Loaded Plyometrics and Short Sprints with Change-of-Direction Training Enhance Jumping, Sprinting, Agility, and Balance Performance of Male Soccer Players

Featured Application: The investigation examined the effectiveness of an 8-week loaded plyometric with short sprints and change-of-direction training program on athletic performance in elite adolescent soccer players. We report that replacing some aspects of typical training with loaded plyometric with short sprints and change-of-direction training improved jump performance, sprinting, agility, and balance performance in soccer players. Abstract: This project investigated effects of 8 weeks of biweekly loaded combined plyometric and short sprint with change-of-direction training an in-season regimen of young soccer players. An experimental (n = 17, age: 16.5 ± 0.5 years, body mass: 64.5 ± 5.2 kg, height: 1.73 ± 0.07 m,) and control group (n = 17, age: 16.7 ± 0.5 years, body mass: 65.6 ± 4.8 kg, height: 1.74 ± 0.07 m) were tested pre- and post-intervention for squat-jump (SJ), countermovement-jump (CMJ), and ﬁve-jump test (5JT), 10 m and 30 m sprint performance, change-of-direction ability, and dynamic balance performance. The experimental group (EG) outperformed the control group (CG) over 8 weeks in jumping ( p < 0.001; d = 0.98–1.17), 10 m sprinting ( p < 0.001; d = 0.94), change-of-direction ability ( p < 0.001; d = 1.24), and dynamic balance performance ( p < 0.001; d = 0.53–0.74). We conclude that adding biweekly loaded plyometric with short sprints and change-of-direction training to habitual training improves athleticism of young soccer players. Therefore, loaded plyometric with short sprints and change-of-direction training should be implemented as a part of research-informed soccer training.


Introduction
Soccer is typified by intermittent exercise bouts with contributions from aerobic and anaerobic metabolism [1][2][3][4]. Therefore, soccer training should mimic this load characteristic in terms of the physiological and biomechanical stimuli in order to induce subsequent adaptation [5]. During match play soccer players cover 10-13 km in distance and completẽ 1350 activities (one every 4-6 s), such as multiple changes of direction, jumps, and sprints, involving acceleration, decelerations, and re-accelerations, which are interspersed with arm swing [CMJA], and the 5 jump test [5JT]), sprint performance (i.e., 10 m and 30 m sprint), change-of-direction ability (S90 • ), repeated change of direction ability (RCOD), and dynamic balance performance (Y balance test).
Prior to the study, participants were well-conditioned as they had completed an 8-week training period of 5-6 training sessions per week. These eight weeks were comprised of resistance training program initially targeting hypertrophy and muscle force. In the final three of these eight weeks, muscular power was targeted by reducing the load and increasing the contraction velocity. Every weekend, participants partook in a noncompetitive training match. Participants participated in five weekly training sessions throughout the championship season.

Experimental Design
Participants did not train in their own time and therefore only trained with the soccer team during the study. Both the experimental and control group trained five times per week (~1.5 h per session), with a competitive fixture at the weekend. Conditioning sessions were undertaken twice per week. The first of these sessions was a high intensity interval training (HIIT) session and small-sided games [31]. The second session comprised resistance training exercises such as half squats, overhead lunges, and countermovement and squat jumps. The other training sessions focused on tactical and technical skills (Table 1). The University Institutional Review Committee for the ethical human experimentation approved the study (reference number: KS00000-KS2021 and date of approval 10 December 2020). Participants (and their guardians, in the case of minors) provided written informed consent. Two familiarization sessions were held 2 weeks before testing, and testing commended 2 months after the start of the competitive season.
The loaded plyometrics and short sprints with change-of-direction training program consisted of 4 principal workshops, twice per week ( Figure 1). The plyometric training included vertical (i.e., hurdle jumps) and horizontal (i.e., bouncy strides and drop jumps) Appl. Sci. 2021, 11, 5587 4 of 12 jumps performed at maximal effort (i.e., maximal height and forward distance with a minimal contact time for vertical and horizontal jumping, respectively) [13]. For workshop n • 3 (6 bouncy strides) the subject must perform a simple take off. The distance between the hurdles (workshop n • 1) and between boxes (workshop n • 4) was approximately 70 cm [32,33]. For workshop n • 1, hoops were spaced approximately 1.8 m apart with the player placing the supporting leg in, after or before the hoop (just keep the same jumping axis). The loaded plyometrics and short sprints with change-of-direction training program consisted of 4 principal workshops, twice per week ( Figure 1). The plyometric training included vertical (i.e., hurdle jumps) and horizontal (i.e., bouncy strides and drop jumps) jumps performed at maximal effort (i.e., maximal height and forward distance with a minimal contact time for vertical and horizontal jumping, respectively) [13]. For workshop n°3 (6 bouncy strides) the subject must perform a simple take off. The distance between the hurdles (workshop n°1) and between boxes (workshop n°4) was approximately 70 cm [32,33]. For workshop n°1, hoops were spaced approximately 1.8 m apart with the player placing the supporting leg in, after or before the hoop (just keep the same jumping axis). Each workshop commenced with plyometric exercises (hurdle jumps, lateral hurdle jumps, bouncy strides, and drop jumps) and ended with short change-of-direction sprints. Details are given in Table 2. All repetitions and workshops were separated by 90-s recovery intervals. Each workshop commenced with plyometric exercises (hurdle jumps, lateral hurdle jumps, bouncy strides, and drop jumps) and ended with short change-of-direction sprints. Details are given in Table 2. The experimental group executed all workshops with a weighted vest (8% body mass) [13,27]. The training protocol was informed by recommendations from Bedoya et al. [34] and recommendations for change of direction ability promotion [35].

Testing Schedule
Tests were conducted ≥3 days following the last competitive fixture and 5-9 days after the final training session. Testing took place on a tartan surface and a standardized Appl. Sci. 2021, 11, 5587 5 of 12 warm-up preceded each test. Tests were completed over three separate testing days the in the following order: anthropometric assessment, SJ and CMJ, S90 • (all day 1), Y balance test, 30 m sprint (both day 2), 5JT and the RCOD (both day 3). The SJ, CMJA [24,36], S90 • [37], Y balance test [38,39], 10 and 30 m sprint performance [40], 5JT [10], and the RCOD [2,41] were all previously described in detail so are not outlined here for brevity and to avoid self-plagiarism [24,39]. The anthropometric characteristics (body mass and body fat percentage) using by were evaluated barefoot in the morning hours (7-8 a.m.) after an overnight fast, with the electrical impedance method (BC−602, Tanita Co., Tokyo, Japan).

Statistical Analysis
All statistical analysis were performed using SPSS version 25.0 for Windows (IBM, Armonk, NY, USA).
Normal distribution was confirmed for all variables using the Shapiro-Wilk test and Levene's test confirmed homogeneity of variance for all variables, justifying parametric data analysis.
Unpaired t-tests tested for between-group differences at baseline. Paired sample t-tests were used to test for within-group differences from pre-to post-intervention. Training-related effects were assessed by 2-way (group × time) mixed factorial analysis of variance (ANOVA). Subsequently, if a main effect of group or time, or an interaction effect (group × time) was observed, Tukey's post hoc procedure located pairwise differences. Alpha level is reported as exact p values as suggested by the American Statistical Association [42].
Cohen's d if provided as a measure of effect size for paired comparisons whereby the difference in means between two samples was divided by the pooled standard deviation (SD). Thresholds of 0.2, 0.50, and 0.85 for Cohen's d [43] were considered small, moderate, and large, respectively.
Reliability was assessed using intra-class correlation coefficients (ICC) [44] with 95% confidence intervals (CI) and coefficients of variation (CV) over consecutive pairs of intraparticipant trials [45]. All measures of jumping, sprinting, change of direction ability, and balance performance had an ICC > 0.80 and a CV < 5%.
Data are reported as mean ± SD, other than reliability data which is given as ICC (95% CI) or CV (95% CI).

Results
Reliability data are summarized in Table 3. Table 3. Interclass correlation coefficient (ICC, 95% confidence intervals (CI)) and coefficient of variation (CV) for all fitness tests.  There were no baseline differences between groups.

Effect of Training on Performance
There was a significant group × time interaction for SJ, CMJA, 5JT, 10 and 30 m sprint, and SJ90 • ( Table 4). There was a significant group × time interaction for RCOD best, RCOD mean, and RCOD total (Table 5). Similarly, there was a group × interaction effect for the anterior, posteromedial, and posterolateral direction r for right support leg during the Y balance test and on the anterior, posteromedial, and posterolateral direction for left support leg.

Discussion
This study examined the effect of substituting some regular in-season training with 8 weeks of biweekly loaded plyometric with short sprints and change-of-direction training in elite adolescent soccer players. In this context, we accept our hypothesis that replacing some regular in-season training by 8 weeks of biweekly loaded plyometric with short sprints and change-of-direction training would improve both horizontal and vertical jump performance, sprint performance, agility, and balance.

Effect of Training on Jump Performance
Both vertical acceleration (jumping power) and horizontal acceleration (sprinting) are important in ball possession, repossession, corner kicks, defense play, and attack on goal [46]. Arnason et al. [47] reported that high level competitive players jump higher than lower performance level players. Here we report training effects on vertical and horizontal jump performance. To the authors' knowledge, this study is the first that investigated effects of loaded plyometric with short sprints and change-of-direction training on jump performance in young soccer players. Our data are in agreement with several previous investigations which have described improved jump performance following unloaded combined plyometric and short sprint with change-of-direction training [12,35,39] and loaded and unloaded combined plyometric and short sprint training [10].
Recently, Hammami et al. [10] studied effects of 10-week loaded and unloaded combined plyometric and short sprint training in junior soccer players (U17 elite males). They observed significant improvement in jump performance (SJ, p < 0.001; CMJ, p < 0.01; CMJA, p < 0.01; and 5JT test, p < 0.01) after both types of training programs. In addition, Michailidis et al. [12] studied the effects of a 6-week plyometric training and change-of-direction exercises in prepubertal male soccer players. These researchers noted significant improvement in vertical and horizontal jumps performance (squat jump and long jump, p < 0.05) but no gain in CMJ and 5JT. Indeed, Hammami et al. [36] studied the influence of 8 weeks of combined plyometric and short sprint with change-of-direction training jump performance in young male handball players (U15) and reported improved jump performance (39%, d = 0.26; small; 29%, d = 0.26; small; 26%, d = 0.25; small; and 13%, d = 0.09; small for SJ, CMJ, CMJA, and 5JT, respectively).
Mechanisms which underpin the improved jump performance following plyometric training are commonly explained by neural factors such as more synergistic muscle activation and less antagonistic muscle activation (i.e., intermuscular coordination) and greater motor unit recruitment (i.e., intramuscular coordination) [48]. Furthermore, Kobal et al. [26] reported that the use of additional load during executed jumps allowed players to apply greater force against the ground in the direction of intended movement (vertical or horizontal axes) on a longer period of time. This mechanical adjustment generates higher impulses (possibly additional overload) during jumps [49], thus producing a greater adaptation of jumping ability in the loaded group [26].

Effect of Training on Sprint Performance
To the authors' knowledge, this study is the first that investigated effects of loaded plyometric with short sprints and change-of-direction training on sprint performance in young soccer players. In our study, we could show relevant improvements in both type of sprints. The interaction effects (d) differed from 0.52 (30 m) to 0.94 (10 m). The improvements in the EG (d range: 1.93-2.51) were markedly higher than in the CG (d range: 0.05-0.70). Improved sprint performance observed herein are in agreement with those of Hammami et al. [10] who examined effects of a 10-week loaded and unloaded combined plyometric and short sprint training in elite junior male soccer players (U17). These researchers reported improvements in sprint performance over 5 (p < 0.001), 10 (p < 0.001), 20 (p < 0.01), 30 (p < 0.001), and 40 m (p < 0.05) sprint times, after both training programs. Similarly, Michailidis et al. [12] reported improvement in sprint performance over distance of 10 m (p < 0.05), but no gain over a distance of 30 m after a 6-week plyometric training and change-of-direction exercises in prepubertal male soccer players. Hammami et al. [50] reported increases in 5, 10, 20, and 30 m sprint times (9%, d = 0.14; 7%, d = 0.21; 7%, d = 0.10; and 9%, d = 0.90, respectively), following combined plyometric and short sprint with change-of-direction training in male U15 handball players. Contrary to our results, few studies [51][52][53] found no gain in sprinting performance following plyometric training in young male soccer players and physical education students. Previous studies have shown that the sprint training charge may increase eccentric strength of extensor muscles during the braking phase of the contact with the ground and increase the rigidity of the springs and leg muscles, potentially increasing the ability of muscle to store elastic energy and improve power output [54,55]. In addition, the contradictory results may be explicated by divergence in training programs and sample populations.

Effect of Training on Change of Direction Ability
To the best of our knowledge, the present investigation is the first to have studied the effects of loaded combined plyometric and short sprints with change-of-direction on change of direction ability, in young soccer players. Our findings of improved ability to change direction are in accordance with Hammami et al. [10] who reported significant improvement in change of direction ability after a 10-week loaded and unloaded plyometric and short sprint training in elite junior male soccer players. We detected the largest performance improvements (interaction effect: d = 1.24) for all investigated parameters in the ability to change direction. The increase in performance in the EG (d = 3.75) was more than four times as high as in the CG (d = 0.82).
Moreover, Michailidis et al. [12] observed improved ability to change direction following combination of plyometric training and change-of-direction exercises, in prepubertal male soccer players. Indeed, Makhlouf et al. [11] observed improved change of direction ability (Illinois agility test (ICODT) without a ball and 4-m × 9-m shuttle run test (Agility 4 m × 9 m), following combined plyometric and short sprint with change-of-direction training, in prepubertal male soccer players. Conversely, Beato et al. [35] examined effects of plyometric and change-of-direction training in elite youth soccer players (U18) for six weeks and observed no gain after training. Loaded sprint training can modify stride length, stride frequency, contact time, and horizontal force production adequately to improve acceleration [56,57], improving the management of change in performance [58]. In addition, dissimilarities in acute program variables (duration, intensity, and frequency) and methodology (period and duration of studies and age, gender, and competitive level of players) could contribute to discrepancies between study results.

Effect of Training on Repeated Change of Direction Ability
This is the first investigation of the effects of loaded combined plyometric and short sprints with change-of-direction training on the ability of youth soccer players to repeated change of direction. The relevant improvements of performance in our study in this ability ranged in the EG from d = 1.94 (fast time) to d = 2.26 (mean time) and in the CG from d = 0.41 (fast time) to d = 0.45 (mean time). In contrast to the medium (fast time; d = 0.73) and large (mean time; d = 0.84) interaction effects, the changes regarding fatigue index were small in both groups (d = 0.03 and 0.17) and not significantly different between groups (p = 0.650). Our results of improved repeated change of direction ability (fast and mean time) corroborate those of Hammami et al. [10] who observed improved repeated change of direction ability parameters except the fatigue index after 10 weeks of loaded and unloaded combined plyometric and short sprint training in elite junior male soccer players (U17). In addition, Hammami et al. [50] examined the influence of combined plyometric and short sprint with change-of-direction training on RCOD in U15 male handball players and reported no improvement. One potential explanation for lack of change in RCOD fatigue index may be its poor reproducibility [59] or differences in the duration of the training protocol, period of the season, and players' level.

Effect of Training on Balance Performance
To our knowledge, our study is the only one that has examined the effects of loaded combined plyometric and short sprints with change-of-direction training program on balance performance in young soccer players. We observed significant and relevant intervention effects for dynamic balance performance. All calculated interaction effects moved on a similar (medium) level (d range: 0.58-0.74). The effect size d in the EG ranged from 1.38 (left support leg, posteriomedial direction) to 2.04 (right support leg, posteriomedial direction). On the other hand, the changes in the CG were only small (d range: 0.34-0.40). Ricotti et al. [60] reported soccer players from a higher competition level (i.e., division) perform better at balance tests than those from a lower competition level. Balance may be important in soccer as Hrysomallis et al. [61] observed poor balance associates with increased ligament injury risk. Our data corroborate results of Makhlouf et al. [8] who examined effects of 8 weeks of plyometric change-of-direction training in prepubertal male soccer players and noted improved dynamic balance (Y-balance test). In contrast, our results disagreed with the results of Hammami et al. [10] who considered the effects of a 10-week loaded and unloaded combined plyometric and short sprint training program, in elite junior male soccer players (U17). These authors reported no gain in dynamic balance performance (Y-balance test) after both types training programs. In addition, Hammami et al. [50] reported no intervention effects for the Y-balance test following combined plyometric and short sprint with change-of-direction training in male U15 handball players. Because balance improvements may reduce lower extremity injury risk in soccer players [62], our results emphasize the worth of this type of training as a strategy to reduce risk of injury. Nevertheless, it seems that the significant increases in balance performance found for both legs, in our study, are mainly due to the improvement of motor coordination: the improvement of the neuromuscular control of the muscle groups of the lower limbs [63].

Practical Implications
This study showed biweekly in-season loaded plyometrics and short sprints with change-of-direction training enhances jump performance, sprinting, change-of-direction ability, the ability to repeatedly change direction, and balance performance, in elite adolescent soccer players. Therefore, strength and conditioning coaches should incorporate loaded plyometrics and short sprints with change-of-direction training into in-season soccer training to enhance fitness.

Conclusions
In conclusion, adding loaded plyometrics and short sprints with change-of-direction training to standard training improves jump performance, sprinting, change-of-direction ability, and balance performance. As such, these data support inclusion of this form of training into in-season soccer training. Future studies may wish to extend these results to other genders, age groups, and competitive level of players. It is also necessary to compare the performance gains in tests with the improvement during competition. Finally, future studies could examine effects of loaded combined plyometric and short sprint with change-of-direction training on other physical qualities. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Data Availability Statement:
The raw data supporting the conclusions of this article will be made available by the authors without undue reservation.