Speed and power are considered success-predicting variables in youth soccer (26). Specifically, sprint ability over short distances (i.e., 15 m), vertical jump, and agility performance have been shown to possess discriminative power between elite and subelite young soccer players (27). Despite the perceived and demonstrated importance of speed and power in soccer (36), to the best of our knowledge only 1 investigation addressed the issue of speed and power development and its relationships with fitness indicators in youth soccer (18).
In competitive soccer, the busy schedules limit the number of training sessions devoted to fitness development during the competitive season (35). As a consequence, training strategies aiming for short-term fitness improvements through a reduced number of weekly training interventions are warranted. Although appealing, low-impact, in-season, short-term training interventions face a number of training-related constraints, mainly associated with volume vs. intensity conflicts of the imposed load. Indeed, on one hand, in-season training interventions should avoid imposing further stress that may limit recovery from matches and training sessions. On the other hand, already-fit players might need important training loads to experience further fitness improvements (3).
Complex training (11) was proposed to induce significant speed and power performance-related improvements (13). This training strategy consists of combining strength/power exercises with subsequent sport-relevant drills (12). The benefits of complex training are considered a consequence of the direct transformation of strength/power gains into specific performance, thus fostering and accelerating strength/power training gains (12). In this context, Taïana et al. (35) suggested the use of soccer drills in complex strength training sequences to induce specific speed and power adaptations with only 1 training session per week.
In light of the aforementioned considerations, the aim of this study was to examine the effects of an in-season, low-impact (short-term and low-volume) complex training program and a traditional sprint-training intervention on speed and power development in a group of youth elite soccer players. It was hypothesized that complex training may induce greater speed and power improvements compared to traditional sprint training (18,35).
Experimental Approach to the Problem
This study adopted a counterbalanced, fully controlled research design with randomly allocated training interventions and pre-post assessments. The training interventions (exercise progression and weekly load) adopted in this study reflected what is usually implemented by soccer coaches and fitness trainers during the competitive season before major competitions or tournaments. This research strategy was used to promote the ecological validity of the possible outcome of this investigation (30). Currently, 2 main speed and power training philosophies are popular in modern soccer, which in this study will be referred to as CONTRAST and SPRINT methods. The rationale of the CONTRAST method in soccer is based on the assumption that alternating heavy-light resistance with soccer-specific drills, speed, and power development is superior to SPRINT training (6,35). With SPRINT training we considered the direct training (i.e., use of line short sprints) of short-term acceleration ability in soccer players. The rationale for this training strategy is based on recent studies that showed that elite soccer players should be trained mainly over the most frequent match-sprint distances such as 10 to 30 m (10,24,25). Protocols of the training interventions are shown in Table 1.
The effects of the training interventions were assessed using a number of field tests that have been previously reported as relevant to soccer (1,15,16,19,20,23,33,34). The experimental procedures took place during the second half of the competitive season after 7 months of uninterrupted training. All subjects were familiar with the experimental procedures. All testing sessions were carried out 3 days after the latest competition match at the same time of the day and under the same experimental conditions. Only 1 player was tested at a time, and each player was instructed, and verbally encouraged, to give a maximal effort for each performance test. Performance testing was initiated after a standardized 15-minute warm-up, including low-intensity forward, sideways, and backward running; several acceleration runs; and jumping at a progressively increased intensity.
Following the initial baseline testing session, players were randomly assigned to either a CONTRAST (n = 10) or SPRINT (n = 10) group. Groups were matched for physical characteristics and performance at the different tests. The training intervention consisted of 6 training sessions over 7 weeks, targeting the improvement of the players' speed and power. The training interventions were initiated the day after the baseline testing session and ended 1 week before the second performance testing session.
All training sessions were directly supervised by a strength and conditioning coach. During SPRINT training, each individual run was timed by means of a photocell gate system, and players were immediately informed of their run performance to stimulate motivation. Total training duration and actual high-intensity training time were kept similar for both interventions.
Twenty-nine highly trained junior male soccer players (5 central defenders, 6 fullbacks, 9 midfield players, and 9 forwards) agreed to participate in this investigation. A written consent was obtained from the subjects (or their parents in the case of players younger than age 18 years) after being thoroughly informed of the purpose and potential risks of participating in the study. All experimental procedures were approved by the Ethics Committee of the Universidad del País Vasco-Euskal Herriko Unibertsitatea. All subjects were members of the development program of the same professional soccer club, had been involved in competitive soccer for at least 7 years, were training four 1.5-hour sessions per week, and were competing at a junior national level at time of the study. Because of injury, illness, or national team commitments, only 20 of the initial 29 players completed the study. Physical characteristics of the subjects are presented in Table 2.
The first session of the CONTRAST training protocol consisted of an introduction session of hill sprinting (8% slope). The second session was dedicated to sled pulling sprint training, towing approximately 18% of body mass (22,32). During the 3 subsequent training sessions (weeks 3 to 5) players performed 3 series of 4 reps of calf rises (approximately 35% of body mass) and parallel squats (approximately 50% body mass) and 2 repetitions per leg of hip flexions (approximately 15% of body mass). According to Wisløff et al. (36) weight training exercises were performed emphasizing maximal concentric mobilization to promote power development. As shown in Table 1, strength and power exercises in sessions 3 through 5 were immediately followed by a variety of soccer-specific activities such as jumps, accelerations, ball kicks, and offensive and defensive actions.
The first 2 sessions of SPRINT training consisted of 2 sets of 4 repetitions of individually timed maximal straight line sprints. Recovery between repetitions was 90 seconds, and between sets it was 180 seconds. In SPRINT sessions 3 and 4 the number of sets was increased to 3, and in sessions 5 and 6 it increased to 4, whereas recovery times remained constant.
Performance testing took place before and after the training intervention, 7 weeks apart. Except for the intervention itself, players in both groups followed the same training program established by their team coaches, and they were familiar with all testing procedures. Testing sessions were carried out between 17:00 and 19:00, at least 24 hours after the last training session and 2 hours after the last intake of food. All fitness tests were performed in an indoor facility and under similar environmental conditions (temperature 19.0-20.3°C, relative humidity 60.9-64.4%; Kestrel 4000 Pocket Weather Tracker, Nielsen Kellerman, Boothwyn, Pennsylvania, USA). Testing sessions started with the assessment of players' body mass, height, and sum of 6 skinfolds (triceps, subscapular, suprailiac, abdominal, front thigh, medial calf; Holtain Ltd., Crymych, United Kingdom). All jumping tests took place on a cement surface with the players wearing running shoes, whereas all running tests were performed on a synthetic soccer pitch with players wearing studded soccer boots. Players were blinded about their test performances until completion of the study. Intraclass correlation coefficients (ICC) of the tests used in this investigation were assessed in separate sessions before the commencement of this study. The ICC of the vertical jump tests ranged from 0.92 to 0.95 and those for sprint and agility tests were 0.94 and 0.92, respectively (p < 0.001).
Lower-limb explosive strength was assessed using a variety of vertical jumps according to Bosco et al. (4). After a standardized warm-up, all the players performed in the following order a countermovement jump (CMJ) test, a countermovement jump with arm swing (ACMJ) test, and 15-second countermovement jump (CMJ-15s) test. Vertical jump tests were separated by 5 minutes of passive recovery. Each player performed 2 maximal CMJ and ACMJ interspersed with 3 minutes of passive recovery, and the best performance of each test was retained for calculations. All vertical jump tests were performed on a contact platform (Ergo Tester, Globus Italia, Codognè, Italy). Flight times were registered by a digital timer connected to the contact platform and converted to height jumped according to Bosco et al. (4). All jumps were initiated from a stationary standing position by allowing a preparatory countermovement consisting of a 90-degree knee flexion. Players were instructed to keep their hands on their waists during the preparatory and jump phases of the CMJs. During the ACMJs players were allowed to swing their arms for impulse during preparatory and jump phases. Jumping conditions during the CMJ-15s test were identical to the CMJ test, with players required to jump for maximal height repeatedly for 15 seconds. Each player performed CMJ-15s only once and mean height jumped (cm) was recorded (4).
Fifteen minutes after completion of the CMJ-15s, players performed the Sprint-15m test. In this test, velocity in a 15-m straight-line dash was measured by mean of photocell gates placed 1.0 m above ground level (Timer S4, Alge-Timing, Lustenau, Austria). Each sprint was initiated from an individually chosen standing position, 3 m behind the photocell gate, which started a digital timer. When players crossed the second set of photocell gates placed at a distance of 15 m, velocity in m·s−1 was displayed and recorded. Each player performed 2 maximal Sprint-15m interspersed with 3 minutes of passive recovery, and the fastest velocity achieved was retained. Sprint distance was chosen according to average sprint distance reported in soccer match-analysis studies (33).
Five minutes after the Sprint-15m, players performed the Agility-15m (Figure 1). In this test, players' velocity in a 15-m agility run was measured with the aforementioned photocell gates system. As in the Sprint-15m test, players started running 3 m behind the initial set of gates. After 3 m of line running players entered a 3-m slalom section marked by 3 sticks 1.6 m high and placed 1.5 m apart, and then cleared a 0.5-m hurdle placed 2 m beyond the third stick. Players finally run 7 m to break the second set of photocell gates, which stopped the timer. Each player performed 2 maximal Agility-15m interspersed with 3 minutes of passive recovery, and the fastest velocity in m·s−1 achieved was retained.
Results are presented as mean ± standard deviation. All reported values concern the 20 subjects who completed the study. Data sets were checked for normality using the Shapiro-Wilk normality test and visual inspection. Comparisons between CONTRAST and SPRINT groups were performed using factorial analysis of variance (ANOVA) (2 × 2 design, time × training intervention). Student's t-test for unpaired samples indicated that there were no baseline differences between groups. Post hoc analyses were carried out using the Bonferroni test. Reliability of tests was assessed using ICC. Calculations were performed using the STATISTICA statistical package (Version 6.0, StatSoft, Inc, Tulsa, Oklahoma, USA).
There were no significant differences in anthropometrical characteristics between the SPRINT and CONTRAST groups (Table 2). At baseline no difference between physical test performance was evident between the 2 experimental groups (p > 0.05).
No time × training group effect was found for any of the vertical jump (Table 3) and agility-15m variables (p > 0.05, Figure 2). Post-training Sprint-15m performance was significantly better in the CONTRAST group compared to the SPRINT group (7.23 ± 0.18 vs. 7.09 ± 0.20, p < 0.01, Figure 2).
This is the first study that addressed the issue of in-season speed and power development in young elite soccer players. Different from previous training studies that addressed fitness development in soccer, this research determined the effect of short-term speed and strength training interventions typically used by coaches in an attempt to produce quick fitness gains in their players (33). This study's main finding was the superiority of COMPLEX training in inducing speed improvement (i.e., 15-m sprint performance).
The results of this study are in line with those reported by Kotzamanidis et al. (18), who found significant improvements in 30-m sprint performance in nonelite soccer players submitted to a progressive-load combined training program conducted over 13 weeks. However, in that study soccer players were not observed for solo speed training effects, limiting the ecological validity of the findings. Additionally, in the Kotzamanidis et al. (18) study heavy-load strength training (i.e., from 8 to 3 repetition maximum loads) was performed 10 minutes before the sprint training session. Although the study reported significant improvement in sprint performance, the training protocol and the training outcome (30-m sprint time) may be limited in their applicability to youth soccer. Indeed, heavy-load training (6-3 repetition maximum) is rarely possible during the soccer competitive season because it usually requires long training sessions (∼60-90 minutes) and training facilities that are not always available. Furthermore, this type of training does not allow players to undertake effective ball practice after this form of concurrent training (18).
In contrast to Kotzamanidis et al. (18), the training protocols used in this study were typical short-term training interventions usually considered by soccer coaches and fitness trainers prior to main competitions (35). Consequently, the CONTRAST training protocol used avoided the limitations previously reported and warrants ecological validity for soccer. Indeed, after the experimental protocols the players of this study undertook the usual technical-tactical soccer practice to prepare for the upcoming championship matches.
Of interest, classic speed training consisting of repeated bouts of short sprints did not result in any improvement in sprint performance and in vertical jump performance (2). This finding was unexpected and challenges the principle of specificity of sprint training (8,9,29). The reason for that observation is difficult to explain with this research design. However, it could be hypothesized that a lack of specificity of the sprint training distance (i.e., 30 m) or ineffective training loads may have contributed to the observed results. It could be argued that more aggressive sprint training protocols may possibly result in significant short-sprint performance enhancement (7). Further studies investigating the most effective sprint training load in soccer are warranted.
Agility is considered a complex physical ability and can be defined as the ability to rapidly change direction. Recent studies reported that agility is influenced by explosive strength, balance, muscular coordination, and flexibility (31). Reilly et al. (27) suggested that agility performance is a physiological prerequisite in soccer, given that players are frequently involved in sudden directional changes in order to be effective during the game. Furthermore, agility was shown to be related to total match distance in female varsity soccer players (17). Several studies have previously shown that agility performance should be regarded as an independent physiological variable in male soccer players (5,21,23,36).
In this study, sequencing strength and soccer-specific fitness drills enabled players to sprint faster after the training intervention without affecting agility. This finding is in line with those studies that addressed agility in team sports (31,37), confirming the specificity of agility responses to training interventions. It is surprising that the use of small-sided game drills after strength exercise did not develop any training adaptation in the agility domain. However, in this study agility was considered as performance (i.e., velocity) over a well-known 15-m slalom test, which is probably less sport-specific than the reactive agility that is required during ball games (14), and this may be considered among the possible reasons for this finding.
Results showed that contrast or complex training (i.e., sequencing strength and explosive-strength sport-specific exercise) resulted in improved sprint performance in subjects who were already sprint trained (10) with a very short-term training program. Although no information is available about the effects of improved sprint ability on actual match play, improved sprint performance has been reported to constitute an advantage in game positioning and ball conquest in soccer (33,36). Consequently the proposed effective progressive-load contrast training proposed in this study may be considered as a resource for those soccer coaches and fitness trainers looking for additional sprint ability in the short term.
Usually for logistic and match fixture reasons soccer coaches and fitness trainers tend to limit in-season heavy-load weight training to improve muscular performance in elite soccer players. Indeed heavy-load strength training requires specific facilities and devices that demand important investments in both economical and logistic terms. Therefore, the use of set weight (approximate percentage of body mass) may represent an evidence-based effective strategy to enhance sprint performance in soccer players.
The ability to sprint over short distance (10-15 m) is considered a relevant component in elite soccer performance (26,28). This study showed that when urgent enhancements in sprint performance are required, the sequence of loaded strength-power exercises (15-50% body mass) and unloaded exercises (jumps and sprints) or drills (small-sided games) may prove effective. The use of exercises aiming at the development of the strength and power spectrum of hip-knee-ankle extensor muscles (8) appeared to be an effective choice for contrast training. In light of the findings of this study, CONTRAST training should be preferred to line sprint training in the short term (4-6 weeks, with 1-2 sessions a week) in young elite soccer players.
The authors gratefully acknowledge the assistance of Txus Pinedo and Xabi Clemente during the implementation of the training interventions and the participating players and their coaches for their enthusiasm and cooperation.
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