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Abstract

BACKGROUND:

Reactive performance is an important component of rhythmic gymnastics. So far, it is unclear whether additional plyometric training in female gymnasts shows an increase in performance.

OBJECTIVE:

The aim of the study was to examine the effect of additional plyometric training in rhythmic gymnastics on the reactive jumping performance and strength of the lower leg muscles.

METHODS:

Fifteen rhythmic gymnasts (age: 12.3 $\pm$ 2.6 years, height: 1.47 $\pm$ 0.12 m, body weight: 37.3 $\pm$ 9.3 kg, BMI: 16.7 $\pm$ 2.1 kg*m ${}^{-2}$ ; competition level: national and international championships, Tanner stages I–III) participated in the study. The athletes were assigned to an experimental (EG) and a control group (CG). The EG performed plyometric exercises three times per week in addition to the regular training. Before and after six weeks of training the reactive jump performance, the work of dorsi flexors and plantar flexors performed during isokinetic plantarflexion, as well as the performance in two sport-specific tests were measured.

RESULTS:

In contrast to the CG, in the EG the jump height (pre: 24.8; post: 27.25 cm; $p<$ 0.05) and the reactive-strength-index (pre: 1.01; post: 1.19; $p<$ 0.01) increased significantly. The EG achieved significant improvements in the counter movement jump test (pre: 27.0 cm; post: 31.5 cm; $p<$ 0.01) and in the sport specific double rope jump test (jumps per minute, pre: 18.0; post: 23.0; $p<$ 0.01). Furthermore, a significant increase in work performed during plantarflexion was found in the EG for the right leg (pre: 24.9 J; post: 29.7 J; $p<$ 0.01) and a tendency to increase for the left leg (pre: 26.4 J; post: 37.7 J; $p=$ 0.05).

CONCLUSION:

Both reactive strength and dynamic force can be efficiently increased by plyometric training. It may be recommended to include plyometric exercises in the training regime of rhythmic gymnasts.

Keywords

Plyometric exercise jumping reactive strength drop jump

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1. Introduction

Rhythmic gymnastics (RG) consists of an artistic manifestation of exercises with musical accompaniment. High demands on coordination, agility, bounce, balance, and control of sports equipment are required [1]. The elements are categorized into jumps, leaps, balance, rotations and working with the handset [2]. For many of these elements, the bounce is performance-determining [3]. In addition to stands and turns, jumps represent the basic requirement of the elements in an exercise [3]. The most frequently chosen techniques during the flight phase in the jump are splits, ring, and “deer” [4]. The jump combinations can consist of jump series and require rapid reactive jumps. Thus, plyometric jump loads in training are of great importance [5]. These jump variants are among the most difficult movements within the basic techniques and require a high degree of coordination, flexibility and strength [6]. The number of different jumps with acyclic technique, are in principle unlimited in the choreography. They involve a long phase of jumping flight, which is realized exclusively by a powerful impression of the legs by utilizing the reactive force [7]. Due to these requirements in the performance structure of the RG, it is necessary to integrate exercises in training that improve the plyometric performance. Various studies investigated the effect of plyometric exercises on athletic performance using different exercise programs. Bogdanis et al. [8] showed a significant increase in sprint performance and agility during 6 weeks of additional plyometric training in gymnastics. Furthermore, Hall et al. [9] found that a 6 week additional plyometric training of specific muscles used within the handspring vault significantly improved hand jump performance in gymnasts. Marina and Jemni [10] investigated a block-wise combined plyometric training with resistance training over 2 training periods on female gymnasts. They observed significant improvements of flight time and mechanical power during a drop jump. Similarly, Nitzsche et al. [11] found increased jumping height, reactive-strength-index, and increased muscle work during isokinetic plantarflexions in rock’n’roll dancers after 6 weeks of plyometric training.

Plyometric training can include e.g. stretch- shortening cycles in an isokinetic dynamometer [12, 13] or plyometric jumps. Plyometric jumps are a quick combination of eccentric and concentric muscle actions. This is called the stretch-shortening cycle (SSC). In SSC, the power and work during the shortening phase are greater than without previous eccentric contraction [14, 15]. Plyometric training increases maximum strength [16] and reactive power [17], thus contributing to increased jumping performance [11]. As mechanisms for this performance enhancement, “force enhancement” [18], activation dynamics, contribution of stretch reflexes and storage and release of elastic energy [19, 20] are discussed. During the eccentric phase, the monosynaptic stretching reflex leads to muscular activation [21]. The size of the muscular activation in the eccentric phase and the speed of movement at the reversal point are decisive for the performance of the SSC [22]. Various studies have shown the effect of plyometric training (also called reactive strength training) on jumping height and strength [23, 17]. Other but inconsistent findings are available on the influence of plyometric training on the H-reflex [24]. Overall, it can be assumed that plyometric training effects adaptations of the muscle as well as neuromuscular adaptations [25, 26, 27, 28] and thus sport performance in RG.

Thus, the aim of the study was to examine the effect of additional plyometric training in rhythmic gymnastics on the reactive jumping performance and performance of the lower leg muscles. It is hypothesised that integrating a plyometric program to the training of gymnasts leads to improvements of leg power and strength. We expect, that the plyometric training increases reactive jumping performance as well as maximal strength and power during isokinetic tests of the lower leg muscles.

Table 1
Descriptions of the anthropometric data (mean $\pm$ standard deviation) of the experimental (EG) and control group (CG)

Group	N	Age	Weight	Height	BMI
EG	11	12.1 ${\pm}$ 2.9 y	37.2 ${\pm}$ 10.2 kg	147.6 ${\pm}$ 12.8 cm	16.6 ${\pm}$ 2.1 kg*m ${}^{-2}$
CG	7	12.6 ${\pm}$ 2.3 y	37.4 ${\pm}$ 6.7 kg	148.0 ${\pm}$ 9.1 cm	17.0 ${\pm}$ 1.8 kg*m ${}^{-2}$

BMI: body mass index.

2. Methods

2.1 Participants

Eighteen female rhythmic gymnasts (age: 12.3 $\pm$ 2.6 years, body height: 1.47 $\pm$ 0.12 m, body mass: 37.3 $\pm$ 9.3 kg, body mass index (BMI): 16.7 $\pm$ 2.1 kg*m ${}^{-2}$ , training experience ranged from 2 to 6 years, competition level: national and international championships, Tanner stages I–III; D/E-squad) were recruited from two RG-clubs in Saxony/Germany. One club represented the experimental group (EG, $N=$ 11), which carried out plyometric training in addition to regular training. The other club was the control group (CG, $N=$ 7), which carried out a similar regular training without any further training content (Table 1). Both training groups followed the framework training plan of the German Gymnastics Federation (https://www.dtb.de/rhythmische-sportgymnastik/downloads/). The coaches of both training groups adhered to the implementation of the basic training content. All athletes were informed together with their parents about the goals and risks of the study and agreed to participate. Excluded were athletes who had injuries in the last six months prior to the start of the study. The study (V-267-17-HS-RSG-2404202018) was approved by the ethics committee of the University of Chemnitz and was conducted according to the Declaration of Helsinki (latest version).

2.2 Experimental design

Both training groups trained three times a week. There was a day pause between every training day. At the weekend there was always a competition. The experimental group performed a plyometric training over six weeks in addition to the normal training regime in rhythmic gymnastics. Each training session started with a general warming up. This involved short runs, static and dynamic stretching of the trunk and leg muscles. Then, jumps with low intensity (no maximum jump height) were performed.

The additional training program of the experimental group included various plyometric jumping exercises. These were increased progressively in their level of difficulty and repetitions of jumps (Table 2). The plyometric training program included squat jumps, lateral jumps, tuck jumps, and hurdle jumps (Fig. 1). In the first training week, the athletes completed 42 additional jumps per training session. By the sixth week of training, the number increased to 81 additional jumps per training session [29].

Table 2
The additional training program performed by the experimental group

Week	Repetition	Sets	Jump drill	Jumps total
1	6	3	Squat Jumps	42
	8	3	Lateral Jumps
2	8	3	Squat Jumps	54
	10	3	Lateral Jumps
3	10	3	Squat Jumps	66
	12	3	Lateral Jumps
4	8	3	Tuck Jumps	51
	3 $\times$ 3	3	Hurdle Jumps
5	10	3	Tuck Jumps	66
	4 $\times$ 3	3	Hurdle Jumps
6	12	3	Tuck Jumps	81
	5 $\times$ 3	3	Hurdle Jumps

Figure 1.

Illustration of the four plyometric jump exercises. A: counter movement jump. B: tuck jump. C: lateral jump. D: drop jump. Starting form an athletic position, the participants initiate the tuck jump with a slight crouch downward while extending their arms behind them. Then they swing their arms forward as they simultaneously jump straight up and pull their knees up as high as possible. At the highest point of the jump, the athletes are in the air with thighs parallel to the ground [51]. During lateral jumps participants performed double leg jumps over a pole (height 30 cm) with both hands on their hips. For the description of counter movement jump (CMJ) and drop jump (DJ), please see the text.

All participants were tested before and after the end of the training intervention. After a standardized warm-up three different jump tests were performed. First, the double rope jump test (DRJ) was performed as a sport specific performance test. It was conducted in order to assess the effect of training on the maximal number of double rope jumps which can be performed within one minute (suppl. material DRJ_video.mp4). This sport specific motor test is a main criterion for D/E-squad norm for gymnastics in Germany. A double punch consists of a vertical jump upwards with two turns of the rope. The subjects should do as many jumps as possible with double punches without a break. The second jump test measured the bilateral counter-movement jump (CMJ) height. The starting position is an upright position. After a quick pull-out movement by a slight squat (knee angle up to 45 ${}^{\circ}$ , eccentric mode of operation), a maximum jump is made in the vertical direction (concentric mode of operation). The arms are fixed to the pelvis/hip throughout the jump. This will minimize coordinative influences during movement execution. The task was to jump upwards without any lifting motion. The third jumping test was a bilateral drop jump (DJ) to determine the reactive jump performance [30]. This jump is useful for testing the stretch-shortening cycle [31, 32]. Participants stood on a box (height: 30 cm) until jump off [33]. They were asked to touch down and jump off as fast as possible and hence, to jump as high as possible [34]. Participants performed 5 jumps (30 s rest between jumps). The mean of the 5 repetitions was evaluated. In the CMJ and DJ the ground contact time ( $t_{gc}$ ) and the flight time ( $t_{\textit{flight}}$ ) were assessed using a contact mat (Fusion Sport, Nottingham, United Kingdom). The jumping height ( $H$ ) was calculated based on the flight time (Eq. (1)):

$\displaystyle H=\frac{1}{8}*g*t^{2}_{\textit{flight}},$ (1)

where $g$ is the gravitational acceleration and $t_{\textit{flight}}$ is the flight time in seconds. Furthermore, a reactive strength index (RSI) (Eq. (2)) was calculated as follows [35, 33]:

$\displaystyle\textit{RSI}=\frac{H}{t_{gc}}$ (2)

where $t_{gc}$ is the ground contact time. Five single drop jumps were conducted (30 s rest). The data of the jump with the best RSI was used for further analyses.

The strength of dorsiflexors and plantarflexors muscles of the ankle joint was measured using an isokinetic diagnostic system (Physiomed CON-TREX MJ System, Schnaittach, Germany). Participants lyed supine. The foot was attached to a pedal. The hip and knee angle were completely straight and fixed with straps. Ankle motion during plantarflexion started at 20 ${}^{\circ}$ dorsiflexion and ended at 35 ${}^{\circ}$ plantarflexion and resulted in a range of motion by 55 ${}^{\circ}$ . Dorsiflexion was measured with same parameters in the opposite movement direction. The angular velocity was set at 240 ${}^{\circ}$ /s with eight repetitions in a concentric-concentric contraction pattern. Both work and power were calculated for each repetition and then averaged.

2.3 Data processing and analysis

The calculation of the sample size (G*Power 3.1.9.7) resulted in 20 subjects. This was based on an effect size of 0.6 on the jump height, which was slightly lower than in a similar plyometric strength training study (Nitzsche et al. 2015). The assumed power is 0.8 with an error probability of 5%. All results are presented as mean $\pm$ standard deviation (SD) and confidence intervals 95% as well as median with interquartile ranges (IQR). Due to heterogeneity and small sample size, the data were tested for statistically significant differences using non-parametric testing. Pre-post comparisons within the group were made using the Wilcoxon rank test. Differences between groups were tested by the Mann Whitney U test. The level of significance was set at 5%. Cohen’s $d$ was used to determine the effect size. The effects size $d$ was calculated using z-statistics (Wilcoxon) for pair wise comparison. The u-statistics for $d$ was used by comparisons between both groups (Mann-Whitney U test) [36, 37]. The calculated effect sizes were interpreted as follows: 0–0.1 no effect, 0.2–0.4 small effect, 0.5–0.7 medium effect, ${\geqslant}$ 0.8 large effect [37]. As the number of analysed subjects was lower than required by the initial power analysis, a post-hoc power analysis ( $\varepsilon$ ) was conducted.

3. Results

Fifteen out of the 18 originally recruited athletes were ultimately included in the analysis. One athlete from the experimental group and two athletes from the control group were excluded from the study due to injuries, which were not related to the training. A total of 1080 plyometric jumps were performed in the EG in addition to the regular training.

3.1 Jumping tests

The analysis of the drop jump data revealed significant improvements in jump height ( $d=$ 2.22; $p<$ 0.05) and RSI ( $d=$ 2.85; $p<$ 0.01) but not in the contact time in the experimental group ( $d=$ 1.25; ${p}=$ 0.09) (Table 4).

The control group revealed no significant changes in these drop jump parameters (RSI: $d=$ 0.63, ${p}=$ 0.50; contact time: ${d}=$ 2.51, $p=$ 0.08; jump height: $d=$ 1.28, $p=$ 0.22). A group*time effect was significant on the changes in jump height ( $d=$ 1.38; $p<$ 0.05; $\varepsilon=$ 0.62), but not for contact time ( $d=$ 0.32; $p=$ 0.59;

Table 3
Results of the drop Jump (DJ), counter movement jump (CMJ) and double rope jump (DRJ)

	DJ						CMJ		DRJ
	Jump height (cm)		Contact time CT (ms)		RSI		Jump height (cm)		Counts (n/minute)
	Pre	Post	Pre	Post	Pre	Post	Pre	Post	Pre	Post
EG ( $N=$ 10)
Mean ${\pm}$ SD	25.3 ${\pm}$ 3.4	27.7 ${\pm}$ 5.7	263.2 ${\pm}$ 59.8	237.3 ${\pm}$ 23.4	1.01 ${\pm}$ 0.3	1.20 ${\pm}$ 0.3	27.5 ${\pm}$ 3.9	31.80 ${\pm}$ 6.1	23.7 ${\pm}$ 12.8	29.1 ${\pm}$ 11.9
CI 95% Median (IQR)	22.68–27.75 24.80 (5.80)	23.65–31.83 27.25 (8.58) ${}^{*}$	220.40–305.99 257.3 (80.4)	220.50–254.05 241.7 (41.2)	0.81–1.25 1.01 (0.47)	1.00–1.38 1.19 (0.40) ${}^{*}$	24.73–30.26 27.0 (5.25)	27.43–36.17 31.50 (8.25) ${}^{*}$	14.53–32.68 18.00 (25.0)	20.55–37.64 23.00 (22.7) ${}^{*}$
CG ( $N=$ 5)
Mean ${\pm}$ SD	27.9 ${\pm}$ 4.30	26.4 ${\pm}$ 4.59	261.1 ${\pm}$ 40.3	236.4 ${\pm}$ 30.4	1.10 ${\pm}$ 0.30	1.14 ${\pm}$ 0.27	30.00 ${\pm}$ 4.3	29.8 ${\pm}$ 3.9	14.6 ${\pm}$ 12.3	13.40 ${\pm}$ 11.7
CI 95% Median (IQR)	22.61–33.30 27.50 (6.85)	20.76–32.16 24.80 (7.45)	211.09–311.15 254.4 (69.4)	198.66–274.06 222.2 (51.8)	0.73–1.48 1.04 (0.43)	0.81–1.48 1.11 (0.42)	24.65–35.34 30.00 (8.0)	24.88–34.71 29.00 (7.0)	$-$ 0.62–29.82 15.00 (21.0)	$-$ 1.12–27.92 12.00 (18.5)

Table 4

Results of the ankle isokinetic strength test

		Group		Experimental group (EG)		Control group (CG)
		Test		Pre	Post	Pre	Post
Left leg	Plantar-flexion	Work (J)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	13.94 ${\pm}$ 5.63 (9.91–17.97) 14.22 (9.83)	18.64 ${\pm}$ 7.46 (13.31-23.98) 20.05 (12.86)	15.36 ${\pm}$ 7.53 (6.01–24.71) 18.93 (13.77)	18.43 ${\pm}$ 6.18 (10.76–26.10) 17.62 (12.11)
		Mean power (W)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	25.56 ${\pm}$ 10.90 (17.76–33.35) 26.44 (18.42)	34.66 ${\pm}$ 14.37 (24.38–44.93) 37.67 (24.32)	28.24 ${\pm}$ 14.81 (9.85–46.63) 35.31 (26.72)	34.61 ${\pm}$ 11.06 (20.24–48.97) 33.12 (22.65)
	Dorsi-flexion	Work (J)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	3.0 ${\pm}$ 0.95 (2.32–3.68) 2.95 (1.09)	3.11 ${\pm}$ 1.62 (1.95–4.27) 3.09 (2.95)	3.55 ${\pm}$ 1.34 (1.88–5.21) 2.33 (2.56)	3.48 ${\pm}$ 2.08 (0.89–6.07) 4.55 (3.78)
		Mean power (W)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	5.43 ${\pm}$ 1.84 (4.11–6.74) 5.47 (2.17)	5.74 ${\pm}$ 3.05 (3.56–7.92) 5.79 (5.54)	6.43 ${\pm}$ 2.79 (2.96–9.89) 5.57 (5.43)	6.53 ${\pm}$ 3.95 (1.63–11.44) 8.55 (7.21)
Right leg	Plantar-flexion	Work (J)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	12.89 ${\pm}$ 4.75 (9.94–16.29) 13.93 (8.41)	18.00 ${\pm}$ 6.98 (13.00–22.99) 16.32 (11.65) ${}^{*}$	17.14 ${\pm}$ 4.05 (12.10–22.17) 17.08 (6.65)	21.13 ${\pm}$ 7.50 (11.82–30.45) 19.13 (13.32)
		Mean power (W)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	22.79 ${\pm}$ 8.84 (16.46–40.06) 24.87 (15.48)	32.60 ${\pm}$ 13.03 (23.27–41.92) 29.67 (21.56) ${}^{*}$	30.94 ${\pm}$ 7.34 (21.82–40.06) 30.97 (12.10)	38.74 ${\pm}$ 13.88 (21.51–55.96) 34.83 (24.62)
	Dorsi-flexion	Work (J)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	3.18 ${\pm}$ 1.32 (2.24–4.13) 3.36 (1.72)	3.45 ${\pm}$ 1.17 (2.61–4.29) 3.43 (2.07)	3.81 ${\pm}$ 0.64 (3.01–4.61) 3.77 (1.01)	3.24 ${\pm}$ 1.11 (1.86–4.61) 3.62 (2.08)
		Mean power (W)	Mean ${\pm}$ SD (lower-upper) Median (IQR)	5.65 ${\pm}$ 2.40 (3.93–7.36) 5.95 (3.36)	6.26 ${\pm}$ 2.19 (4.69–7.83) 6.27 (3.81)	6.89 ${\pm}$ 1.19 (5.41–8.37) 6.78 (1.80)	5.92 ${\pm}$ 2.02 (3.42–8.42) 6.58 (3.81)

$\varepsilon=$ 0.08) and RSI ( $d=$ 0.94; $p=$ 0.09; $\varepsilon=$ 0.34). Furthermore, the experimental group showed significant improvement in the CMJ height ( $d=$ 3.94, $p<$ 0.01) and in the DRJ count number ( ${d}=$ 3.94, ${p}<$ 0.01). The control group showed no significant changes in the CMJ height ( ${d}=$ 0.52, ${p}=$ 0.56) and DRJ count number ( ${d}=$ 1.43, ${p}=$ 0.19). A significant group*time effect was found by CMJ height ( ${d}=$ 2.45; ${p}<$ 0.01; $\varepsilon=$ 0.97) and DRJ count number ( $d=$ 2.58; $p<$ 0.01; $\varepsilon=$ 0.98).

3.2 Isokinetic test

The experimental group showed a significant increase in plantarflexion work in the right leg ( ${d}=$ 3.81; ${p}<$ 0.01) and a tendency to increase for the left leg ( ${d}=$ 1.48; ${p}=$ 0.05) (Table 4). The mean power during plantarflexion of the right leg increased significantly in the experimental group ( ${d}=$ 3.81; ${p}<$ 0.01) but not for the left leg ( ${d}=$ 1.25, ${p}=$ 0.09). The work in during dorsiflexion showed no significant changes (left leg: ${d}=$ 0.12; ${p}=$ 0.79; right leg: ${d}=$ 0.75; ${p}=$ 0.26).

The control group showed no significant changes in plantarflexion work (left leg: ${d}=$ 2.51; ${p}=$ 0.08; right leg: ${d}=$ 0.93; ${p}=$ 0.34) and dorsiflexion work (left leg: ${d}=$ 0.36; ${p}=$ 0.69; right leg: ${d}=$ 1.72; ${p}=$ 0.14). No significant change was detected in the power for ankle plantarflexion (left leg: ${d}=$ 2.51, ${p}=$ 0.08; right leg: ${d}=$ 1.28; ${p}=$ 0.22) and dorsiflexion (left leg: ${d}=$ 0.36, ${p}=$ 0.68; right leg: ${d}=$ 1.77, ${p}=$ 0.14) (Table 4).

The isokinetic ankle test revealed no significant group*time effect for the work of dorsi-flexors (left leg: ${d}=$ 0.12; ${p}=$ 0.86; $\varepsilon=$ 0.05; right leg: ${d}=$ 1.07; ${p}=$ 0.08; $\varepsilon=$ 0.42) and plantar-flexors (left leg: ${d}=$ 0.06, ${p}=$ 0.95; $\varepsilon=$ 0.05; right leg: ${d}=$ 0.12; ${p}=$ 0.86; $\varepsilon=$ 0.05). The same results were detected for the mean power of dorsi-flexors (left leg: ${d}=$ 0.19, ${p}=$ 0.77; $\varepsilon=$ 0.06; right leg: ${d}=$ 1.17; ${p}=$ 0.05; $\varepsilon=$ 0.48) and plantar-flexors (left leg: ${d}=$ 0.12, ${p}=$ 0.86; $\varepsilon=$ 0.05; right leg: ${d}=$ 0.12; ${p}=$ 0.86; $\varepsilon=$ 0.05).

4. Discussion

The aim of the study was to determine whether changes in jumping performance and ankle strength could be observed in female gymnasts as a result of this plyometric training program. We observed changes of voluntary performance parameters during a six week plyometric training intervention. In general the jump performance, work and mean power during maximal isokinetic plantarflexion increased significantly in the experimental group but not in the control group. The small effect sizes between the training groups are comparable to Agostini et al. [38]. Agostini et al. [38] showed only small differences in jump height between the groups in the first 12 weeks of training. However, they reported very high effects between the training groups after 6 months of training.

Brown et al. [39] investigated the effects of a 6 week lasting plyometric training in dancers and found increased jump height and force of the lower body. According to Brown et al. [39], it was concluded that a specific plyometric training installed in a regularly RG training improves the voluntary strength and power. Based on improved sport-specific jump performance higher technical skills can be trained in RG.

In the present study the jump height of the drop jump increased while the ground contact time remained unchanged. This finding is in agreement with data reported in literature [28]. The results might be based on changes of the muscle-tendon complex (e.g. muscle fiber type, tendon, muscle architecture) or changes of the muscle coordination. The calf muscles contain different muscle fiber types [40, 41]. With a physiological view, after a 6 week sprint training the portion of type IIa fibers did increase while the portion of type I fibers decreased [42]. However, Kyröläinen et al. [28] did not find any changes in muscle fiber types after 15 week of plyometric training. Type II fibers have higher propagation velocities, more Ca ${}^{2+}$ channels, higher contraction rates and thus, we would expect decreased ground contact times. Since, the ground contact times remained unaltered in our study, alterations of the muscle fiber type ratio might be unlikely to explain the increased jump height and RSI in the present study. However, there might be other possible mechanisms contributing to performance enhancements in gymnastics after plyometric training.

Adaptations of the Achilles tendon properties were discussed in literature in response to plyometric training. Komi suggested that during the stretch shortening cycle (SSC) the tendon has spring-like properties. i.e. elastic energy will be stored during the stretch phase and will be subsequently recoiled [43]. Thus, mechanical properties like muscle stiffness and energy dissipation might change. However, the results reported in the literature are ambiguous. After 12 week plyometric training Kubo et al. [26] observed no increase in tendon stiffness, while Fouré et al. [44] demonstrated a significant increase after 14 weeks of plyometric training. Furthermore, Fouré et al. [44] observed a decreased dissipation of the Achilles tendon. It seems that adaptations of the Achilles tendon properties need longer than 12 weeks of plyometric training. The present study lasted only 6 weeks. Thus, other adaptation mechanism occurred. It was also suggested that muscle activation might change due to plyometric training [34]. Kyröläinen et al. [28] reported increased muscle activity of the M. soleus and M. gastrocnemius within 10 weeks of plyometric training. This increase was highly correlated to MVC force increase of the plantar flexors. In the present study the work and power of the plantar flexors during maximal isokinetic plantarflexions increased, too.

Furthermore, it has been speculated that force and thus performance during the SSC is in part associated with mechanism of force enhancement [14, 15]. Force enhancement has been described firstly by Abbott et al. [18] and means that the muscle force after a stretch is higher than the isometric force at the same muscle length. Underlying mechanism of force-enhancement are not completely understood so far [45]. However, interaction of the giant muscle protein titin [46] with actin during muscle activation is suggested to contribute to enhanced forces during muscle stretch [47, 48]. Titin-actin interaction and thus eccentric muscle performance might be adjustable by eccentric training [49, 47, 13]. Similar as in the present study, vertical jump height was increased in a group of basketball players performing a 6 week high-force eccentric cycle ergometry training [49]. Increased performance was attributed to the higher leg stiffness and an increased storage and release of elastic energy [50]. It was speculated that titin may significantly contribute to the force production during muscle stretch and that titin may function as an adaptable muscle spring [50, 49]. Hence, training induced adaptations in the specific titin isoform might contribute to the observed increases of jumping performance of rhythmic gymnasts after six weeks of plyometric training. Compared to other plyometric training protocols used on female gymnasts, this protocol with only four different exercises showed an increase in performance [8, 9]. This is important in practice, as such training content must be integrated economically into the existing training. Therefore, it remains to be examined in the future how extensively this content should be placed in the training.

As low and unequal sample size may limit this study, conservative statistical tests were used to circumvent a possible $\alpha$ error. In Saxony (Germany), the two training groups included in this study were the only ones with cadre status. The recruitment of further subjects would only have been possible with considerable logistical effort. Since both training groups essentially implemented the framework training concept, it is assumed that the general training contents were comparable. Only the additional plyometric training increased the training volume in the experimental group. This methodological approach was also used in other training studies with plyometric training [8, 9]. Consequently, it is unclear how a homogenised training volume in both groups would have changed the results. Regarding the differences in training volume, future studies should try to balance the addition of plyometric training in an experimental group with neutral training content in the control group.

Because of his low sample size, the influence of the girls’ age on performance changes cannot be examined. The effectiveness of plyometric training in addition to conventional training can only be related to the test methods and the training program (training exercise and intensity, number of repetitions) used here.

5. Conclusion

Improvement in jump height is a specific athletic requirement which needs to be incorporated and learned in RG. Both reactive strength and maximal dynamic force output are important factors influencing jump height. Plyometric training improves jump performance and isokinetic strength of youth RG athletes. We propose that this training program should be integrated in the traditional training regime in order to improve the competition performance of youth RG athletes.

Author contributions

CONCEPTION: Nico Nitzsche and Norman Stutzig.

PERFORMANCE OF WORK: Nico Nitzsche.

INTERPRETATION OR ANALYSIS OF DATA: Nico Nitzsche, Norman Stutzig and Tobias Siebert.

PREPARATION OF THE MANUSCRIPT: Nico Nitzsche, Norman Stutzig and Tobias Siebert.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Henry Schulz and Tobias Siebert.

SUPERVISION: Henry Schulz and Tobias Siebert.

Ethical considerations

All athletes were informed together with their parents about the goals and risks of the study and agreed to participate. The study (V-267-17-HS-RSG-2404202018) was approved by the ethics committee of the University of Chemnitz and was conducted according to the Declaration of Helsinki (latest version).

Funding

The authors report no funding.

Footnotes

Acknowledgments

The authors would like to thank Anastasya Lobanova and Steffen Gro $\beta$ mann for their support in data collection and coordination in training implementation.

Conflict of interest

The authors have no conflicts of interest to report.

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Effect of plyometric training on dynamic leg strength and jumping performance in rhythmic gymnastics: A preliminary study

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

Table 1 Descriptions of the anthropometric data (mean ± standard deviation) of the experimental (EG) and control group (CG)

2.1 Participants

2.2 Experimental design

Table 2 The additional training program performed by the experimental group

3. Results

3.1 Jumping tests

Table 3 Results of the drop Jump (DJ), counter movement jump (CMJ) and double rope jump (DRJ)

4. Discussion

5. Conclusion

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Descriptions of the anthropometric data (mean $\pm$ standard deviation) of the experimental (EG) and control group (CG)

Table 2
The additional training program performed by the experimental group

Table 3
Results of the drop Jump (DJ), counter movement jump (CMJ) and double rope jump (DRJ)