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Original Research

Relationships Between Repeated Sprint Ability, Mechanical Parameters, and Blood Metabolites in Professional Soccer Players

Morcillo, Jose A.1; Jiménez-Reyes, Pedro2; Cuadrado-Peñafiel, Victor3; Lozano, Emilio4; Ortega-Becerra, Manuel5; Párraga, Juan1

Author Information
The Journal of Strength & Conditioning Research: June 2015 - Volume 29 - Issue 6 - p 1673-1682
doi: 10.1519/JSC.0000000000000782
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Abstract

Introduction

High-intensity performance of soccer player is considered as key factor in elite soccer (37). For this reason, different researchers have aimed to clarify the importance of the main aspects from different repeated sprints protocols (6,12,13,17,42), and the player's ability on this type of tests has shown to be related with different physical and physiological requirements.

Previous studies have examined physical performance, especially high-intensity activities in competitive soccer match play (19). Although these analysis were influenced by different variables such as game location (19) and player's competitive level (37), the ability of soccer players to repeat high-intensity actions is considered as a key factor in elite soccer (37,45).

Soccer is a complex sport requiring the repetition of many different activities such as jogging, sprinting, and jumping. Players are often required to repeatedly produce maximal or near-maximal sprints of short duration (1–7 seconds) with brief recovery periods, thus the ability to repeat sprint are deemed relevant fitness prerequisites in competitive soccer players and for soccer physical performance (37). Indeed, in elite players, Carling et al. (11) reported in match play more high-intensity actions interspersed by short recovery times in the players who showed lowest performance decrements in a repeated-sprint test, and Rampinini et al. (36) reported moderate negative correlations (0.60–0.65) between RSA and the distance covered above 19.8 km·h−1.

The majority of protocols have used short-duration sprints (<10 seconds) interspersed with recovery periods (<60 seconds) (22). However, top-class players perform a high-intensity run (>19.8 km·h−1) every 72 to ∼90 seconds (45), and when the analysis was focused on very high-intensity activities (>25 km·h−1), the elite soccer player performed from 17 to 30 sprints during an official match according to their playing positions (19).

Moreover, metabolic factors directly related with the energetic contribution in this kind of efforts play an important role. The blood lactate concentration as consequence of glycolytic energetic contribution has been related with the reduction in strength or power output, and the repeated sprint ability (RSA) in soccer players (37). Other specific metabolite recently used as indicator of neuromuscular fatigue in high-intensity exercises is the blood ammonia level (39), but its influence in RSA has not been yet studied. Together with blood metabolites, another parameter could be useful and has not been used by the moment in RSA protocols is the jumping capacity during an RSA protocol. Recent evidence suggests that the incorporation of movements involving the stretch-shortening cycle (SSC) (30) provides a more specific examination of neuromuscular fatigue (33). For instance, in RSA protocols have been verified a high relationship between countermovement jump (CMJ) and sprint ability (15,46), as well as that the CMJ is a good indirect indicator of power for the lower body (10), and, therefore it seems clear from this body of research that loss of CMJ height could be used as an indicator of neuromuscular fatigue (39).

The information provided by field tests of RSA should ensure physiological responses similar to those occurring during intense periods of play in actual matches. Mechanical variables could provide a better understanding of mechanisms underlying differences in fatigue and performance, leading to coaches in establishing performance oriented test batteries to enhance fitness assessment and training prescription (37).

The recording of the current physiological demands of world-class elite male soccer players has scientific interest and direct practical applications because it is very rare due to limited access to such subjects, given the finite nature of the population, and because it can offer a comprehensive picture of the upper limits of the acute effects during training in this particular sport. This should help to highlight the limiting factors on a specific sport such as soccer. Furthermore, the direct practical application of the study is that this knowledge could assist coaches to make evidence-based practice decisions because it provides new normative physiological data and can be used for soccer player selection and for profiling players. Finally, these data can be used by national elite and subelite soccer coaches and conditioners to highlight the demands of higher-level competitions and for the design of training sessions, which may assist players in the transition to elite competition. We hypothesized that a soccer player suffer an important decrement of physical qualities related to sprint and CMJ as a consequence of a specific RSA protocol, influencing on mechanical and metabolic responses associated to intermittent sprint specific efforts in highly trained soccer players. Considering the aforementioned above highlighting main aspects of RSA, the main purpose of this study was to investigate the changes through RSA sequences in sprint and jumping ability, and metabolic response (lactate and ammonia) in professional Spanish soccer players.

Methods

Experimental Approach to the Problem

A correlational and descriptive study was conducted to determine whether mechanical and metabolic responses are associated to a specific RSA protocol in an elite male soccer team on competitive period, and to determine the changes through RSA protocol checking if CMJ loss could use for monitoring specific training sessions and providing actual information about fatigue. This elite soccer team was tested in the competitive period, during an official concentration of the team in one of the rest-weeks of league, thus, nutritional intakes, hydration sleep, and rest conditions were properly controlled by physical coach. All tests were conducted at the same time of day, from 6:00 PM to 8:00 PM, and were completed at the end of the first competitive midseason and were carried out on a week where they had no official match play. After preliminary familiarization and pretesting, participants completed all testing. In the first session, a battery of tests were performed in the following order: (a) 12 × 30-m all-out running sprints with 30 seconds of recovery and (b) countermovement vertical jumps pre a post RSA protocol (CMJ). Sessions took place at a neuromuscular research laboratory under the direct supervision of the investigators, at the same time of day (±1 hour) for each subject and under constant environmental conditions (20° C, 60% humidity). Before the tests were completed, participants executed a standardized warm-up directed by the primary researcher along with the coach. During the execution of these tests, the players were verbally encouraged to give their maximal effort. The tests executed for the measurement of performance are explained in detail below.

In the preceding 2 weeks, 4 preliminary familiarization sessions were undertaken with the purpose of emphasizing proper execution technique in the different tests assessed. Anthropometric and medical examinations were also conducted during these sessions.

Subjects

Eighteen soccer players (mean ± SD; age: 26.8 ± 3.6 years; height: 1.80 ± 0.05 m; body mass: 78.15 ± 4.73 kg) volunteered to participate in this study. Participants played in a professional team of the First Division of Spanish National League division and trained 4–5 training sessions plus 1 official game per week. After being informed about the purpose, testing procedures and potential risks of the investigation, all participants gave their voluntary written consent to participate. No physical limitations, health problems, or musculoskeletal injuries that could affect testing or training were found after a medical examination. None of the participants was taking drugs, medications, or dietary supplements known to influence physical performance. This investigation was approved by the Research Ethics Committee of University of Jaen and was conducted in accordance with the Declaration of Helsinki.

Procedures

Measurements of Jump Ability

Jump height was calculated at the nearest 0.1 cm from flight time measured with an infrared timing system (Optojump; Microgate, Bolzano, Italy). Participants completed 3 maximal CMJs with their hands on their hips, and the average of these jumps was recorded (CMJbest). One minute of rest for complete recovery was given between jumps. After the 3 jumps, the players executed the RSA protocol. After the RSA protocol, players jump again to know the jump decrement produced by the RSA protocol considering the percent decrement of CMJ jump height (SdecCMJ) and calculated as follows: (1 − CMJmean/CMJbest) × 100.

Measurement of Repeated Sprint Ability

Twelve 30-m sprints, separated by a 30-second rest, were performed in an indoor running track. Photocell timing gates (Polifemo Radio Light; Microgate, Bolzano, Italy) were placed at 0, 20 m (T20), and 30 m (T30). A standing start with the lead-off foot placed 1 m behind the first timing gate was used. Participants were required to give an all-out maximal effort in each sprint. The mean sprint time (RSAmean1–12) expressed in seconds and the percent sprint decrement (Sdec) calculated as follows: (RSAmean/RSAbest × 100) − 100 (42) were recorded as a measure of repeated-efforts performance. These sequences have been shown to be reliable, coefficient of variation (CV) = 0.8%, 90% confidence limit (CL) (0.6–1.0) for mean shuttle-sprints time (27), and CV = 1.0%, 90% CL (0.7–1.6) for mean shuttle-sprints time with a CMJ performed between sprints (9).

Blood Lactate and Ammonia Measurement

Capillary blood samples for the determination of lactate and ammonia concentrations were obtained from the fingertip before exercise (pre-exercise), and 30 seconds after the last sprint, 3 and 5 minutes after the end of the RSA protocol. The Lactate Pro LT-1710 (Arkray, Kyoto, Japan) portable lactate analyzer was used for lactate measurements. The suitability and reproducibility of this analyzer has been previously established throughout the physiological range of 1.0–18.0 mmol·L−1 (35). Ammonia was measured using PocketChem BA PA-4130 (Menarini Diagnostics, Florence, Italy). Both devices were calibrated before each exercise session according to the manufacturer's specifications.

Fatigue Index Formulas

Different computations were made to determine the “Fatigue Index,” using the formulas proposed and used previously by Glaister et al. (23).

  • Formula 1 (F1): Fatigue = % increase between fastest and slowest sprint time.

  • Formula 2 (F2): Fatigue = % decrease of total test result.

where total sprint time = all sprint times sum, and ideal sprint time = number of sprints × fastest sprint time.

  • Formula 3 (F3): Fatigue = % increase between the 2 fastest and slowest sprint times.

  • Formula 4 (F4): Fatigue = difference between the 2 fastest and slowest sprint times.

Statistical Analyses

Standard statistical methods were used for the calculation of mean values and SD. The normal distribution of the data was checked by the Kolmogorov-Smirnov test, and data were normally distributed for variables of study. The relationships between the variables studied were determined by calculating the Pearson correlation coefficients (r). A related samples t-test was used to analyze CMJ height pre-post changes as well as to compare pre- and post-exercise lactate and ammonia levels. The effect size (ES) was calculated for CMJ and sprint changes pre-post RSA protocol, according to the procedure proposed by Cohen (10), considering the following criteria: >0.2 (small), >0.5 (medium), and >0.8 (large). The probability level of statistical significance was set at p ≤ 0.05, and the CLs of 95% were calculated for all measures. Analyses were performed using SPSS software version 15.0 (SPSS, Chicago, IL, USA).

Results

Mean values and SD of the different variables assessed are reported in Table 1. Likewise, postexercise CMJ height and sprint were significantly different (p ≤ 0.001; ES: 1.88 and 1.47 for CMJ and sprint, respectively; large ES) than pre-exercise after the RSA test (Table 1).

Table 1
Table 1:
Values in selected performance parameters related to RSA.*†

Statistically significant differences in pre-post exercise for lactate concentration and ammonia were found (p ≤ 0.01) (Table 1).

Relationships Between Mechanical and Metabolic Measures of Fatigue

A nearly perfect correlation between CMJ height loss value and the lactate concentration value after finishing the RSA test was found for all subjects (r = 0.97; p < 0.001) (Figure 1), and very high correlation was also found between CMJ height loss value and post-RSA ammonia value for all subjects (r = 0.92; p < 0.001) (Figure 2).

Figure 1
Figure 1:
Relationship between relative loss of CMJ height pre-post RSA test and blood lactate concentration for each one of the subjects. CMJ = countermovement jump.
Figure 2
Figure 2:
Relationship between relative loss of CMJ height pre-post RSA test and blood ammonia concentration for each one of the subjects. CMJ = countermovement jump.

A moderate correlation between speed loss value and the lactate concentration value after finishing the RSA test was found for all subjects (r = 0.65; p < 0.01) (Figure 6), and also a medium correlation was also found between speed loss value and post-RSA ammonia value for all subjects (r = 0.61; p < 0.01) (Figure 7).

In the same way, we found important correlations between mechanical parameters between themselves and metabolic, respectively. A high correlation between best CMJ and sprint time was found (r = −0.8; p < 0.01) (Figure 3) and also a moderate correlation after finishing the RSA test, between CMJ height loss and speed loss (r = 0.62; p < 0.01) (Figure 5). Regarding metabolic associations, a very high relationship between blood ammonia and lactate concentrations for each one of the subjects after RSA was found (r = 0.94; p < 0.001) (Tables 2 and 3, and Figures 4–7).

Figure 3
Figure 3:
Relationship between the best CMJ pre-RSA test and the best time in 30 m during the RSA test. CMJ = countermovement jump; RSA = repeated sprint ability.
Table 2
Table 2:
Fatigue index obtained with the different formulas.
Table 3
Table 3:
Relationships between fatigue index formulas and CMJbest and 30 mBest-Time.*
Figure 4
Figure 4:
Relationship between blood ammonia and lactate concentrations for each one of the subjects in RSA test. RSA = repeated sprint ability.
Figure 5
Figure 5:
Relationship between relative loss of CMJ height pre-post RSA test and speed loss for each one of the subjects. CMJ = countermovement jump; RSA = repeated sprint ability.
Figure 6
Figure 6:
Relationship between blood lactate concentration and speed loss for each one of the subjects.
Figure 7
Figure 7:
Relationship between blood ammonia concentration and speed loss for each one of the subjects.

Discussion

To the best of our knowledge, this is the first study to analyze the acute response of an RSA regarding the mechanical and metabolic effects in relation to some proposed formulas of speed loss. Although some studies have examined acute effects of a RSA, nevertheless, the number of repetitions performed were lower in the literature (27), and the mechanical and metabolic response to this specific RSA test inducing different fatigue index had not been previously analyzed. In this study, a detailed examination of a typical and specific RSA session was conducted under controlled conditions to assess whether loss of vertical jump height could be used as an objective indicator of the extent of neuromuscular fatigue induced by the aforementioned RSA session. Our results indicate that by monitoring vertical jump height during a specific conditioning soccer training, it is possible to reasonably estimate the metabolic stress and neuromuscular fatigue induced by RSA actions. A unique finding of this study is that ammonia and lactate show a linear response to loss of vertical jump height during RSA. The RSA leading to a significant level of fatigue (change pre-post—p < 0.001; Table 1 and Figures 1, 2, 6, and 7) was equivalent to a 8% of speed loss or 6% of vertical jump height decrease, what caused ammonia to significantly rise above resting values, which likely indicates an accelerated purine nucleotide degradation and loss of total adenine nucleotides from muscle, thereby suggesting that these decrements in mechanical variables such as speed or vertical jump are probably not recommended since they induce excessive fatigue and could compromise recovery for subsequent specific soccer training sessions.

In agreement with previous studies (46), we have found an important association between soccer player's strength and both sprint and jump performance (r = −0.8; p < 0.01), but a more interesting and important finding of this investigation, and in accordance with previous studies (6,12,31), was the relationship between RSA, 30mBest-Time and performance impairment. A plausible explanation might be that better sprinters use more of their available PCr stores than sprinters of lesser ability (4), and moreover, faster subjects are more sensitive to fatigue than slower subjects because faster subjects tend to have a higher percentage of fast twitch fibers (14,25). Thus, in this study, an important issue is highlighted as the need to consider the speed loss in repeated sprints in different sports modalities, such as soccer, whose performance depends largely on these type of actions and performance is widely related to high and maximum intensity actions and repeated over time (5,23,24,43).

Previous studies have examined the metabolic response and fatigue development during repeated intense exercise (2). Some studies have demonstrated a strong correlation between low levels of muscle glycogen and pH and the decrement of force and power (18). Besides, exercise induces an increase in the blood ammonia level during high-intensity exercise, when the ATP/ADP ratio is low (3). The use of ammonia is important in high-intensity exercise as indicator of muscle fatigue because of its negative effect produced during exercise that alters neuromuscular activity and may contribute to local muscle fatigue (47), and may reach the brain and cause detrimental effect on central nervous system function (3). Therefore, blood muscular lactate concentration (32) and blood ammonia level (3) can be used as indicators to determine exercise intensity.

Neuromuscular fatigue has been described in humans as any exercise-induced reduction in the maximal voluntary force or power produced by a muscle or muscle group (21) and is dependent by the type of muscle contraction, the intensity of exercise, and the duration of the exercise (20). Traditionally, neuromuscular fatigue has been examined using isolated forms of isometric, concentric, or eccentric movements (21). However, recent evidence suggests that the incorporation of movements involving the SSC (30) provides a more specific examination of neuromuscular fatigue (33). For instance, in sprint training sessions have been verified a high relationship between CMJ and sprint ability (16), as well as that the CMJ is a good indirect indicator of power for the lower body in professional soccer (10); therefore, it seems clear from this body of research that loss of CMJ height could be used as an indicator of neuromuscular fatigue (39).

In essence, all models of fatigue entail 2 components: fatigue induction, and fatigue quantification (29). In this study, fatigue was quantified using 2 different methods: (a) percent decline in speed loss over all the repetitions performed in the RSA test and (b) percent change in CMJ height pre-post RSA test. Since fatigue has been traditionally defined as a loss of force-generating capability with an eventual inability to sustain exercise at the required or expected level (20), the postexercise vertical jump height loss experienced during the RSA test can be considered as a good expression of neuromuscular fatigue. Indeed, in addition to force reduction, other aspects of neuromuscular performance that are affected by fatigue are muscle shortening velocity (decreases) and relaxation time (increases) (1). Due to fatigue, these factors will be affected, and considering the decline in vertical jump height observed when analyzing loss of CMJ height pre-post a resistance training session (39), we could consider the loss of CMJ height as equivalent to loss of muscle shortening velocity.

Several studies have used measurements of vertical jump height pre-post exercise to quantify the extent of fatigue. Gorostiaga et al. (25) examined CMJ height loss after typical sprint workouts in 400-m elite runners. They found reductions of 5–19% in CMJ height pre-post exercise, with no clear relationship to sprint distance. Furthermore, vertical jump height loss has been used in resistance training in some studies (34,38–41). And also have been used in team sports to determine the effect of competitive team match play on neuromuscular fatigue. Comparing our findings with those of these investigations is difficult since the protocols used to induce fatigue, the samples, and even the type of actions and movement velocities greatly differed between studies. Nevertheless, it seems clear from this body of research that loss of CMJ height can be used as an indicator of neuromuscular fatigue.

In this study, medium and significant correlation (r = 0.62; p < 0.01) was found between the 2 different types of mechanical measures used to assess neuromuscular fatigue (Figure 5). This relationship is an important finding for the quantification and monitoring of training load during RSA. The fact that there exists such a close relationship between loss of speed in RSA and loss of CMJ height pre-post RSA session is a novel finding, which emphasizes the validity of using percent loss of vertical jump height during an RSA-specific session as an indicator of neuromuscular fatigue.

The validity of using percent vertical jump height loss in CMJ to quantify neuromuscular fatigue during RSA is further supported by the relationships observed between mechanical measures of fatigue and metabolic stress (acute lactate and ammonia responses) (Figures 1, 2, 6, and 7), and the decrease between pre-post CMJ in RSA session (6%; p < 0.001). Lactate showed an extremely high correlations (r = 0.97; p < 0.001) with loss of CMJ height over all repetitions performed (Figure 1). The highest peak lactate values (∼13 mmol·L−1) obtained after the RSA test were close to values reported during a 100-m sprint of a national competition (12.5 mmol·L−1) (8).

A unique and interesting finding of this study is that ammonia response shows a linear relationship to loss of CMJ height (Figure 3). Peak postexercise ammonia, as lactate response, showed also an extremely high correlations (r = 0.92; p < 0.001) with loss of CMJ height over the RSA session (Figure 2). Blood ammonia increased above twice basal resting levels, and an increase in blood ammonia levels during short-term high-intensity exercise is usually interpreted as indicative of an accelerated ammonia production by muscle resulting from the deamination of AMP to IMP. The purine nucleotide cycle serves, among other functions, to maintain a high ATP/ADP ratio (26) and acts as an urgency mechanism to prevent muscle ATP from falling to critical levels under conditions of high metabolic stress. It is also possible that a frequent loss of purines will exceed the rate of purine salvage, leading to a chronic reduction of resting muscle ATP content (44). Therefore, the observed relationship in our study between the increase of blood ammonia concentration and decrease in RSA performance suggests that the decrease of the availability of ATP or PCr levels would limit the maintenance of performance in RSA, especially since an increase of blood ammonia concentration is interpreted as an indication of net degradation of adenine nucleotides in the muscle (7). According to the results of this study, the number of repetitions performed that resulted in blood ammonia significantly higher than resting levels were the equivalent of a 6% of speed loss or 8% of CMJ height loss. Therefore, it seems plausible to suggest that to speed up the recovery process between training sessions and avoid loss of purines from muscle, it is not advisable to use the aforementioned threshold in most RSA programs training settings, especially when important competitions are close.

In addition to these relationships and changes pre-post RSA, we analyzed the proposed Fatigue Index by Glaister et al. (23) with the same computations. We found similar results with the 4 formulas proposed (7.10 ± 1.69 vs.7.72 ± 3.22 years, 3.67 ± 1.55 vs. 4.43 ± 1.79 years, 6.10 ± 1.69 vs. 6.72 ± 2.81 years, and 4.12 ± 0.05 vs. 5.43 ± 2.71 years, respectively). Differences may be due to the different populations (professional soccer players vs. physical education students). For Glaister et al. (23), the most reliable and useful formulas are 2 and 4. In our case, for formula 2, we did not found any relationship between parameters related to performance in RSA and this formula, by the contrary, for formula 4, we found moderate relationships between these determinant performance parameters in RSA and this formula. Therefore, considering best and worst sprint times and speed loss is likely to be a good indicator for fatigue in this kind of workouts.

Practical Applications

This study has practical importance because it shows that (a) there is a very high correlation between loss of speed in RSA and loss of CMJ height pre-post RSA session, (b) an important metabolic stress and neuromuscular fatigue was induced by RSA actions, leading to a significant level of fatigue, and (c) any of the proposed formulas could be used to quantify the extent of physiological demands of a specific RSA protocol in professional soccer teams. These findings observed provide new physiological data for these populations and can contribute to talent selection and identification. Men's soccer coaches should apply repeated sprint sequences in strength and conditioning training programs and evaluate players accordingly, so that they may receive appropriate training stimuli to match the physiological demands of their level of competition. Thus, the assessment of mechanical (i.e., CMJ and speed losses) and metabolic (i.e., blood ammonia and lactate) responses induced by a specific RSA protocol during the competitive period is the first step preceding the design of specific conditioning programs. Collectively, these findings provide important information for prescription of training aimed at developing physiological qualities specific to the demands of competitive elite men's soccer players and physical fitness testing adapted to the specific requirements of each position.

The present data show that an RSA protocol in a training session should not be fixed beforehand as well as is an important aspect to take into account when prescribing RSA, since the CMJ height loss and metabolic stress clearly differs between subjects when performing the same number of repetitions imposed by a RSA protocol and not a given same level of performance decrease or fatigue, which would be more useful for individualizing the training. Likewise, the high correlations found between mechanical (CMJ height loss) and metabolic (lactate and ammonia) measures of fatigue support the use of CMJ height for monitoring RSA and further suggest that the training effect is determined by the magnitude of CMJ loss incurred (28). This study is expected to contribute to the field of exercise science by allowing a more rational characterization of the RSA stimulus.

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Keywords:

fatigue; blood lactate; ammonia; RSA; CMJ

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