Fiedler et al. BMC Research Notes          (2022) 15:351  
https://doi.org/10.1186/s13104-022-06247-1
RESEARCH NOTE
Daytime fluctuations of endurance 
performance in young soccer players: 
a randomized cross-over trial
Janis Fiedler1*  , Stefan Altmann1,2, Hamdi Chtourou3,4, Florian A. Engel5, Rainer Neumann6 and 
Alexander Woll1 
Abstract 
Objectives: Fluctuations of physical performance and biological responses during a repetitive daily 24-h cycle are 
known as circadian rhythms. These circadian rhythms can influence the optimal time of day for endurance perfor-
mance and related parameters which can be crucial in a variety of sports disciplines. The current study aimed to 
evaluate the daytime variations in endurance running performance in a 3.000-m field run and endurance running 
performance, blood lactate levels, and heart rate in an incremental treadmill test in adolescent soccer players.
Results: In this study, 15 adolescent male soccer players (age: 18.0 ± 0.6 years) performed a 3.000-m run and an 
incremental treadmill test at 7:00–8:00 a.m. and 7:00–8:00 p.m. in a randomized cross-over manner. No significant 
variations after a Bonferroni correction were evident in endurance running performance, perceived exertion, blood 
lactate levels, and heart rates between the morning and the evening. Here, the largest effect size was observed for 
maximal blood lactate concentration (9.15 ± 2.18 mmol/l vs. 10.64 ± 2.30 mmol/l, p = .110, ES = 0.67). Therefore, 
endurance running performance and physiological responses during a field-based 3.000-m run and a laboratory-
based test in young male soccer players indicated no evidence for daytime variations.
Keywords: Circadian rhythm, Soccer, Aerobic exercise, Endurance, Lactate, Heart rate
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Introduction
Circadian rhythms describe periodic changes in physi-
ological parameters for an approximately 24-h cycle [1]. 
They are well established for a range of biological param-
eters like core body temperature, heart rate (HR), blood 
pressure, and different hormones [1] and are also pre-
sent in physical performance and related responses [2, 3]. 
These circadian rhythms are influenced by other parame-
ters like age, light hours, sleeping pattern, or type of exer-
cise but are overall stable [2]. For coaches and athletes 
(i.e. soccer players), it might be important to consider 
circadian rhythms as determinants of exercise capacity 
as well as performance for the best results in competi-
tions [4]. As endurance running performance is related 
to overall performance in soccer players, and elite play-
ers run about 10 km during one game, this motor fitness 
parameter is of particular interest [5]. Previous research 
including soccer players found heterogenic results con-
cerning the presence of daytime variation for endurance 
performance and related physiological responses like lac-
tate or HR [2, 4, 6–13].
Therefore, this study aimed to examine potential day-
time variation (morning vs. evening) in i) endurance 
running performance during a 3.000-m field run and an 
incremental treadmill test; and ii) blood lactate concen-
tration and HR during the incremental treadmill test.
Open Access
BMC Research Notes
*Correspondence:  Janis.fiedler@kit.edu
1 Institute of Sports and Sports Science, Karlsruhe Institute of Technology, 
Engler-Bunte-Ring 15, 76131 Karlsruhe, Germany
Full list of author information is available at the end of the article
Page 2 of 6
Fiedler et al. BMC Research Notes          (2022) 15:351 
According to the literature, we hypothesized that (i) 
endurance running performance during the 3.000-m 
run and the treadmill test would be higher in the even-
ing than in the morning [7, 12, 14, 15] and (ii) that both 
blood lactate levels and HR during the incremental tread-
mill test would be different between the morning and 
evening [4, 16–21].
Material and Methods
Participants
Fifteen male soccer players (age = 18.0 ± 0.6  years; 
height = 178.7 ± 5.3  cm; weight = 71.1 ± 6.6  kg), with 
a regular training volume of three training sessions per 
week and one soccer match on the weekend, volunteered 
to participate in this study.
Procedures
All 15 participants performed a 3.000-m run and an 
incremental treadmill test (see [22]) on two occasions 
at two-day times (one in the morning between 7:00 and 
8:00 a.m. as well as one in the evening between 7:00 and 
8:00 p.m.). Using a cross-over design, the participants 
were randomly assigned to two groups. Both groups per-
formed the 3.000-m run first and the incremental tread-
mill test trials second. However, group 1 performed the 
first trial for both tests (3.000-m run and incremental 
treadmill test, respectively) in the morning and the sec-
ond in the evening, while the timing was switched for 
group 2. All tests were separated by 36 h.
In a first step, all participants performed the two field-
based 3.000-m runs on a 400 m running track. The par-
ticipants were familiar with the 3.000-m run and were 
instructed to perform the whole 3.000-m run as fast as 
possible. Time to completion and ratings of perceived 
exertion (RPE) [23] were recorded after each trial in 
order to control for exhaustion criteria [24].
In a second step, all participants performed two labo-
ratory-based incremental treadmill tests on a Woodway 
treadmill (Woodway GmbH, Weil am Rhein, Germany) 
with a slope of 1%. Each trial started at a running speed 
of 6 km/h, increasing by 2 km/h every 3 min. After each 
3  min-stage, participants rested for 30-s for collection 
of capillary blood from the earlobe; two participants 
provided no consent for blood withdrawal and lactate 
thresholds were estimated using the Ergonizer Software 
(Ergonizer, Freiburg, Germany). HR was monitored 
using a Polar system (Polar Electro Oy, Kempele, Fin-
land) throughout the whole test. Athletes were instructed 
to complete as many stages as possible, and the test 
was finished at volitional exhaustion. Blood lactate 
concentration for each stage was analyzed utilizing Bio-
sen C-Line Sport (EKF-diagnostic GmbH, Barleben, 
Germany).
Data analysis
Time to completion and RPE were recorded as param-
eters for the 3.000-m run. Regarding the incremental 
treadmill test, the following measurement points were 
chosen to measure one or multiple of the following 
parameters: blood lactate concentration, HR, and run-
ning speed (see [25]):
• rest: immediately before the beginning of the test in 
a standing position
• individual aerobic threshold (LT): running velocity 
at which blood lactate concentration begins to rise 
above baseline levels
• individual anaerobic threshold (IAT): running 
velocity at LT + blood lactate concentration of 
1.5 mmol/l
• maximal running speed (max): running velocity at 
the point of volitional exhaustion
The following parameters were included for the incre-
mental treadmill test:
• blood lactate concentration (at rest, LT, IAT, and 
max)
• HR (at rest, LT, IAT, and max)
• running speed (LT, IAT, and max)
Statistical analysis
Because of the cross-over study design, the existence of 
possible sequencing effects was calculated by perform-
ing an independent t-test between the sum scores (day 
1 + day 2 group 1 vs day 1 + day 2 group 2) for each 
parameter in addition to a sufficient washout period [26]. 
All Data are available in the Additional file 1.
Daytime variations in all measured variables were cal-
culated using paired t-tests. To correct for multiple test-
ing, the results were adapted by multiplying the p-value 
with the number of comparisons of the parameter follow-
ing the Bonferroni correction [27]. In addition, Cohen’s 
d effect sizes (ES) were calculated to quantify the mag-
nitude of differences between the morning and even-
ing trials: 0.2 ≤ ES < 0.5 was considered a small effect; 
0.5 ≤ ES < 0.8 was considered a moderate effect; ES ≥ 0.8 
was considered a large effect [28]. Statistical analyses 
were performed using SPSS statistical software version 
26.0 (SPSS, Inc., Chicago, IL). The level for significance 
was set a priori to 0.05 after the Bonferroni correction.
Page 3 of 6
Fiedler et al. BMC Research Notes          (2022) 15:351 
 
Results
The investigation of potential sequencing effects, ana-
lyzed using an independent t-test, showed no signifi-
cant differences between the two groups.
For the 3.000-m run, neither time for completion 
(see Fig. 1a) nor RPE (see Fig. 1b) differed significantly 
between the morning and evening trials. For the incre-
mental treadmill test, no significant differences after 
the Bonferroni correction were found for blood lac-
tate (maximal blood lactate concentration see Fig. 2a) 
or running speed (maximal running speed see Fig. 2b) 
between the morning and evening trials (see Table  1 
for detailed results).
Discussion and conclusion
This study aimed to evaluate daytime variation in aerobic 
endurance performance in a 3.000-m run and an incre-
mental treadmill test in young soccer players. Addition-
ally, blood lactate concentrations and HR during the 
incremental treadmill test were analyzed for daytime 
differences. Hypothesis (i) that aerobic endurance per-
formance would be better in the evening than in the 
morning could not be verified for the 3.000-m run and 
the incremental treadmill test. Hypothesis (ii) that blood 
lactate levels and HR during exercise would be higher in 
the evening could also not be verified.
Fig. 1 The individual values of all participants (lines) and the mean value (box) for the parameters a time to completion, and b perceived exertion 
during the morning and evening trial of the 3.000-m run
Fig. 2 The individual values of all participants (lines) and the mean value (box) for a maximal blood lactate concentration, and b maximal running 
speed during the morning and evening trial of the incremental treadmill test
Page 4 of 6
Fiedler et al. BMC Research Notes          (2022) 15:351 
Aerobic endurance performance in the incremen-
tal treadmill test indicated no evidence for differences 
between the evening and the morning. This is in line with 
some previous studies in untrained participants [20] and 
competitive cyclists [29] while others reported increased 
endurance performance in an incremental cycle ergome-
ter test in students [15] and a Yo-Yo intermittent recovery 
test in young soccer players [12]. While there is a good 
theoretical basis for performance differences due to hor-
monal control of glucose metabolism [13], results from 
laboratory and field studies yield heterogenic findings. 
Additionally, no differences in endurance performance 
and RPE were found for the field test (i.e., 3.000-m run). 
One possible explanation for the results of the 3.000-m 
run is that the self-selected pacing is a crucial factor for 
maximum performance in the 3.000-m run [30]. This 
is supported by the reported mean RPE which did not 
reach the range of exhaustion criteria (RPE  > 16) in the 
3.000-m run, while exhaustion criteria were reached 
(mean max lactate  > 9  mmol/l) [24] in the incremental 
treadmill test.
Furthermore, no evidence for a daytime variation in 
any physiological parameter was found in our study. Con-
trasting, previous studies found higher blood lactate lev-
els for various exercises [4, 21]. Additionally, one study 
reported higher blood lactate levels at rest in the morn-
ing compared to the afternoon and evening [20], and 
another study found no differences in blood lactate levels 
throughout the day [29]. Reasons for the different results 
between the aforementioned studies and the results of 
the present study can be found in different test proce-
dure and population. Concerning daytime variations of 
HR during endurance exercise, the overall results seem 
to be inconsistent [30]. While some studies reported evi-
dence for the presence of daytime variation in HR [16–
18], Chtourou and Souissi described equivocal results for 
daytime variation of HR in their recent review [30].
Overall, our hypotheses that daytime variations are 
present in endurance performance and related physi-
ological parameters of youth soccer players could not 
be confirmed by this study. While circadian rhythms are 
considered an important factor related to physical per-
formance and physiological parameters in competitive 
sports, the importance of circadian rhythms for aerobic 
endurance performance remains unclear.
Limitations
Some limitations must be acknowledged concerning 
this study. First, the use of only two times of the day (i.e., 
morning and evening) might not be sufficient because 
the time window for optimal performance differs for each 
individuum [2]. However, the choice of the selected times 
of the day in our study did incorporate the optimum time 
of day for soccer players’ performance between 04:00 
Table 1 Results for endurance running performance, blood lactate levels, and heart rate differences between morning and evening
Means (standard deviations) and results of the paired t-tests for daytime differences at the incremental treadmill test before the start (rest), at the onset of lactate 
accumulation (LT), the individual anaerobic threshold (IAT), and immediately after volitional exhaustion (max) and at the end of the 3.000-m run for time to 
completion (Time) and rating of perceived exertion (RPE). p-values were corrected using the Bonferroni method
Incremental treadmill test
Parameter
Morning
Evening
Mean difference
corrected p-value 
(original)
Cohen’s d (t-value)
df
Heart rate [1/min]
 Rest
86.60 (9.68)
85.73 (10.88)
0.87 (10.33)
1.00 (0.750)
− 0.09 (0.35)
14
 LT
150.93 (12.13)
153.47 (10.29)
− 2.53 (9.81)
1.00 (0.334)
0.23 (− 1)
14
 IAT
177.47 (7.85)
179.27 (6.31)
− 1.80 (4.87)
0.872 (0.174)
0.25 (− 1.43)
14
 Max
197.13 (6.29)
198.73 (6.08)
− 1.60 (4.21)
0.814 (0.163)
0.26 (− 1.47)
14
Lactate concentration [mmol/l]
 Rest
0.84 (0.21)
0.83 (0.31)
0.01 (0.31)
1.00 (0.930)
− 0.04 (0.09)
12
 LT
1.52 (0.67)
1.66 (0.61)
− 0.13 (0.40)
1.00 (0.250)
0.22 (− 1.20)
12
 IAT
3.02 (0.67)
3.16 (0.61)
− 0.14 (0.40)
0.962 (0.241)
0.22 (− 2.51)
12
 Max
9.15 (2.18)
10.64 (2.30)
− 1.49 (2.15)
0.110 (0.028)
0.67 (− 2.51)
12
Running speed [km/h]
 LT
8.67 (1.17)
9.00 (1.10)
− 3.30 (0.74)
0.429 (0.107)
0.29 (− 1.72)
14
 IAT
11.94 (1.28)
12.12 (1.18)
− 0.19 (0.68)
1.00 (0.317)
0.15 (− 1.04)
13
 Max
15.81 (1.62)
16.31 (1.59)
− 0.49 (0.76)
0.100 (0.025)
0.31 (− 2.51)
14
3.000-m test
 Time [min:sec]
12:59:00 (1:29)
13:06:00 (1:30)
− 0:07 (0:22)
0.228
0.08 (− 1.26)
15
 RPE
15.31 (1.82)
15.44 (1.09)
− 0.13 (1.78)
0.783
0.09 (− 0.28)
15
Page 5 of 6
Fiedler et al. BMC Research Notes          (2022) 15:351 
 
p.m. and 08:00 p.m. [6] to compensate for this shortcom-
ing. Secondly, the RPE used in the 3.000-m run has not 
been used in the incremental treadmill test, while blood 
lactate testing has only been performed during the incre-
mental treadmill test and not after the 3.000-m run and 
therefore limits the interpretation concerning exhaus-
tion criteria. Other important factors might be that this 
study did not control for sleeping patterns, sleep dura-
tion, naps, and morning or evening type of participants 
which is known to influence the circadian rhythm [2, 31]. 
Here, the relation between the chronotype and the per-
formance of athletes at certain daytimes is particularly 
interesting but evidence in the literature is heterogenic 
[32, 33]. Finally, a larger sample size would have reduced 
the beta error and would lead to more robust results.
Future studies should address these shortcomings by 
adding physiological parameters to control for exhaus-
tion criteria with parameters like blood lactate, HR, 
and RPE. Additionally, sleep related variables, and 
chronotype of participants should be considered. This 
may enable researchers to distinguish between physio-
logical and psychological aspects of aerobic endurance 
performance and to better determine if and why day-
time variations are present for the different outcome 
parameters. Finally, if studies aim to determine sport-
specific (i.e., soccer) daytime variation, a field test rep-
resenting the sport-specific requirements seems more 
appropriate compared to generic endurance tests like 
the 3.000-m run.
Abbreviations
ES: Effect size (Cohen’s d); HR: Heart rate; IAT: Individual anaerobic threshold; LT: 
Individual aerobic threshold; max: Maximal; RPE: Rate of perceived exertion.
Supplementary Information
The online version contains supplementary material available at https:// doi. 
org/ 10. 1186/ s13104- 022- 06247-1.
Additional file 1. Exercise data of soccer players.
Acknowledgments
The authors would like to thank the participants for their enthusiastic 
participation and the students for their support during data collection. We 
acknowledge support by the KIT-Publication Fund of the Karlsruhe Institute of 
Technology.
Author contributions
Conceptualization, JF, SA, FE; Data curation, JF; Formal analysis, JF; Investi-
gation, RN, FE, SA; Methodology, JF, SA, RN, FE, SA; Writing—original draft, 
JF; Writing—review & editing, SA, HC, FE, RN, and AW All authors read and 
approved the final manuscript.
Funding
Open Access funding enabled and organized by Projekt DEAL. No funding 
was provided for this study.
Availability of data and materials
All data generated or analysed during this study are included in this published 
article [and its Additional file 1].
Declarations
Ethics approval and consent to participate
All participants provided written informed consent before the start. The study 
was approved by the Institutional Reviewer Board of the Institute of Sport and 
Sport Science at the Karlsruhe Institute of Technology and all methods were 
carried out according to the countries guidelines and regulations.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Author details
1 Institute of Sports and Sports Science, Karlsruhe Institute of Technology, 
Engler-Bunte-Ring 15, 76131 Karlsruhe, Germany. 2 TSG ResearchLab gGmbH, 
74939 Zuzenhausen, Germany. 3 Institut Supérieur du Sport et de l’Education 
Physique de Sfax, Université de Sfax, 3000 Sfax, Tunisie. 4 Activité Physique, 
Sport et Santé, UR18JS01, Observatoire National du Sport, 1003 Tunis, Tunisie. 
5 Institute of Sport Science, Integrative & Experimental Exercise Science & Train-
ing, Würzburg University, 97070 Würzburg, Germany. 6 Institute of Movement 
and Sport, University of Education Karlsruhe, 73133 Karlsruhe, Germany. 
Received: 18 July 2022   Accepted: 11 November 2022
References
 1. 
Minors DS, Waterhouse JM. Circadian Rhythms and the Human. Oxford: 
Elsevier Science; 2014.
 2. 
Reilly T, Atkinson G, Waterhouse J. Biological rhythms and exercise. 
Oxford: Oxford University Press; 1997.
 3. 
Drust B, Waterhouse J, Atkinson G, Edwards B, Reilly T. Circadian rhythms 
in sports performance–an update. Chronobiol Int. 2005;22(1):21–44.
 4. 
Hammouda O, Chtourou H, Chaouachi A, Chahed H, Bellimem H, 
Chamari K, et al. Time-of-day effects on biochemical responses to 
soccer-specific endurance in elite Tunisian football players. J Sports Sci. 
2013;31(9):963–71.
 5. 
Stølen T, Chamari K, Castagna C, Wisløff U. 2005 Physiology of soc-
cer: an update. Sports Med. 35(6):501–36. https://link.springer.com/
article/https:// doi. org/ 10. 2165/ 00007 256- 20053 5060- 00004.
 6. 
Reilly T, Atkinson G, Edwards B, Waterhouse J, Farrelly K, Fairhurst E. Diur-
nal variation in temperature, mental and physical performance, and tasks 
specifically related to football (soccer). Chronobiol Int. 2007;24(3):507–19.
 7. 
Hammouda O, Chtourou H, Chahed H, Ferchichi S, Chaouachi A, Kallel 
C, et al. Diurnal variations in physical performances related to football in 
young soccer players. Int J Sports Med. 2012;33(11):886–91.
 8. 
Hammouda O, Chahed H, Chtourou H, Ferchichi S, Miled A, Souissi N. 
Morning-to-evening difference of biomarkers of muscle injury and 
antioxidant status in young trained soccer players. Biol Rhythm Res. 
2012;43(4):431–8.
 9. 
Chtourou H, Hammouda O, Souissi N, Chaouachi A. Temporal specific-
ity of training: an update. J Athl Enhancement. 2014. https:// doi. org/ 10. 
4172/ 2324- 9080. 10001 53.
 10. Chtourou H, Chaouachi A, Driss T, Dogui M, Behm DG, Chamari K, et al. 
The effect of training at the same time of day and tapering period on the 
diurnal variation of short exercise performances. J Strength Cond Res. 
2012;26(3):697–708.
 11. Hill DW, Cureton KJ, Collins MA. Circadian specificity in exercise training. 
Ergonomics. 1989;32(1):79–92.
 12. Hammouda O, Chtourou H, Farjallah MA, Davenne D, Souissi N. The effect 
of Ramadan fasting on the diurnal variations in aerobic and anaero-
bic performances in Tunisian youth soccer players. Biol Rhythm Res. 
2012;43(2):177–90.
Page 6 of 6
Fiedler et al. BMC Research Notes          (2022) 15:351 
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Ready to submit your research ?  Choose BMC and benefit from: 
?  Choose BMC and benefit from: 
 13. Kusumoto H, Ta C, Brown SM, Mulcahey MK. Factors contributing to 
diurnal variation in athletic performance and methods to reduce within-
day performance variation: a systematic review. J Strength Cond Res. 
2021;35(Suppl 12):S119–35.
 14. Souissi W, Hammouda O, Ammar A, Ayachi M, Bardiaa Y, Daoud O, et al. 
Higher evening metabolic responses contribute to diurnal variation of 
self-paced cycling performance. Biol Sport. 2021. https:// doi. org/ 10. 5114/ 
biols port. 2021. 102930.
 15. Hill DW, Cureton KJ, Ma Collins, Grisham SC. Diurnal variations in 
responses to exercise of morning types and evening types. J Sports Med 
Phys Fitness. 1988;28(3):213–9.
 16. Cohen CJ, Muehl GE. Human circadian rhythms in resting and exercise 
pulse rates. Ergonomics. 1977;20(5):475–9.
 17. Cohen CJ. Human circadian rhythms in heart rate response to a maximal 
exercise stress. Ergonomics. 1980;23(6):591–5.
 18. Wahlberg I, Åstrand I. Physical work capacity during the day and at night. 
Work, Environment, Health. 1973;10(2):65–8.
 19. Winget CM, DeRoshia CW, Holley DC. Circadian rhythms and athletic 
performance. Med Sci Sports Exerc. 1985;17(5):498–516.
 20. Dschenes MR, Sharma JV, Brittingham KT, Casa DJ, Le Armstrong, Maresh 
CM. Chronobiological effects on performance and selected physiological 
responses. Eur J Appl Physiol. 1998;77(3):249–56.
 21. Forsyth JJ, Reilly T. Circadian rhythms in blood lactate concentra-
tion during incremental ergometer rowing. Eur J Appl Physiol. 
2004;92(1–2):69–74.
 22. Altmann S, Neumann R, Woll A, Härtel S. Endurance capacities in profes-
sional soccer players: are performance profiles position specific? Front 
Sports Act Living. 2020;2:549897.
 23. Borg G. Perceived exertion as an indicator of somatic stress. Scand J 
Rehabil Med. 1970;2(2):92–8.
 24. Dickhuth H-H, Röcker K, Gollhofer A, König D, Mayer F. 2011 Einführung in 
die Sport-und Leistungsmedizin. Überarbeitete und aktualisierte Auflage. 
Schorndorf: Hofmann
 25. Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid 
are they? Sports Med. 2009;39(6):469–90.
 26. Wellek S, Blettner M. On the proper use of the crossover design in clinical 
trials: part 18 of a series on evaluation of scientific publications. Dtsch 
Arztebl Int. 2012;109(15):276–81.
 27. Shaffer JP. Multiple hypothesis testing. Annu Rev Psychol. 1995;46:561–84.
 28. Cohen J. 1988 Statistical Power Analysis for the Behavioral Sciences. L. 
Erlbaum Associates, Hillsdale, NJ.
 29. Dalton B, McNaughton L, Davoren B. Circadian rhythms have no effect on 
cycling performance. int J Sports Med. 1997;18(7):538–42.
 30. Chtourou H, Souissi N. The effect of training at a specific time of day: a 
review. J Strength Cond Res. 2012;26(7):1984–2005.
 31. Lastella M, Halson SL, Vitale JA, Memon AR, Vincent GE. To nap or not to 
nap? a systematic review evaluating napping behavior in athletes and 
the impact on various measures of athletic performance. Nat Sci Sleep. 
2021;13:841–62.
 32. Rae DE, Stephenson KJ, Roden LC. 2015 Factors to consider when 
assessing diurnal variation in sports performance: the influence of 
chronotype and habitual training time-of-day. Eur J Appl Physiol. 
115(6):1339–49. https://link.springer.com/article/https:// doi. org/ 10. 1007/ 
s00421- 015- 3109-9.
 33. Vitale JA, Weydahl A. 2017 Chronotype, Physical Activity, and Sport Per-
formance: A Systematic Review. Sports Med. 47(9):1859–68. https://link.
springer.com/article/https:// doi. org/ 10. 1007/ s40279- 017- 0741-z. Decla 
ratio ns
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Daytime fluctuations of endurance performance in young soccer players: a randomized cross-over trial.
Fiedler, Janis,Altmann, Stefan,Chtourou, Hamdi,Engel, Florian A,Neumann, Rainer,Woll, Alexander