SEM=5.5%, MDC95=14.0 cm). 2.3.2. Sprint and COD Tests Ten- and twenty-meter linear sprint performances were assessed electronically (Microgate, SARL, Bolzano, Italy). Participants were asked to stand in an upright stride stance with the preferred leg forward, 0.3 m before the rst infrared photoelectric gate, which was placed 0.75 m above the ground to ensure it captured trunk movement and avoided false limb motion signals. The intraclass correlation coe cient for the 10 m sprint was 0.95 (95% CI: 0.90–0.97) (SEM=2.1%, MDC90=0.14 s) and for the 20 m sprint ICC was 0.96 (95% CI: 0.91–0.98) (SEM=1.9%, MDC90=0.21 s). COD tests were performed over two distances: 5+5 m and 10+10 m with a 180 turn. Cones were placed at 0, 5, and 10 m on a gymnastics vault runway. Participants were instructed to accelerate as quickly as possible along the 5 m distance, pivot 180 around the cone, and return as quickly as possible to the starting line. The same procedure was repeated for the 10+10 m COD. The total time to run the COD tests was measured electronically (Microgate, SARL, Bolzano, Italy). The ICC for5+5 m COD was 0.88 (95% CI: 0.78–0.93) (SEM=2.6%, MDC90=0.23 s) and for the 10+10 m COD, it was 0.94 (95% CI: 0.89–0.97) (SEM=1.6%, MDC90=0.22s). All performance tests were completed in the same session, 48 h after the last training and following a 10 min standardized warm-up, including 5 min of light jogging, dynamic stretches, and 2 short accelerations. For the 10 and 20 m of linear sprint tests and the COD tests, athletes performed 2 trials for each distance interspersed by 1

(SEM=1.6%, MDC90=0.22s). All performance tests were completed in the same session, 48 h after the last training and following a 10 min standardized warm-up, including 5 min of light jogging, dynamic stretches, and 2 short accelerations. For the 10 and 20 m of linear sprint tests and the COD tests, athletes performed 2 trials for each distance interspersed by 1 min of rest and the best time was recorded for further analysis. Sprint and COD tests were separated by 3 min of passive rest. After that, and following 3 min of recovery, the gymnasts performed the jump tests. Two trials were performed for each jump and the average value was used for analysis. Trials were separated by 30 s and there was a 2 min rest between the di erent jump tests. 2.3.3. Plyometric Training At the start of the Monday and Wednesday training session, and immediately after the 10 min standardized warm-up, the athletes of the PG performed plyometric training while the athletes of the CG performed choreography movements. Following the plyometric program all gymnasts continued their regular gymnastics training. The plyometric program was designed to include two 4-week training blocks. Six plyometric exercises of progressive di culty were performed in each training block according to the stages of plyometric load proposed by Lloyd et al. [20]. In each session, athletes performed two rounds of six exercises in a circuit form, with 30 s rest between exercises and 5 min of rest between rounds (Table). Plyometric exercises were performed on the surface of a gymnastics carpet with the gymnasts wearing gymnastics shoes. Plyometric training was supervised by an experienced coach and proper technique of movement was emphasized at every training and testing session. Also, an experienced coach recorded all the gymnastics elements with a plyometric component for the lower limbs (e.g., handsprings, round-o , vaulting) during all training sessions, to ensure that all participants received similar plyometric training load. In the rst week of training gymnasts in both groups performed ~100 gymnastics elements with a plyometric component in each training session and increasing to ~130 gymnastics elements by study

the gymnastics elements with a plyometric component for the lower limbs (e.g., handsprings, round-o , vaulting) during all training sessions, to ensure that all participants received similar plyometric training load. In the rst week of training gymnasts in both groups performed ~100 gymnastics elements with a plyometric component in each training session and increasing to ~130 gymnastics elements by study completion.

Sports2019,7, 116 5 of 10 Table 2. Plyometric training program executed two times per week for eight weeks by the athletes in the plyometric training group (PG). Athletes performed two rounds of six exercises in a circuit form, with 30 s rest between exercises and 5 min of rest between rounds. Weeks: 1 to 4 (Total: 180 Jumps Per Session) Weeks: 5 to 8 (Total: 190 Jumps Per Session) 1. rolling backward and forward on a 10 cm mat to jump up (10 repetitions) 1. rolling backward and forward on a 10 cm mat to jump up (15 repetitions) 2. 3. (6 repetitions per leg) 3. (10 repetitions per leg) 4. box (6 repetitions) 4. box (8 repetitions) 5. (12 repetitions) 5. (12 repetitions) 6. Lateral double-leg line jumps to a half-squat position (10 repetitions) 2.4. Statistical Analyses Statistical analyses were carried out using SPSS (IBM SPSS Statistics, Version 22.0, IBM corporation, Armonk, New York, NY, USA). Data are presented as means and standard deviations. The 90% con dence intervals were also calculated for the mean di erences reported. The normality of data distributions was checked using a Kolmogorov–Smirnov test. One-way analyses of variance (ANOVA) were applied to determine signi cant di erences in baseline values between groups. A two-way ANOVA (group (plyometric/control) time (pre/post-training)), with repeated measures on the time dimension, was conducted to examine the e ect of plyometric training on all examined variables. When a signi cant main e ect or interaction was observed (p<0.05) a Tukey's post hoc test was performed. E ect sizes (ES) for the ANOVA were determined by partial eta squared ( 2). For pairwise comparisons, ES was determined by Cohen's d. Magnitude-based inference (MBI) tests were performed, as described by Hopkins et al. [21]. The threshold value for the smallest worthwhile change was set in this study at 0.2 of the between subject standard deviation. The scale used to interpret the probabilities was as follows: 25–75%, possible; 75–95%, likely; 95–99%, very likely;>99.5%, most likely. Additionally, the standard error of measurement (SEM) and the meaningful detectable change at 90% con dence interval (MDC90)

al. [21]. The threshold value for the smallest worthwhile change was set in this study at 0.2 of the between subject standard deviation. The scale used to interpret the probabilities was as follows: 25–75%, possible; 75–95%, likely; 95–99%, very likely;>99.5%, most likely. Additionally, the standard error of measurement (SEM) and the meaningful detectable change at 90% con dence interval (MDC90) were calculated. Statistical signi cance was set atp<0.05. 3. Results 3.1. Anthropometric Measurement There were no statistical di erences in age, training experience, maturity o set, and anthropometric characteristics between the PG and the CG at baseline (Table). After training, there was a statistically signi cant increase in standing height in both groups (by 0.6 0.3 cm and 0.6 0.4 cm, respectively, p=0.001), with no di erence between groups (p=0.777). All other anthropometric variables remained unchanged after training (p>0.5). 3.2. Performance Variables There was no statistical di erence between groups in all baseline values (p>0.3; Table). No group time interaction was shown for all other parameters examining various forms of jumping performance (SJ, CMJ, DJ, SLJ, CT, RSI), as well as for the 5+5 COD test (Table). For all these parameters, except for CT, a main e ect for time was observed (p<0.01, 2>0.07), indicating similar improvements in both groups. However, the improvement in single- and double-leg CMJ in PG had a greater e ect size than that in CG (0.72 and 0.67 in PG vs. 0.34 and 0.18 in CG, Table). Comparison of the changes in performance between the PG and CG showed a small to moderate e ect size, favoring greater

Plyometric training improved sprint and change of direction performance in child female gymnasts.