EFFECT OF UNILATERAL, BILATERAL, AND COMBINED PLYOMETRIC TRAINING ON EXPLOSIVE AND ENDURANCE PERFORMANCE OF YOUNG SOCCER PLAYERS RODRIGO RAMI´REZ-CAMPILLO,1 CARLOS H. BURGOS,2 CARLOS HENRI´QUEZ-OLGUI´N,2 ´ LVAREZ,4 MAURICIO CASTRO-SEPU´LVEDA,5 DAVID C. ANDRADE,2 CRISTIAN MARTI´NEZ,3 CRISTIAN A 6,7 8,9 MA´RIO C. MARQUES, AND MIKEL IZQUIERDO 1

Department of Physical Activity Sciences, University of Los Lagos, Osorno, Chile; 2Laboratory of Exercise Sciences, MEDS Clinic, Santiago, Chile; 3Department of Physical Education, Sport and Recreation, University of La Frontera, Temuco, Chile; 4 Health Promotion Program, Family Health Center of Los Lagos, Los Lagos, Chile; 5Exercise Science Laboratory, School of Kinesiology, Faculty of Medicine, Finis Terrae University, Santiago, Chile; 6Research Center in Sports, Health and Human Development, Vila Real, Portugal; 7Department of Sport Sciences, University of Beira Interior, Covilha˜, Portugal; 8Faculty of Cultura Fı´sica, Department of Recreation, Santo Tomas University, Bogota´ D.C, Colombia; and 9Public University of Navarra, Department of Health Sciences, Pamplona, Spain ABSTRACT Ramı´rez-Campillo, R, Burgos, CH, Henrı´quez-Olguı´n, C, Andrade, DC, Martı´nez, C, A´lvarez, C, Castro-Sepu´lveda, M, Marques, MC, and Izquierdo, M. Effect of unilateral, bilateral, and combined plyometric training on explosive and endurance performance of young soccer players. J Strength Cond Res 29(5): 1317–1328, 2015—The aim of the study was to compare the effects of bilateral, unilateral, or combined bilateral and unilateral plyometric training (PT) on muscle power output, endurance, and balance performance adaptations in young soccer players. Four groups of young soccer players (age 11.4 6 2.2 years) were divided into control group (CG; n = 14), bilateral group (BG; n = 12), unilateral group (UG; n = 16), and bilateral + unilateral group (B + UG; n = 12). Players were measured in unilateral and bilateral countermovement jump with arms, 5 multiple bounds test, 20-cm drop jump reactive strength index, maximal kicking velocity, sprint and agility test time, endurance, and balance performance. The PT was applied during 6 weeks, 2 sessions per week, for a total of 2,160 jumps. After intervention, all PT groups showed a statistically significant (p # 0.05) change in all performance measures, with no statistically significant differences between treatments. Among the 21 performance measures, the B + UG showed a significantly (p # 0.05) higher performance change in 13 of them vs. the CG, Address correspondence to Dr. Mikel Izquierdo, mikel.izquierdo@ gmail.com. 29(5)/1317–1328 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association

whereas the UG and BG showed only 6 and 3, respectively. The current study showed that bilateral, unilateral, and combined bilateral and unilateral PT ensured significant improvement in several muscular power and endurance performance measures in young soccer players. However, the combination of unilateral and bilateral drills seems more advantageous to induce superior performance improvements.

KEY WORDS explosive strength, stretch-shortening cycle, football, children, strength training

INTRODUCTION

S

occer is the world’s most popular sport, and because of its enormous popularity, multiple researches have been conducted to understand the fundamental skills required by a soccer player. During the past 2 decades, soccer is becoming progressively more athletic, and the capacity of soccer players to produce varied forceful and explosive actions, such as sprinting, jumping, tackling, kicking, turning, and changing pace, has become crucial in many game situations (5,39). In modern football approach, these considerations are increasingly essential to optimal performance not only in adults but also in young players because it has been previously reported in notational game analyses (5,44). For instance, high-speed sprinting contributes up to 3% of the total distance covered in children’s games (5) and most crucial moments of the game, such as winning ball possession, scoring, or conceding goals, depend on it (39). Such explosive actions are integral elements for success in soccer and have to be trained independently from aerobic power with an optimal training stimulus (13). VOLUME 29 | NUMBER 5 | MAY 2015 |

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Plyometric Training in Young Soccer Players Stretch-shortening cycle (SSC) muscle actions like plyometric training (PT) exercises do provide such training stimuli and are well-established techniques for enhancing athletic performance in strength and power events (24,40,41). Here, the effectiveness of a plyometric method could be attributed to factors such as eccentric overloading, segmental coordination, muscle power, and specificity according to joint angle and angular velocities (42). However, to the best of the authors’ knowledge, a limited number of studies have established optimum PT design for varied forceful and explosive actions, such as sprinting, jumping, kicking, turning, and changing pace (7). Additionally, as far as the effect of PT on soccer player’s motor performance is concerned, research has focused more upon determining its influence in adult soccer athletes rather in young children soccer players (17,28). As specificity is important for training-induced adaptations, performance changes in explosive neuromuscular actions may require specific training strategies and must consider the multi-planar and unilateral nature of most competitive soccer actions (29). Therefore, there is a need for soccer-specific strength training interventions that incorporate multidirectional unilateral force production exercises. Although a background knowledge of single exercise characteristics is helpful when planning for training programs (23), it would be important to know the isolated and combined effect of unilateral and bilateral exercises because these modes of muscle action are present during soccer games. Recently, it has been reported a significantly higher performance change in power and jumping ability after 6 weeks of unilateral PT but not after bilateral PT (23). Similarly, unilateral PT induced a higher increase in jumping performance vs. bilateral PT (8). On the other side, compared with bilateral training, unilateral exercises were equally effective to induce improvement of unilateral and bilateral leg strength and power (27) or even induce significantly larger contact times, lower vertical ground reaction forces, and rate of force development (4). All these previous results may suggest that bilateral exercises can be a more specific power training drill for many reported sprint and jump actions that occur during soccer games. Aside from this lack of results agreement, the effects of combined unilateral and bilateral PT exercises are not clear. Some have hypothesized that combined unilateral and bilateral plyometric drills may offer higher benefits (3); however, to the best of the authors’ knowledge, no studies have established the effect of unilateral, bilateral, or the combination of both types of plyometric exercises on explosive and endurance performance of young soccer players. Thus, our objective was to determine the effect of unilateral, bilateral, and combined unilateral + bilateral plyometric drills on muscle power output, endurance, and balance performance adaptations in young soccer players.

per week), which incorporates unilateral, bilateral or unilateral plus bilateral exercises, affects jumping, sprinting, kicking, endurance, agility, and balance performance measures in young soccer players. To do this, we compared the effects of 6 weeks of plyometric treatment in 4 groups of subjects. Some initial tests were executed to establish a baseline. After the initial measurements, subjects were randomly assigned to a control group (CG, n = 14) or to 3 other groups that completed bilateral jumps (BG, n = 12), unilateral jumps (UG, n = 16), or combined bilateral and unilateral jumps (B + UG, n = 12). Our study design (4 groups) permits us to determine the effect of the independent variables PT and type of PT (i.e., bilateral, unilateral, or combined bilateral + unilateral). Subjects

METHODS

This study involved a group of sub-elite level (3 training sessions plus a competition game per week) young (aged between 10 and 15 years) male soccer players (descriptive characteristics depicted in Table 1). Participants have a mean of 3–4 years of systematic soccer training and competition experience and showed continuous soccer training in the 6month period before intervention. None of the subjects had any background in regular strength training or competitive sports that involved any of the training methods used in the investigation. Subjects were reminded during each training session to maintain their usual physical activity habits during the experiment. The CG did not perform the PT but performed their usual soccer training, including 40 minutes of technical-tactical exercises (e.g., ball control, ball pass, ball conduction and dribbling, ball kicking, ball heading, defensive drills, offensive drills, corner kicks situations, penalty kicks), 20 minutes of smallsided games and basic conditioning exercises, and 30 minutes of simulated competitive games. Exclusion criteria included subjects with (a) potential medical problems or a history of ankle, knee, or back pathology in the 3 months before the study, (b) medical or orthopedic problems that compromised their participation or performance in the study, and (c) any lower extremity reconstructive surgery in the past 2 years or unresolved musculoskeletal disorders. Individuals were not eliminated if they had been taking vitamins, minerals, or related natural supplements (other than creatine monohydrate). Institutional Review Board of University of Los Lagos approval for our study was obtained, and all subjects (and their parents or guardians) were carefully informed about the experimental procedures and about the possible risk and benefits associated with participation in the study, and an appropriate signed informed consent document has been obtained pursuant to law before any of the tests and training was performed. We comply with the human and animal experimentation policy statement guidelines of the American College of Sports Medicine.

Experimental Approach to the Problem

Procedures

This study was designed to address the question of how a short-term PT program of moderate frequency (2 sessions

The subjects were carefully familiarized with the test procedures during several submaximal and maximal actions

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TABLE 1. Descriptive data of the CG, BG, UG, and B + UG.* CG (n = 14) Age (y) Height (cm) Sit height (cm) Body mass (kg) Body mass index (kg$m22) Predicted age of peak height velocity (y) Predicted years from age of peak height velocity (y) Predicted adult height (cm) Current percentage of predicted adult height (%) Body fat (%) Body water (%) Body muscle (%) Body muscle (kg) Bone mass (kg) Trunk fat (%) Legs fat (%) Session rating of perceived exertion Soccer experience (y) Weekly non-soccer sport practice (h)

11.2 143 75.6 41.8 19.4 13.8 22.0

6 6 6 6 6 6 6

2.4 17.7 9.2 12.7 1.7 0.8 2.2

173 6 7.2 83.4 6 9.1 10.8 66.4 55.3 20.0 8.7 13.1 8.1 360 3.9 1.1

6 6 6 6 6 6 6 6 6 6

3.0 4.7 8.4 5.1 1.9 1.5 1.9 98 2.4 0.9

BG (n = 12) 11.0 146 78.3 43.5 19.0 13.6 22.0

6 6 6 6 6 6 6

2.0 13.7 8.6 14.9 3.3 0.2 2.0

177 6 5.4 84.8 6 7.6 10.3 65.4 52.7 22.0 9.4 13.1 8.1 380 3.8 1.5

6 6 6 6 6 6 6 6 6 6

3.7 6.2 7.4 4.5 1.7 3.4 3.2 140 3.3 2.3

UG (n = 16) 11.6 147 77.4 45.0 20.7 13.6 22.0

6 6 6 6 6 6 6

1.7 11.1 6.0 9.3 2.7 0.9 1.2

175 6 7.3 83.1 6 5.1 11.5 63.7 50.4 20.9 9.2 13.8 8.9 375 3.4 1.3

6 6 6 6 6 6 6 6 6 6

3.6 5.4 5.9 3.1 1.3 3.4 3.7 216 1.8 1.8

B + UG (n = 12) 11.6 144 74.9 42.2 19.4 13.7 22.2

6 6 6 6 6 6 6

2.7 17.5 9.4 16.9 3.2 0.9 2.4

174 6 5.4 82.8 6 8.8 10.9 64.4 53.7 20.4 8.7 13.0 8.8 320 3.8 1.0

6 6 6 6 6 6 6 6 6 6

4.2 7.4 9.2 5.0 1.9 4.2 4.7 98 3.4 2.5

*CG = control group; BG = bilateral plyometric training group; UG = unilateral plyometric training group; B + UG = combined bilateral + unilateral plyometric training group.

a few days before the performance measurements were taken (4 learning sessions during 2 weeks) to reduce any learning effects. Each subject also completed several explosive-type actions to become familiar with the exercises used during training. Measurements were applied 1 week immediately before and 1 week immediately after intervention. All tests were administered in the same order before and after intervention by the same investigators. Anthropometric measurements were completed between 10:00 and 12:00 hours, whereas performance measurement was conducted between 12:00 and 18:00 hours. Testing sessions were scheduled .48 hours following a competition or hard physical training to minimize the influence of fatigue. All subjects (and their parents or guardians) were instructed to (a) have a good night’s sleep ($8 hours) before each testing day and (b) have a meal rich in carbohydrates and to be well hydrated before measurements. The participants were motivated to give their maximum effort during performance measurements. Tests were completed in 3 days. On day 1, the following tests were completed: standing height, sitting height, body mass, body composition, unilateral and bilateral countermovement jump with arms (CMJA) for maximal vertical and horizontal distance (in centimeters), 20-cm drop jump reactive strength index (RSI20), and the multiple 5 bounds test (MB5) for maximal horizontal distance (in centimeters). On day 2, the 15-m sprint test for acceleration and

30-m sprint test for maximal speed, the maximal kicking velocity (MKV) test, and a soccer-specific agility test (30) were carried out. On day 3, bilateral balance and the Yo-Yo intermittent recovery test level 1 (Yo-Yo L1) were completed. Ten minutes of standard warm-up (5 minutes of submaximal running with several displacements and 2 submaximal jump exercises of 20 vertical and 10 longitudinal jumps) were executed before each testing day. Subjects were instructed to use the same athletic shoes and clothes during the preintervention and postintervention testing. All tests were conducted indoors on a wooden surface. Anthropometric and Maturity Measures. Anthropometric measurements were taken a week before performance measurements, using a stadiometer (Bodymeter 206; SECA, Hamburg, Germany) and an electrical bio-impedance scale (BF 100_Body Complete; Beurer, Ulm, Germany). Corporal composition, standing height (in centimeters), sitting height (in centimeters), and mass (in kilograms) were measured, and the athletes’ maturity status was determined using predicted years from age of peak height velocity (i.e., PHV offset) (32). Bilateral and Unilateral Vertical and Horizontal Countermovement Jump With Arms. Bilateral and unilateral vertical and horizontal CMJA tests were used to assess maximal jump height performance requiring slow SSC action ($250 milliseconds). VOLUME 29 | NUMBER 5 | MAY 2015 |

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Plyometric Training in Young Soccer Players The CMJA tests were performed using an electronic contact mat system (Globus Tester, Codogne, Italy) with a precision of 0.01 m during vertical CMJA, and for horizontal CMJA, a fiber glass metric tape 5 m long endorsed to a wooden floor was used. Subjects were instructed to use their arms to aid in the jump, positioning their foot shoulders wide apart for the bilateral test, and using 1 foot stand (right and leg) for the unilateral test. In addition, subjects were instructed to perform a fast downward movement (approximately 1208 knee angle) followed by a maximal effort jump. All subjects were instructed to land in an upright position during vertical jumps and to bend their knees after landing.

after dropping down from a 20-cm drop box. The RSI20 was calculated as previously reported (22). Multiple Five Bounds Test. The MB5 was started from a standing position. Subjects performed a set of 5 forward jumps with alternative left and right leg contacts to cover the longest distance possible. The distance of the MB5 was measured to the nearest 0.5 cm using a fiberglass tape measure 20 m long. Sprint and Agility Test. The 15- and 30-m sprint time were measured to the nearest 0.01 second using single beam infrared photoelectric cells (Globus Italia, Codogne, Italy). The starting position was standardized to a still split standing position with the toe of the preferred foot forward and behind the starting line. Sprint start was given by a random sound, which triggers timing. The photoelectric signal was positioned at 15 and 30 m and set ;0.7 m above the floor (i.e., hip level) to capture the trunk movement rather than a false trigger from a limb. The agility test has been described

Twenty-Centimeter Drop Jump Reactive Strength Index. The RSI20 was determined on a mobile contact mat (Ergojump; Globus, Codogne, Italy) with arms akimbo. Take-off and landing were standardized to full knee and ankle extension on the same spot. The participants were instructed to maximize jump height and minimize ground contact time

TABLE 2. Six-week plyometric training program.* Set 3 repetitions (mode of execution) Group

Exercises†z

BG (n = 12)

Bilateral vertical Bilateral horizontal

UG (n = 16)

Vertical right leg Vertical left leg Horizontal right leg Horizontal left leg

B + UG (n = 12)

Horizontal left leg Horizontal right leg Vertical left leg Vertical right leg Bilateral vertical Bilateral horizontal

Week 1

Week 2

Week 3

Week 4

Week 5

Week 6

6 3 5 (C) 6 3 5 (A) 6 3 5 (C) 6 3 5 (A) 3 3 5 (C) 3 3 5 (A) 3 3 5 (C) 3 3 5 (A) 3 3 5 (C) 3 3 5 (A) 3 3 5 (C) 3 3 5 (A) 2 3 5 (C)

6 3 6 (C) 6 3 6 (A) 6 3 6 (C) 6 3 6 (A) 3 3 6 (C) 3 3 6 (A) 3 3 6 (C) 3 3 6 (A) 3 3 6 (C) 3 3 6 (A) 3 3 6 (C) 3 3 6 (A) 2 3 6 (C)

6 3 7 (C) 6 3 7 (A) 6 3 7 (C) 6 3 7 (A) 3 3 7 (C) 3 3 7 (A) 3 3 7 (C) 3 3 7 (A) 3 3 7 (C) 3 3 7 (A) 3 3 7 (C) 3 3 7 (A) 2 3 7 (C)

6 3 8 (C) 6 3 8 (A) 6 3 8 (C) 6 3 8 (A) 3 3 8 (C) 3 3 8 (A) 3 3 8 (C) 3 3 8 (A) 3 3 8 (C) 3 3 8 (A) 3 3 8 (C) 3 3 8 (A) 2 3 8 (C)

6 3 9 (C) 6 3 9 (A) 6 3 9 (C) 6 3 9 (A) 3 3 9 (C) 3 3 9 (A) 3 3 9 (C) 3 3 9 (A) 3 3 9 (C) 3 3 9 (A) 3 3 9 (C) 3 3 9 (A) 2 3 9 (C)

6 3 10 (C) 6 3 10 (A) 6 3 10 (C) 6 3 10 (A) 3 3 10 (C) 3 3 10 (A) 3 3 10 (C) 3 3 10 (A) 3 3 10 (C) 3 3 10 (A) 3 3 10 (C) 3 3 10 (A) 2 3 10 (C)

2 3 5 (A) 2 3 5 (C) 2 3 5 (A) 2 3 5 (C) 2 3 5 (A) 2 3 5 (C) 2 3 5 (A) 2 3 5 (C) 2 3 5 (A) 2 3 5 (C) 2 3 5 (A)

2 3 6 (A) 2 3 6 (C) 2 3 6 (A) 2 3 6 (C) 2 3 6 (A) 2 3 6 (C) 2 3 6 (A) 2 3 6 (C) 2 3 6 (A) 2 3 6 (C) 2 3 6 (A)

2 3 7 (A) 2 3 7 (C) 2 3 7 (A) 2 3 7 (C) 2 3 7 (A) 2 3 7 (C) 2 3 7 (A) 2 3 7 (C) 2 3 7 (A) 2 3 7 (C) 2 3 7 (A)

2 3 8 (A) 2 3 8 (C) 2 3 8 (A) 2 3 8 (C) 2 3 8 (A) 2 3 8 (C) 2 3 8 (A) 2 3 8 (C) 2 3 8 (A) 2 3 8 (C) 2 3 8 (A)

2 3 9 (A) 2 3 9 (C) 2 3 9 (A) 2 3 9 (C) 2 3 9 (A) 2 3 9 (C) 2 3 9 (A) 2 3 9 (C) 2 3 9 (A) 2 3 9 (C) 2 3 9 (A)

2 3 10 (A) 2 3 10 (C) 2 3 10 (A) 2 3 10 (C) 2 3 10 (A) 2 3 10 (C) 2 3 10 (A) 2 3 10 (C) 2 3 10 (A) 2 3 10 (C) 2 3 10 (A)

*BG = bilateral plyometric training group; C = cyclic; A = acyclic; UG = unilateral plyometric training group; B + UG = combined bilateral + unilateral plyometric training group. †The order of exercises execution was randomized in each training session. zAll exercises were executed with the technique described as countermovement with arms.

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size 5 footballs (Nike Seitiro; FIFA certified). A previously described protocol was used for the test (2). Basically, participants performed a maximal instep kick with their dominant leg after a run-up of 2 strides (the length of run-up was self-determined) directed toward a goal net. Participants

TABLE 3. Training effects (with 90% confidence limits) for the jump performance variables for the CG (n = 14), BG (n = 12), UG (n = 16), and B + UG (n = 12).*†z Baseline mean 6 SD Vertical right leg CMJA (cm) CG BG UG B + UG Vertical left leg CMJA (cm) CG BG UG B + UG Vertical CMJA (cm) CG BG UG B + UG Horizontal right leg CMJA (cm) CG BG UG B + UG Horizontal left leg CMJA (cm) CG BG UG B + UG Horizontal CMJA (cm) CG BG UG B + UG 20-cm drop jump reactive strength index (mm$ms21) CG BG UG B + UG Multiple 5 bounds test (cm) CG BG UG B + UG

Performance change (%)

Magnitude of training effect

19.1 20.0 18.9 19.3

6 6 6 6

1.8 2.4 1.6 7.3

1.8 10.1 13.7 11.9

(20.2 to 3.8) (5.7 to 14.6)a (6.9 to 21.0)c (5.9 to 16.2)c

0.15 0.56 1.93 1.55

(20.02 to 0.32) (0.32 to 0.79)§ (1.01 to 2.86)k (0.88 to 1.96)k

19.0 19.9 19.3 19.1

6 6 6 6

2.8 5.6 1.7 4.5

1.5 7.8 14.4 12.9

(21.3 to 4.5) (22.1 to 18.7)a (7.0 to 22.3)c,d (5.7 to 16.2)c,d

0.13 0.25 1.78 1.27

(20.12 to 0.38) (20.07 to 0.57)§ (0.90 to 2.66)k (0.80 to 1.54)k

28.9 31.1 29.5 30.5

6 6 6 6

7.6 2.0 4.3 9.3

1.6 18.7 7.9 15.4

(212.7 to 18.2) (4.8 to 34.3)c,d (3.9 to 12.1)a (6.4 to 22.9)c,d

0.05 1.68 0.58 1.47

(20.45 to 0.55) (0.46 to 2.90)k (0.29 to 0.88)§ (0.22 to 1.72)k

131 135 130 131

6 6 6 6

17 16 20 32

0.0 6.6 14.2 12.9

(25.9 to 6.3) (3.2 to 10.0)a (9.2 to 19.3)c,d (6.8 to 17.3)c,d

0.0 0.41 0.80 0.65

(20.33 to 0.33) (0.21 to 0.62)§ (0.53 to 1.07)¶ (0.36 to 0.94)¶

133 142 132 130

6 6 6 6

23 33 26 32

0.2 7.3 15.2 13.1

(213.2 to 15.6) (20.8 to 16.1)a (6.7 to 24.5)b,d (4.1 to 23.3)b,d

0.01 0.30 0.94 0.69

(20.62 to 0.64) (20.04 to 0.63)§ (0.59 to 1.39)¶ (0.30 to 1.08)¶

152 166 153 153

6 6 6 6

21 33 22 41

1.7 17.4 8.9 14.6

(25.3 to 9.2) (9.8 to 25.6)b,d (5.3 to 12.6)a (8.4 to 21.2)b,d

0.11 0.83 0.55 0.64

(20.35 to 0.56) (0.48 to 1.18)¶ (0.33 to 0.77)§ (0.43 to 0.85)¶

1.4 1.3 1.2 1.2

6 6 6 6

0.4 0.5 0.3 0.5

3.3 18.9 10.0 16.6

(26.6 to 14.1) (5.0 to 34.6)c,d (5.2 to 15.1)a (5.6 to 28.6)c,d

0.12 0.80 0.44 0.63

(20.25 to 0.48) (0.44 to 1.26)¶ (0.23 to 0.64)§ (0.42 to 0.83)¶

810 858 804 819

6 6 6 6

150 159 123 222

1.1 5.8 11.5 10.4

(24.7 to 7.3) (0.8 to 11.0)a (9.6 to 13.4)c,d (8.0 to 12.8)c,d

0.06 0.28 0.73 0.64

(20.24 to 0.36) (0.04 to 0.53)§ (0.53 to 0.93)¶ (0.46 to 0.81)¶

*CG = control group; BG = bilateral plyometric training group; UG = unilateral plyometric training group; B + UG = combined bilateral + unilateral plyometric training group; CMJA = countermovement jump with arms. †Values in brackets represent 90% confidence limits. za, b, and c: denote significant difference pretraining to posttraining (p # 0.05, p , 0.01, and p , 0.001, respectively); d and e: denote significant difference with the CG posttraining (p # 0.05 and p , 0.01, respectively). §Small standardized effect. kLarge standardized effect. ¶Moderate standardized effect.

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Plyometric Training in Young Soccer Players were specifically instructed to focus only on the maximum kicking velocity, directing their kicks toward a vertical square target (length 2 m 3 width 2 m; the “bull’s eye” placed in its centre) positioned over the goal net. The distance between the ball and the target was 4 m. Each participant was given 2 practice trials and 3 valid maximal trials. The best performance trial was used for the subsequent statistical analysis. Al least 1 minute of rest was permitted between trials. The ball velocity was measured by a radar gun (Sports Radar Speed Gun SR3600; Homosassa, FL, USA) positioned 2 m behind the target and 0.7 m above the floor. Bilateral Balance. As previously reported (1), all participants completed 4 stability tests performed on a balance platform at a sample rate of 1,000 Hz (Bertec BP5050 balance plate platform; Bertec Corporation, Columbus, OH, USA): (a) normal stance, eyes open (N-EO), (b) normal stance, eyes closed (N-EC), (c) perturbed stance, eyes open (P-EO), and

(d) perturbed stance, eyes closed (P-EC). The perturbed stance condition was defined as that obtained while standing on a piece of foam 3 cm thick. The average of 2 balance trials for each test was used for subsequent analysis and was expressed in centimeters. Both anterior-posterior and mediallateral data were collected during each trial. Yo-Yo Intermittent Recovery Test Level 1. The test was executed as previously described (18). Basically, in the Yo-Yo L1, 2 markers were positioned at a distance of 20 m and the players perform repeated 20-m shuttle runs interspersed with 10 seconds of active recovery. The time allowed for a shuttle was progressively decreased. Test result was determined as the distance covered during the test. Before the test, all subjects carried out a specific warm-up period consisting of the first 4 running bouts in the test. Throughout all testing procedures, an investigator to subject ratio of 1:1 was maintained. Subjects were instructed to achieve maximal effort

TABLE 4. Training effects (with 90% confidence limits) for the soccer-specific explosive and endurance performance variables for the CG (n = 14), BG (n = 12), UG (n = 16), and B + UG (n = 12).*†z Baseline mean 6 SD Maximal kicking velocity (km$h21) CG BG UG B + UG 15-m sprint time (s) CG BG UG B + UG 30-m sprint time (s) CG BG UG B + UG Agility test time (s) CG BG UG B + UG Yo-Yo intermittent recovery test level 1 (m) CG BG UG B + UG

Performance change (%)

Magnitude of training effect

59.1 59.2 59.9 61.8

6 6 6 6

14.2 18.4 10.8 19.6

1.0 8.4 14.0 12.0

(210.2 to 13.6) (2.6 to 14.5)a (9.8 to 18.3)a,d (8.0 to 16.1)a,d

0.04 0.26 0.92 0.62

3.5 3.3 3.5 3.5

6 6 6 6

0.2 0.3 0.2 0.2

20.2 23.8 25.1 25.9

(24.0 to 3.8) (26.6 to 20.9)b (211.9 to 2.1)c (213.0 to 1.7)c

20.02 20.42 20.47 20.56

(20.55 (20.74 (21.14 (21.27

to to to to

0.51) 20.10)§ 0.19)§ 0.16)§

6.1 5.7 6.1 6.0

6 6 6 6

0.6 0.5 0.4 0.6

0.4 23.2 26.2 26.5

(24.2 to 5.3) (210.4 to 4.5)a (211.4 to 20.6)c (211.6 to 21.2)b

0.03 20.31 20.61 20.53

(20.33 (21.04 (21.17 (20.98

to to to to

0.40) 0.42)§ 20.06)k 20.09)§

5.4 5.4 5.3 5.2

6 6 6 6

0.6 0.5 0.5 0.6

0.8 23.9 28.3 28.3

(22.9 to 4.7) (210.1 to 2.8)a (212.7 to 23.7)c,d (213.8 to 22.4)c,d

0.06 20.42 20.80 20.66

(20.23 (21.13 (21.25 (21.13

to to to to

0.36) 0.29)§ 20.35)k 20.18)k

711 703 813 797

6 6 6 6

252 185 477 645

1.6 12.7 17.4 19.0

(24.1 to 6.3) (2.9 to 23.5)b (11.4 to 23.8)c (21.9 to 44.4)c

0.11 0.35 0.26 0.21

(20.40 to 0.47) (0.08 to 0.43)§ (0.74 to 1.09)k (0.21 to 0.73)k

(20.12 to 0.21) (0.08 to 0.62)§ (0.17 to 0.34)§ (20.02 to 0.44)§

*CG = control group; BG = bilateral plyometric training group; UG = unilateral plyometric training group; B + UG = combined bilateral + unilateral plyometric training group. †Values in brackets represent 90% confidence limits. za, b, and c: denote significant difference pretraining to posttraining (p # 0.05, p , 0.01, and p , 0.001, respectively); d and e: denote significant difference with the CG posttraining (p # 0.05 and p , 0.01, respectively). §Small standardized effect. kModerate standardized effect.

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TABLE 5. Training effects (with 90% confidence limits) for the balance performance variables for the CG (n = 14), BG (n = 12), UG (n = 16), and B + UG (n = 12).*†z Balance condition Anterior-posterior normal stance eyes open (cm) CG BG UG B + UG Medial-lateral normal stance eyes open (cm) CG BG UG B + UG Anterior-posterior normal stance eyes closed (cm) CG BG UG B + UG Medial-lateral normal stance eyes closed (cm) CG BG UG B + UG Anterior-posterior perturbed stance eyes open (cm) CG BG UG B + UG Medial-lateral perturbed stance eyes open (cm) CG BG UG B + UG Anterior-posterior perturbed stance eyes closed (cm) CG BG UG B + UG Medial-lateral perturbed stance eyes closed (cm) CG BG UG B + UG

Baseline mean 6 SD

Performance change (%)

Magnitude of training effect

0.50 0.50 0.45 0.51

6 6 6 6

0.15 0.18 0.20 0.17

21.5 28.1 29.8 219.6

(22.0 to 1.0) (29.5 to 26.7)a (211.2 to 28.4)a (226.4 to 212.1)b

20.01 20.40 20.21 20.61

(20.05 (20.47 (20.24 (20.86

to to to to

0.02) 20.33)§ 20.18)§ 20.36)k

0.34 0.31 0.32 0.34

6 6 6 6

0.11 0.11 0.11 0.14

22.9 212.5 29.1 216.3

(28.7 to 3.1) (214.6 to 210.3)b (211.9 to 26.3)a (217.7 to 214.9)c

20.07 20.28 20.23 21.17

(20.20 (20.33 (20.30 (21.18

to to to to

0.07) 20.23)§ 20.15)§ 20.97)k

0.70 0.70 0.74 0.66

6 6 6 6

0.20 0.15 0.24 0.22

22.5 215.3 211.5 221.1

(210.9 (218.0 (216.1 (227.9

20.06 20.57 20.35 20.62

(20.27 to 0.15) (20.8 to 20.54)§ (20.49 to 20.2)§ (20.85 to 20.38)k

0.34 0.30 0.34 0.31

6 6 6 6

0.10 0.13 0.07 0.08

21.3 214.4 27.1 217.8

(25.4 to 3.0) (217.7 to 211.0)b (29.9 to 24.3)a (224.9 to 210.1)c

20.04 20.29 20.35 20.77

(20.15 (20.37 (20.49 (21.12

to to to to

0.08) 20.22)§ 20.21)§ 20.42)k

0.58 0.55 0.63 0.59

6 6 6 6

0.19 0.20 0.24 0.26

21.1 213.1 29.9 218.7

(212.3 (218.4 (215.4 (227.0

11.5) 27.3)b 23.9)a 29.5)c,d

20.04 20.33 20.24 20.47

(20.46 (20.47 (20.39 (20.56

to to to to

0.38) 20.18)§ 20.09)§ 20.18)§

0.52 0.52 0.52 0.53

6 6 6 6

0.13 0.23 0.16 0.21

25.6 216.1 215.2 224.9

(28.0 to 23.2)a (220.6 to 211.5)c (220.9 to 29.1)c (230.4 to 219.0)c

20.20 20.35 20.51 20.68

(20.29 (20.46 (20.73 (20.85

to to to to

20.11)§ 20.24)§ 20.30)§ 20.50)k

0.92 0.67 0.90 0.91

6 6 6 6

0.25 0.33 0.19 0.37

23.3 213.2 212.9 224.6

(212.4 (217.8 (215.8 (231.0

to to to to

6.6) 28.4)c 29.8)b 217.5)c,d

20.1 20.69 20.44 20.61

(20.4 to 0.2) (20.95 to 20.42)k (20.55 to 20.33)§ (20.80 to 20.42)k

0.65 0.68 0.64 0.64

6 6 6 6

0.21 0.43 0.27 0.28

25.5 215.2 210.9 220.6

(215.2 (220.5 (215.5 (226.8

to to to to

10.2) 29.5)a 26.1)a 213.8)c

20.12 20.22 20.21 21.12

(20.44 (20.30 (20.29 (21.43

to to to to

to to to to

6.7) 212.5)a 26.8)a 213.6)c,d

to to to to

0.31) 20.13)§ 20.11)§ 20.81)k

*CG = control group; BG = bilateral plyometric training group; UG = unilateral plyometric training group; B + UG = combined bilateral + unilateral plyometric training group. †Values in brackets represent 90% confidence limits. za, b, and c: denote significant difference pretraining to posttraining (p # 0.05, p , 0.01, and p , 0.001, respectively); d: denotes significant difference with the CG posttraining (p # 0.05). §Small standardized effect. kModerate standardized effect.

VOLUME 29 | NUMBER 5 | MAY 2015 |

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Plyometric Training in Young Soccer Players during testing (610% of predicted maximal heart rate). Data of subjects who did not achieve this condition were excluded from final analysis. Internal Training Load Determination. To assure that all soccer players receive the same soccer training load during intervention, the session rating of perceived exertion was determined as previously described (16). In this study, the Chilean translation of the 10-point category ratio scale modified by Foster et al. (10) was used. This scale was modified to better reflect the Chilean idiomatic English. Treatment

The BG and UG executed bilateral and unilateral exercises, respectively, whereas the B + UG group combined bilateral and unilateral jumps. The plyometric interventions were created based on previous research experience from our research team (37). The basic execution technique for the different exercises was the one described previously as CMJA, combining cyclic and acyclic jumps. Subjects were asked to produce maximal intensity vertical height and horizontal distance for acyclic jumps and with minimum ground contact time for cyclic jumps. Maximal intensity during plyometric drills execution was verified in a randomly assigned subsample of soccer players (2 from each PT group; n = 6) during 2 randomly assigned training sessions, by measuring contact times, height and distance of jumps drills using an electronic contact mat system (Globus Tester) with a precision of 0.01 m. A detailed description of the training program is depicted in Table 2. The current experiment was completed during the mid portion of their competition period. Previous to the competitive period, subjects completed 6 weeks of summer pre-season training. The PT groups performed plyometric drills as a substitute for some technical-tactical soccer drills within the usual 90-minute practice twice per week for 6 weeks. Because players did not have any history of formal plyometrics, before beginning the training period, subjects were instructed to properly execute all the exercises to be done during this 6-week period. Aside from the formal training intervention, all participants attended to their regular physical education classes. All groups completed the same amount of total jumps during intervention, using the same surface and time of day for training, with the same rest intervals between jumps, series, and training sessions. Statistical Analyses

All values are reported as mean 6 SD. Relative changes (in percentage) in performance and standardized effects (SEs) are expressed with 90% confidence limits. Normality and homoscedasticity assumptions for all data before and after intervention were checked, respectively, with KolmogorovSmirnov and Levene tests. To determine the effect of intervention (i.e., PT) on performance adaptations, a 2-way variance analysis with repeated measurements (4 groups 3

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2 times) was applied. When a significant F value was achieved across time or between groups, Sheffe post hoc procedures were performed to locate the pairwise differences between the means. The a level was set at p # 0.05 for statistical significance. All statistical calculations were performed using STATISTICA statistical package (Version 8.0; StatSoft, Inc., Tulsa, OK, USA). In addition to this null hypothesis testing, these data were also assessed for clinical significance using an approach based on the magnitudes of change. Threshold values for assessing magnitudes of SE (changes as a fraction or multiple of baseline SD) were 0.20, 0.60, 1.2, and 2.0 for small, moderate, large, and very large, respectively (15). We obtained high intraclass correlation coefficients for the different performance measurements, varying between 0.80 and 0.94.

RESULTS Despite not pair-matching individuals based on an independent variable, there were no significant differences between groups’ descriptive data (Table 1). Before training, no significant differences were observed between groups in CMJA, RSI20, MB5 (Table 3), MKV, 15and 30-m sprint time, agility time, Yo-Yo L1 endurance (Table 4), or balance (Table 5) test performance. No significant changes in the CG were observed, except for an increase in 1 (of 8) balance performance measure (Table 5), with a small clinically significant change (20.20 SE). After training, all experimental groups showed a significant (p # 0.05) increase and a similar small-to-large SE in CMJA, RSI20, MB5 (Table 3), MKV, 15- and 30-m sprint, agility, Yo-Yo L1 endurance (Table 4), and balance performance (Table 5), with no statistically significant differences between groups. In comparison with the CG, the UG and B + UG showed a significantly (p # 0.05) higher performance change in vertical CMJA with left leg, horizontal CMJA with right and left leg, MB5 (Table 3), MKV, and agility test (Table 4). Similarly, the BG and B + UG revealed a significantly (p # 0.05) superior performance change in vertical CMJA, horizontal CMJA, and RSI20 test vs. the CG (Table 3). Only the B + UG expressed a significantly (p # 0.05) higher performance change in anterior-posterior balance performance vs. the CG (Table 5).

DISCUSSION Our results indicate a specificity training effect, where the use of unilateral PT exercises induced a significantly greater increase in performance tests where unilateral dominance was preferred, whereas the use of bilateral PT exercises induced a significantly greater increase in performance tests where bilateral dominance was preferred. Also, the results indicate that compared with soccer training alone, only the combination of bilateral and unilateral PT exercises induce a significantly greater increase in balance performance. Finally, our results showed that the combination of soccer

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Journal of Strength and Conditioning Research drills and specific power training with no additional training time in-season optimize general and soccer-specific explosiveness and endurance performance in young soccer players. All PT groups show a significant increase in CMJA performance, with a small-to-large SE (Table 3). In the present study, the magnitude change in CMJA was similar or even higher than previously reported for similar slow SSC muscle actions (SE = 0.50–0.87) (9,30,46) after explosive training with young soccer players using interventions of similar duration or number of sessions. It was interesting to note that the PT groups that incorporate unilateral jump drills (i.e., UG and B + UG) showed a significantly higher performance change in unilateral CMJA performance vs. CG, in both dominant and nondominant leg, and a higher SE (Table 3). Similarly, PT groups that incorporate bilateral jump drills (i.e., BG and B + UG) show a significantly (p # 0.05) higher performance change in bilateral CMJA performance vs. CG, in both vertical and horizontal plane, and a higher SE (Table 3). These results can be highly supported because of the fact that the effectiveness of PT depends on factors such as eccentric overloading, segmental coordination, muscle power, and specificity (42). All PT groups show a significant increase in RSI20 performance, with a small-to-moderate SE (Table 3). The magnitude change in RSI20 was similar than previously reported (SE = 0.46–0.68) (36) after PT with young soccer players. Considering the necessity to produce a high rate of force development in explosive actions, the improvement in RSI may have enhanced physical parameters of game performance. The improvement observed could have been induced by various neuromuscular adaptations (25); however, because no physiological measurements were made, only speculations are possible. It is interesting to note that PT groups that incorporate bilateral jumps (i.e., BG and U + BG) show a significantly (p # 0.05) higher performance change in RSI20 performance vs. CG (Table 3), which can be attributed to the training principle of specificity (38). All PT groups show a statistically significant increase in MB5 performance, with a small-to-moderate SE (Table 3). The magnitude change in MB5 in this study was similar than that previously reported (SE = 0.44–0.86) (9,30) after explosive training with young soccer players. An increase in MB5 after PT may be achieved by motor coordination adaptations that can be related to the specificity of movements used during training (9), therefore the PT effects are higher for the jumps in which athletes have been specially trained. The greater magnitude of training effect in MB5 in the UG and B + UG vs. BG would support such contention, considering the unilateral nature of the MB5. To the best of the authors’ knowledge, this is the first study to compare the effects of unilateral, bilateral, and combined unilateral + bilateral PT in MKV in young soccer players. All PT groups show a statistically significant increase in MKV performance, with a small-to-moderate SE (Table 4).

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Although differences in type of training program applied make comparisons between different studies difficult, others have found similar significant increases in kicking performance after PT incorporating unilateral and vertical jumps in preadolescent soccer players (31). Interestingly, only the UG and B + UG demonstrated a significantly higher performance change vs. CG (Table 4); in addition, the UG and B + UG show a moderate SE, whereas the BG achieves only a small SE. These results suggest that PT incorporating unilateral exercises may induce higher MKV performance changes in young soccer players. Because PT may induce motor coordination adaptations, related to the specificity of movements used during training (9), the unilateral nature of the instep kick may help explain the higher PT effects observed in athletes who were specially trained with unilateral drills (i.e., UG and B + UG). The increased MKV performance in young soccer players after PT may be attributed to increased strength and power of legs’ extensor muscles and these changes could be attributed solely to neuromuscular adaptations (31). It may be that these neuromuscular and strength-power adaptations had an effect on the biomechanical factors related to kicking performance, such as maximum linear velocity of the toe, ankle, knee, and hip at ball contact (20), which may have cumulatively or individually contributed to a higher ball kicking velocity. Sprint performance may not only be important at youth level (12,45) but also at a later stage of a player’s career (19) and need to be developed from a young age. Our results show that all PT groups show a significant increase in 15and 30-m sprint performance, with a small-to-moderate SE (Table 4). To the best of the authors’ knowledge, this is the first study to compare the effects of unilateral, bilateral, and combined unilateral + bilateral PT in 15- and 30-m sprint times in young soccer players submitted to a PT intervention that replace some of their soccer drills during the in-season period. Although differences in type of training program applied make comparisons between different studies difficult, others have found similar significant increases in sprint performance after PT incorporating unilateral and vertical jumps in preadolescent soccer players (31). As the training stimulus in our study incorporates horizontal stimulus, this might have increase the chances to gain adaptations, considering the importance of horizontal force production and application in sprint performance (33). This agrees with previous studies where vertical PT fails to improve sprint performance in young soccer players (36). In contrast to the positive explosive adaptations observed in the experimental groups, the CG did not exhibit a significant improvement in their sprint performance. Other studies have shown that during the in-season period, young soccer players submitted to soccer-only training may even reduce their sprint performance (36). These observations reinforce the value of an independent power training program to enhance acceleration and maximal sprint ability of young soccer players during their in season. VOLUME 29 | NUMBER 5 | MAY 2015 |

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Plyometric Training in Young Soccer Players Although some evidence show no difference in explosive strength (i.e., jump) between elite and recreational youth soccer players (6), multiple studies show that power-related qualities (like agility) may differentiate youth elite and subelite players vs. non-elite players (12,39,45). Our results show that all PT groups achieve a significant increase in agility performance, with a small-to-moderate SE (Table 4). However, only the experimental groups that incorporate unilateral jump drills (i.e., UG and B + UG) showed a significantly (p # 0.05) higher performance change in agility performance vs. CG. Also, the UG and the B + UG show a moderate SE, whereas the BG shows only a small SE. The important differences between agility tests used among studies difficult a comparative analysis with our results; however, the agility performance change in this study (at least in the groups that incorporate unilateral jump drills) was similar than previously reported (29.6%) (30), where the same agility test was used to asses this quality after explosive training with young soccer players. It must be acknowledged that PT groups completed a training program with several plyometric exercises designed to induce short contact times; and a reduction in contact time with PT may increase RSI, which may predict the ability to change directions while running (47). In addition, an improved agility performance may be related to power development changes or an increase in eccentric strength of the lower limbs, which can impact changes of direction performance during the deceleration phase (43). These observations suggest that compared with bilateral exercises, unilateral exercises may induce greater adaptations in agility performance in young soccer players. Our results show that all PT groups achieve a significant increase in Yo-Yo L1 endurance performance, with a small SE (Table 4), whereas the CG shows no changes. Although we did not determine the reliability of the Yo-Yo test, subjects achieve a mean of 207 b$min21 at the end of the test, suggesting maximal effort during testing. In young soccer players, PT may not induce a significant increase in V_ O2max (31) or lactate threshold (11) but may increase the performance during an intermittent test that implicates repeated and sudden changes of direction (46), increase that may be related to improved neuromuscular and anaerobic characteristics. Also, an increased neuromuscular ability (i.e., RSI) may be transferred into improved running economy (26) that, independent of V_ O2max, lactate threshold, among others “aerobic” indicators, may affect endurance (35). To the best of our knowledge, this is the first study that analyzed the effect of unilateral, bilateral, and unilateral + bilateral PT on balance performance in young soccer players. Also, another unique feature of our study was the isolated application of PT in young soccer players, which expand the limited knowledge available about this topic. Our results show that the CG achieves a statistically significant increase only in medial-lateral P-EO, with a small SE. On the other side, our results show that all PT groups achieve a statistically significant increase in all measures of medial-lateral and

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anterior-posterior balance, with a small-to-moderate SE (Table 5). Improvement in balance has been observed in previous PT interventions that incorporate unilateral and bilateral exercises in young athletes (34) and intervention periods of 6 weeks seem to be appropriate to induce these changes (48), including PT interventions with young athletes (17,34). Because balance improvement may not only result in an increased athletic performance but also in reduced lower extremity injury risk in soccer players (48), our results reinforce the value of PT as an effective strategy to reduce injury risk in young athletes. From the specificity point of view, it is not rare to observe in our results that the B + UG show a significantly higher performance change vs. CG in anterior-posterior balance (Table 5) but not in mediallateral balance because during PT intervention, plyometric drills were executed in the former direction but not in the latter. Also, it is worth of noting that the significantly higher balance performance change in the B + UG vs. CG may be related with the fact that the former group incorporates an integrated training protocol (i.e., a combination of unilateral and bilateral plyometric drills) that may be the most effective training model (3). This observation may reinforce the necessity to include combination of plyometric drills to induce optimum training adaptations not only of unilateral and bilateral nature but also in different planes in young soccer players. The improvement in balance performance may be related to improved cocontraction of lower extremity muscles (21) or changes in proprioception and neuromuscular control (14). In conclusion, although BG, UG, and B + UG achieve statistically significant change and small-to-large SE in explosive and endurance performance measures, because the B + UG showed a significantly (p # 0.05) higher change in 13 of 21 performance measures vs. the CG, whereas the UG and BG showed only 6 and 3, respectively, it is concluded that the combination of unilateral and bilateral jump drills seems more advantageous to induce significant performance improvements during high-intensity, short-term explosive training in young male soccer players.

PRACTICAL APPLICATIONS The replacement of some soccer drills with high-intensity plyometric exercises may positively affect jump, sprint, kicking, agility, endurance, and balance performance in young soccer players during the in-season period. These adaptations can be achieved in the short term and may potentially increase competitive performance and may reduce injury risk. However, to maximize adaptations, it is recommended that during training soccer players combine unilateral and bilateral drills, executed in different planes (i.e., vertical, horizontal, lateral). Although because training adaptations demonstrated a specificity effect, in sports where performance depend mostly on unilateral or bilateral movements, then a high portion of drills should be executed with this movement pattern. Also, a combination of cyclic and

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Journal of Strength and Conditioning Research acyclic jumps (with a moderate increase in volume across time) may optimize performance adaptations. Also, although PT can induce an increase in explosive, endurance, and balance performance in young soccer players, to optimize training adaptations, this training strategy should be adequately applied in a more complex training plan that incorporates other explosive (e.g., sprints), endurance, technical, and tactical-oriented training methods.

ACKNOWLEDGMENTS The authors disclose professional relationships with companies or manufacturers who will benefit from the results of this study. The results of this study do not constitute endorsement of the product by the authors of the National Strength and Conditioning Association. The authors disclose funding received for this work from any of the following organizations: National Institutes of Health, Welcome Trust, Howard Hughes Medical Institute, and other(s).

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