Analysis of vertical jump, rating of perceived exertion, delayed-onset muscle soreness, and muscular peak power in young male Brazilian football players submitted to plyometric and semi-squat training with weights

. This study analyzed the effects of plyometric and strength training on vertical jump (VJ), rating of perceived exertion (RPE), delayed-onset muscle soreness (DOMS), and absolute (APP) and relative (RPP) muscle peak power in young male football players. Twenty-five participants were randomly divided into semi-squat training group (SSTG), plyometric training group (PTG), and control group (CG). The duration of the intervention was six weeks. VJ was analyzed with a computerized jumping platform. DOMS and RPE with the Borg’s Visual Analogue Scale (VAS) and the Adapted Borg Scale (ABS), respectively. The SSTG showed improvements ( p < 0.05) in countermovement jump (CMJ), squat jump (SJ), APP (3190.67 ± 338.49 W), and RPP (47.75 ± 5.01 W/kg). PTG showed improvements ( p < 0.05) in SJ. In the intragroup comparations, SSTG, PTG, and CG showed an increase ( p < 0.05) in RPE and DOMS. Between groups, PTG presented an increase ( p < 0.05) on RPE and DOMS compared with SSTG and CG. CMJ presented strong correlations between APP and VJ, RPP, and VJ, and APP and RPP. SJ showed a higher positive correlation between all the physical variables. Only SSTG promoted an increase in both types of jumps, with a greater APP and RPP and a lower RPE and DOMS.

. Thus, improving power is a primary focus of many strength and conditioning programs. There are ways to improve power and speed. One recommendation is to improve both absolute and relative strength through a combination of lower-body resistance training, weightlifting, and plyometric training (Dawes & Lentz, 2012).
The implementation of well-structured methods of training may improve the strength, speed, change of directions, and skill performance of young football players (Hammami et al., 2018;Hoyo et al., 2016;Sáez de Villarreal, Suarez-Arrones, Requena, Haff & Ferrete, 2015). However, high demands of physical effort tend to promote a greater risk of muscle injuries, which are associated with sports that promote accelerations, rapid decelerations, jumps, cuts, spins, or kicks. This can produce a physical performance reduction, as observed in football. Fatigue involves a decrease in the capacity to generate maximal strength or muscular power, which is associated with continuous physical activity, and it results in a reduction in sports performance (Torreblanca-Martínez, Otero-Saborido & González-Jurado, 2017).
Football is an example of a sport in which clubs and academies gather their young players in age-related groups or teams, assuming that they are in equal conditions to participate in the same training and development programs (Peña González, Sarabia & Moya-Ramón, 2022). According to Peña González et al. (2022), there are some negative effects resulting from the football player grouping based on the age of the participants and possible implications in player conditioning and sports development.
Nevertheless, it is not clear yet, that young football players demonstrate the same favorable changes in the age groups chosen in this study, between sixteen and eighteen years old, in some physical performances and physiological responses to strength and plyometric training programs, separately. Therefore, the present study aimed to analyze the effects of plyometric and semi-squat training on VJ, rating of perceived exertion (RPE), delayed-onset muscle soreness (DOMS), and absolute and relative peak power in young male football players. We hypothesized that 6 weeks of plyometric and ST programs would improve the VJ and absolute and relative peak power of the young male football players.

Participants
This is an experimental study with two different types of intervention applied to two distinct training groups. The volunteers belonged to a second-division football club in Rio de Janeiro, Brazil, and had no experience in any type muscle power and strength are critical physical factors for successful participation. Plyometric activities are widely implemented as a training methodology for enhancing functional sports performance (Bianchi, Coratella, Dello Iacono & Beato, 2019).
Among the most important vertical jumps to assess neuromuscular capacity are the countermovement jump (CMJ), squat jump (SJ), and drop jump (DJ) (Moreno, 2020). Typical plyometric exercises include the CMJ, which is a jump that uses the eccentric and concentric phases of muscle contraction, and the DJ, which is an exercise where an athlete drops from a pre-determined height and attempts to jump immediately upon landing (Makaruk, Czaplicki, Sacewicz & Sadowski, 2014;Maloney, Richards & Fletcher, 2018).
Participation in strength training programs seems to induce large gains in strength and moderate improvements in speed and other factors related to football performance (Hammami et al., 2018;Hoyo et al., 2016;Panagoulis et al., 2020;Sander, Keiner, Wirth & Schmidtbleicher, 2013). Strength training is also usually applied to the development of the musculoskeletal system of young football players, including the squat jump (SJ), which uses only the concentric phase of muscle contraction (Panagoulis et al., 2020).
The SJ is one of the exercises most frequently used to develop the quadriceps muscle's concentric strength since it can improve both lower extremity power and specific sports physical performances (Hoyo et al., 2016). Previous investigations have explored the relationships between absolute strength and several predictors, such as relative strength, muscle resistance, and muscle power of athletic performance in male football players (Hammami et al., 2018;Hoyo et al., 2016;Styles, Matthews & Comfort, 2016).
Maximal strength, as measured by the one-repetition maximum (1RM) back squat, was highly correlated with improved sprint performance, change of direction speed, and VJ height in adolescent male football players (Hammami Before the procedures, 2 young football players were removed from the study due to muscle injuries. One of them was in the SSTG and the other was in the CG. Hence, a total of 25 football players were included in the selection of the groups. The groups were thus divided into PTG (n = 9), SSTG (n = 8), and CG (n = 8). Table I presents the characteristics of the groups.

Data collection
The subjects were evaluated in two different types of jumps: CMJ and SJ, with three consecutive jumps during each test, totaling six vertical jumps for each voluntary. The instruction given to the volunteers was as follows: "jump as high as you can" for both tests. At first, the young football players performed the CMJ, followed by the SJ, both with hands fixed on the hips. They performed 3 jumps, with an intWerval of two minutes between each attempt, with an interval of five minutes between the CMJ and SJ test (Markovic, 2007).
The initial knee flexion angle was not specified, but all the subjects performed the CMJ test between 60 e 90° of knee flexion. The athletes were instructed to execute a downward movement followed by a complete extension of the knees to determine the countermovement amplitude to avoid changes in jumping coordination. In the SJ, athletes were required to remain in a static position with a 90° knee flexion angle for 3-second before jumping, without any preparatory movement. The highest jump from the three attempts was used for data analysis. The VJ height, the flight time, as well the absolute and relative power were evaluated. Pre-and post-training measurements were made two days before and one day after the completion of the programs (Markovic, 2007).
The football players performed the tests on a contact platform with a sampling rate of 1,000 Hz (60 × 60 cm; Elite Jump, S2 Sports, São Paulo, Brazil). The validation of the Elite Jump contact was done after using a force platform as the criterion to measure VJ height (Loturco et al., 2017).

Intervention procedures
Regarding the plyometric training, the evaluators received training one week before the beginning of the intervention to minimize the occurrence of errors during of strength training programs. The following inclusion criteria were adopted: (a) be male; (b) age between 16 and 18 years old, which corresponds to the final stage of maturation, level 4, classified as the organization level, evaluated in the period between September and October of 2019; (c) have not suffered any serious muscle injury in the last 2 years, according to the club's medical department report; (d) never having participated in any systematic plyometric and muscle ST program before, according to the club´s football department report; (e) not to be competing throughout the study period. The exclusion criteria were: (a) to be considered physically unfit by a medical evaluation; (b) present any type of acute or chronic pathological condition that could be aggravated during the battery of tests or the proposed intervention; (c) using antiinflammatory or hormone replacement drugs; (d) obtain less than 75% presence in activities during the intervention period. The participants were asked to abstain from any physical conditioning program during this study. None of the subjects were taking any medications or nutritional supplements.
The G*Power 3.1 software estimated the need for a total of 27 football players, divided into 3 groups of 9 participants each. As input, were informed statistical test: ANOVA with repeated measurements and within-between interaction; effect size of 0.25; α of 0.05; power of 0.80; number of groups = 3; number of measurements = 2; correlation coefficient among repeated measures = 0.7.

Ethical procedures
All procedures were conducted according to the Declaration of Helsinki and Resolution 466/12 of the Brazilian National Health Council. This study was approved by the Research Ethics Committee of the Rio de Janeiro State University (process number: 4.626.823). Before participation in the study, the parents of the young football players were informed about the procedures of the study (including risks and benefits) and that the authors were committed to maintaining the confidentiality of the data. The parents signed an informed consent form and the young football players signed an assent form.

Randomization process
The groups were randomly distributed through a simple draw using the random function of the Excel software. The participants were divided according to blinding criteria. The first drawn was allocated in the plyometric training group (PTG), the second in the semi-squat training group (SSTG), and the third in the control group (CG), alternately and consecutively, until all participants were allocated in one of the groups. this load with the bar on the same position, extending completely the knees without any help, determined the 1RM load for each player. An interval of 3 minutes was performed between the attempts until the highest lifted load was achieved (Chiu et al., 2003).
In the program, the participants completed 1 repetition only when the 2 lower limbs were alternately directed forward during the half-squat, completing 1 roundtrip performed at low speed by the members of the SSTG. The semi-squat training program is presented in Table 3.
The volunteers of the CG performed only low-intensity technical training, such as passing, dribbling, and ball controlling, without performing any physical training involving strength or muscle power training during the study period.
To evaluate RPE and DOMS, the researchers were submitted to a training period to correctly apply the anchoring of the Borg's Visual Analogue Scale (VAS) and the Adapted Borg Scale (ABS) (Vieira et al., 2020). The participants were submitted to a program of sensitization to the responses of DOMS and RPE for one week before answering the questionnaires. DOMS and RPE were carried out 48h after the third session of each training, respectively. The VAS has a score ranging from 0 to 10, with the following grading: 0 = absence of pain; 1-2 = minimal pain; 3-5 = moderate pain; 7-8 = severe pain; 9-10 = extreme pain. The RPE has a score from 0 to 10, where: 0 = rest; 1 = extremely light; 2 = very light; 3 = too light; 4 = light; 5 = mild-moderate; 6 = moderate; 7 = moderate-intense; 8 = intense; 9 = very intense; 10 = exhaustive (Vieira et al., 2020).

Statistical Analysis
The data were presented in a descriptive way using mean and standard deviation. Normality and sphericity of data were determined using Shapiro-Wilk and Bartlett tests, respectively. Repeated measures ANOVA was applied to SSTG, PTG, and CG (pre and post-test) for intra-and intergroup comparisons, followed by the adjusted Bonferroni post hoc test to identify possible differences. The Pearson correlation test was used to the procedures. All sessions of the interventions were done in the morning after all the players have informed that followed all the guidelines in the food recall given by the researchers before the start of the training.
The football players used two benches with a height of 50 cm each. The benches were positioned at 1 meter between each other. Before the start of the intervention, PTG performed 30 minutes of a brief adaptation, with the accomplishment of 1 set of 10 repetitions of DJ. The hands were supported in the pelvic girdle region, during the jumps, which were performed from an upper plane towards the ground. The jumps were performed from a superior plane, over one of the benches directing the body towards the ground, through the initial projection of one of the limbs, to contact both feet at the same time on the ground, for the immediate execution of VJ towards the other bench ahead (DJ). The same procedure was performed from there towards the initial bench, completing one repetition (Fonseca et al., 2017).
The program lasted 6 weeks for each experimental group and was performed twice a week, where the number of repetitions was counted after the round-trip cycle (from the number of foot contacts on the ground) performed at the highest possible speed. The program was designed based on a systematic review, which involved the analysis of the 22 studies with DJ exercises (Markovic, 2007). The plyometric training program is described in Table 2.
Concerning the semi-squat training, the determination of maximum dynamic voluntary muscle strength was performed using the 1 repetition maximum (RM) in the high bar parallel back squat, according to the procedures before the intervention (Chiu et al., 2003).
In the 1 st practice session's test, the SSTG performed a general warm-up of five minutes, with the high bar parallel back squat, where the players performed 2 sets of 5 repetitions of underloaded parallel squats (around 50% of body mass). On the 2 nd practice session's test, the subjects were tested to determine the maximum load for each player in this group. The maximum load that the volunteer could support with the bar resting on the shoulders, starting from the standing position up to 90º of hips and knee flexion, and immediately after it, to rise 12 Table 2 Plyometric training program with a progressive increase in jumping load and intensity control throughout the training interval.
Week   Table 7 present the correlations on the CMJ test. It was found a significant correlation between the APP with VJ and RPP. It was found a higher significant correlation between the RPP with VJ and APP.
The correlations on the SJ test are presented in table 8. There was a significant correlation between the APP with VJ and RPP. It was found a strong significant correlation between the RPP with VJ and APP.
analyze the associations between the variables of the study. The effect size (d) was calculated to analyze the magnitude of the results. It was used for interpretation: < 0.2: weak; 0.2-0.79: moderate; ≥ 0.8: strong (Cohen, 1988). The significance level was set at p < 0.05 for all tests. Data were analyzed by the SPSS Statistics 23.

Results
The CMJ comparisons are presented in Table 4. Only SSTG presented significant increases in VJ, APP, and RPP.

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In this study, it could be observed a significant increase in RPE and DOMS in the members of the PTG when compared with the SSTG and the CG. In another study about the effects of lower body muscular fatigue on VJ, it was identified that muscle fatigue typically occurs after high-intensity exercise or prolonged bouts of physical activity (Cooper, Dabbs, Judith & Nicole, 2020). In our study, the CG showed an increase in RPE and DOMS, although the football players did not perform any plyometric or semi-squat training. This increase may have happened because, despite the members of the CG performed a lower intensity technical training, they did all the sessions during the same period of six weeks, as PTG and SSTG.
Probably, the lower result in VJ observed in the PTG in the CMJ test, compared with the SJ test, might have been influenced by fatigue due to the great increase in the result of the RPE and DOMS found in PTG, probably due to the greater landing impact force produced by the DJ, which also occurred in another study that used this same type of jump to verify the effects of aquatic (APT) and land plyometric training (LPT). The group that performed the LPT presented a higher subjective perception of DOMS compared with the APT group (Fonseca et al., 2017).
The results in this study could probably explain one of the main reasons that the plyometric training did not produce the expected effect on the CMJ, due to the performance of the PTG has been done through DJ on the land surface, where some studies showed that this method is one of the causes of exercise-induced muscle damage (EIMD) in the knee extensors, on eccentric contractions (Gribble & Hertel, 2004;Jakeman, Byrne & Eston, 2010;Macaluso, Isaacs, Di Felice & Myburgh, 2014).
Fatigue can be defined as a decline in tension capacity or force output after repeated muscle contractions (Gribble & Hertel, 2004). This may result in negative effects on an individual's overall performance. The VJ height is one of the most sensitive markers of fatigue to monitor elite athletes (Nakamura et al., 2015;Oliver, Lloyd & Whitney, 2015).
For instance, the CMJ height has shown to be progressively reduced over a 7-week training block in youth rugby players, who were considered "overloaded" by their concomitant participation in college and representative teams (Oliver, Lloyd & Whitney, 2015). In the present study, although the CMJ height, after 6 weeks, did not show a reduction in PTG, there was not a significant increase between pre and post-test.
In another study, significant correlations were found between lower-body strength and performance,

Discussion
This study compared the effects of a plyometric and a semi-squat training program on VJ, RPE, and DOMS correlated with the absolute and relative muscle peak power in young male football players. It was observed better results on VJ after the semi-squat training program compared with the plyometric training. Only in SSTG occurred a significant increase in CMJ. In another study about the changes in explosive strength performance in sub-20 Brazilian football players, with an intervention period similar to our study (i.e., after 8 weeks of training), it was possible to also observe an increase in CMJ (Hespanhol, Maria, Silva Neto, Arruda & Prates, 2006).
In the current study, the SSTG produced a similar increase in CMJ after 6 weeks of training. The higher increase in CMJ in our study compared with the results of Hespanhol et al. (2006), may have been produced by the half-squat movements, performed by the SSTG, which is like the biomechanical movement of the CMJ.
In this study, the SSTG showed an increase in the APP and the RPP, which seems to demonstrate a greater efficiency of the semi-squat training on muscle peak power. In another study about the velocity-based training of lower limbs to improve absolute and relative power outputs in the concentric phase of half-squat in football players, the training protocol increased RPP and APP, too, after 2 training sessions per week for 10 weeks (Ramírez, Núñez, Lancho, Poblador & Lancho, 2015). These results corroborate with the increase of the post-test values, founded on the RPP and the APP in SSTG in the current study, even with a shorter training time.
Some studies have reported significant decreases in VJ ability on CMJ height after inducing fatigue in the lower limbs by requesting the subjects to sequentially extend/ flex their knees in some programs of training (Rodacki, Fowler & Bennett, 2002;Smilios, 1998). In the present study, the higher number of jumps in the PTG compared to the SSTG program may have contributed to not having a significant increase in CMJ among PTG volunteers, probably due to the greater RPE observed through the ABS scale and the DOMS collected through the VAS.

Conclusions
The semi-squat training produced an increase in VJ, through the CMJ and SJ, while the plyometric training promoted a significant elevation only in SJ. The semi-squat training promoted a greater APP and RPP and lower RPE and DOMS in the young football players. In both types of jumps, positive correlations were found between APP and VJ, RPP and VJ, as between APP and RPP. The results obtained by the SSTG in CMJ and SJ were an interesting training alternative to develop a greater APP and RPP among young football players, too. Different methods of plyometric training must be tested, with a lower impact on the land to obtain higher results on VJ with lower responses of RPE and DOMS to prevent injuries among young football players. especially the relative muscle strength of football players (Andersen, Lockie & Dawes, 2018). The present study found strong correlations between absolute and relative muscle peak power with VJ in the SSTG. Other scientific studies (Fonseca et al., 2021;Turner & Stewart, 2014) also reported significant correlations between maximal strength and an increase in VJ performance in male football athletes. Based on the results of these investigations, it appears that improving absolute muscle strength might bring more benefits if it also enhances the relative muscle strength in this population.
In the present study, it was used DJ plyometric training, with a height of 50 cm, which requires the execution of a greater countermovement depth. The results showed lower values of absolute and relative power in PTG. In addition, a lower VJ after the CMJ test when compared with the SSTG that used the semi-squats movements, did not perform a greater countermovement depth, and showed greater values of absolute and relative power with significative higher VJ in CMJ and SJ.

Study limitations
New studies must be conducted with different training protocols and including biochemical responses of muscle injury markers to verify other correlations with RPE and DOMS. In our study, we collected blood samples with this intention, however, due to the coronavirus pandemic, it was not possible to access the analyses.
Furthermore, new studies should be conducted with other types of plyometric and strength training, besides the concurrent drills involving the simultaneous application of strength and power to observe the effects on vertical impulsion and fatigue and muscle pain responses in young football players.