Soccer is a high-intensity intermittent team sport played for >90 minutes that consists of two 45-minute halves, which are separated by a half-time recovery period. Consistent observations confirm that players cover in excess of 10 km in a match (8,25,33); consequently, soccer is a primarily aerobic sport. However, success during soccer match play is associated with high-intensity activities (20), such as sprinting and performing technical actions. Elevated blood lactate concentrations have been observed during both simulated (16,28,32) and actual (10,29) match play. However, an inevitable consequence of energy production by anaerobic glycolysis is that in addition to lactate formation, hydrogen ions are also produced; therefore, the increased rate of muscle lactate production that occurs during high-intensity periods of match play has the potential to influence exercise performance.
The mechanisms of match-related fatigue are likely to be multifaceted in origin, including compromised fiber-specific muscle glycogen levels (16), dehydration (4), and a reduction in blood glucose to concentrations that might impair cognitive functioning (4,10). However, low muscle pH has also been shown to reduce muscle contractility (11,19) and to inhibit glycolytic activity (30). Although the role of acidosis in fatigue remains a subject of debate (for a review see ), the ability to maintain acid-base balance during exercise might be an important factor in multisprint team sports such as soccer (23). Thus, information about the transient changes of acid-base balance indices during soccer-specific exercise could have application to professionals involved in the sport who are responsible for dictating future training direction and acute supplementation strategies. However, to date, relatively few studies have examined the effects of soccer-specific exercise on indices of acid-base balance in soccer players.
The depression in blood bicarbonate concentrations observed after exercise in soccer players (14,23) suggests that buffering capacity is compromised toward the end of a match. However, such information lacks temporal resolution and is only applicable to players about to enter a period of extra time. Moreover, the ecological validity of these findings to soccer match-play scenarios is compromised because previous authors have implemented exercise protocols that replicate the demands of soccer training sessions (14), use protocols that replicate other intermittent team sports (e.g., hockey ), and use exhaustive performance tests (23), all of which fail to adequately replicate the demands of soccer match play. Consequently, little evidence exists concerning the effects of 90 minutes of soccer-specific exercise on acid-base balance throughout simulated or actual match play.
The half-time break is often considered crucial in team sports for primarily tactical reasons; however, it can also be viewed as a period of recovery after the first half of exercise, a period of preparation before the second half, and a transition between the 2 halves of play. Despite the potential for a period of recovery between 2 bouts of exercise to influence physiological responses (e.g., blood glucose concentrations ), it has not been common practice for exercise simulations that aim to replicate the demands of high-intensity intermittent sports, such as soccer, to include a half-time recovery period (e.g., Loughborough Intermittent Shuttle Test [LIST (21)]). The validity and test-retest reliability of the performance, physiological and metabolic responses to a newly developed soccer match simulation (SMS) that incorporates the performance of technical actions throughout standardized soccer-specific exercise has recently been confirmed (28,29). However, the acid-base response to this protocol remains to be investigated.
In summary, transient changes in markers of acid-base balance have the potential to influence performance during soccer-specific exercise; however, the time course of these changes during soccer-specific exercise is unclear. Consequently, the effects of soccer-specific exercise on markers of acid-base balance remain to be fully elucidated. Therefore, the aim of this study was to investigate the changes in markers of acid-base balance that occur throughout the full duration of a soccer match. The null hypothesis associated with this study was that soccer-specific exercise would have no effect on indices of acid-base balance.
Experimental Approach to the Problem
To investigate the effects of soccer-specific exercise on acid-base balance, familiarized academy soccer players completed a simulation of soccer match play with serial physiological measurements taken throughout. The dependent variables included in this study were measures of exercise intensity (heart rate and rating of perceived exertion [RPE]), measures of hydration status (plasma osmolality and changes in plasma volume), and indices of acid-base balance (blood lactate concentration, blood pH, blood bicarbonate concentration, and base excess).
Sixteen soccer players (age: 18 ± 1 years, height: 1.77 ± 0.01 m, mass: 70.5 ± 2.3 kg, estimated V[Combining Dot Above]O2max: 55.8 ± 0.9 ml·kg−1·min−1) from a British Championship club, which represents the second tier of professional soccer within the United Kingdom, volunteered to participate in the study. The potential risks of the study were explained, and written informed consent was obtained from the players (and parents and guardians if the players were <18 years) before participation. The players were recruited on the basis that they had no injuries, were nondiabetic, did not smoke, and had regularly participated in training with a soccer team of at least academy standard for 12 months before the start of the study.
This study took place in the second half of the 2008–2009 competitive season while players were in the maintenance phase of their competition period. In addition to weekly matches, the players participated in approximately 6 hours of training per week (consisting of 3 predominantly tactical-based sessions that were 2 hours in duration) during this period. The players were encouraged to rest on cessation of training and competition. The participants who completed the main trial attended at least 4 preliminary visits before completing the SMS (29). Maximum oxygen uptake was estimated on the first visit using the multistage fitness test (MSFT) (24) to calculate the running speeds used in the main trial. The remaining visits were used to habitualize the participants with the exercise and skilled components of the SMS.
The participants were asked to refrain from strenuous physical activity and caffeine consumption in the 2 days preceding all the testing sessions. Additionally, the players recorded all food consumed in the 2 days before the main trial. Food records were subsequently analyzed using commercially available software (CompEat version 5.8.0; Nutrition Systems, Grantham, United Kingdom). All the players gave verbal confirmation that they had complied with these instructions upon the completion of the study.
Main Trial Procedures
The participants were paired according to the estimated aerobic capacity (within 0.5 decimal levels on MSFT), and they all arrived at the testing site at approximately 08:15 AM, after an overnight fast. Upon arrival, the participants proceeded to empty their bowels and bladder before a standardized breakfast that consisted of a 1,470-kJ meal (energy content: 62% carbohydrates, 25% fats, 13% proteins) and 500 ml of a fluid-electrolyte beverage was consumed. Body mass (model 770; Seca Ltd., Birmingham, United Kingdom) and stature (Portable Stadiometer; Holtain Ltd., Wales, United Kingdom) were then measured. The participants then rested for approximately 100 minutes before a preexercise blood sample was taken. A 20-minute standardized warm-up (consisting of running, dynamic stretching, and ball skills) was completed before the exercise and skills protocol commenced. After completing the exercise and skills protocol, blood samples were taken, and body mass (BM) was measured. A schematic of the procedures for both main trials is presented in Figure 1.
Soccer Match Simulation
The specific details of the demands of the SMS are presented elsewhere (29); however, briefly, the SMS required the participants to perform soccer shooting, passing, and dribbling skills throughout two 45 minutes halves of soccer-specific activity that was separated by a 15-minute passive half-time period (Figure 1). The exercise protocol, which is a modification of the LIST (21) that has been adapted to include additional components that further replicate the movement demands of soccer match play (15), has previously been validated against the demands of match play (29) and has been shown to demonstrate test-retest reliability (28). Movements were dictated by audio signals from CDs, and a total distance of 10.1 km was covered. Additionally, 93 on-the-ball actions were completed throughout the SMS, incorporating: 56 passes, 16 shots, and 21 dribbles.
Exercise was made up of 4.5-minute blocks that consisted of 3 repeated cycles of three 20-m walks, 1 walk to the side, an alternating timed 15-m sprint (Brower timing gates; UT, USA) or an 18-m dribble, a 4-second passive recovery period, five 20-m jogs at a speed corresponding to 40% V[Combining Dot Above]O2max, one 20-m backward jog at 40% V[Combining Dot Above]O2max, and two 20-m strides at 85% V[Combining Dot Above]O2max. A 2-minute period incorporating the performance of soccer passing (1 minute) and recovery (1 minute) followed all blocks of exercise (Figure 1). Seven blocks of intermittent activity and skills were completed during each half of the exercise. Moreover, the reproducibility of responses to the SMS, as represented by the coefficient of variance (CV) values, has previously been determined as 2.6 and 2.3% for the mean and peak HR, respectively (28).
Although the outcomes for the skill tests involved in the SMS were not determined for this study, the participants were unaware of this and were therefore required to perform the passing, shooting, and dribbling skills throughout the exercise. The shooting and passing skill tests required the participants to kick toward 1 of 4 randomly determined targets (identified by a custom lighting system); consequently, the players were required to carry out visual searching and decision making during each attempt (similar to a soccer match when looking for space or other players). The participants were instructed to aim passes at the center of the target that was illuminated. When shooting, the participants were instructed to kick the ball as accurately as possible at the illuminated target within the goal at match pace. The bouts of passing and shooting consisted of 4 attempts, and to enhance ecological validity, no prior touches were allowed to control the ball, and the participants chose to kick the ball with the foot that they felt was most suitable to successfully complete the task.
The layout of the dribbling task was similar to that employed by McGregor et al. (18) with start and finish lines placed 20 m apart. The participants were instructed to dribble the ball as fast and as accurately as possible between all cones. For the players of this standard completing the dribbling task, CV values have previously been determined as 2.6, 4.1, and 1.6% for the speed, precision, and success of dribbling, respectively (27).
Throughout the trial, a carbohydrate-free electrolyte beverage, which was flavored with a commercially available fruit cordial (carbohydrate content <0.1 g·L−1), was provided. An initial bolus of fluid (500 ml) was consumed with the standardized meal, and additional fluid was consumed during the trials at a rate of 14 ml·kg−1·h−1 BM. Equal volumes were consumed 10 minutes before commencing each half of the exercise and once every 15 minutes throughout exercise. In the month before the practice sessions began, the players ingested water at a rate of 14 ml·kg−1·h−1 BM during training sessions to promote gastric tolerance to this rate of fluid ingestion.
Blood Sampling and Analyses
Fingertip blood samples were taken before exercise (preexercise), 10 minutes into half time (half time), and every 15 minutes during exercise (first half: 15, 30, 45 minutes; second half: 60, 75, 90 minutes). Blood pH, lactate concentrations, and hematocrit (Hct) were determined immediately by means of an automated blood gas analyzer (GEM Premier 3000, Instrumentation Laboratory, Warrington, United Kingdom) from 170-μl heparinized capillary tubes at each time point. Using measurements from specific sensors that determine pH, PCO2, PO2, and blood lactate concentrations, indices of acid-base balance (i.e., standard and actual blood bicarbonate and base excess) were retrospectively calculated in accordance with the manufacturer's instructions (eqs. 1–6) and using an approach congruent with that of previous studies (23). Additionally, hemoglobin (Hb) concentrations (HemoCue AB, Angelholm, Sweden) were also measured at each time point. The CV values for pH, Hct, and Hb were 1.2, 3.2, and 2.7%, respectively.
where PO2pp is the partial pressure of oxygen in the blood at a pH of 7.4 and HCO-3, HCO-3 std, and BE are in millimoles per liter.
Changes from resting plasma volume were estimated before and after exercise using previously described methods (9). Additional blood samples at preexercise and at 90 minutes were centrifuged at 4,000g for 15 minutes, and the osmolality of 50 μl of plasma was measured by freezing point depression (Gonotec Cryoscopic Osmometer Osmomat 030; YSI Limited, Hampshire, United Kingdom: Intraassay CV = 0.2%).
Statistical analyses were carried out using SPSS software (Version 19.0; SPSS Inc., Chicago, IL, USA). All the results were reported as the mean ± SEM, and the level of statistical significance was set at p ≤ 0.05. Paired sample t-tests were used to examine data with 2 time points, whereas 1-way repeated measures analysis of variance (ANOVA; within-participant factor: time of sample) was used where data contained >2 time points. Mauchly's test was consulted, and Greenhouse–Geisser correction was applied if the assumption of sphericity was violated. Significant main effects of time (time of sample) were further investigated using a Tukey honestly significant difference post hoc test. For specific variables, the rate of change between successive time points was calculated as a percentage.
Environmental conditions were 20.3 ± 0.3° C, 761 ± 2 mm Hg, and 65 ± 2%, for ambient temperature, barometric pressure and humidity, respectively. The calculated daily diet did not contain the use of any supplements (either health-specific or performance enhancing in nature) and comprised 11.6 ± 0.9 MJ·d−1, of which 51 ± 2, 29 ± 2, 17 ± 1% of energy intake was obtained from carbohydrates, fats, and proteins, respectively.
Physiological Demand and Exercise Intensity
The mean and peak HR values throughout the SMS were 166 ± 2 and 199 ± 3 b·min−1, respectively. The RPE increased significantly over time reaching 16 ± 1 units at 90 minutes of exercise (time of sample effect: F(1,21) = 156.425, p < 0.001, partial-eta2 = 0.912), and sprint speed reduced by 2.8 ± 0.8% over the course of the protocol (time of sample effect: F(5,75) = 6.518, p < 0.001, partial-eta2 = 0.303, Figure 2). Although the mean volume of fluid ingested was 1,981 ± 47 ml, the BM declined from initial values, and the average mass losses were 2.2 ± 0.1 kg (p = 0.007).
Throughout the protocol, blood lactate concentrations were elevated above preexercise values from 15 minutes of exercise (time of sample effect: F(2,34) = 42.685, p < 0.001, partial-eta2 = 0.740, Figure 3), with the greatest rate of change occurring within the first 15 minutes of exercise (453 ± 100% increase). However, lactate concentrations in the second half of exercise were lower than in the first half (first half: 7.23 ± 0.68 mmol·L−1, second half: 5.72 ± 0.71 mmol·L−1, p < 0.001).
The pH response was influenced by exercise (time of sample effect: F(3,38) = 17.717, p < 0.001, partial-eta2 = 0.542, Figure 3) with all the values throughout the first and second halves of the SMS being depressed from preexercise values; however, half time negated this effect (half time: 7.41 ± 0.01 units, p= 0.122). Additionally, the pH was significantly higher during the second half of exercise (first half: 7.36 ± 0.01 units, second half: 7.38 ± 0.01 units, p < 0.001), and the greatest rate of change in pH occurred within the first 15 minutes (1.4 ± 0.2% decrease).
Base excess (time of sample effect: F(2,37) = 29.603, p < 0.001, partial-eta2 = 0.664), standard blood bicarbonate (time of sample effect: F(2,36) = 30.042, p < 0.001, partial-eta2 = 0.667), and actual blood bicarbonate (time of sample effect: F(2,29) = 31.829, p < 0.001, partial-eta2 = 0.680) were depressed from preexercise at all time points during exercise (Figure 3). However, at half time, this exercise-induced reduction in base excess, standard blood bicarbonate, and actual blood bicarbonate was attenuated and returned to preexercise values (p > 0.05). Additionally, base excess (first half: −1.43 ± 0.30 mmol·L−1, second half: −1.01 ± 0.29 mmol·L−1, p = 0.013), standard blood bicarbonate (first half: 22.92 ± 0.24 mmol·L−1, second half: 23.29 ± 0.23 mmol·L−1, p = 0.006), and actual blood bicarbonate (first half: 21.00 ± 0.66 mmol·L−1, second half: 22.27 ± 0.57 mmol·L−1, p < 0.001) increased in the second half of the SMS. The greatest percentage change in base excess, standard blood bicarbonate, and actual blood bicarbonate occurred in the first 15 minutes of exercise (156 ± 55% decrease, 12 ± 1% decrease, and 27 ± 3% decrease, respectively).
The estimated changes in plasma volume were not different from preexercise values at any time point (p > 0.05), and plasma osmolality was not influenced by exercise (p = 0.994), being 288 ± 8 and 288 ± 4 mosmol·kg·H2O−1 at rest and after exercise, respectively.
The primary finding of this study was that a SMS, which incorporated high-intensity activities, such as sprints and soccer skills throughout exercise, influenced markers of acid-base balance during the SMS. Specifically, blood bicarbonate (actual and standard), base excess, and pH were influenced by exercise; however, these variables were restored to preexercise values at half time. Therefore, acute interventions that improve the buffering capacity of soccer players throughout exercise might help buffer acidity and enhance performance.
Using methods similar to those used in this study, Rampinini et al. (23) reported that blood bicarbonate is depressed from preexercise values after intermittent exercise. The findings from this study support this observation (Figure 3); in addition, this is the first study to provide information concerning the transient changes in indices of acid-base balance that occur during simulated soccer match play because previous authors have compared the acid-base balance of soccer players before and after exercise (14,23). Moreover, the SMS has previously been found to replicate the physiological demands of soccer match play (29), and all the participants began exercise having refrained from strenuous activities for 2 days, were fed a preexercise meal, and commenced exercise in a hydrated state; consequently, the ecological validity of these findings are high. It is therefore highly plausible that the changes observed in this study are representative of those that occur during actual soccer match play.
It has been suggested that H+ accumulation can inhibit the enzymes involved in oxidative phosphorylation and glycolysis, reduce Ca2+ binding to troponin C, and also inhibit the sarcoplasmic reticulum enzyme Ca2+-ATPase (1,12). The transient nature of the normal physiological response to changes in H+ is regulated by a series of intracellular (i.e., protein and phosphate groups) and extracellular (i.e., proteins, Hb, and the bicarbonate pool) buffering mechanisms, which act to minimize the effects of H+ on metabolism by removing hydrogen ions when the pH declines and by releasing hydrogen ions when the pH increases (22). Although specific substances associated with intracellular buffering were not measured in this study (e.g., carnosine concentrations), the significant reductions in blood bicarbonate and base excess observed throughout the SMS indicate that soccer-specific exercise compromises total buffering capacity; specifically, extracellular buffering capacity as bicarbonate ions (HCO-3) are unable to permeate the cell membrane (17).
Figure 3 shows that blood bicarbonate concentrations and pH levels were depressed throughout the full duration of the exercise; however, the passive half-time recovery period allowed a restoration of these markers of acid-base balance to preexercise values. Data concerning the time course of recovery in markers of acid-base balance between successive exercise bouts are limited; this is somewhat surprising because most team sports include a half-time break and, therefore, follow this pattern of activity. Nevertheless, these findings contradict those of Stringer et al. (31) whereby 30 minutes of recovery did not attenuate the reductions in arterial pH induced by a 6-minute square wave bout of heavy-intensity cycling exercise. That said, the differences between the modes of activity used in the 2 studies (i.e., continuous vs. intermittent) are likely to explain the lack of agreement because the turnover rates of muscle and blood lactate during the different modes of exercise cannot be considered similar (3).
In agreement with previous data from our laboratory, the transient changes in markers of acid-base balance that occurred at half time provide additional evidence that this period of recovery can influence certain physiological responses to soccer-specific exercise (e.g., blood glucose concentrations ). Consequently, if ecological validity is desired, researchers are encouraged to implement exercise simulations that include a period of recovery between the 2 halves of exercise rather than rely on those that omit this period (e.g. ).
It has been proposed that elevated blood lactate concentrations, which are often seen in soccer, might not represent a high lactate production in a single action during the game but, rather, an accumulated, balanced response to a number of high-intensity activities (4). The SMS required 15-m sprints and technical actions (passing, shooting, and dribbling skills) to be performed throughout 90 minutes of exercise. In agreement with the findings of previous studies, sprint speed reduced throughout exercise and elevated blood lactate concentrations were observed (28). However, although blood lactate concentrations were elevated above preexercise values throughout exercise, it is important to acknowledge that the design of the SMS meant that blood samples were taken within 2 minutes of high-intensity activity and that blood lactate concentrations collected during soccer-specific exercise are largely dependent on prior activity in the immediate presampling period (5). Nevertheless, blood lactate concentrations elicited by the SMS reflected those that have been previously observed in soccer players (16).
The participants covered a distance (10.1 km) that corresponds to the distances covered by players during soccer matches (8) and the responses of HR, sprint speed, blood lactate concentration, and blood pH were reflective of match play (16). Moreover, the SMS has previously been validated against the demands of match play in the same subject population (29) and has also been shown to demonstrate test-retest reliability in performance, physiological and metabolic responses (28). Consequently, despite partial recovery at half time, results from this study indicate that markers of acid-base balance were influenced throughout the SMS, and thus the ergogenic potential of extracellular buffering agents in soccer players remains to be determined because this is the first study to provide data concerning the transient nature of the acid-base response throughout soccer-specific exercise.
In addition to the established effects of sodium bicarbonate on indices of physical performance (for a review see ), supplementation of this alkalinizing agent has recently been found to exert positive effects on muscle fiber conduction velocity and force output (13) and to attenuate reductions in skilled performances after a simulated tennis match (34). The skilled response to the SMS has previously been investigated in a similar population of players whereby the quality of the second half performance of shooting and passing skills was depressed compared with performances in the first half (28). Consequently, the transient reductions in markers of extracellular buffering capacity that occurred throughout exercise in this study could theoretically imply that changes in acid-base balance might relate to technical proficiency in soccer players. Although this statement is somewhat speculative and warrants further investigation, indices of acid-base balance have previously been found to discriminate between playing standard (23).
This study provides new information concerning the acid-base balance responses of familiarized professional soccer players to simulated soccer match play. In the players who were in the maintenance phase of their competitive season, blood pH, actual bicarbonate, standard bicarbonate, and base excess were compromised by exercise that simulated the demands of 90 minutes of match play. Passive recovery during the half-time interval also influenced all these indices. Moreover, the greatest rates of change in the acid-base balance occurred in the initial 15 minutes of the simulation; therefore, interventions that alleviate such reductions throughout the full duration of exercise might prove beneficial to soccer players.
This study was partly supported by a grant from The Sugar Bureau, United Kingdom.
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