Participants

At the beginning, the study group comprised 28 semi-professional soccer players. During the tests, 4 players who were unable to complete the tests or who experienced medically certified conditions were excluded, which constituted 14% of the group. Finally, 24 players were qualified for the analysis. The inclusion criteria were complete test results of all players without cognitive alterations, recent surgeries or injuries. Furthermore, only outfield players were considered. All players were instructed not to consume alcohol or take any drugs for at least 24 hours before the tests, and to maintain normal diets. Tested players were aged 21.31 ± 3.63 years from a Polish 4th League club. According to The Participant Classification Framework a participants were classified as a Tier 2: Trained / Developmental [24]. The players’ mean body height was 179.77 ± 6.16 cm and mean body mass was 76.02 ± 5.64 kg. All players had 9.0 ± 1.3 years of training experience, and trained five times a week. The study was carried out in November 2018 during the 2017/2018 league season.

All participants were provided with a detailed explanation of the study purpose and requirements. They were informed of potential risks and gave their written consent to participate. Additionally, parental or guardian consent was obtained from participants under 18 years of age. All players were free to withdraw from the study at any time. The study protocol had been approved by the Research Ethics Committee Wroclaw University of Health and Sport Sciences (no. 937/17) and authors obtained informed written consent from the participants. The study complied with the Declaration of Helsinki requirements as well as all relevant health and safety procedures.

Procedures

The study protocol comprised 10 sets of 6-second repeated efforts at maximum intensity, with 90-second rest intervals between the sets [25–28]. The tests were performed on a MONARK LT2 cycloergometer (Vansbro, Sweden). The participants were to attain maximum anaerobic power on the cycloergometer in each repetition. Toe clips and heel straps at the pedals were used for foot fixation. Strong verbal motivation was provided during each exercise. All sprints were performed from the same starting pedal position with the right crank arm positioned 45° forward to the vertical axis. The participants remained on the cycloergometer for the 90-second rest after each even sprint, during which capillary blood was drawn, or the players performed a psychomotor test (in selected repetitions, respectively). Next, the power-related, physiological, psychomotor, choice reaction time, blood-related and electrolyte-related variables were compared between the hermoneutral and conditions predicted for the 2022 World Cup in Qatar. The values at each subsequent repetition were also pair compared with those at the 1st repetition.

The tests were performed in a Weiss Technik WK-26 climatic test chamber in two different settings: 1) in thermo-neutral conditions (NC—ambient temperature of 20.5°C, relative humidity of 58.7%) [29, 30]; and 2) in climatic conditions corresponding to the average maximum ambient temperature (QSC—28.5 ± 1.92°C, relative humidity of 58.7 ± 8.64%). To predict the 2022 World Cup climatic conditions, the authors together with climatologists calculated climate data over the past 10 years (from 2008 to 2018 https://en.tutiempo.net/) on the first day of the tournament in Doha, Qatar.

Before the start of each exercise series, after entering the chamber each player spent the first 10 minutes adapting at rest to the chamber conditions [31], followed by a 5-minute warm-up on the cycloergometer reaching a heart rate of 150 bpm [32, 33]. Five minutes after warm-up completion, the actual measurements began. The tests took place from 9 am to 2 pm from Monday to Friday. The test series were separated by 7 days.

Measured variables

Power related values were recorded with an ergometer connected to an IBM compatible computer system to enable data collection to calculate the power generated at each revolution of the hand wheel and the work performed during each individual sprint repetition (Lab-VIEW, National Instruments Corp., Austin, Texas). Previously, this procedure was used in many other studies [7, 27, 33, 34], in which the individual stages of data registration and data collection were discussed in detail. The following power-related variables were recorded: peak power (W), mean power (W), time to peak power (s), total work (J), fatigue slope (W/sec), rate of fatigue (%).

Physiological respiratory parameters were recorded using a breath-by-breath measurement system (Cosmed Quark CPET, Cosmed Srl, Italy). The flow meter and gas analyzers were calibrated before each test, according to the manufacturer’s instructions (Nieman et al., 2013) [35]. The standard mask fitting procedure was followed before, during, and after the test [36]. The studied parameters included V’E (l/min), V’O2/kg (ml/min/kg), RER, and HR (bests/min). RPE (au) and choice reaction time (ms) were also recorded. RPE was recorded using a Borg scale (0–10) [37, 38]. Psychomotor performance was assessed on the basis of CRT measurements using the APR reaction measuring instrument (UNI-PAR, Warsaw, Poland). The procedure of collecting psychomotor test data corresponded to the procedures in Chmura et al. [39].

Blood- and electrolyte-related variables were collected by qualified personnel in a professional laboratory. All safety precautions were strictly followed during blood collection. Data was collected according to the procedure described by [40]. The analysed parameters included PCO2 (mmHg), PO2 (mmHg), PH, LAC (mg/dl), GLU (mg/dl), HCT (%), NA+ (mmol/l), K+ (mmol/l), CL- (mmol/l), and CA2+ (mmol/l).

Statistical analysis

The data were analysed as a time series of repeated measurements using a spreadsheet developed by Hopkins [41]. Firstly, the mean change values of each of the 22 parameters in each repetition, as well as in rest and post-test status from NC to QSC were evaluated. Secondly, the mean change values of power-related, physiological and psychomotor variables (Peak Power, Mean Power, Time to Peak Power, Fatigue slope, Rate of Fatigue, Total Work, V’E, V’O2/kg, RER, HR, RPE) from the 1st repetition to the 2nd to 10th repetition were assessed. Moreover, the mean change values of the Choice Reaction Time from the rest status to the 3rd, 5th, 7th, 10th repetitions were evaluated. Furthermore, the mean change values of the blood-related and electrolytes-related variables (PCO2, PO2, PH, LAC, GLU, HCT, NA+, K+, CL-, CA2+) from the rest status to the 2nd, 4th, 6th, 8th, 10th repetition and post-test status were assessed.

Changes in the means of raw values as well as their 90% compatibility intervals were assessed in standardised units. For the within-subject change from NC to QSC, the mean change values were divided by the standard deviation (SD) in NC, while for the within-subject change from the 1st repetition or rest status to the 2nd to 10th repetitions or post-test status, the mean change values were divided by the SD in the 1st repetition or in the rest-status [42, 43]. The magnitudes of the mean changes were then evaluated qualitatively using the following scale: < 0.2—trivial, 0.2–0.6—small, 0.6–1.2—moderate, 1.2–2.0—large, 2.0–4.0—very large, > 4.0—extremely large. Decisions about magnitudes accounting for the uncertainty were based on hypothesis tests for significant and trivial effects [44]. Hypotheses of significant decrease and increase in the means were rejected if their respective p values (p–and p+) were less than 0.05. If only one hypothesis was rejected, the p value for the other hypothesis corresponded to the posterior probability of magnitude of the true effect in a reference-Bayesian analysis with a minimally informative prior [45] and was interpreted using the following scale: > 0.25—possibly; > 0.75—likely; > 0.95—very likely; > 0.995—most likely [46]. If neither hypothesis was rejected, the effect is described as unclear. Only very likely and most likely effects were discussed in the study.

Comparison between the NC vs QSC conditions

Exercising in warm to hot (25–45°C) environments poses significant challenges to the human regulatory system [47, 48] inhibiting the performance of repeated maximal efforts or sprints in team games or endurance sports [49]. The detrimental effects of environmental temperatures above 30°C, compared with cool conditions, on repeated maximal efforts have been documented in tests using cycle ergometry [50], treadmill [51], and field runs [49, 52]. Considering power-related variables, the studied soccer players performing the protocol in QSC achieved higher RME values in each repetition. A higher ambient temperature also influenced shorter / faster generating power. This is consistent with the results of the most recent study by Chodor et al. [7] which demonstrated similar dependencies. The comparison with other RME protocols is difficult because in many cases the length of the rest break was different, more often shorter than longer. For example, Girard et al. [31] found that the average maximal power (Pmax) was 3.1% higher in HOT (35°C) than in NC (24°C). Moreover, Yamaguchi et al. [53] reported Pmax in HOT (35°C) 3% higher than in NC (20°C). Frikha et al. [54] noted that the higher mean power-output values in HOT (30°C) conditions may be related to the higher increase in core temperature [55]. This increase was shown to be a major factor responsible for improving nerve conduction velocity, enzymatic activities, oxygen delivery to muscles as well as for decreasing muscular viscous resistance [56]. Similarly, an increase in muscle temperature was shown to be responsible for a ~ 4% improvement of muscular leg power for each elevated 1°C [54].

Cardiopulmonary fitness is also often cited as a significant characteristic of success in soccer due to the large distances and continual dynamic movement patterns players are required to perform during a game [57]. Maximal or very high-intensity exercise in the heat causes a relevant impairment in oxygen delivery to the exercising muscles related to cardiac and muscle blood flow decreases [47, 54]. Drust et al. [29] reported lower accumulated oxygen consumption during repeated sprints in HOT (40°C) than in NC (20°C) conditions. Decreased maximal oxygen consumption was previously observed in hyperthermia [58], which was associated with the lowering of cardiac output and mean arterial pressure and their associated effects on skeletal muscle blood flow and oxygen uptake and delivery [47]. Little attention has been paid so far to ventilatory capacity. Studies into ventilatory response to exercise in soccer players were conducted by di Paco et al. [59, 60] but in less specific conditions, e.g. progressive tests, not using the RME protocols. The present study, however, did not reveal many significant changes between the conditions in this group of variables. Périard et al. [61] who studied 6-second repeated maximal efforts in NC (20°C) and HOT (40°C) conditions reported no significant differences in RPE. The completion of the intermittent protocol in HOT conditions was associated with changes in glycogen utilization, blood glucose, noradrenaline, heart rate and RPE, compared with exercise in NC. Some of these physiological changes could potentially lead to alterations in participants’ physiological state prior to the repeated sprints that may impair their ability to perform high intensity exercise [29]. Studies on psychomotor performance, including choice reaction time in heat stress, revealed a deterioration of performance on a central executive task (random movement generation) but not on verbal and spatial recall, and choice reaction time tasks [62]. Central executive tasks are inhibited by heat stress and moving away from the source of the heat and rehydration have some effect on cognition returning to normal [62]. Also, not many changes to these variables were observed in the present study protocol. The only observation worth emphasizing was the substantially shorter CRT in the middle of the protocol, after the fifth repetition in QSC conditions as compared to NC.

The analysis of blood-related variables in the present study revealed no significant differences between the climatic conditions, i.e. the levels of blood-related were very similar in both sets of conditions. Similarly, Yamaguchi et al. [53] found no significant differences in the RSA protocol between NC (20°C) and HOT (35°C) in blood acid-base balance parameters such as LAC, PH, HCO3 and PO2. Furthermore, Mohr et al. [9] when comparing NC (21°C) and HOT (43°C) match conditions noted no significant blood lactate concentration differences in studied players. In the present study these parameters did not differ significantly between QSC and NC either. In fact, Yamaguchi et al. [53] revealed a 15°C difference between HOT and NC, while Mohr et al. [9]–a 22°C difference; however, no difference between climatic conditions was found. It can be concluded that the temperature difference does not modify blood-related variables for short (6-second) repeated maximal efforts. In support of this thesis Drust et al. [29] stated that the concentrations of metabolic parameters were not significantly different between the experimental trials at the end of the intermittent protocol. This would indicate that the changes in performance were not associated with the accumulation of these peripheral fatigue products. Furthermore, Yamaguchi et al. [53] noted that there is still insufficient research on the responses of electrolyte levels between different simulated conditions during repeated maximal efforts. The present study shows that more changes were recorded in the level of electrolytes (in K+ and CL–) comparing QSC and NC. This indicates that players should be alert to regular fluid replenishment, isotonic drinking, and that training staff should control players’ dehydration status. More studies should also be designed to monitor changes in electrolyte concentration in players performing intermittent efforts in team games in different climatic conditions, especially in high temperatures.

Analysis of trial time between the first and subsequent repetitions

Heat stress is commonly associated with a shorter time to exhaustion [63, 64] or longer trial completion time [61, 65]. There have been, however, few studies regarding the effects of hot ambient conditions on the performance of repeated maximal exercise. In their analysis of even repetitions of a 10-trial protocol Chodor et al. [7] observed that players’ peak power increased in QSC and decreased in NC conditions; however, the changes were non-significant, i.e. the players maintained their power values in the protocol with a 90-second break. In turn, Girard et al. [31] noted a decrease in Pmax from 1 to 10 repetitions, in 6—second repeated sprints, in both NC and HOT conditions, with a similar rate of power decrease. They also registered a higher Fatigue Index in NC (19.7%) than in HOT conditions (16.5%). The present study revealed more significant changes in NC between 1 and 10 repetitions, most likely because the players’ muscle temperature was not high enough to generate power in the 1^st repetition. On the other hand, Drust et al. [29] found greater power decreases in HOT than in NC. Aldous et al. [51] claim that performance decrements were accompanied by earlier and greater increases in core temperature under heat stress, exceeding 39.5°C. Similarly, a strong correlation was found between the rate of core temperature rise and heat-induced reductions in performance (r = 0.90) [52]. Girard et al. [49] also indicated that “normo-thermic” individuals may not necessarily demonstrate lower performance levels in hotter climate conditions. Therefore, despite a higher cardiovascular and perceptual strain in the environmental temperature of 40°C vs 24°C in the absence of hyperthermia the performance levels did not differ between the two environments [66]. In Yaicharoen et al. [67] the final values for core temperature may not have been high enough to unduly affect performance in 36°C as compared with 23°C. Furthermore, enhanced short (< 30 s) power output or single-sprint performance resulting from transient heat exposure (rise in muscle temperature) can be attributed to improved muscle contractility. There is also compelling evidence suggesting that poorer intermittent-sprint performance in hotter conditions occurs only when exercise induces marked hyperthermia (core temperature above 39°C) [49]. This was not found in the present study since the applied protocol was shorter than 30 minutes.

Respiratory parameters and ventilatory response can play a key role in qualitative and quantitative evaluation of professional soccer players’ performance [59]. During the performance of all-out 6-second sprints, oxygen consumption increases rapidly at the onset of sprinting [68] and rises in subsequent sprints [69] to the levels that may exceed 70% VO₂max [70]. In the present study the players obtained similar levels of this variable in both sets of climatic conditions. During intense intermittent exercise the oxygen consumed between sprints has been associated with enhanced PCr restoration [71], which should result in superior power maintenance on subsequent sprints. Also the higher VO₂ during the sprints should increase the amount of energy available for muscle contraction by supplementing anaerobic energy, affecting the overall sprinting performance. In support of this Hamilton et al. [70] found a relationship between the aerobic response during repeated sprint-recovery intervals and fatigue in peak power (r = -.60) [72]. Therefore, a higher maximal aerobic power is associated with enhanced aerobic contribution and superior power maintenance during repeated supramaximal cycle sprints in female recreational soccer players [72]. However, Aziz et al. [73] as well as others [74] suggest a weak association between aerobic fitness (VO₂max) and running RSA performance in team-sports players. Then, Kerhervé et al. [20] observed that a significant increase in RPE levelled off after the fifth bout, and that maximum RPE was reached at the 10th bout of exercise. In our research players reported substantially higher RPE in both conditions after 3 repetitions, and also the highest value of this variable was recorded in the final 10th repetition. A similar body response in soccer players was observed following HR measurements in both climatic conditions. As far as choice reaction time is concerned, it is important to emphasize that in sport successful performances strongly depend on the ability to simultaneously meet cognitive and physical demands. It has been established that acute moderate exercise enhances cognitive functions [75–77]. Davranche and McMorris [78] also showed that most of this improvement is due to better discharge synchronization of the motor units, and, to an extent, it is due to greater efficiency of the peripheral sensorial processes. Chmura et al. [79] showed an inverted-U effect during the performance of a choice reaction time test following the epinephrine (E) and norepinephrine (NE) thresholds being faster than at rest and during maximal intensity exercise. However, McMorris et al. [80] demonstrated a significant improvement at the E threshold and during exercise at maximum power output (:W max). Whereas, McMorris et al. [81] demonstrated no significant effect of exercise. The research results in this regard remain contradictory. Our study confirms the two-phase course of changes in QCS conditions. After exceeding the anaerobic threshold the fastest response occurs, however, a further increase in fatigue reduces psychomotor performance. This is one of the most interesting observations in the entire experiment, which requires further research. Recent studies have examined the effect that undertaking a cognitively fatiguing task for ≤ 90 min has on subsequent physical performance. Cognitive fatigue is claimed to affect subsequent physical performance by inducing energy depletion in the brain, depletion of brain catecholamine neurotransmitters, or changes in motivation [82].

One of the most significant training stimuli for maximizing the effects of repeated maximal exercise is also metabolite (lactate and hydrogen ion) accumulation in working muscles [83]. A single bout of repeated maximal efforts can decidedly increase muscle lactate content and decrease muscle glycogen content [84]. With regard to the blood-related variables in the present study, the PH was shown to decrease and LAC to increase significantly in subsequent maximal exercises in both sets of climatic conditions. The magnitude of these changes was considerable, and their real effect most likely. Since metabolite production and clearance are both closely associated with local blood flow in surrounding muscle tissues, restricted blood flow would enhance metabolite accumulation [85]. The attenuated power output during sprint exercise may be associated with the ability of the neuromuscular system to produce maximal force [86] and/or a relationship between force and velocity during repeated sprints [85]. It had been demonstrated that exercise in the heat leads to greater reliance on muscle glycogen, anaerobic metabolism, and muscle and blood lactate accumulation [87], which generate fatigue and a decline in force production. Fatigue development during high-intensity intermittent exercise may also be caused by a complex interplay between intra and extracellular levels and gradients of K+, Na+, Cl–, H+ and Mg+ [88, 89]. Although contentious [90], critical rises in hydrogen cation (H+) accumulation can be linked to a reduction in the release and uptake of calcium ions (Ca2+) from the sarcoplasmic reticulum [91], disruption of glycolytic pathway key enzymes [92], and lowered muscle excitability and action potentials by decreasing strong ion difference [93]. In turn, this may hamper subsequent performance by reducing the capability for muscle force production [94, 95]. When analyzing the level of selected electrolytes in the repeated maximal effort protocol, many meaningful changes were observed, which is not surprising. This indicates that short-term but repeated efforts have an impact on the level of these variables. These variables should be monitored over time/during a longer protocol, and we should be aware that their disturbances will reduce exercise capacity, especially accompanied by high temperatures.

One limitation of this study was that it was conducted in a climate test chamber, i.e. in a non-specific environment for soccer players. The exercise tests were also performed on a cycle ergometer (not a soccer-specific exercise) without a game-related stressor. Moreover, despite the examination of 4 different groups of parameters, such as power-related, physiological, psychomotor, blood-related and electrolytes-related variables (22 parameters in total), the internal body temperature was not taken into account. The study conclusions should therefore be used with caution. Further research should involve the examination of body responses in players of different ages and at different levels of training experience, in conditions typical for soccer: on a grass pitch, with plenty of running exercises, and with rest breaks of different length. Such tests could be carried out under a special canopy providing specific climatic conditions within it. Prospective studies in varied climatic conditions are necessary to provide a more in-depth analysis of effects of heat stress on repeated maximal exercise performance. It is also suggested to consider the measurement of muscle core temperature, players’ hydration status—especially in longer study protocols—and the WBGT index, which involves more components related to weather conditions.