Over the last hundred years, soccer has become 1 of the most popular sports in the history of humanity. Coupling dynamic tactical combinations with players' physical fitness makes the outcome of the bouts of the whole game often unpredictable.
Currently, professional soccer competitors are defined by a high level of motor abilities allowing them to overcome the interleaving of high-level and low-level intensity physical efforts (23). The varying intensity of a soccer game requires the use of the appropriate energy sources: aerobic and anaerobic. During the maximal intensity loads under anaerobic conditions, adenosine triphosphate (ATP) synthesis is accompanied by phosphocreatine (PCr) (21) whose shuttle mechanism is essential in the normal functioning of muscles during high activity periods (29). The analysis of short time high-intensity bouts revealed that incomplete resynthesis of PCr leads to increase of blood lactate (LA) concentration to over 12 nmol·L−1 (5). The concomitant of this phenomenon is increase in intracellular and extracellular [H+] resulting in a decrease of the blood pH (12,14).
It is obvious that interplay of physiological parameters reflects homeostasis, which resultantly influences the fitness level of a competitor (4,15,19). It also became clear that monitoring of specific physiological parameters can be useful for the assessment of different types of myopathies, which may lead to injuries (9).
Although the assessment of physiological response in elevating exhaustion among young soccer players should be, in our opinion, of extreme importance to the soccer coaches, this subject does not attract a lot of attention from the scientific community. It is rather strange because adult players are molded during the early stages of their professional career.
It has already been shown that individualized training (IT) has a positive influence on a player's capabilities (24). Thus, to angle and monitor a player's abilities, we recorded motor type, specific physiological parameters, and match season preparation phases, as a function of the IT. Because pH (6,27,28), lactate (10,31), lactate dehydrogenase (LDH) (25), and creatine kinase (CK) (7-9) are widely used for assessment of physiological performance, the study presented is performed under the null hypothesis that IT should differentiate young players' motor types with regard to these parameters studied.
Experimental Approach to the Problem
In this work, we analyzed the effect of IT on changes in the specific physiological parameters of young soccer players in response to the IT and the macrocycle's preparation phase. We analyzed changes in the pH, the lactate concentration, the LDH, and the CK activities for a specific motor type for 2 physical conditions, that is, before and after speed effort during the macrocycle's preparation phase. The differences among specific physiological parameters as a function of the preparation phase for a defined motor type are referred to throughout the text as longitudinal analysis. The differences in the studied physiological parameters among the defined motor types for a defined preparation phase are referred to throughout the text as orthogonal analysis. The longitudinal analysis was performed by means of a 1-way generalized linear model (GLM) for repeated measurements and the orthogonal analysis by means of the GLM for independent data. To ensure that observed differences are because of the applied IT, all the studied subjects practiced soccer for 4 years and were guided by the same coach.
Nineteen subjunior soccer players with the following phenotypic properties, presented as mean ± SD, participated in the study: age 13.5 ± 0.4 years, height 159.69 ± 8.5 cm, and weight 48.06 ± 8.42 kg. All participants underwent soccer training for 4 years, and all games were played in the Fourth League. During the matches, the competitors played “total football” (22). The experiment was conducted during a 6-month macrocycle (January-June) comprising 3 8-week phases: phase 1—preparation period, phase 2—match season, and phase 3—recuperation period. All measurements were performed at the beginning of the respective phases. The participants and their parents were informed of the purpose, the procedures, the benefits, and the time involvement expected with this study. Informed written consent was obtained from the parent or the guardian of a child before testing, and the University Ethics Committee approved the study protocols.
Determination of Motor Type
The motor type was determined indirectly by means of a speed run test. The differentiation between the speed and the endurance components was obtained by means of 2 sprints at the progressing distances. The better of 2 timings for runs from standing starts over distances of 20, 30, and 40 m with recuperation times between runs equal to 2, 3, and 4 minutes was used to calculate the arithmetic mean (3,11). The value obtained was used as a threshold defining the specific motor type. Players performing above the threshold were classified as the speed type and those below and equal to the threshold as the endurance type. All measurements were performed with 1-millisecond accuracy by means of photocells.
To simulate the number of sprint bouts performed by a player under game conditions, each training unit included 3 series of 6 repetitions of speed and coordination elements. Different distances and active recuperation times for different motor types were also administered. Thus, the speed-type footballers ran distances over 9-17 m and the endurance-type 4-9 m; recuperation time was equal to 40-60 and 25-50 seconds, respectively (3).
All measurements were performed before speed work, ante intentio (ai), and after speed work, post intentio (pi). Based on previously presented research, we defined speed work as a run over 30 m from a standing start with the maximum speed (3).
Blood Serum Analysis
Blood samples for the analysis of pH, lactate concentration (LA), LDH activity, and CK activity were collected 3 minutes after the speed-work test. The procedure of blood collection followed the guidelines specified by the World Health Organization, MONICA manual (30) Section 2, Chapters 2 and 3. Blood pH was analyzed using the glass pH electrode. The lactate concentration (mmol·L−1) was measured using the lactate assay kit based on colorimetric measurement at λ = 570 nm. The LDH activity (U·L−1) was assessed using the LDH kit based on colorimetric kinetic determination of the LDH activity. The creatine kinase (U·L−1) activity was measured using the creatine kinase assay kit based on colorimetric determination of the creatine kinase activity at 340 nm.
Strict rules were applied for the assessment of the normality of distributions of the studied parameters. Each sample was tested by means of the Shapiro-Wilk (26), Kolmogorov-Smirnov (17), Cramer-von Misses (1), and Anderson-Darling (2) tests. Because of the small sample size, the normality of all samples was also confirmed by means of a quantile-quantile plot. The analysis of the differences among the means of the studied parameters (pH, LA, LDH, and CK) as a function of the macrocycle's phase for a specific motor type, referred to throughout the text as longitudinal analysis, was performed by means of 1-way GLM for repeated measurements and followed by post hoc Tukey's studentized range (honestly significant difference [HSD]) method. The differences in the studied physiological parameters among the defined motor types for the defined preparation phase, referred to throughout the text as orthogonal analysis, was performed by means of a 1-way GLM for independent measurements followed by post hoc Tukey's studentized range (HSD) method. Differences between ai and pi measurements for the specific motor type and the preparation phase were analyzed by means of a t-test for matched pairs. For all the analyses, a significance level α equal to 0.05 was used.
The mean values of the parameters studied, the pH (−log[H+]), the lactate concentration (mmol·L−1) and the LDH (U·L−1), and the creatine kinase (U·L−1) activities are presented in Table 1.
The longitudinal analysis of pH changes reveals the lack of statistically significant differences. The observation holds for the both pHai and pHpi alike (Figures 1A, B). The orthogonal analysis also yields the absence of statistically significant differences. The change in the pH pattern as a function of the preparation phase is clear for the control group (Figure 1C). The analysis of differences between pHai and pHpi reveals the presence of statistically significant differences between the means for each motor type at each preparation phase (Figure 1C).
Longitudinal analysis of LAai for speed-type players reveals the presence of statistically significant differences between phases 2 and 3 (Figure 2A). The LApi analysis results indicate the absence of statistically significant differences (Figure 2B). The endurance-type player is described by the absence of statistically significant differences for both the LAai and LApi. The analysis of the control group presents a different picture; phase 1 is significantly different from phases 2 and 3, and this observation holds for the LAai and the LApi (Figures 2A, B). Orthogonal analysis reveals the lack of the statistical differences for phase 1. However, LAai in phase 2 for speed- and endurance-types differs significantly from the control group; further, the LApi for speed-type players differs from that of the endurance-type players. Phase 3 is a mirror of phase 1. The analysis of the differences between LAai and LApi for the specific motor type as a function of the preparation phase yields statistically significant differences for each phase (Figure 2C).
Longitudinal analysis of the LDH activity returns the lack of statistically significant differences for the studied motor types. The observation refers to both the LDHai and the LDHpi (Figures 3A, B). Furthermore, orthogonal analysis yields the absence of statistically significant differences. The analysis of the differences between LDHai and LDHpi, for the specific motor type, indicates physiologically but not statistically significant differences for the speed and the endurance type. The control group, however, is defined by the presence of statistically significant differences among for all phases of the macrocycle (Figure 3C).
Creatine Kinase Analysis
Longitudinal analysis of the CK activity reveals the absence of significant differences for all the studied motor types. The observation holds for both the CKai and the CKpi (Figures 4A, B). Orthogonal analysis of the CKai and the CKpi reveals the lack of statistically significant differences for phases 1 and 2. Phase 3, in turn, is characterized by significant differences between the speed-type players and the control group for CKpi but not for CKai (Figures 4A, B). The analysis of the differences between the CKai and the CKpi reveals the presence of statistically significant differences for the speed-type player phases 1 and 2, the endurance-type player phase 3, and the control group phase 2 (Figure 4C).
The improvement of the speed, endurance, and agility of a player while preserving homeostasis is 1 of the main goals when preparing for a contemporary soccer game. Our study revealed the influence of IT on changes in the level of a set of physiological parameters among young soccer players. In the previous studies, we have shown that under match conditions, IT allowed the speed-type players to complete a greater number of sprints than the endurance-type players and that the average distance covered by the speed-type players during thirteen consecutive matches was less than that of the endurance-type players (3). Our results interleaved with those presented by Kaplan et al. (16), abetting the appropriateness of the motor type classification. They also confirm the previous findings (6), indicating a significant decrease in pH after short anaerobic effort bouts. Data analysis reveals that phase and effort-dependent pH changes have similar patterns for the speed-type and endurance-type players. A drop in the serum pH of ∼0.3 pH units corresponding to 0.7 nmol H+ L−1 was also observed. Because only extracellular pH changes were measured, we assume a significantly greater intracellular pH drop in consequence, leading to reduction in adenosine monophosphate (AMP) deamination and thus acidification of myocytes (18). Because the observed pH decrease is most profound among the speed-type players (Figure 1C, phase 2), we surmise that they are well prepared for the short anaerobic effort bouts. The analysis of the lactate metabolism reveals that lactate concentration in phase 2 for speed- and endurance-type players is higher than that observed for the control group. Because lactate biosynthesis is essential for cystolic nicotinamide adenine dinucleotide (NAD+) production supporting ATP regeneration from glycolysis (25), this observation indicates the higher fitness level of speed-type and endurance-type players than that observed for the control group. Thus, the above-adduced analysis results clearly support the practical relevance of IT.
The examination of the LDH metabolism dynamics shows significant changes after speed effort for the speed-type and the endurance-type players. These changes allow continuing glycolytic ATP regeneration and buffering against cellular proton accumulation leading to retardation of metabolic acidosis and assist in proton removal from the cell (25). In consequence, this phenomenon prevents acidification of myocytes. Ergo, the analysis of the LDH metabolism also argues for IT.
We observe that the distribution of the statistical differences for the pH, the LA, and the LDH are rather obscure (Figures 1AB, 2AB, 3AB, and 4AB). However, in daily practice, the trend analysis is much more appropriate (Figures 1C, 2C, 3C), and this reveals the clear differences between the studied motor types as a function of a preparation phase.
It has been shown that athletes opposite to untrained subjects are characterized by a higher level of the CK (19) and that aerobic workout induces CK activity (20). In our study, we show a statistically significant increase in the activity of the serum CK after short anaerobic bouts with the following motor type dependent pattern: speed type > endurance type > control group. We also observe that the speed-type players are defined by continuous increase in the CK activity across all the phases, that is, preparation period, match season, and recuperation period. This is the sign of mounting physical fatigue and overtraining caused by an inadequate recuperation period, which in consequence may lead to a silent myopathy (13). This observation, although not directly coupled with the usability of IT, is extremely important from the practical point of view. It clearly shows that to prevent injuries, coaches ought to monitor the CK activity on a regular basis.
Summarizing, we may state that the dynamics of the pH, the LA, and the LDH changes support the appropriateness of the IT application on the proposed motor types. It is also clear that these parameters can be used to monitor the level of physical fitness acquired and attained by a competitor. The CK analysis may also serve as the means to injury prevention.
In this report, we analyzed the usefulness of individualized speed exercises combined with coordination elements to shape the physiological performance of specific motor type players. We show that IT differentiates the specific motor types. This observation should enable a coach to assign the player to a specific tactical position on a field. We postulate that to attain the highest competitive level, IT should be combined with the analysis of the blood serum pH, the lactate concentration, and the LDH activity. Although this study pertains to young competitors, we are convinced that this approach may also be applied to adult competitors of both the sexes.
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