Effects of 1 vs. 2 sessions per week of equal-volume sprint training on explosive, high-intensity and endurance-intensive performances in young soccer players

The study aimed to evaluate the effects of 1 vs. 2 sessions per week of equal-volume sprint training on explosive, high-intensity and endurance-intensive performances among young soccer players. Thirty-six young male soccer players were randomly divided into 2 experimental groups that performed either a single weekly sprint training session (ST1, n = 18, age: 17.2 ± 0.8 years) or two weekly sprint training sessions (ST2, n = 18; age: 17.1 ± 0.9 years) of equal weekly and total volume, in addition to their regular soccer training regimen. Linear sprinting (10 m, 20 m, 30 m, and flying 10 m), T-test agility, countermovement jump (CMJ) and maximal oxygen consumption were assessed one week before (T1), in the middle (T2) and immediately after the 10 weeks of training (T3). A large magnitude and statistically significant main effect for time was found in all the assessed variables after both training interventions (all p < 0.001; ES ≥ 0.80). No main effect was observed between the 2 groups at any time in linear sprinting, T-test or CMJ test (p > 0.05; ES < 0.20). A significant interaction effect (F = 4.05; p = 0.04, ES = 0.21) was found for maximal oxygen consumption with ST2 inducing better performance than ST1 (p = 0.001; ES = 1.11). Our findings suggested that the two sprint training frequencies were effective in enhancing explosive, high-intensity and endurance-intensive performances. However, it is recommended for coaches and fitness coaches to use a biweekly sprint training modality as it was found to be more effective in improving endurance-intensive performance.


INTRODUCTION
Soccer is the most popular sport worldwide [1]. It has been reported that during soccer matches, players are required to perform multiple actions demanding muscles strength, power, speed, agility and endurance-intensive fitness components [2,3,4], suggesting therefore that physical conditioning for soccer players is a complex process [5]. Moreover, studies investigating the time structure of soccer matches reported that players covered about 9-12 km during a 90-min game, with high-intensity running or sprinting representing only 8-12% of the game [6,7,8]. In this context, it has been reported that each player performed 17 to 81 sprints, with each action lasting 2 to 4 seconds, covering then a distance shorter than 20 m [6,8,9]. Likewise, peak sprint velocity during matches was found to be around 31 to 32 km.h -1 , most often performed without a ball [7]. For more details, it was found that 45% of the scored goals were preceded by straight sprints while 16% are preceded by a jump and only 6% by a change of direction [10]. Furthermore, straight line sprints, agility skills and repeated sprint ability (RSA)

Effects of 1 vs. 2 sessions per week of equal-volume sprint training on explosive, high-intensity and endurance-intensive performances in young soccer players
before the beginning of the experimentation. All participants were screened and were safe from injuries prior to preliminary testing. They had been involved in competitive soccer for at least 6 years, were training 5 times per week (1.5 hours per session) and were competing regularly at a junior regional level. Goalkeepers were not included in this study due to the potential differences in their morphological characteristics and motor ability [24]. The study was fully approved by a local research ethics committee and the protocol was conducted according to the Declaration of Helsinki [25].

Experimental design
This study adopted a repeated measures design with a randomized allocation to training intervention. Subjects were divided into 2 training groups that performed either a single weekly sprint training session (GST1) or two weekly sprint training sessions (GST2) of equal weekly and total volume, in addition to their regular soccer training regimen. As the independent variable was "training type", no control group was used. The study was conducted during the soccer in- the 20-m multi-stage shuttle run test [26].
The testing schedule included 4 similar sets of tests performed 1 week before the initiation of the study, the week before and the week after the 10-week training period, and the 6th week of the training period. The first set was conducted with the aim of getting the subjects familiarized with the testing procedures. In addition, test results of sets 1 and 2 were also used for assessing the test-retest reliability of the measures. All tests were administered on 2 nonconsecutive days separated by 72 hours. On the first test day, after the anthropometric assessment, sprinting, agility and jumping tests were performed. On the second day, the 20-m multistage shuttle run test was assessed. All tests were performed on a synthetic soccer pitch under similar environmental conditions (temperature: 17-22°C, humidity: 67 ± 2%) and at the same time of the day.
Before the tests, participants completed standardized warm-up sessions of 15 min consisting of 10 min of jogging, and dynamic stretching, followed then by 2 sets of 3 exercise sprints (5 m, 10 m and 15 m). All subjects performed each test with at least 1 min of rest between all trials to ensure sufficient recovery [27].

Exercise and measurements
Anthropometric measurements. Anthropometric variables of height (cm) and body mass (kg) were measured 2 times for each subject, and the mean of each measure set was calculated. Stature and body mass measurements were made on a digital scale (OHAUS, Florhman Park, NJ) with an accuracy of 0.1 cm and 0.1 kg, respectively.
In this regard, several contributing factors (e.g., specificity, individualization) have been reported by different researchers to be paramount for the development of speed in soccer [11]. For that reason, sprint training programmes should be gradually progressive, in terms of intensity and the number of sprint repetitions [11]. Furthermore, due to its anaerobic nature, speed training targets the neuromuscular system, and therefore intensities and recovery periods must be calibrated with precision [23]. During the in-season period, professional soccer players are required to play 1 to 3 official matches per week, and consequently have limited time available for athletic training. Therefore, it is important that coaches optimize the in-season sprint training frequency so that athletic performance can be maintained and acute training-related fatigue avoided. Usually, sprint training for soccer is conducted specifically once per week, eliciting positive effects on specific soccer fitness-related performance among young elite soccer players [17]. However, the optimal number of training sessions per week remains undefined [11]. To the current knowledge of the authors, no studies have investigated the efficacy of different sprint training frequencies at the same training volume on the soccer players' physical fitness. For that reason, the purpose of this study was to evaluate the effects of 1 vs. 2 sessions per week of equal-volume sprint training on explosive (linear sprint speed and vertical jumping), high-intensity (agility T-test) and endurance-intensive (maximal oxygen consumption) performances in young soccer players.
We hypothesized that two sprint training sessions per week would be more effective than one session per week to improve physical fitness in young soccer players.  64.9 ± 7.1 kg, body mass index: 21.2 ± 2.0 kg/m 2 ). Written consent was obtained from the participants and their parents after being thoroughly informed about the purpose and potential risks of participating in the study. During the intervention period, the participants were requested to refrain from performing any strenuous exercises 48 hours

Training interventions
The sprint training programme was designed by the investigators, consisting of adding one or two sessions a week, throughout 10 weeks, variation. An independent samples t-test was applied to compare the RPE scores between the two groups. Statistical significance was set at p < 0.05.

RESULTS
RPE scores collected at each training session during the whole training period were not different between the two group conditions.
Absolute values and qualitative outcomes resulting from the within and between-group analyses are displayed in Tables 2 and 3.
Trivial magnitude and non-statistically significant differences were observed between conditions in S10 (F   [36] reported that 6 weeks of resisted sprint training were effective to In-season sprint training in youth soccer intervention. According to these authors, training programmes designed to improve agility should be specific and independent from speed training programmes [19,38]. Since agility movements are more dependent on motor control factors than maximal strength or muscular power [39], agility performance improvement for both groups in the present study may be related to the nature of speed training where players performed change of direction drills throughout the whole training period, which may have positively affected the agility performance by improving motor control and coordination.
Nonetheless, no significant difference was found between the two groups in the present study for agility performance, suggesting that agility performance enhancement was not dependant on training sessions' frequency when athletes were performing with the same training load.
For jump height performance, CMJ performances increased significantly for ST1 and ST2 with 1.8 and 3.7 cm, respectively. Dawson et al. [40] and Markovic et al. [41] reported that repeated sprint training was able to increase jumping height in physically active subjects after 6-and 10-week interventions, respectively. The improvement of CMJ performance during the present study may possibly be explained by the fast and efficient utilization of elastic energy in the stretch-shortening cycle as well as increased strength and power of knee extensor muscles, strongly engaged in the actions during fast running [41]. Moreover, CMJ performance improvement may be related to a concomitant speed performance enhancement as it was reported that CMJ performance has been related to sprint performance [42]. During the present study, although not significant, the improvement for ST2, which was twice as high as that recorded for ST1, is encouraging to investigate the effect of a longer training programme.
Finally, for endurance-intensive performance, both groups showed significant improvement for VO 2  Another finding of this study was the significantly greater improvement in VO 2 max performance after the biweekly sprint training modality compared with once weekly sprint training. It was reported that the magnitude of oxygen consumption increase depends on the training programme design, which can be determined through the exercise intensity, duration, recovery and frequency of sessions [45].
In this regard, it was reported that only two sessions per week may be a sufficient training session frequency to increase VO 2 max [46].
The results of the present study are consistent with those reported by Baquet et al. [46], who showed that two sessions per week over 7 weeks of short intermittent exercises (10 or 20 s) at velocity ranging from 100 to 130% of maximal aerobic speed, performed on a short track, were able to increase absolute VO 2 max (9.1%) and VO 2 max relative to body mass (8.2%) in prepubertal children, which reinforces the usefulness of using more than one session a week to enhance aerobic fitness in prepubertal and adolescent participants.
It seems that under volume-equated conditions, 2 sessions/week of sprint training resulted in better VO 2 max improvement compared with 1 session/week. Our findings suggest that exposure to only 1 weekly sprint training session in combination with specific soccer training can be a sufficient stimulus to enhance performance in youth soccer players while guaranteeing the reduction of injury risk [11].
Thus, from a practical point of view, 1 sprint training session may be more useful for coaches when programming training content during congested periods, especially during soccer competition, which implies limited training days/sessions due to the necessity of recovering from matches and traveling [47]. Moreover, one training session allows coaches to spend more time in improving their technicaltactical abilities while preserving the same physical fitness development.
Finally, we acknowledge some limitations in the study. First, the study period lasted only 10 weeks, whereas longer periods of training may be required especially when investigating the effects of training on related muscular performances (i.e., speed, agility, power). Although this duration was sufficient to achieve significant increases in explosive, high-intensity and endurance-intensive performances in both groups, it is conceivable that explosive and high-intensity differences between groups may be achieved when a longer training programme is investigated. Also, the absence of a control group in which subjects would have completed the regular training sessions and played the official matches without participating in any of the experimental protocols limits the conclusions from this study. Moreover, larger number of subjects could have more relevant effects. Finally, the results of our investigation are specific to young soccer players from the regional level and therefore cannot necessarily be generalized to other populations including senior males and females with a high level of practice.

CONCLUSIONS
Both of the training regimens seem to be effective for soccer-related fitness improvement in youth players during the in-season period.
Indeed, our results showed that the prescription of 1 or 2 weekly sprint training sessions during the in-season period contributed to improving coaches and strength and conditioning professionals to develop more personalized training and rehabilitation for both children and adults.
However, additional training modalities could be included (e.g., plyometric training, strength training, RSA training) if the goal is to further improve specific soccer fitness-related performance.