The effect of complex contrast training with different training frequency on the physical performance of youth soccer players: a randomized study

Introduction

Stimulating muscular strength through resistance training (RT) during the biological maturation phase benefits young athletes (Malina et al., 2015; McQuilliam et al., 2020), with evidence suggesting that increased strength can improve agility, speed, and power output, crucial actions to develop athletic performance (Cormier et al., 2020; Seitz et al., 2014); resistance training effectively enhances these abilities (Hammami et al., 2017; Maio Alves et al., 2010). RT is directly associated with improvements in specific sports drills (SSD) such as change of direction, acceleration, deceleration, sprints, jumps, and kicks (Paul, Gabbett & Nassis, 2016; van den Tillaar & Marques, 2009). Building upon prior work by Maio Alves et al. (2010), who introduced a similar training program, we conducted a training method known as complex contrast training (CCT). In other words, this means simultaneous training (in the same training session).

In current sports science and strength training research, strategies that incorporate both heavy and light-to-moderate loads, known as complex training (CPX), have gained prominence. However, there has been variability in the terminology used to describe these approaches. Cormier et al. (2022) propose a new terminology, introducing the umbrella term CPX, encompassing four distinct implementations: Contrast training (CNT), a specific type of CPX involving alternating high-load and low-load (high-velocity) exercises in a circuit performed within the same training session; Ascending training (AT), a subset of CPX in which several sets of low-load, high-velocity exercises are completed before subsequent sets of high-load exercises within the same training session; Descending training (DT), a type of CPX consisting of several sets of high-load exercises (e.g., back squat) completed before the execution of subsequent sets of low-load, high-velocity exercises (e.g., vertical jump) within the same training session; and French contrast training (FCNT), a subset of CPX-CNT involving exercise sets performed in the following sequence: heavy compound exercise, SSC plyometric exercise, light-to-moderate load compound exercise with high velocity, and a plyometric exercise (often assisted). These definitions were developed to bring clarity to the diverse range of training methods in the literature. The combination of exercises using high and low loads, as well as incorporating plyometric exercises and SSD, has shown potential to improve athletes’ performance in various sports (Abade et al., 2020; Argus et al., 2012; Dodd & Alvar, 2007; Maio Alves et al., 2010).

CCT strategies have demonstrated to utilize a post-activation potentiation enhancement (PAPE) in the neuromuscular system of the muscles engaged in the task. In training, the PAPE effect is achieved by employing a conditioning activity (CA) with a relatively high load (submaximal), followed by an explosive post-activation exercise with a biomechanically similar movement pattern (Seitz & Haff, 2016). This refers to a potential enhancement of synaptic conduction and number of recruited motor units that results in an increased rate of force development (RFD) and power output required in a specific sport skill, caused by the CA (Docherty & Hodgson, 2007; Freitas et al., 2017; Lagrange et al., 2020). Considering the above, a positive correlation between RFD and sports skills suggests that PAPE might improve performance in sports in which explosive skills are required (Gołaś et al., 2017; Kilduff et al., 2007). Moreover, it is important to consider the time sensitivity of PAPE. The rest interval between the CA and subsequent explosive activities is crucial for maximizing its effect. The optimal rest interval duration for inducing the desired PAPE response is still being investigated, with studies exploring a range of intervals from 0 to 21 min (Ciocca, Tschan & Tessitore, 2021; Guo et al., 2022; Maroto-Izquierdo, Bautista & Martín Rivera, 2020).

Furthermore, CCT has been extensively investigated with regard to training variables (Cormier et al., 2020); however, training frequency remains an important variable that requires further study. Recently, Kumar et al. (2023) showed that CCT with a different weekly frequency (2 vs. 3 training sessions per week) and equalized volume load had similar effects on measures of physical fitness in active adult males. Conversely, CCT performed in a group of athletes with a weekly frequency of 3 days elicited greater improvements in agility T-tests compared to CCT with a weekly frequency of 2 days. Maio Alves et al. (2010) found that CCT led to increased performance in S5, S15 and SJ, but the number of CCT sessions per week (1 vs. 2) did not affect the level of improvement in these skills. Brito et al. (2014) reported no significant differences in sprint and agility performances after 9 weeks of twice-per-week complex training. However, Hammami et al. (2017) found significant improvements in many important components of athletic performance relative to standard in-season training (e.g., sprint and agility) of youth athletes (~16 years old) after 8 weeks of twice-per-week contrast training. The inconsistency in findings may be due to a high heterogeneity in the studies’ methods regarding the age of the populations, exercises, training frequency, and other variables of comparison.

The aim of the present study was to analyze the effects of 6 weeks of CCT programs applied in different training frequencies (sessions per week) on the performance of youth soccer players. We compared soccer players’ speed, agility, jumping ability, and free kick speed after regular training versus CCT performed 2 and 3 times per week during the preseason. Our hypothesis was that incorporating CCT into regular soccer training sessions would enhance the athletes’ physical performance. In addition, we anticipated that the degree of performance improvement would be independent of the number of CCT training sessions conducted in a weekly microcycle.

Materials and Methods

Experimental approach to the problem

This study was conducted during a 12-week pre-season training period. The soccer players were randomly assigned into two experimental groups with different volumes of CCT in addition to their regular training routines, and one control group. Within- and between-factors were assessed to compare the effects of different CCT frequency on the players’ sprint, agility, vertical jump, and free-kick speed. The study was conducted as: week-1; presentation of methods, week-2-3; familiarization with the CCT protocol, week-4; 1RM evaluation, week-5; performance tests, week-6-12; CCT program intervention, week-13; performance tests. Participants were instructed not to consume food or drinks with caffeine or engage in any vigorous activity other than those of the training program during the experiment (Fig. 1).

Complex contrast training

Groups G2 and G3 had the CCT inserted at the beginning of their training routines (after warm-up) in 2 and 3 sessions per week, respectively. A specific warm-up was also included for these subjects’ preparation for CCT, and CCT sessions were organized into five exercise stations including a general exercise, a multiform exercise, and a specific exercise as follows (Fig. 2):

Station 1–6 repetitions of squat (80% of 1RM) + 5-m high skipping + 5-m sprint.

Station 2–6 repetitions of seated calf raise (90% of 1RM) + 6 jumps + 3 jump headers.

Station 3–6 repetitions of knee extensions (80% of 1RM) + 6 jumps from a 60-cm bench (FIJU7944, São Paulo, Brazil) to floor + 3 jump headers from the bench.

Station 4–6 repetitions of hip adductions on a chair (80% of 1RM) + 6 jumping jacks + 5-m change of direction.

Station 5–6 repetitions of unilateral hip flexion (85% of 1RM) + 6 kicking simulations + 3 × 11-m free kicks.

After every 2 weeks of CCT training, 5% of the 1RM load was added.

Results

Twenty-one players completed the study (G0 = 8; G2 = 6; and G3 = 7). No differences in time point vs. group were found for body mass [F(_2,18) = 0.988; p = 0.392; η² = 0.099], height [F_(2,18) = 0.158; p = 0.855; η² = 0.017], BMI [F_(2,18) = 1.184; p = 0.329; η² = 0.116], and body fat percentage [F_(2,18) = 1.882; p = 0.181 η² = 0.173]; or between the groups’ age [F_(2,18) = 0.305; p = 0.741], body mass [F_(2,18) = 0.065; p = 0.938], weight [F_(2,18) = 0.861; p = 0.439], BMI [F_(2,18) = 0.230; p = 0.797], and body fat percentage [F_(2,18) = 1.418; p = 0.268]. Fourteen participants (67%) were at maturational stage four in Tanner’s scale (Tanner, 1986). Fisher’s test did not identify a significant difference in participants’ maturational stages (p = 0.480). Participants’ characteristics are presented in Table 1.

No group-time interaction was found for S5 [F_(2,18) = 0.058; p = 0.944; η² = 0.06], S15 [F_(2,18) = 0.732; p = 0.495; η² = 0.075], agility [F_(2,18) = 0.684; p = 0.517; η² = 0.071] and SJ [F_(2,18) = 1.544; p = 0.241; η² = 0.146]. For the free kick [F_(2,18) = 5.598; p = 0.013; η² = 0.383] and the CMJ [F_(2,18) = 4.701; p = 0.023; η² = 0.343] a significant group-time interaction was found.

No effect of time point was found for S5 [F_(1,18) = 0.005; p = 0.943; η² = 0.00], S15 [F_(1,18) = 0.056; p = 0.816; η² = 0.003], agility [F_(1,18) = 2.804; p = 0.111; η² = 0.135], free kick [F_(1,18) = 3.657; p = 0.072; η² = 0.169], and SJ [F_(1,18) = 2.022; p = 0.172; η² = 0.101], however, there was an effect of time point for CMJ [F_(1,18) = 10.303; p = 0.005; η² = 0.364]. Pre- and post-CCT analyses revealed a statistically significant improvement, only in G3, for the CMJ (p = 0.001–Fig. 3).

There was no statistically significant difference between the groups for S5 [F_(2,18) = 0.381; p = 0.689; η² = 0.041], S15 [F_(2,18) = 0.625; p = 0.547; η² = 0.065], agility [F_(2,18) = 0.315; p = 0.733; η² = 0.034], free kick [F_(2,18) = 0.097; p = 0.908; η² = 0.011], SJ [F_(2,18) = 0.455; p = 0.642; η² = 0.048], and CMJ [F_(2,18) = 0.259; p = 0.774; η² = 0.028]. Table 2 shows the difference in means (95% confidence interval) and Sidak’s post-hoc values for the mentioned variables.

Discussion

The main finding of this study is that CCT training resulted in a significant improvement in the youth soccer players’ performance, but only when three training sessions were performed (in addition to regular soccer training). Significant improvements were observed in CMJ and free kick speed. This demonstrates that improvements in players’ physical abilities following CCT require a minimum amount of training frequency (volume). For well-trained athletes, further performance improvements may necessitate a more skill-specific approach and exercises that exhibit a dose-dependent effectiveness.

The significant increase in CMJ performance observed exclusively in players who practiced CCT 3 times per week may be attributed to a chronic enhancement in the neuromotor recruitment of muscle fibers and calcium release by the sarcoplasmic reticulum (Tillin & Bishop, 2009). These findings suggest that a high stimulation frequency is indispensable for eliciting substantial improvements in physical performance. Previous studies utilizing training protocols of 2–3 sessions per week have demonstrated improvements in athletes’ CMJ results. However, these improvements were observed for periods longer than 6 weeks (Cormier et al., 2020; Kotzamanidis et al., 2005; de Villarreal et al., 2015; Tricoli et al., 2005). It seems that significant improvements in performance of youth soccer players require a frequency of at least 3 CCT sessions per week, or a longer CCT intervention period (Kotzamanidis et al., 2005; Maio Alves et al., 2010). Nevertheless, in this study we have demonstrated that a 6-week CCT protocol is effective in improving CMJ in youth male soccer players from U-15 and U-17 categories, which shows to be a feasible strategy for pre-season preparations.

Although no time point effect was found for the free kick (0.072), Sidak’s post-hoc test found a difference only for the G3 group when the different times points were compared (0.003). The sample size may not have been large enough for the primary effect to reveal the difference, but the improvement seems to have taken place. The improvement in free kick speed in the group that performed 3 CCT training sessions per week was consistent with a previous study that observed improvements in the physical fitness of young soccer players, particularly in increasing free kick efficiency (Cavaco et al., 2014). These improvements could be attributed to mechanisms such as improved motoneuron firing rate, increased neural coordination, and the positive transfer of muscle demands through the implementation of complex training (Cronin, McNair & Marshall, 2001; Robbins, 2005). In contrast with the findings of Cavaco et al. (2014), where an improvement in athletes’ free kick efficiency was found after a 6-week training program with two sessions per week, in the present study the group that included 2 CCT training sessions per week did not show such an improvement. This difference in outcomes may be attributed to variations in the protocols used, such as the specific training method employed and the number of stations utilized by Cavaco et al. (2014), which may have impacted the results differently. Before the preseason period, all athletes underwent a 2-week familiarization phase with RT, followed by 6 weeks of CCT alongside regular soccer training for the experimental groups. The control group continued with RT alongside their regular soccer training. It is worth noting that there was no sudden increase in training loads during the preseason period, which could explain the observed improvements. The training programs were designed to ensure consistency and controlled progress throughout the study.

In terms of squat jump (SJ) performance, no improvement was observed in any of the studied groups following the training intervention. When comparing our study to previous studies on CCT, Maio Alves et al. (2010) implemented an 8-week protocol comprising 2 weeks of RT adaptation followed by a 6-week CCT. They reported improvements in SJ performance that were independent of the training frequency (1 vs. 2 times per week). In another experiment, Kotzamanidis et al. (2005) observed changes in SJ after 13 weeks of training. The absence of SJ improvement in our study may be attributed to various factors, including the need for the nervous system to adapt to control and transfer additional force after resistance training, as reported by Bobbert & Van Soest (1994). The lack of SJ improvement in our study could also be due to insufficient training volume or adaptations in the muscle-tendon unit. Considering these factors, the lack of SJ improvement may be influenced by learning effects, training frequency, and rest intervals. The development of SJ performance is intricately linked to the frequency of resistance training sessions (Hoffman et al., 1990). Our study, which varied training frequency (2 vs. 3 times per week), may not have sufficient training volume to provide SJ improvements, despite previous findings suggesting the positive impact of combining resistance training with running performance on SJ (Baker, 1996; Fatouros et al., 2000). While speed training was included, SJ results did not improve, possibly due to inadequate adaptations in the muscle-tendon unit (Young, Wilson & Byrne, 1999), which are increased by resistance training (Kubo, Kanehisa & Fukunaga, 2002). Therefore, lack of SJ improvement observed in our study may be influenced by several factors, including the learning effect, training frequency, and rest intervals utilized.

The present study showed no improvement in agility levels after 6 weeks of CCT training when compared to the group that did not undergo CCT, aligning with previous findings (Maio Alves et al., 2010; Tricoli et al., 2005). Despite incorporating recommended exercises that simulate directional changes in agility tests (Maio Alves et al., 2010), the complexity of agility tasks suggests that motor control factors have a greater impact on agility performance than muscle strength or power capacity (Tricoli et al., 2005). In contrast, García-Pinillos et al. (2014) observed improvements in agility among youth soccer players after a 12-week CCT protocol, without concurrent changes in body composition. Their study emphasized the influence of various factors such as training status, age, gender, testing methods, and program characteristics on agility outcomes. However, comparing agility gains across studies is challenging due to the diversity of evaluation methods used. Considering the complexity of agility tasks and the interplay of factors like age, training status, and testing methods, designing tailored training programs that account for individual characteristics becomes crucial for optimizing agility enhancement. Further research is warranted to explore effective strategies for improving agility in diverse populations.

In our study, consistent with various previous research (Brito et al., 2014; Hammami et al., 2017; Kotzamanidis et al., 2005; Maio Alves et al., 2010), no improvements in sprint performance (S5 and S15) were shown among any of the groups. In the study by Brito et al. (2014), an improvement in the S20 sprint was observed after 9 weeks of CCT, albeit with a longer training program compared to our study. Additionally, Maio Alves et al. (2010) reported reductions in sprint performances (S5 and S15), but the participants in the study had an average age of 17.4 years, which is higher than the average age (15.3) of the participants in our study. During maturational stage five (post-PHV), athletes will experience an increase in testosterone and growth hormones, which leads to improved muscle strength (Costa et al., 2021; Till et al., 2014) and peak velocity (Fernández-Galván et al., 2022).

Cavaco et al. (2014) in a study with a similar age and protocol duration to ours, also found no changes in S15 sprint performance among youth athletes. Notably, a substantial portion of our sample (67%) consisted of youth soccer players in maturational stage four (on the Tanner scale), suggesting that neuromuscular maturation might be a contributing factor in sprint performance. Factors such as neural function, muscle stiffness, testosterone concentration, and muscle strength can influence the impact of maturational stage on performance (Costa et al., 2021; Mendez-Villanueva et al., 2011). Fernández-Galván et al. (2022) propose that lower sprinting performance in youth athletes at maturation stage four could be attributed to a decrease in relative strength due to increased body mass and height, as previously reported (Comfort et al., 2014). Another possible explanation is that young athletes in maturational stage four may experience a phase of “motor clumsiness” as their motor coordination is disrupted by torso and limb growth. These factors highlight the potential influence of maturational stage on sprint performance.

A limitation of our study is that the CCT protocol was implemented during the preseason, potentially introducing methodological conflicts. It might be advisable to prioritize CCT interventions during the preparatory season and possibly segment players by position (e.g., defenders, forwards, and midfielders). Another limitation relates to sample size; our study included a modest group of 21 players, potentially limiting the generalizability of the findings. Additionally, we did not conduct a sample size calculation to determine the necessary number of participants. Notably, we lack the 1 RM values for the players in each group. It is worth noting that PAPE is influenced by training background and strength/power levels. Therefore, future research should aim to include larger and more diverse samples to enhance the external validity of results.

Conclusions

The implementation of CCT in regular soccer training for youth athletes, in a regime of three sessions per week for 6 weeks, was effective in improving the athlete’s CMJ and free kick speed. However, this intervention did not yield significant changes in other aspects, highlighting its limitations. Notably, the G2 volume exhibited ineffectiveness in producing desired outcomes. While further research is needed to explore optimal training protocols for enhancing other abilities such as agility and sprint performance, it is important to consider factors beyond training volume, such as maturational stage, which may play a role in reducing the dose-response and frequency while increasing the effectiveness of the intervention.

Supplemental Information