Introduction

In modern football, players are required to developed both aerobic and anaerobic capacities enabling them to meet the physical demands of football1. Additionally, adequate aerobic and anaerobic capacities are crucial in football, as they enable players to sustain high-intensity activities throughout the match, facilitating rapid changes in speed, endurance, and recovery, ultimately influencing overall performance on the field2,3. High-intensity interval training (HIIT) may play a crucial role in football by enhancing players’ ability to meet the physical demands of matches and training3. These demands include covering significant distances, executing frequent accelerations and decelerations, and performing repeated high-intensity efforts such as sprints and directional changes2,4. Rather than increasing the match demands themselves, improvements in physical conditioning through HIIT enable players to sustain performance over prolonged durations, recover effectively between intense actions, and reduce fatigue during critical periods of the game5. This underscores the importance of HIIT as a targeted method for addressing the specific physiological and neuromuscular challenges inherent to football performance. Additionally, as the modern game demands higher levels of intensity, it is crucial for young players to develop the physical capacity to withstand the physiological stress of both training and competition6.

Although small-sided games (SSG) are a widely utilized and common type of training in football7, recent meta-analytical findings suggest that SSG, whether intermittent or continuous, may not be sufficient to improve certain physical performances, such as sprinting8. HIIT represents a time-efficient strategy for achieving adaptive goals in the training process9. HIIT, through its repeated bouts of high-intensity effort, closely mimics the intermittent physical demands characteristic of team sports such as football10. To select an appropriate HIIT format that leads to improvement in physical performance, it is necessary to consider the requirements of the sport itself, the athlete’s condition, desired long-term adaptations, as well as periodization11. In football, four types of HIIT training are mostly utilized. The most common is game-based HIIT, followed by short-interval HIIT, as well as long intervals and repeated short sprints12.

It is well documented that HIIT is a significant component for the optimal progression of performance in young athletes whose sports demand a combination of prolonged endurance and repeated high-intensity efforts, including maximal-speed actions13. Considering that HIIT meets the demands of the game itself, besides linear movement, changes of direction (COD) are often performed. HIIT training with changes of direction is considered a crucial performance characteristic as it adapts to the game’s demands due to alternating periods of low and high intensity14. This was confirmed in a study demonstrating that an increased number of directional changes during intermittent shuttle running significantly impacts physiological, perceptual, and neuromuscular responses, highlighting the importance of incorporating COD elements in HIIT programs15. When incorporating COD, it’s necessary to account for the additional time required for executing turns in calculations to ensure similar stress compared to linear running12.

Previous studies have investigated the effects of HIIT protocols utilizing repeated linear efforts on physical performance in football players16,17,18,19. Notably, one study in adolescent football players highlighted that repeated sprint COD training improved COD performance, but the initial state of fitness must be considered as a factor influencing these outcomes20. Only one study compared these two types of HIIT training in male football players, but it’s worth noting that the training program in that study consisted of repeated sprints21. Moreover, aforementioned study has conducted training program during competitive period, lasting only two weeks.

Having in mind that speed and agility are paramount in soccer due to their significant impact on players’ performance across various aspects of the game, it was of great importance to analyze the effects of HIIT programs with and without turns on the mentioned parameters. The transition period, typically occurring between competitive seasons, is often dedicated to recovery and general physical preparation. However, prolonged reductions in training intensity during this time can lead to detraining effects, including declines in speed, agility, and neuromuscular performance21. To mitigate these losses, time-efficient and targeted training approaches, such as HIIT, may be beneficial. This study therefore aims to examine the effectiveness of a 4-week HIIT program, with and without COD, implemented during the transition period, on speed and agility in football players aged 15–16 years. Based on results of similar study22, we have hypothesized that both types of HIIT will have similar effects on speed, COD speed, and agility in young soccer players.

Methods

Participants

The F test and a priori type power analysis was used to asses required number of 54 participants using an effect size of 0.25, a power of 0.95 and α error probability at 0.05 using G*Power (Version 3.1.9.4, University of Dusseldorf, Germany). Initially, the participants for this research were 59 young football players (aged 15–16 years). Two of the players were excluded based on the exclusion criteria. All of them compete at the highest level of football competitions in Serbia as part of the professional football club “Spartak” from Subotica. The participants were randomly assigned to an experimental group that performed one type of HIIT training (HIITlinear; N = 28) and another group that performed another type of HIIT training (HIITCOD; N = 29). The players were divided into two groups in a 1:1 random manner following a baseline measurement, taking into account their positions to ensure balance relative to the demands of the football match. This approach was implemented to maintain the ecological validity of the study while accounting for potential positional differences. Additionally, this ensured that each group had an equal chance of receiving either training protocol, thereby minimizing selection bias and promoting balanced baseline characteristics between the groups. Table 1 shows the demographic characteristics. The inclusion criteria were outlined as follows: male adolescents aged 15–16 years, with advanced level of training experience, actively engaged in top football competition in country, and undergoing a minimum of five weekly training sessions. Participants screened for any pre-existing health conditions or injuries that could affect their ability to participate in high-intensity training. Exclusion criteria encompassed individuals with cardiovascular, respiratory, or other medical conditions, those undergoing rehabilitation, individuals with recent musculoskeletal injuries within the last three months, as well as participants who underwent anterior cruciate ligament surgery within the preceding year. The club management and participants were familiarized with the experimental treatment, the implementation and the aim of this research. Each participant has parental consent, which they have signed. Additionally, the club management has signed consent for the experimental design. Through this consent, the club and participants confirm that they are familiar with all the details of the experimental program, the testing procedures, and that the data obtained through testing will be used solely for scientific purposes. The study was approved by the Ethical Committee of the Faculty of sport and physical education, University of Nis (04/651-2) and in accordance with the Declaration of Helsinki.

Table 1 Descriptive characteristics of participants.
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Testing procedures

The entire data collection process for basic anthropometric measurements (body height and mass), speed, COD and agility time was conducted in the training center of the Football club “Spartak” from Subotica. In addition to researchers, coaches from the mentioned club also participated in collecting the data, due to the fact that they were familiarized with the procedures as well as with the players. All the measurements were performed on one day. At first, according to the protocol, body height and body mass were measured in the morning on fasted state following speed, COD speed and agility tests. After the body height and body mass measurement, all players completed a 15-minute standardize warm-up, that consisted of jogging (5 min), stretching (3 min), progressive running (5 min) and COD speed runs (2 min). Speed, COD and agility testing was carried out between 9 am and 1 pm. The procedure for conducting measurements during the testing intervals was the same for both initial and final measurements. The testing was conducted 2 days after the last match or/and the last training session. All parameters on the initial and final testing were measured by the same person, in the same order, and using the same instruments. The data was automatically stored in a computer, primarily through direct input from the measuring instruments in most cases. Where no software solution was available, the results were manually entered on pre-prepared measurement sheets.

Body height was measured according to the IBP (International Biological Program) protocol23. The instrument used for measurement was the Martin Anthropometer. Body mass was measured using The InBody 230 (Biospace Co. Inc., Seoul, South Korea) Bioelectrical Impedance Analyzer (BIA).

To assess the acceleration and running speed of players, 5, 10 and a 20 m linear sprint effort was used with photocell gates (Microgate, Polifemo Radio Light, Bolzano, Italy) placed 0.4 m above the ground, with an accuracy of 0.001 ms24. The timing mechanism initiated automatically as participants passed the initial gate positioned at the starting line, recording split times at 5 m and 10 m. Participants were instructed to perform a maximal 20-meter sprint from a stationary, crouched position 0.5 m behind the photocell gates.

The following tests were used to evaluate COD and agility time:

COD, also known in the literature as the Y agility test, evaluates the sprint time of an athlete’s change of direction to the left and right at a 45-degree angle. This test provides a measure of how quickly an athlete can alter their course, simulating movements common in football. Photocell gates (Microgate, Polifemo Radio Light, Bolzano, Italy) were used to obtain results in this test. The test area is set up with cones or markers to form a Y-shape, with the starting point at the bottom of the Y and two branches at 45-degree angles to the left and right. Photocells are placed at specific points to record the athlete’s sprint time accurately. The athlete starts at the base of the Y. On the signal, the athlete sprints forward and, upon reaching the decision point, changes direction to either the left or right branch at a 45-degree angle. According to the protocol, the athlete has 3 attempts to the left and right side, and the best time was recorded at the end.

The Illinois Agility Test (IAT) is a standardized assessment used to evaluate an individual’s ability to accelerate, decelerate, and change direction quickly in a pre-planned manner. The course is set up on a 10 × 5-meter rectangle, with four cones marking the corners and four additional cones placed 3.3 m apart along the center line for weaving. The participant starts lying prone behind the starting line, sprints 10 m, weaves through the central cones in a slalom pattern, and then completes the course by sprinting back along the opposite side. Timing begins at movement initiation and ends upon crossing the finish line, measured using electronic timing gates (Microgate, Polifemo Radio Light, Bolzano, Italy). The test is performed without a ball, and participants complete three trials, with the best time recorded25.

The Reactive Agility Test (RAT) assesses reactive agility, which involves both the ability to quickly change direction and the athlete’s motor-cognitive domain (Fig. 1)24. This ability is crucial for success in the game, particularly in elite sports26. For COD assessments, participants were briefed on the prescribed movement sequence beforehand, whereas reactive agility involved altering direction in response to stimuli presented during the assessment27. The test begins with the participant positioned at the starting line, where the first photocell is located. As the participant begins running, they cross the second photocell positioned 5 m away, which triggers a signal on the monitor. This signal, in the form of a light or arrow pointing left or right, indicates the direction the athlete must immediately react to and continue moving accordingly. The athlete must see and process the signal during execution and, with the fastest reaction possible, continue moving in the specified direction. Witty SEM photocells (Microgate, Bolzano, Italy) were used to obtain results in this test. Witty SEM is a cognitive reactive training system used together with photocells to measure the reaction of players based on the light stimuli. The reactive agility test using a light stimulus is a common method used to assess this aspect of performance24. Using a light stimulus is a practical and effective way to evaluate a soccer player’s ability to anticipate and react swiftly to changing game situations, contributing to their overall performance on the field28. In the current study, the RAT involved a reaction time element provided by a light stimuli thanks to the Witty SEM lights. According to the protocol, the athlete has 3 attempts, and the best time, as well as the reaction time from the moment the signal appears to cross the finish line, are recorded at the end.

Fig. 1
figure 1

Schematic representation of RAT test.

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The training program

The experimental groups represented active groups during the transition period and conducted the program for four weeks. Training sessions for both groups were held twice a week (Tuesday and Thursday). On other days, players were subjected to the same practice (Table 2).

Table 2 Training program across the 4-week training cycle during transition period.
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The training sessions were designed in accordance with the developmental characteristics of 15–16-year-old adolescents, considering their physical maturity, training experience, and tolerance for high-intensity workloads. Exercise volume, intensity, and recovery intervals were adapted to align with established recommendations for youth athletes at this stage of biological and athletic development29. During the study, players performed two specific training (HIITlinear or HIITCOD) sessions per week, in addition to their sport-specific team training for a total of 4 consecutive weeks. Both groups had the same training program beside two types of HIIT during the week (Table 2). Each HIIT session lasted approximately 20 to 25 min, depending on the number of intervals performed, and was conducted twice per week. Considering that players participated in five other training sessions per week (technical, tactical, and strength-based), the HIIT intervention accounted for approximately 20–25% of the total weekly training volume during the 4-week intervention period. Training sessions started with a 15-minute standardized warm-up, comprising light jogging, stretching exercises, and the inclusion of soccer-specific movements. Both groups performed running-based HIIT. The first group (HIITlinear) performed linear HIIT (without a change of direction), while the second group (HIITCOD) performed HIIT with a change of direction. To establish the running speed for the high-intensity interval training (HIIT), the 30 − 15 intermittent fitness test was conducted30. The recorded speed corresponds to the velocity obtained during the final completed stage of the 30–15 intermittent fitness test (VIFT). The intensity of the HIIT sessions was adjusted based on individual VIFT scores, aiming to challenge the players while ensuring they can maintain the prescribed work-rest ratios and complete the sessions effectively. Each participant performed the running intervals in a designated lane with set distances determined by their individualized VIFT. Although training intensity was based on the VIFT, the overall training workload was expressed in arbitrary training units (ATU) (ATU = [work intensity + rest intensity)/2]×number of repetitions×number of series)31. Since COD sprints in the training were included, authors adjusted the total distance covered to compensate the increased energy demands of the turns. In this regard, for the second group, the length of the distance to be covered in a given time is reduced because it has been proven that a change of direction by 180 degrees requires additional energy. Therefore, the recommendation is to compensate for the excess energy expenditure by reducing the distance by 2–3% for each change of direction (180 degree turn)9. To ensure that players maintained the intended individualized intensity during each HIIT session, the distance to be covered in each 15-second repetition was calculated based on 90–95% of each player’s individual VIFT. Markers (cones) were placed at the corresponding distance endpoints for each player, and verbal cues from the coaches were used to ensure timely arrival within the 15-second interval. Failure to reach the marker was used as an indicator of reduced effort or fatigue, and corrective feedback was provided in real time. In the HIITCOD group, a 180-degree COD refers to a full reversal of running direction (i.e., shuttle runs). Each 15-second repetition included one 180-degree COD, meaning the player sprinted to the halfway cone, turned 180 degrees, and sprinted back. In sessions where 2 or 3 CODs were required (weeks 2–4), this indicated 2 or 3 changes of direction within a single 15-second repetition, resulting in shuttle-style runs with multiple turnarounds per repetition. Distances were proportionally reduced (by 3%, 5%, or 7%) to compensate for the added mechanical and metabolic demands of each additional turn, as recommended in previous research9. The total distances and number of CODs per repetition were pre-defined and monitored by coaches. A detailed description of the weekly experimental treatment is shown in Table 3.

Table 3 Detailed overview of the experimental program for both groups lasting 4 weeks.
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Statistical analysis

The data was processed using the Statistical Package for Social Sciences (SPSS) software (v20.0, SPSS Inc., Chicago, IL, USA). The mean ± standard deviation were calculated for all variables. We have employed the Kolmogorov–Smirnov test to assess the normality of the distribution. A two-way repeated measures ANOVA [(group (HIITlinear and HIITCOD) and time (initial and final)] was used to assess the differences between two different programs, effectively capturing both main and interaction effects to evaluate their impact on speed, COD speed and agility. The effect size (ES) for within group differences was showed as follows: <0.2 as trivial; 0.2–0.6 as small; 0.6–1.2 as moderate; 1.2–2.0 as large; 2.0–4.0 as very large; and > 4.0 as extremely large32. ES was calculated using Jamovi computer software (version 2.6). An alpha level of p < 0.05 was considered statistically significant and marked as *p < 0.05.

Results

The Kolmogorov–Smirnov test showed all data were normally distributed. The total training load for the 4-week HIIT program was calculated by summing the weekly training loads across all sessions (Table 3). Over the 4-week period, the total training load amounted to 9.400 ATU, reflecting the progressive increase in intensity and volume across the sessions. Table 4 presents the descriptive statistics for tests assessing speed, COD, and agility time in pre- and post-testing for both groups, along with the differences between them. No differences between the groups at the initial measurement were found for all variables (p > 0.05). A two-way repeated measures ANOVA revealed no significant group × time interaction effects for any variable (p > 0.05). However, a significant main effect of time was observed for the 5 m sprint (F1, 55 = 48.2, p < 0.01, η² = 0.47), 10 m sprint (F1, 55 = 36.5, p < 0.01, η² = 0.40), 20 m sprint (F1, 55 = 42.8, p < 0.01, η² = 0.44), and Illinois Agility Test (F1, 55 = 33.1, p < 0.01, η² = 0.38) in both HIITlinear and HIITCOD groups. Effect sizes for these improvements were very large (ES > 2), and percentage change (%diff) showed meaningful improvements (Table 4).

Table 4 Pre-post differences in HIITlinear and HIITCOD training groups.
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Discussion

This study aimed to investigate the effects of transition period HIITCOD compared to HIITlinear on linear sprint, COD speed and agility in young football players. According to the authors’ knowledge, this study is the first to demonstrate the effects of training programs in two groups during the transitional period in adolescent football players. The main finding were that four weeks of training contributed to improvements in linear sprint and Illinois agility test in both groups (HIITCOD and HIITlinear). However, COD speed and RAT results did not revealed any significant changes. The hypothesis can be confirmed since both types of HIIT training contributed equally.

Linear sprinting has been recognized as the primary action most commonly observed in goal-scoring situations, executed by both the scoring player and the assisting player, with sprint distances varying based on tactical demands and player positioning33,34. Therefore, the training process must elicit specific movements that will enable athletes to enhance their acceleration and speed abilities. Participants in both groups showed statistically significant improvements in linear sprint tests (5 m, 10 m and 20 m). The results of the present study align with findings from similar training programs conducted with male participants35,36. In the current study, the HIITlinear achieved a reduction in time at 5 m by 24.2%, 10 m by 10% and at 20 m by 8.8%, while the HIITCOD achieved a reduction at 5 m by 16.7%, 10 m by 10% and at 20 m by 9.1%. In the study by Taylor et al.22, results showed slightly lower improvements during competitive season but also without significant differences between groups. Compared to our results, they showed large improvements in 5 m (linear: 9.6% and COD: 9.4%), 10 m (linear 6.6% and COD: 6.7%) and 20 m (linear 3.6% and COD: 4.0%). Additionally, it must be noted that aforementioned study lasted only two weeks. Most recent study37, but in female football players, observed positive but more modest improvements compared to our results in 10 and 20 m sprint in both groups, with increases in HIITlinear (5.3–9.7%) compared to HIITCOD (5.4–8.7%). It could be speculated that training with COD may improve speed more than training without changes of direction due to higher systemic load. However, the results of this and aforementioned studies did not yield such an outcome. Based on the current results, it is not clear whether training with changes of direction has a greater impact on improving speed compared to training without changes of direction. Additionally, we cannot confidently assert that this type of training has a greater effect on speed in participants of higher average age compared to those of lower average age. Furthermore, it should be noted that in football players competing at the youth level, speed can be improved using both types of training from this study. It is important to acknowledge inter-individual variability in training response, which may be influenced by differences in physical fitness, body composition, and the proportion of fast-twitch muscle fibers—traits that are partly genetically determined20,38. Future studies should consider stratifying participants based on initial fitness levels to better understand training responsiveness. Nevertheless, these findings have practical implications for coaches and practitioners, highlighting that both HIITlinear and HIITCOD protocols can be integrated into transition-period training to improve speed and pre-planned agility without compromising recovery.

We observed no significant changes in reactive agility performance in the current study, which could be expected given that neuromuscular factors mainly determine the players’ ability to change direction rapidly and efficiently22. The only significant change was in Illinois test where both groups showed similar improvements. These results are consistent with previous research by Taylor et al.22, who also reported no significant changes in agility following a two-week HIIT intervention, regardless of the inclusion of directional changes. On the other hand, Attene et al.39 demonstrated significant improvements in agility when comparing training with two changes of direction to one, although their study involved basketball players, which may limit the generalizability to football. Furthermore, recent findings by Stanković et al.37 in female football players indicated significant improvements in COD speed following both HIITlinear and HIITCOD interventions, highlighting the potential for positive adaptations depending on training design, population characteristics, and outcome measures.

The better effects of HIITlinear and HIITCOD on COD sprint time compared to reactive agility can be attributed to several factors. Firstly, COD speed involves executing predetermined movement patterns, allowing players to anticipate and mentally prepare for the required actions. This distinction is important when interpreting training effects, as improvements in COD sprint time may result from enhanced technique and movement efficiency rather than solely from reactive ability. This type of agility relies on performing well-practiced movement patterns with precision and speed40. While sprinting can be used as an exercise modality within HIIT protocols, HIITlinear is primarily designed to improve aerobic and anaerobic endurance, rather than to directly enhance maximal sprint speed. HIITCOD, on the other hand, emphasizes the ability to quickly change direction while maintaining speed and control12. Secondly, COD speed often relies on muscle memory and well-established motor patterns, which can be developed and refined through repetitive training, such as linear sprinting and directional drills. The consistent practice of specific movements during HIIT sessions reinforces these motor patterns, leading to greater proficiency in executing pre-planned agility tasks. On the other hand, reactive agility requires rapid decision-making, perceptual skills, and the ability to adapt to unpredictable stimuli33. While HIIT can enhance general athletic attributes such as speed, power, and COD ability, it may not directly influence the cognitive and perceptual demands involved in reactive agility. For instance, the Illinois Agility Test measures pre-planned COD speed and does not involve perceptual cues or decision-making under pressure. This helps explain why both HIITlinear and HIITCOD protocols led to improvements in Illinois Agility Test performance, yet did not significantly impact reactive agility outcomes. Reactive agility requires athletes to respond to unpredictable stimuli—such as an opponent’s movement or a sudden visual signal—which involves perceptual processing and rapid decision-making. Therefore, specific training methods targeting these cognitive and perceptual elements may be necessary to improve reactive agility beyond what traditional HIIT can offer24.

Based on the results of the present study, it remains inconclusive whether incorporating COD into HIIT provides greater benefits for improving sprint speed, COD speed, and agility compared to HIITlinear alone. However, it is noteworthy that in our sample of youth football players, both HIIT modalities led to improvements in sprint performance and pre-planned agility. The absence of significant between-group differences may be related to the relatively short duration of the intervention, the moderate training load, or the timing of the program within the transition period, which typically emphasizes recovery. Moreover, both HIITlinear and HIITCOD involved exercises of same intensity and total volume, which could result in comparable improvements in COD sprint time and sprint performance. Moreover, the duration and frequency of the training programs were identical. Baseline fitness levels of players have been such that high-intensity training may yield similar improvements. Beginners or less conditioned athletes often experience rapid improvements from a variety of training stimuli41,42. In addition, the tests used to measure COD speed might not have been sensitive enough to detect small differences in improvements between these two types of HIIT training. Future research should include different protocols, athlete profiles, and tests.

One of the limitations is that we did not have a control group. The presence of the control group could give more information regarding the effects of technique and tactic training on physical performance compared to HIIT groups. Moreover, the absence of a control group limits our ability to attribute the observed sprint improvements solely to the specific effects of the HIIT interventions. Future research should focus on manipulating the HIIT protocols (duration and type) for the concurrent development of anaerobic and aerobic capacity during the transition period. Additionally, maintaining optimal physical fitness during the competitive season is also crucial, raising questions about the optimal type, intensity, and frequency of HIIT during this period. However, it must be stated that if not properly monitored, HIIT may result in negative consequences and cause possible risks in young athletes43.

Conclusion

This study demonstrated that both HIITlinear and HIITCOD are effective methods for improving linear speed and Illinois agility test in young football players over a four-week transition period. However, neither training modality elicited significant improvements in reactive agility, suggesting that reactive agility might require additional or complementary training methods. The findings highlight that while both HIIT modalities can be integrated into training regimens to enhance specific performance attributes, their effects are limited to the targeted physical qualities and do not encompass broader agility components. Coaches and researchers are encouraged to consider the distinct advantages and limitations of these HIIT protocols when designing training programs, emphasizing the importance of tailoring training to address the multifaceted demands of football performance.