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Caffeine Increases Strength and Power Performance in Resistance-trained Females During Early Follicular Phase


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The effects of 4 mg·kg‐1caffeine ingestion on strength and power were investigated for the first time, in resistance‐trained females during the early follicular phase utilizing a randomized, double‐blind, placebo‐controlled, crossover design. Fifteen females (29.8±4.0 years, 63.8±5.5 kg [mean±SD]) ingested caffeine or placebo 60 minutes before completing a test battery separated by 72 hours. One‐repetition maximum (1RM), repetitions to failure (RTF) at 60% of 1RM, were assessed in the squat and bench press. Maximal voluntary contraction torque (MVC) and rate of force development (RFD) were measured during isometric knee‐extensions, while utilizing interpolated twitch technique to measure voluntary muscle activation. Maximal power and jump height were assessed during countermovement jumps (CMJ). Caffeine metabolites were measured in plasma. Adverse effects were registered after each trial. Caffeine significantly improved squat (4.5±1.9%, effect size [ES]: 0.25) and bench press 1RM (3.3±1.4%, ES: 0.20), and squat (15.9±17.9%, ES: 0.31) and bench press RTF (9.8±13.6%, ES: 0.31), compared to placebo. MVC torque (4.6±7.3%, ES: 0.26), CMJ height (7.6±4.0%, ES: 0.50) and power (3.8±2.2%, ES: 0.24) were also significantly increased with caffeine. There were no differences in RFD or muscle activation. Plasma [caffeine] was significantly increased throughout the protocol and mild side‐effects of caffeine were experienced by only 3 participants. This study demonstrated that 4 mg·kg‐1 caffeine ingestion enhanced maximal strength, power and muscular endurance in resistance‐trained and caffeine‐habituated females during the early follicular phase, with few adverse effects. Female strength and power athletes may consider using this dose pre‐competition and ‐training as an effective ergogenic aid.
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Scand J Med Sci Sports. 2020;30:2116–2129.
Received: 6 December 2019
Revised: 28 June 2020
Accepted: 9 July 2020
DOI: 10.1111/sms.13776
Caffeine increases strength and power performance in resistance-
trained females during early follicular phase
Linn ChristinRisvang1,2
Per OlaRønning2
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original
work is properly cited.
© 2020 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd
Martin Norum and Linn Christin Risvang should be considered joint first authors.
1School of Science and Technology,
London Sport Institute, Middlesex
University, London, UK
2Department of Mechanical, Electronics
and Chemical Engineering, Faculty
of Technology, Art and Design, Oslo
Metropolitan University, Oslo, Norway
3Department of Sport Science and
Physical Education, Faculty of Health
and Sport Sciences, University of Agder,
Kristiansand, Norway
4Norwegian Olympic and Paralympic
Committee and Confederation of Sports,
Oslo, Norway
5Department of Natural Sciences, School
of Science and Technology, Middlesex
University, London, UK
6Department of Life Sciences and
Health, Faculty of Health Sciences, Oslo
Metropolitan University, Oslo, Norway
7Department of Physical Performance,
Norwegian School of Sport Sciences, Oslo,
Linn Christin Risvang, Department of
Mechanical, Electronics and Chemical
Engineering, Faculty of Technology, Art
and Design, Oslo Metropolitan University,
Oslo, Norway.
The effects of 4mg·kg−1caffeine ingestion on strength and power were investigated
for the first time, in resistance-trained females during the early follicular phase utiliz-
ing a randomized, double-blind, placebo-controlled, crossover design. Fifteen females
(29.8±4.0years, 63.8±5.5kg [mean±SD]) ingested caffeine or placebo 60min-
utes before completing a test battery separated by 72hours. One-repetition maximum
(1RM), repetitions to failure (RTF) at 60% of 1RM, was assessed in the squat and
bench press. Maximal voluntary contraction torque (MVC) and rate of force devel-
opment (RFD) were measured during isometric knee extensions, while utilizing in-
terpolated twitch technique to measure voluntary muscle activation. Maximal power
and jump height were assessed during countermovement jumps (CMJ). Caffeine me-
tabolites were measured in plasma. Adverse effects were registered after each trial.
Caffeine significantly improved squat (4.5±1.9%, effect size [ES]: 0.25) and bench
press 1RM (3.3± 1.4%, ES: 0.20), and squat (15.9± 17.9%, ES: 0.31) and bench
press RTF (9.8±13.6%, ES: 0.31), compared to placebo. MVC torque (4.6±7.3%,
ES: 0.26), CMJ height (7.6±4.0%, ES: 0.50), and power (3.8±2.2%, ES: 0.24) were
also significantly increased with caffeine. There were no differences in RFD or mus-
cle activation. Plasma [caffeine] was significantly increased throughout the protocol,
and mild side effects of caffeine were experienced by only 3 participants. This study
demonstrated that 4mg·kg−1 caffeine ingestion enhanced maximal strength, power,
and muscular endurance in resistance-trained and caffeine-habituated females dur-
ing the early follicular phase, with few adverse effects. Female strength and power
athletes may consider using this dose pre-competition and -training as an effective
ergogenic aid.
caffeine supplementation, female athletes, muscular activation level, muscular endurance, strength
and power performance
NORUM et al.
Caffeine (1,3,7-trimethylxanthine) is the most widely used
legal drug in the world, by the general as well as athletic pop-
ulations,1 and researchers’ interest in the effects of caffeine
on exercise performance is apparent in light of multiple re-
views of the literature published in the recent years.2-5 These
reviews currently agree that caffeine is a potent ergogenic aid
for a variety of exercise performances; however, the effects of
caffeine on maximal strength and power performance are less
clear. Meta-analyses by Warren et al6 and Polito et al7 showed
that caffeine ingestion can increase isometric strength and
muscular endurance performance. However, Polito et al7
could not observe improved dynamic strength with caffeine
supplementation, and a recent meta-analysis by Grgic et al4
only found increased performance in upper but not lower
body dynamic strength. On the other hand, increased mus-
cular endurance has been demonstrated with larger effect
sizes in lower body rather than upper body exercises.7 The
conflicting results could be due to varying effect of caffeine
on different types of contractions, as the contribution of cor-
tical and spinal centers to the neural drive changes with the
contraction type.8 Hence, further research is warranted to in-
vestigate the effect of caffeine on maximal isometric versus
dynamic strength, power, and muscular endurance, as well as
comparing lower and upper body muscle groups.
A recent review of the caffeine literature found that only
~13% of the total sample in research on the ergogenic effect
of caffeine between 1978 and 2018 were women and that
the number of women in studies investigating caffeine ef-
fects on speed and muscle power is very low.9 A likely ex-
planation for this difference in representation of the sexes
is that females can be a slightly more challenging cohort
to conduct caffeine research on. The use of oral contracep-
tives10 and the large variations in hormone concentrations
between phases of the menstrual cycles11 can alter caffeine
metabolization speeds,12 which in turn may alter the ergo-
genic effects of caffeine. Indeed, significant sex differences
have been reported in caffeine concentrations post-exercise
with ingestion of 3mg/kg caffeine, with females having a
greater amount. This suggests that females do not metab-
olize caffeine as rapidly as males. Furthermore, variations
in strength and power have been demonstrated through-
out the menstrual cycle,13 which can cause noise in per-
formance data and affect overall results. Taken together,
although there are a number of studies demonstrating that
caffeine clearly has an ergogenic effect in females,9,14-16 the
information about the effect of caffeine on muscle perfor-
mance in women is uncertain, especially in strength and
power performance. As an example, a recent meta-sub-
group analysis examined the effects of caffeine on muscle
power in females for the first time.4 However, only three
studies examining vertical jumps were included and neither
controlled for potential metabolic alterations across the
menstrual cycle, making it difficult to conclude on the ef-
fects of caffeine on power in females. Moreover, a recent
study found differences in the effect of caffeine on power
performance between the phases in the menstrual cycle.17
It, therefore, seems important to control for stages in the
menstrual cycle to further establish clear recommendations
for the use of caffeine in females. The early follicular phase
of the menstrual cycle has shown the lowest variability in
oestradiol and progesterone concentration,18 and the sex
hormone levels in this phase are similar to the levels in
females using hormone contraceptives.19 Furthermore, a
recent study found that the fluctuations in sex hormones
throughout the menstrual cycle affect neuromuscular func-
tion.20 Hence, conducting caffeine research on females
would benefit from being performed at the same stage of
the menstrual cycle and can reliably be performed during
the early follicular phase.
The underlying mechanisms by which caffeine may aid
maximal strength and power are likely increased motor unit
recruitment and voluntary muscle activation of the involved
muscles.6,21,22 However, there seem to be discrepancies in
the caffeine effect on strength and power that corresponds
to varying degree of baseline voluntary activation. Larger
lower body muscles such as knee extensors seem to have a
relatively low (85%-95%) muscle activation level compared
to the small upper body muscles (90%-99%),6 such as elbow
flexors.23 These differences in baseline muscle activation
may influence the magnitude of the caffeine effect. As
Warren et al6 discuss in their meta-analysis, logically there
will be more to improve with lower baseline muscle activa-
tion levels, that is, larger lower body muscles might have
a greater effect of caffeine. Correspondingly, strength and
power improvements with caffeine have been reported in
this pattern.6 However, one study in females shows the quite
opposite pattern, that is, caffeine-induced improvements of
upper body but not lower body maximal strength, although
this needs further investigation.16 Perceived pain and exer-
tion during exhaustive resistance work have been thought
to be reduced, and thereby improving performance, through
caffeine's inhibitory binding to adenosine receptors.21
However, caffeine's effect on intra-set ratings of perceived
exertion seems under-investigated compared to post-fatigue
ratings, although Doherty et al's meta-analysis24 observed
that a ~5% reduction in intra-set ratings of perceived ex-
ertion (RPE) explained about a third of the variance in
exhaustive work between caffeine and placebo. Moreover,
the contribution of muscle activation to increased strength
and power, comparison of upper and lower body maximal
strength and effects on RPE and pain has to the authors’
knowledge, not been investigated specifically with moder-
ate caffeine doses in resistance-trained females while con-
trolling for menstrual cycle.
NORUM et al.
Only three studies have investigated the effects of caf-
feine doses <6 mg·kg−1 on strength performance in fe-
males.16,25,26 Goldstein et al27 and others4 have specifically
proposed that future research should examine the ergogenic
effects of lower doses of caffeine. Several studies have
reported severe side effects such as “intense emotional
responses,” tremor, heart palpitations, and tachycardia
when supplementing with relatively high doses of caffeine
(6-11 mg·kg−1).27-29 A lower caffeine dose could induce
similar performance enhancements but with fewer adverse
events, which would be an advantage, especially to compet-
ing strength and power athletes.
Thus, the main purpose of the present study was to in-
vestigate, for the first time, the effects of 4mg·kg−1 caffeine
on various strength and power measures in resistance-trained
females during the early follicular phase. We hypothesized
a caffeine-induced increase in maximal strength and muscu-
lar activation levels, vertical jump height, as well as in mus-
cular endurance, compared to placebo ingestion. Secondary
outcomes of the study were intra-set ratings of perceived
exertion, perceived pain, plasma caffeine concentration, ha-
bituation, and adverse effects.
Fifteen caucasian female volunteers (age: 29.8±5.5years;
stature: 165.8 ± 4.8 cm; body mass: 63.8 ± 5.5 kg
[mean±SD]) completed this study (Table 1). Nine of the
25 recruited participants dropped out after randomization due
to logistical issues, and one was excluded due to intake of a
source of caffeine unknown to participant and researchers.
Resistance-trained participants (recreational lifters, personal
trainers, and functional fitness athletes) were recruited fol-
lowing these inclusion criteria: (a) 18-45years old; (b) resist-
ance-trained for minimum 12months, 2-3 sessions/week and
currently resistance training; (c) ability to perform squat and
bench press with a load corresponding to 110% and 80% of
their current body mass, respectively, and (d) familiar with
the bench press and back squat exercises (performed at least
one time/wk). Participants were excluded if they were smok-
ers, pregnant, or lactating, were adversely affected by caf-
feine, used medicines and/or other ergogenic supplements,
had history of recent injury, illness or other diseases that
could affect measurements. Participants signed a written in-
formed consent and completed a Physical Activity Readiness
Questionnaire (PAR-Q). Ethical approval was obtained from
the research ethics committee of London Sports Institute,
Middlesex University (London, UK) and the Norwegian
School of Sport Science (Oslo, Norway). The project was ap-
proved by the Norwegian Centre for Research Data.
Study design
A randomized, double-blind, placebo-controlled crossover
design was used to investigate the effects of 4mg·kg−1 caf-
feine on strength and power performance. The participants
attended four sessions; two familiarizations to all procedures
(except blood sampling) and to the test battery, and two tri-
als. However, three familiarization sessions were performed
when the variation between the two first familiarization ses-
sions exceeded a coefficient of variation (CV) of 10% (total
number of participants completing three familiarizations for
one of the tests, n=8). Participants were instructed to re-
frain from alcohol, caffeine, and vigorous physical activity
48hours prior to the trials and were provided with a detailed
list of items containing caffeine, such as coffee, chocolate,
tea, soda, and energy drinks. All participants recorded their
weekly intake of these products using a caffeine frequency
questionnaire to calculate their habitual caffeine intake
(Table 1), and were classified as low, medium, or high caf-
feine consumers based on habitual intakes (<1.5, 1.5-5.0 and
>5.0mg·kg−1·d−1, respectively).30 They also completed a 24-
hour food diary (MyFitnessPal®, MyFitnessPal, Inc) prior to
the first trial and replicated the food intake prior to the second
trial to ensure minimal variation in hydration level and en-
ergy intake. Body composition was assessed by bioelectrical
TABLE 1 Participant characteristics.
Mean±SD Range
Fat-free mass (kg)a 52.3±5.2 44.4-63.2
Fat mass (kg)a 11.3±4.0 4.9-21.2
Fat mass (%)a 17.7±5.8 8.1-32.3
Hormone contraceptive use (n - %) 10 66.7
RE experience (y) 7±5 2-16
RE frequency (sessions·wk−1) 4±1 2-5
Squat 1RM (kg)b 97±13 75-115
Squat 1RM (kg·bw−1) 1.5±0.2 1.2-1.8
Bench press 1RM (kg)b 66±10 50-82
Bench press 1RM (kg·bw−1) 1.0±0.2 0.8-1.3
Energy (kcal)c 2208±509 1473-
Protein (g·d−1)c 143±37 67-210
Carbohydrate (g·d−1)c 209±54 130-301
Fat (g·d−1)c 84±39 40-182
Caffeine (mg·d−1)d 341±184 54-692
Note: Range: min-max.
Abbreviations: 1RM, one-repetition maximum; RE, resistance exercise.
aMeasured with InBody720.
bBased on the maximal 1RM across the two familiarizations.
cMean habitual intakes from a 24-h food diary prior to each test day.
dHabitual caffeine intake questionnaire.
NORUM et al.
impedance analysis (InBody 720, InBody Co., Ltd) follow-
ing (a) 24 hours without vigorous exercise, (b) minimum
2hours fasting, and (c) emptying the bladder.
Both trials were performed at the same time of day, ap-
proximately 1 week after familiarization. The participants
performed the caffeine and placebo trials at individually
standardized test times, which was self-selected to corre-
spond with the participant's habitual training schedule. The
trials were interspersed by 72hours to ensure treatment wash-
out, allow for recovery and for both trials to be completed
within the early follicular phase of the menstruation cycle
as previously used by Chen et al15 This is when the concen-
tration and variation in estrogen and progesterone are low-
est as compared to other the phases of the menstrual cycle.18
Participants using hormone contraceptives were included, as
these show very similar levels of estrogen and progesterone
to the levels during the early follicular phase.19 Confirmation
of a new menstruation cycle was obtained from each partici-
pant prior to confirming trial day 1.
Experimental protocol
All participants performed the test battery in the same order
each day within the set amount of time of 210minutes, in-
cluding rest intervals and breaks, estimated from pilot testing
(Figure1). Upon arrival, participants provided a urine sample
for visual assessment of hydration status (The Urine Colour
Chart®, Human Hydration, LLC). If the urine color chart in-
dicated a score of 5 or below, the participants were provided
250-500mL of water to improve hydration levels prior to con-
tinuing the protocol. In addition, 4mL blood was collected
from the cubital fossa veins (Vacuette® Multiple use draw-
ing needle; Vacuette® tube, 4mL K2EDTA, Greiner Bio-One
GmbH). Blood was further collected at 60 and 270minutes
following treatment ingestion. Subsequently, height and body
mass were measured (SECA stadiometer, Model 213; SECA
weight scale 876, respectively). All participants received a
standardized meal 45minutes prior to testing, consisting of
0.4 g·kg−1 whey protein powder (0.36 g·kg−1 protein) and
1.5g·kg−1 banana (0.35g·kg−1 carbohydrate). All participants
performed a standardized warm-up for 10minutes by cycling
on a stationary bicycle at ~100W at 80-90 RPM (Monark,
Ergomedic 828E), followed by a standardized 5minutes rest,
and were equally verbally encouraged to perform to the best
of their abilities during all tests. The participants completed
questionnaires about their preparation adherence, withdrawal
symptoms, and the Brunel Mood score (BRUMS) 24-item
questionnaire31 prior to the protocol and an end-of-trial ques-
tionnaire about adverse effects and blinding, where the par-
ticipants were asked to state if they believed they had received
caffeine, placebo or were unsure, after completion of the test
battery and final blood sampling.
Treatment was given 60minutes prior to testing, allowing
peak plasma levels of caffeine to coincide with testing.1 The
treatments were administered as 150 mL non-caloric Fun
Light© cordial concentrate from an opaque bottle. To pre-
pare the caffeine treatment, 4 mg·kg−1 anhydrous caffeine
(Caffeine, ReagentPlus®, Sigma-Aldrich) was dissolved in
the cordial concentrate with heat to ensure complete disso-
lution of the caffeine. Both treatments were equal in color,
taste, and volume due to not diluting the cordial. The drink
was rapidly ingested immediately followed by another 150-
mL cordial from a separate cup to conceal any potential
bitter taste and rinse the mouth of caffeine residues. An inde-
pendent researcher randomized treatment order, mixed, and
administered the treatments and held the key to the randomi-
zation until the end of the study.
Countermovement jump
Participants performed the countermovement jump (CMJ)
to assess jump height (cm), maximal power (W) and
FIGURE 1 Experimental protocol
timeline. Overview of the experimental
protocol. In addition, urine was observed at
arrival for visual hydration status estimation
with the urine color chart. 1RM, one-
repetition maximum; BP, bench press; CMJ,
countermovement jump; ITT, interpolated
twitch technique; MVC, maximal voluntary
isometric contraction; SQ, squat; RTF,
repetitions to failure
NORUM et al.
maximal force (N). Participants were instructed to stand
on a force plate (FP4, HUR Labs OY, Hur AB) with hands
kept on their hips with legs shoulder width apart while ex-
ecuting a maximal vertical jump, from an upright position
to a self-selected depth immediately prior to jumping. To
warm-up, three submaximal CMJ trials with approximately
50%, 75%, and 90% intensity were performed with 1-min-
ute breaks. After another 2 minutes rest, maximal effort
CMJ trials with 2 minutes rest between each trial were
performed for at least 3 sets. If the third set resulted in
an improved jump height compared to the second, the par-
ticipants were allowed to continue until a set resulted in a
decline in performance. Jump height was determined as the
center of mass displacement, calculated from take-off force
development and force plate-measured body mass with the
provided software (Force Platform Software Suite, Version
2.6.51). The single best result was noted and used for sta-
tistical analysis. Test-retest measurements revealed a CV
of 9.7%, 5%, and 6% for jump height, maximal power, and
maximal force, respectively.
Maximal isometric strength,
muscular activation level, and RFD
Peak torque, muscular activation level, and RFD were
measured by maximal voluntary isometric contractions
(MVC) of the right knee extensor muscles, while seated
in a knee extension machine (Knee extension, Gym2000;
Software: Acq Knowledge 4.4, Biopac systems Inc) in-
strumented with a load cell (U2A, Hottinger Baldwin
Messtechnik GmbH). The seat was adjusted to 100- and
90- degrees hip and knee flection, respectively, the mo-
ment arm pad proximal to the ankle and the knee axis of
rotation coincided with that of the apparatus. The partici-
pants were strapped across the hip, chest, and ankle of the
right leg to minimize any joint movement. Adjustments
were recorded to ensure consistent positioning between tri-
als. All participants were instructed to contract as hard and
as rapidly as possible. After three submaximal warm-up
contractions (~50%, 75%, and 90%), five MVCs were per-
formed with 60seconds rest intervals. Peak torque, defined
as the maximum voluntarily achieved value across the five
MVCs, was used in the data analyses. RFDmax, defined as
the maximum positive change of force over 10ms intervals
from initiation of contraction, as well as torque at 100ms
(from initiation of contraction) was extracted from the soft-
ware. The recordings had a sampling frequency of 1000Hz
and were smoothed with a moving average of 10 samplings
before analyses.
Of the five MVCs, three were un-evoked and two were
evoked utilizing the interpolated twitch technique (ITT).32
The MVCs were performed in an alternating fashion,
beginning and ending with an un-evoked contraction. The
maximal voluntary activation level across the two attempts
is presented. The peak torque of un-evoked MVCs controlled
whether the evoked were in fact maximal contractions and
contractions with torque prior to stimulus below 80% of peak
torque were defined as submaximal and excluded from fur-
ther calculation of activation level (n=6). Two self-adhesive
surface electrodes (Veinoplus, 8×13cm, Oval shape, Ad
Rem Technology) were positioned over the quadriceps of the
right leg, one proximally and one distally, in a medial-lateral
position to target as many muscle bellies as possible. An in-
tensity test was performed in rested state after the warm-up,
to determine the stimulus output level for the ITT. The stim-
uli were given as 200µs, 400V single-imposed signals from
a digitimer (Digitimer DS7AH HV Constant current stimu-
lator, Digitimer Ltd.), with successive increments until the
evoked force amplitude was no larger than the previous. To
ensure maximal evoked force, a 10% increase was added to
the stimulus output, equating totally to 660-990mA. Four
“singlet” stimulations about 5seconds apart and one dou-
ble-imposed stimulus at this output were given as familiar-
ization with the stimuli. The “doublet” was given as a 10ms,
100 Hz-stimulus (Digitimer DG2A Train/Delay Generator,
Digitimer Ltd.) and was used during the evoked MVC.
The MVC was evoked at the peak of contraction, about
0.5seconds after initiation, and again as the quadriceps had
relaxed and the force curve had returned to baseline. The
percentage muscle activation level was determined with the
following equation32:
where D is the difference between the voluntary and evoked
If submaximal voluntary force was achieved during the
evoked contractions, the calculated muscle activation % was
corrected by replacing Peak forceMVC in Equation (1) with the
peak force across the un-evoked contractions. Test-retest mea-
surements revealed a CV of 9.7%, 7.1%, and 18.3% for peak
torque, muscle activation level, and RFDmax, respectively.
1-repetition maximum
The participants completed 1-repetition maximum (1RM) in
the squat followed by bench press (T-100G, Eleiko Sport).
A standardized warm-up was performed consisting of three
Muscle activation %
Mean forceMVCpre stimulus
Peak forceMVC
Peak forceEvoked at rest
Peak forceEvoked MVC
Mean forceMVC pre
NORUM et al.
sets with gradually increasing load (50-75-90% of maximal
familiarization 1RM) and declining number of repetitions (8-
4-1). After 2minutes rest, the first attempt was performed at
95% of maximal familiarization 1RM. After each success-
ful attempt and 3-minute rest periods, the load was increased
by 0.5%-5% (smallest increment 0.5kg) until the participant
reached voluntary failure. If the lift was unsuccessful, the
load was decreased (0.5%-5%) for another attempt until 1RM
was determined. The bench press 1RM test was performed
in the same manner with a preceding 5-minute rest follow-
ing the squat RTF test (Figure1). A Smith rack was used to
prevent substantial change in the technique during the squats.
Intra-individual control of equipment utilized (limited to
weight lifting shoes, belt, wrist support, and knee sleeves),
squat stance and bar position, bench press set up, and grip
distance that the participants were accustomed to were noted
and reproduced in the second trial. The CV for this test was
2.3% for squats and 2.4% for bench press, and number of at-
tempts were 4-6 and 3-5, respectively.
Muscular endurance and perceived
exertion and pain
Repetitions to failure (RTF) were performed with 60% of
maximal familiarization 1RM to ensure equal absolute load.
The repetitions were counted out loud and a smart phone
metronome application (Tap Metronome v1.2.1, Daniel
Soper) was set to 15 BPM/4-seconds intervals to standard-
ize the repetitions. The technical requirements were (a) depth
equating to hips below parallel and maintaining an upright
torso position, and (b) a controlled change of direction and
fully extended arms in the top position, for squats and bench
press, respectively. If unable to complete a repetition within
the two metronome signals, the following repetition had to
be completed in time, otherwise the previous repetition was
counted as the last. Failure was otherwise defined as failure
to complete the repetition at all. The CV for this test was
2.0% for squats and 2.4% for bench press.
From pilot testing and previous studies at 60% of 1RM,25,27
it was expected that the participants would complete over
20 repetitions in both the squat and bench press RTF test.
Following repetition 10, the participants gave ratings of per-
ceived exertion from the 11-point Borg RPE C-10 scale (0
[rest] to 10 [maximal exertion]). Perceived pain was rated
from the 11-point NRS perceived pain scale (0 [no pain] to
10 [worst imaginable pain]) immediately after the RTF tests.
Plasma analysis
All samples were centrifuged for 10minutes at 3000rpm,
1700 g, and 4°C (Heraeus Megafuge 16R, ThermoFisher
Scientific, Thermo Electron LED GmbH) before transferring
plasma to two 1.5mL micro tubes (MCT-150-C, Axygen, Inc
for storage at −80°C until further preparation and analyses.
Samples were analyzed in duplicate with reverse phase
LC-MS (Dionex Ultimate HPLC 3000 system; Agilent TOF
6230, positive electrospray ionization [ESI]), based on the
method used by Chen et al.33 We were not able to separate
paraxanthine and theophylline; hence, all paraxanthine anal-
yses included small contributions (~4% of total caffeine me-
tabolites concentration) from theophylline.34 Individually
prepared quality control samples at three concentration levels
and a blank sample were included in each run of the plasma
analyses. Limit of detection (LOD) and limit of quantifica-
tion (LOQ) were determined based on signal-noise ratio to
be <0.008 μg·mL−1 and <0.05µg·mL−1, respectively. In all
samples where the analytes were non-detected or estimated
<LOQ, values were substituted with worst case scenarios
equal to LOD and LOQ, respectively, that is biased high,
to enable statistical analyses comparing baseline to 60- and
Statistical analyses
The sample size was calculated using a priori t tests for
paired samples to ensure sufficient statistical power in
the main analyses (G*Power version 3.1, Heinrich-Heine
University).35 With α-level set at 0.05 for the main outcomes
and a 1-β error probability of 0.8, we used the mean and
SD from Goldstein et al27 to calculate the sample size. Ten
participants were needed to detect a true mean difference in
1RM strength of 0.8kg (1.54% difference). Due to an ex-
pected drop out of 25%, we aimed to recruit a minimum of 15
subjects for the present study.
All variables’ distributions were tested with the Shapiro-
Wilks normality test and assessing skewness, kurtosis, and
histograms. Paired sample t tests and Wilcoxon signed-rank
tests were performed on paired differences with Gaussian
and non-Gaussian distribution, respectively, and P < .05
was considered statistically significant. Values are given as
mean±SD and median (confidence interval) for parametric
and non-parametric tests, respectively. To assess “practical”
significance, Hedge's g values were calculated with weighted
and pooled SD’s and adjustment for samples n<50. Effect
size cutoffs were defined as <0.25, 0.25-0.5, 0.5-1.0, and
>1.0 for trivial, small, moderate, and large effect sizes, re-
spectively.36 Values are given as mean±SD and as median
(confidence interval) for parametric and non-parametric tests,
respectively. The ergogenic effects of caffeine dependent of
order of trials and caffeine identification were assessed with
unpaired t tests. Pearson r correlation was assessed between
habitual caffeine intakes and delta caffeine effects. CV for the
main outcomes was calculated from the two familiarizations
NORUM et al.
and the last familiarization and the placebo trial, and the
largest was consistently chosen throughout. Statistical anal-
yses were performed using GraphPad Prism 7.0 (GraphPad
Software, Inc).
There were no significant differences in the macronu-
trient intake prior to each of the trials (carbohydrate
[P=.39], fat [P=.62], protein [P =.59]) and overall
energy intake (P=.77), or in withdrawal symptoms (all
P>.16) or BRUMS mood score on commencement of
either trials (all P>.42). On the post-trial question about
which treatment the participants thought they received,
seven participants (44%) correctly guessed the treatment
order (ie, correctly guessed both conditions), stating rest-
lessness, heart palpitations, and or increased energy and
motivation as reasons for guessing caffeine. However,
10 participants (66%) total correctly identified caffeine
independent of identifying placebo. No differences were
observed in the effects of caffeine between the identifiers
and non-identifiers of the caffeine condition (all P>.20,
see Appendix Table A1) or by the order of trials (all
P>.13, see Appendix TableA2). All performance and
plasma caffeine concentration data are shown in Tables 2
and 3, respectively.
Countermovement jump
The mean CMJ jump height, maximal power, and maximal
force across the two trials were 33±2 cm, 2893 ±74 W,
and 1570±26N, respectively. Jump height and peak power
increased by 2.3±1.1cm (7.6±4.0%) and 105 ± 63 W
(3.8±2.2%), respectively (Table 2; Figure3). No difference
was observed in peak force.
Maximal isometric strength, rate of
force development, and muscle activation level
The mean peak torque, RFDmax, and activation level across
the two trials were 177±6 Nm, 19 ±1 Nm·10ms−1, and
86 ± 1% muscle activation, respectively. Caffeine signifi-
cantly increased peak torque of the knee extensors by 11 Nm
(CI: 2-18Nm), corresponding to 4.6±7.3%, compared to
placebo (Figure 2). No difference was observed with caf-
feine on muscle activation level (−2±4%, n=9, Figure2),
RFDmax (1.1±4.9 Nm·10ms−1 [9.2±26.5%], Figure3), or
torque at 100ms (−2.9±26.2Nm, Table 2). Six participants,
in one or both of the trials, had a substantially lower force
output during the evoked MVC than the unevoked MVC. The
force output during the evoked MVC was 26%-78% of the
peak torque contraction in these six participants, whom were
excluded from the statistical analyses.
TABLE 2 The effect of caffeine on performance outcomes
Performance outcomes Placebo Caffeine
Mean of
Δ±SD 95% CI P-value
size- Magnitude
CMJ jump height (cm) 32.0±4.7 34.3±4.5 2.3±1.1 1.7, 2.9 <.001 0.44 - Small
CMJ peak power (W) 2840±430 2946±430 105±63 71, 140 <.0001 0.21 - Trivial
CMJ peak force (N) 1550±247 1588±247 37±96 −16, 91 .16 0.13 - Trivial
MVC peak torque (Nm) 173±29 181±31 11a 2, 18 .02 0.23 - Trivial
MVC activation level (%) [n=9] 87±5 85±5 −2±4 −5, 1 .16 −0.35 - Small
MVC RFDmax (Nm.10ms−1) 15±5 17±6 2±5 −0.5, 4.5 .10 0.34 - Small
MVC Torque100ms (Nm) 75±24 72±29 −3±26 −17, 12 .67 −0.09 - Trivial
1RM Squat (kg) 96±14 100±13 4±1 3, 5 <.001 0.27 - Small
RTF Squat (repetitions) 39±17 45±17 5.8±6.2 2, 9 .003 0.27 - Small
RPE Squat rep 10 6±1 6±1 −1a −1, 1 .67 0.05 - Trivial
PP Post-squat 8±1 9±2 0a −1, 0 .60 0.07 - Trivial
1RM Bench press (kg) 66±10 68±11 2±1 2, 3 <.001 0.18 - Trivial
RTF Bench press (repetitions) 21±6 23±6 2±3 0, 3 .01 0.27 - Small
RPE Bench press rep 10 7±1 7±1 0a −1, 1 >.99 0.09 - Trivial
PP Post-bench press 8±2 7±1 0a −1, 0 .14 0.27 - Small
Note: Values are presented as mean±SD or mediana and 95% confidence intervals.
Abbreviations: 1RM, one repletion maximum; CI, 95% confidence interval; CMJ, countermovement jump; Δ, difference between trials; MVC, maximal voluntary
contractions; PP, perceived pain; RPE, rating of perceived exertion; RTF, repetitions to failure.
aNon-Gaussian distributed paired differences tested with Wilcoxon paired rank test.
NORUM et al.
1-repetition maximum
The mean absolute weight lifted across the two trials
was 98.4 ± 2.4kg and 66.6±1.5kg for squat and bench
press, respectively. Compared to placebo, caffeine inges-
tion increased 1RM in the squat and in the bench press by
4.1±1.4kg (4.5±1.9%) and by 2.2±1.0kg (3.3±1.4%)
(see Table 2 and Figure2).
Muscular endurance and perceived
effort and pain
The mean absolute weight lifted during the RTF test (60%
of familiarization 1RM) was 58±8kg and 39±6kg in
squats and bench press, respectively. Caffeine significantly
increased squat RTF by 5.8±6.2 repetitions (15.9±17.9%)
and bench press RTF by 1.8±2.5 repetitions (9.8±13.6%),
compared to placebo (Table 2; Figure4). No differences be-
tween trials were found in intra-set RPE at repetition 10 or in
at-failure perceived pain (Table 2).
Plasma caffeine concentration
Upon arrival on both trial days, plasma caffeine concentra-
tions were negligible, that is, not detected or <LOQ in all
participants except two in the placebo trial and one in the
caffeine trial (all 0.4 μg·mL−1). At baseline, theobromine
was significantly higher in the placebo compared to the
caffeine trial (P=.03); however, 8 and 9 of the individual
values, respectively, were below LOQ. Due to the choco-
late protein powder administered all participants, theobro-
mine was significantly increased from baseline to 60 and
TABLE 3 The effect of caffeine on plasma concentrations
Caffeine Placebo
Baseline 60 min 270 min Baseline 60 min 270 min
Caffeine (μg·mL−1) 0.0±0.1 3.6±0.8a,c 3.1±0.9a,b,c 0.1±0.2 0.1±0.1 0.0±0.0
Paraxanthine (μg·mL−1) 0.1±0.1 0.8±0.4a,c 1.7±0.8a,b,c 0.1±0.2 0.1±0.1 0.2±0.2
Theobromine (μg·mL−1) 0.0±0.0 0.5±0.1a 0.7±0.1a,b 0.2±0.3c 0.6±0.4a 0.8±0.4a
TC (μg·mL−1) 0.2±0.3 4.9±0.9a,c 5.6±1.0a,b,c 0.4±0.6c 0.8±0.6 1.0±0.6a
Note: All baseline and placebo mean values are based on several substituted values for non-detected and non-quantifiable measurements equal to limit of detection
and limit of quantification, respectively, and thus, should be interpreted with caution. Paraxanthine concentrations include a small contribution of the metabolite
Values are presented as mean±SD.
Abbreviation: TC, total concentration of metabolites.
aDifferent from within condition baseline (P<.05).
bDifferent from within condition 60 min (P<.05).
cDifferent from between condition corresponding time-point (P<.05).
FIGURE 2 Effect of caffeine on
maximal strength and activation level.
Individual results (dotted lines) and
mean±CI (solid lines) are presented for (A)
squats; and (B) bench press 1RM; (C) MVC
peak torque and (D) MVC activation level
of the knee extensors (n=9). *Significantly
different from placebo (P<.05). CI,
95% confidence interval; MVC, maximal
isometric voluntary contraction
NORUM et al.
270minutes during both trials (all P<.01) with no differ-
ences between trials (P>.05). No other analyte increased
from baseline during the placebo trial (all P>.05). In the
caffeine trial, plasma caffeine concentration increased to
3.6±0.8 (P< .001) and 3.1 ± 0.9µg·mL−1 (P<.001)
60 and 270 minutes following ingestion, respectively,
confirming intention to treat (Table 3). Paraxanthine and
total metabolite concentration significantly increased from
baseline to 60minutes and 270minutes following caffeine
ingestion (all P>.001, Table 3).
The habitual caffeine intake was 341±184mg·d−1, cor-
responding to 5.4±2.9mg·kg−1, while the administered
dose of 4mg·kg−1 equated to 254±20mg. The partici-
pants were moderate to high caffeine consumers (n catego-
rized as low, moderate, high: 2, 5, 8, respectively). Only
the effect of caffeine on muscular endurance was signifi-
cantly correlated with the habitual intakes (Pearson r=.52,
P=.045 and r=.58, P=.024 for squat and bench press
RTF, respectively).
This study investigated the acute effects of 4mg·kg−1 caf-
feine ingestion on maximal isometric and dynamic mus-
cle strength, power, activation level, RFD, and muscular
endurance in resistance-trained females during the early
follicular phase. There were several notable findings in
the present study. Caffeine ingestion increased dynamic
strength measured as 1RM in squat and bench press and
isometric knee extension torque, leg muscle power and
jump height in CMJ, and improved both squat and bench
press muscular endurance measured as repetitions per-
formed until failure at 60% of 1RM. However, no effect of
caffeine was observed on RFD, muscle activation, or affect
perceived exertion and pain.
In this study, caffeine increased maximal upper body
strength, which is in agreement with Grgic et al's recent me-
ta-analysis,4 as well as the study by Goldstein et al27 who found
increased bench press 1RM performance (1.5%) in 15 resis-
tance-trained females. It is suggested that smaller upper body
muscles are less affected by caffeine than larger lower body
muscles,6 which has been implied by studies on for example
elbow flexors, not showing effects on maximal strength with
FIGURE 3 Effect of caffeine on rate
of force development and countermovement
jumps. Individual results (dotted lines) and
mean±CI (solid lines) are presented for
(A) RFD max during MVC of the knee
extensors; (B) CMJ jump height; (C) CMJ
Peak force; and (D) CMJ Peak power.
*Significantly different from placebo
(P<.05). CI, 95% confidence interval;
CMJ, countermovement jump; MVC,
maximal isometric voluntary contraction;
RFD, rate of force development
FIGURE 4 Effect of caffeine on muscular endurance. Individual results (dotted lines) and mean±CI (solid lines) are presented for (A) squats
and (B) bench press repetitions to failure at 60% of familiarization-1RM. *Significantly different from placebo (P<.05). CI, 95% confidence
NORUM et al.
caffeine.23 Moreover, the positive associations seen between
strength and muscle activation with caffeine suggests that mus-
cles with high baseline activation level, such as upper body
muscles like the elbow flexors, would likely be less affected
by caffeine, that is,there is less room to improve.6 However,
in studies examining multi-joint upper body exercises, there
seems to be an overall trend that caffeine has positive effects on
strength.4 This discrepancy might be explained by more mus-
cle mass being recruited as compared to single joint arm exer-
cises, including several muscles with varying activation levels,
which might potentiate the effect of caffeine. The present re-
sults support that multi-joint upper body strength is indeed af-
fected by caffeine, although possibly still less than lower body
strength (3.3% [ES:0.20] vs 4.5% [ES: 0.25] increase in bench
press and squat 1RM, respectively).
A novel finding of this study was that a dose of only
4mg·kg−1 caffeine induced a similar or even greater effect
on bench press 1RM than a dose of 6mg·kg−1 in the study by
Goldstein et al27 (+3.3% vs +1.5%, respectively). The slight
difference in performance between our study and Goldstein
et al27 may partly be explained by severity of adverse events
occurring during the caffeine trial. Three participants felt
“shaky” and the remaining participants reported no adverse
events in the present study, as opposed to three participants
“exhibiting intense emotional responses” in the study by
Goldstein et al,27 who reported habitual caffeine intakes of
only 0-41mg·d−1. The difference in side effects may be ex-
plained by the lower acute dose of caffeine (4 vs 6mg·kg−1)
and possibly due to higher habitual caffeine intakes in the
present trial (341±184mg·d−1).
Even though habitual caffeine intake may influence the
prevalence of adverse events, it might not affect exercise per-
formance. A study,37 although on endurance performance,
found that acutely ingesting 6mg·kg−1 caffeine increased
performance irrespective of whether the daily habitual in-
take was low (0.8mg·kg−1), moderate (1.9mg·kg−1), or
high (4.6mg·kg−1) and that habituation was not correlated
with performance. This is in line with the results of the
present study, and in addition, and contrary to the above
study, we report the same for participants habitually con-
suming more than the acute dose administered (4mg·kg−1
vs 5.4 mg·kg−1·d−1, respectively). Importantly, habitual
caffeine may be consumed in small doses over the day, so
an acute dose of 4mg·kg−1 may induce higher peak plasma
concentration levels than many habitual consumers will
experience by administering 5.4 mg·kg−1·d−1 daily. This
raises the question if the use of high doses is necessary to
achieve an equally or potentially better ergogenic effect as
seen in the example with Goldstein et al's study.27 Thus,
future research should explore optimal caffeine dosage in
relation to habituation.
Squat 1RM increased (+4.5%) significantly in this study,
as opposed to Grgic et al's meta-analysis,4 who observed no
overall effect on lower body maximal strength. However,
very few studies have been conducted examining dynamic,
multi-joint maximal strength in females, indicated by only
three included in the above meta-analysis from 2018.25,27,38
Two of the three studies investigated lower body maximal
strength, in which one observed an effect of caffeine and the
other a trend of increased performance.25,38 Thus, one could
speculate whether females could have a greater effect of caf-
feine on lower body dynamic strength compared to males.
Furthermore, Grgic et al4 discuss that the included studies did
not report the reliability of their strength tests. In the present
study, we report a low CV (2.3%) for the squat 1RM, which
could partly explain why we were able to detect an effect of
Although no sex differences have been reported on the
ergogenic effects of caffeine on exercise performance,2 only
two studies,15,16 to our knowledge, have investigated caf-
feine's effects on sex differences with strength-power modal-
ities, showing similar effects (or lack of effects) of caffeine
in both males and females.15,16 As previously mentioned,
fluctuating hormone levels with the phases of the menstrual
cycles can alter caffeine metabolization speeds,12 as well as
neuromuscular function,20 and ultimately the ergogenic ef-
fects of caffeine. As an example, a recently published study
showed that half squat velocity was increased by 1.4%, 5%,
and 5.3% in the early follicular, late follicular, and mid-luteal
phase, respectively.17 Thus, ensuring caffeine research in fe-
males is conducted during the same menstrual cycle phase
is important and furthermore, which phase could potentially
affect the effect size. Moreover, only one15 of the two stud-
ies comparing effects of caffeine on strength performance
between the sexes controlled for menstruation cycle phase.
Therefore, further research is still warranted to establish
whether sex differences in ergogenic effect of caffeine on
maximal strength occur.
The effect of caffeine on maximal isometric strength ob-
served in this study is in agreement with Warren et al's me-
ta-analysis findings,6 who found caffeine to have a moderate
effect on isometric knee extensor strength. On the other hand,
Ali et al14 found no effects of 6mg·kg−1 caffeine on knee ex-
tensor isometric strength in women. However, their protocol
measured maximal muscle strength between fatiguing blocks
of sprints and consequently, might have masked a caffeine-in-
duced effect on maximal strength.
No effect of caffeine on voluntary muscle activation
of the knee extensors was observed in the present study.
Previous studies such as Behrens et al22 demonstrated that
strength enhancements by caffeine are associated with in-
creased voluntary activation, and the meta-analysis by
Warren et al6 showed that caffeine has an moderate effect
on voluntary muscle activation. On the other hand, Meyers
& Cafarelli39 found no effect of caffeine on muscle activa-
tion level after ingesting 6mg·kg−1 of caffeine. The initial
NORUM et al.
activation level in Meyers & Cafarelli's39 study was ~94%
compared to 70%-80% in the study by Behrens et al,22
which may suggest that baseline muscle activation level
may affect the results, that is, the higher baseline level the
less room to improve. In the present study, the participants
had a muscle activation level of 85%-87%, which could
partly explain why we did not detect any effects of caffeine.
Six participants (excluded from analysis of activation level)
found it especially difficult to maximally contract during
the ITT compared to the un-evoked contractions, indepen-
dent of treatment. These participants’ maximal force output
when knowing they would be stimulated was ~25%-75%
lower than the intra-trial peak force, although they reported
that they felt they were contracting as forcefully as possi-
ble. Thus, there may be a psychological factor (ie, being
afraid of the electrical stimuli) inhibiting the voluntary con-
traction when knowing electrical stimuli would be given.
Potentially, this might be overcome with further familiar-
izations to increase the reliability of the test, that is, more
than two as in the present study. However, this is a well-
known negative effect of stimulus anticipation in the ITT
The main mechanism by which caffeine induces ergo-
genic effects on muscular strength and power is thought to in-
volve supra-spinally-driven increases in muscle activation.14
Surprisingly, we did not observe any difference between
conditions in muscle activation level or RFDmax, despite
demonstrating effects in 1RM strength, isometric strength,
and power. However, the high CV revealed especially for
RFDmax (18.3%) in the present study increases the risk of
a type II error as the statistical power might have been too
low to detect a possible effect. Nevertheless, this is a com-
mon challenge and even higher CVs than demonstrated in
this study are typically reported for RFD in the literature.41
Furthermore, RFD is closer associated to the rate of mus-
cle activation (RMA) rather than just muscle activation per
se, as demonstrated by a recent study showing that the pre-
ceding effective motor neuron drive to the muscle influences
changes in RFD.42 Unfortunately, we did not measure RMA
in the present study. It could be speculated that the influence
of caffeine on changes in RMA is not as profound as with
other strength-power measures.
In parallel to the observed effect on muscle strength but
in contrast to the lacking effect on RFD, caffeine ingestion
improved performance and power measures in the CMJ; the
participants jumped 2.3cm higher with caffeine than in the
placebo trial. In line with previous divergent results of caf-
feine effects on maximal strength, the acute effects of caf-
feine ingestion on strength-power performance and RFD are
inconsistent, but most studies show significant increased
lower body power during countermovement jumps.4 In a
subgroup meta-analysis,4 training status indicated a signifi-
cant effect for athletes, but not for non-athletes. Although our
participants were not athletes, one could speculate that the
training status of our participants might have contributed to
the positive effect of caffeine. Altogether, the evidence sug-
gests that caffeine acutely improves power, which is in line
with our results.
Finally, 4 mg·kg−1 caffeine ingestion also significantly
increased muscular endurance in both lower (~16%) and
upper body (~10%,) exercises in this study. These results
are in agreement with Duncan et al,43 who found 5mg·kg−1
caffeine to increase the number of bench press repetitions to
failure (60% of 1RM) in men. On the other hand, these results
are in disagreement with other studies in females who did not
find any effects on muscular endurance.16,25,27 However, hor-
mone concentration and hormone contraceptive use were not
controlled for16,25,27 and one did not report familiarization,16
while the other two only performed one familiarization ses-
sion.25,27 In the present study, the participants who performed
three familiarization sessions were mainly participants with
CV>9% in the muscular endurance tests. Hence, there could
have been a masking of the caffeine effect in the studies with
only one or no familiarization, due to continued learning ef-
fect in both trials.
Caffeine reducing pain perception and RPE is a possible
mechanism for increased performance,21 and, as mentioned
in the introduction, in a 2005 meta-analysis, Doherty et al,24
observed that a ~5% reduction in RPE during, as opposed
to at-failure, explained about a third of the variance in ex-
haustive work performance between caffeine and placebo.
However, and albeit the analgesic effects might be easier to
observe when assessed intra-set compared to at-failure (due
to an assumed greater relative difference in motor output be-
tween trials when caffeine increases number of repetitions
performed), no difference in intra-set RPE was observed be-
tween the caffeine and placebo trials in the present study. The
fact that RPE was assessed only one time during the set and
that a lower dose was used than most of the included studies
in the meta-analysis (4 vs 6mg·kg−1) could explain why no
difference in intra-set exertion was observed.
Total caffeine concentration and the individual metabo-
lites were significantly higher at 270minutes as compared
to 60minutes after ingestion, whereas caffeine tended to be
lower. Theophylline and paraxanthine can contribute to the
pharmacological effect on the central nervous system as these
also inhibit the adenosine A1 and A2 receptors.44 Theophylline
is considered to be three to five times more potent than caf-
feine, and paraxanthine may be as potent as caffeine.44 Thus,
we can expect that the participants in the present study had
similar effects of caffeine throughout the test protocol (60-
270minutes following ingestion), and we did indeed observe
significant effects both on the first (CMJ), as well as the last
(bench press RTF) test of the protocol.
Controlling for hormone concentrations in the way which
was used in the present study is cost- and time-efficient,
NORUM et al.
when assuming the participants are having no health is-
sues that would affect their hormones around the menstrual
cycle. To our knowledge, this is only the second study on
the effects of caffeine on strength performance to control
for oscillations in reproductive hormones in this way.15
However, we did not confirm the hormone concentrations
in blood samples, which would be a strength of future stud-
ies. Recently, as mentioned, the first study on the effect
of caffeine on half squat velocity during three phases of
menstrual cycle was published.17 Nevertheless, we recom-
mend that further studies compare the effects of caffeine on
female strength and power performance between the men-
strual cycle phases to establish the interaction of female
reproductive hormones on the ergogenity of caffeine. This
is warranted to further optimize personalized recommen-
dations for caffeine use in female athletes and will inform
future research on caffeine in females. Another strength of
this study is the blinding efficacy check, a potential bias
in the caffeine literature, as recently discussed by Painelli
et al45 and Pickering and Grgic.46 Although 66% partici-
pants correctly guessed when they ingested caffeine, no
difference in performance was observed between these and
those that guessed incorrectly in the present study. Thus,
the performance increments observed in the caffeine trial
do not seem to be due to the placebo effect.
A limitation of this study is a skewed counterbalance of
treatment order arising due to dropout after randomization.
Consequently, ten participants received placebo and five
participants received caffeine in the first trial. However, we
could not detect an effect of treatment order. All participants
had an effect of caffeine irrespective of order of trial on CMJ
jump height and power and on maximal strength, and fur-
thermore, 12 of the 15 participants performed better with
caffeine in the muscular endurance and isometric strength
tests. Nevertheless, the low statistical power in the analyses
of treatment order in the latter outcomes increases the risk of
type II error.
In conclusion, ingestion of 4mg·kg−1 caffeine 60minutes
prior to tests improved maximal strength and power in highly
resistance-trained females during the early follicular phase of
menstruation. The caffeine supplementation also increased
muscular endurance in both upper and lower body exercises
without differences in perceived exertion or pain. Furthermore,
very few adverse events were reported, and caffeine-induced
ergogenic effects were observed although the participants ha-
bitually were consuming in excess of the acute dose.
These findings of 3%-5% improvement on maximal
strength and power could potentially be relevant to female
strength and power athletes, where the margins between top
placements in competition can be small. However, within-
individual differences in performance need to be taken into
account and the acute effects of caffeine may be smaller in a
competitive context due to increased arousal. Performance
effects of caffeine during the different menstrual cycle
phases should be investigated further. Establishing whether
menstrual cycle phase affects the ergogenity of caffeine al-
lows optimization of personalized recommendations and
will inform future caffeine research. Furthermore, further
examination of the potential sex differences in the ergo-
genic effect of caffeine on strength and power is warranted.
At the time being, such research should take into account
the effects of menstrual cycle phase. Lastly, the long-term
effects of chronic caffeine supplementation on resistance
exercise adaptations have not been investigated and are thus
All authors declare no conflict of interest. No funding was
received for this study. The authors would like to thank Dr
Hans Kristian Stadheim for the preparation, blinding and ad-
ministering of the treatments, and to all the research partici-
pants for partaking in this study.
MN, LCR, TB, LD, and TR involved in conception and de-
sign. MN, LCR, TB, POR, MB, and TR involved in acquisi-
tion of data, and/or analysis and interpretation of data. MN
and LCR drafted the manuscript. MN, LCR, TB, LD, POR,
MB, and TR revised the manuscript.
Linn Christin Risvang https://orcid.
Thomas Bjørnsen https://orcid.
Lygeri Dimitriou https://orcid.
Per Ola Rønning
Truls Raastad
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Additional supporting information may be found online in
the Supporting Information section.
How to cite this article: Norum M, Risvang LC,
Bjørnsen T, et al. Caffeine increases strength and power
performance in resistance-trained females during early
follicular phase. Scand J Med Sci Sports. 2020;30:2116–
... In recent years, several studies explored caffeine's effects on vertical jumping performance in females (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). This topic appears to be gaining in popularity as only in 2021 and 2022 there have been six studies on the topic (11-13, 16, 21, 22). ...
... However, while there is an increase in the evidence base, the findings between studies remain conflicting. For example, some studies reported an ergogenic effect, while others suggest that caffeine may not enhance vertical jumping performance in females (9,10,15,19). The variation in between-study findings could be due to several reasons, such as the phase of the menstrual cycle at which the data was collected. ...
... Caffeine dose may also impact its ergogenic effect, as a low dose (e.g., 2 mg/ kg) may not yield similar effects as higher doses (10,11). Studies on the topic have varied in their methodological approach (e.g., testing at different phases of the menstrual cycle or times of day), and currently, the influence of these moderator variables on jumping performance in females has yet to be thoroughly examined (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). ...
We aimed to perform a systematic review and meta-analysis of caffeine's effects on vertical jumping performance in females, with subgroup analyses for potential moderators, including phase of the menstrual cycle, testing time of day, caffeine dose, and test type. Fifteen studies were included in the review (n = 197). Their data were pooled in a random-effects meta-analysis of effect sizes (Hedges' g). In the main meta-analysis, we found an ergogenic effect of caffeine on jumping performance (g: 0.28). An ergogenic effect of caffeine on jumping performance was found when the testing was carried out in the luteal phase (g: 0.24), follicular phase (g: 0.52), luteal or follicular phase (g: 0.31), and when the phase was not specified (g: 0.21). The test for subgroup differences indicated that the ergogenic effects of caffeine were significantly greater in the follicular phase compared to all other conditions. An ergogenic effect of caffeine on jumping performance was found when the testing was carried out in the morning (g: 0.38), evening (g: 0.19), mixed morning or evening (g: 0.38), and when time was not specified (g: 0.32), with no subgroup differences. An ergogenic effect of caffeine on jumping performance was found when the dose was ≤3 mg/kg (g: 0.21), or >3 mg/kg (g: 0.37), with no subgroup differences. An ergogenic effect of caffeine on jumping performance was found in the countermovement jump test (g: 0.26) and squat jump test (g: 0.35), with no subgroup differences. In summary, caffeine ingestion is ergogenic for vertical jumping performance in females, and it seems that the magnitude of these effects is the largest in the follicular phase of the menstrual cycle.
... Despite the improvement generally caused by caffeine intake in V mean , V peak , W mean and rate of force development (RFD) in resistance exercises, most studies focused on exploring separately upper (e.g., bench press) or lower body exercises (e.g., squat) [10]. Only in a few studies, upper and lower-body exercises were examined in the same trial at different loads [5,6,9,[18][19][20][21][22][23]. These studies showed that when low caffeine doses were provided (3 mg/kg), the ergogenic effect of this substance was observed in V mean at low-to-moderate loads (25-50%1RM) but not moderate-tohigh loads in upper (e.g., bench press) and lower-body (e.g., back squat) exercises [5]; and that moderate caffeine doses (6 mg/kg of body mass) are required to observe ergogenic effect at moderate loads (75% 1RM) [6]. ...
... These studies showed that when low caffeine doses were provided (3 mg/kg), the ergogenic effect of this substance was observed in V mean at low-to-moderate loads (25-50%1RM) but not moderate-tohigh loads in upper (e.g., bench press) and lower-body (e.g., back squat) exercises [5]; and that moderate caffeine doses (6 mg/kg of body mass) are required to observe ergogenic effect at moderate loads (75% 1RM) [6]. Besides, in muscular endurance, low-to-moderate caffeine doses (2-6 mg/ kg of body mass) improved the number of repetitions of lower-body (e.g., squat) but not upper-body exercises (e.g., bench press) in female athletes at 40%1RM [19] and resistance-trained males at 60%1RM [18], except in one study where resistance-trained females in the early follicular phase improved the total number of repetitions in both exercise types at 60%1RM [20]. However, more research is needed to elucidate if the improvement in the number of repetitions can be translated to an improvement in velocity, power or force production. ...
... RFD has been revealed as an interesting marker of sportspecific performance [29], related to, for instance, vertical jump performance [30]. Although previous studies have reported inconclusive results regarding the effect of caffeine n RFD [20,31], a more recent meta-analysis indicates caffeine increases RFD, however, a small-to-moderate effect was found when 3-5 mg/kg doses of caffeine were used [27]. In our study, 3 mg/kg of caffeine improves RFD at 50% 1RM (14%), 75% 1RM (17%) and 90% 1RM (97%) in back squat but not in the bench press exercise. ...
Full-text available
IntroductionAlthough acute caffeine intake seems to improve muscular strength–power–endurance performance, there is scarce evidence evaluating upper vs lower-body exercises at different loads. Thus, this study aimed to examine the effects of acute caffeine intake on upper and lower-body muscular strength, power and endurance performance at different loads.Methods Twenty resistance-trained athletes (male/female: 10/10; age: 23 ± 4 years; body mass: 70.6 ± 15.1) participated in a double-blind, placebo-controlled, cross-over and randomized study. Participants were provided with either 3 mg/kg of body mass of caffeine or maltodextrin (placebo). Sixty minutes after ingestion, they performed muscular strength and power assessment for bench press and back squat exercise at 25%, 50%, 75% and 90% 1-repetition-maximum (1RM), performing 3, 2, 1 and 1 repetitions respectively, followed by muscular endurance assessment for both exercises at 65% and 85% 1RM performing until task failure. Isometric handgrip, isometric mid-thigh pull and vertical jump tests were also performed.ResultsIn muscular strength and power, compared to placebo, caffeine improved mean velocity (P = 0.045; pη2 = 0.101), mean power (P = 0.049; pη2 = 0.189) and rate of force development (RFD, P = 0.032; pη2 = 0.216), particularly in back squat exercise at 75% and 90% 1RM where mean velocity increased by 5–7% (P = 0.48–0.038; g = 0.348–1.413), mean power by 6–8% (P = 0.050–0.032; g = 0.547–0.818) and RFD by 17–97% (P = 0.042–0.046; g = 1.436–1.196). No differences were found in bench press exercise. In muscular endurance, caffeine improved the number of repetitions in all exercises and loads (P = 0.003; pη2 = 0.206), but only in back squat exercise at 85% 1RM, caffeine increased mean and peak velocity (8–9%, P = 0.006–0.004; g = 2.029–2.075), mean and peak power (10–13%, P = 0.006–0.003; g = 0.888–1.151) and force peak (3%, P = 0.009; g = 0.247).Conclusions Acute caffeine intake (3 mg/kg) improved muscular strength, power and endurance performance, revealing a more pronounced effect at high-loads (≥ 75% 1RM) and in lower-body (back squat) than in upper-body exercise (bench press) according to muscle group size.
... n = 82), 4 with recreationally active females (9.3%, n = 75), 1 with sports students (2.3%, Nutrients 2023, 15, 81 5 of 29 n = 24), 1 with tennis players (2.3%, n = 17) and 1 with kayak athletes (2.3%, n = 5) (see Table 1). Regarding the EAs used to enhance performance, CAF was used alone in 18 studies, (41.9%) [10,29,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54], combined with BJ in 2 articles [55,56], and with taurine (TAU) [57] and sodium phosphate (SP) [58] in another 2 studies. BA was presented alone in five studies [59][60][61][62][63] and combined with CRE in one study [64]. ...
... This capacity was analyzed in nine studies [29,39,40,43,46,[51][52][53]71]. The SMD varied from 0.07 to 0.49, with most of the estimates being positive (100%). ...
... Isometric strength was analyzed in six studies [40,43,47,51,52,60]. The observed SMD ranged from −0.46 to 1.37, with most estimates being positive (83%). ...
Full-text available
Most intervention studies investigating the effects of ergogenic aids (EAs) on sports performance have been carried out in the male population. Thus, the aim of this systematic review and meta-analysis was to summarize the effects in the existing literature of EAs used by female athletes on performance. A literature research was conducted, and a descriptive analysis of the articles included in the systematic review was carried out. Meta-analyses could be performed on 32 of the included articles, evaluating performance in strength, sprint, and cardiovascular capacity. A random-effects model and the standardized mean differences (SMD) ± 95% confidence intervals (CI) were reported. The results showed that caffeine helped to improve jumping performance, isometric strength values, and the number of repetitions until failure. Caffeine and sodium phosphate helped to improve sprint performance. Aerobic tests could be improved with the use of taurine, caffeine, and beta-alanine. No conclusive effects of beetroot juice, polyphenols, or creatine in improving aerobic performance were shown. In terms of anaerobic variables, both caffeine and sodium phosphate could help to improve repeated sprint ability. More studies are needed in female athletes that measure the effects of different EAs on sports performance, such as beetroot juice, beta-alanine or sodium phosphate, as the studies to date are scarce and there are many types of EA that need to be further considered in this population, such as creatine and taurine.
... To our knowledge, only two studies controlled the menstrual cycle phase when investigating the effects of caffeine on resistance-based exercises (Norum et al., 2020;Romero-Moraleda, Del Coso et al., 2019). In the first one, the effect of caffeine was investigated only on muscle power, which increased with caffeine ingestion in the early-follicular but not in the mid-luteal phase, when compared to placebo (Romero-Moraleda, Del Coso et al., 2019). ...
... In the first one, the effect of caffeine was investigated only on muscle power, which increased with caffeine ingestion in the early-follicular but not in the mid-luteal phase, when compared to placebo (Romero-Moraleda, Del Coso et al., 2019). In the second study, the effect of caffeine was investigated only in the early-follicular phase, with a positive effect of caffeine ingestion on muscle strength, muscle power, and muscular endurance, when compared to placebo (Norum et al., 2020). However, the lack of the midluteal phase in the experimental design precluded establishment of whether caffeine is similarly ergogenic in the earlyfollicular and mid-luteal phases (Norum et al., 2020). ...
... In the second study, the effect of caffeine was investigated only in the early-follicular phase, with a positive effect of caffeine ingestion on muscle strength, muscle power, and muscular endurance, when compared to placebo (Norum et al., 2020). However, the lack of the midluteal phase in the experimental design precluded establishment of whether caffeine is similarly ergogenic in the earlyfollicular and mid-luteal phases (Norum et al., 2020). Thus, further studies are needed to provide a deeper understanding of the effects of the menstrual cycle on the influence of caffeine on muscular performance, which might assist women to elaborate their supplementation plan in accordance with the phase of their menstrual cycle. ...
The aim of this study was to compare the effects of caffeine ingestion on muscular performance during the early-follicular and mid-luteal phases of the menstrual cycle. Fourteen resistance-trained naturally menstruating women performed countermovement jump (CMJ), maximal voluntary isometric contraction (MVIC), one-repetition maximum (1-RM), and repetitions-to-failure (RF) at 80% of 1-RM in the half-squat exercise, in early-follicular and mid-luteal phases, after placebo or caffeine ingestion. The early-follicular and mid-luteal phases were identified via calendar-based counting method. The MVIC was lower in the early-follicular than mid-luteal phase (-6.2 ± 15.2 N, p < 0.05) and higher with caffeine than placebo ingestion regardless of the menstrual cycle phase (+16.8 ± 26.7 N, p < 0.05). The magnitude of gains (supplement x phase interaction, p < 0.026) in 1-RM, CMJ, and RF with caffeine ingestion was higher in the early-follicular (+16.6 ± 7.1 kg, +2.5 ± 1.6 cm, and +4.5 ± 2.6 repetitions, respectively) than in the mid-luteal phase (+7.7 ± 4.8 kg, +1.5 ± 2.0 cm, and +2.4 ± 3.1 repetitions, respectively). In conclusion, the greater ergogenic effect of caffeine during the early-follicular phase supports its use to mitigate the decline in muscular performance in this phase of the menstrual cycle.
... Furthermore, the meta-analysis by Grigic et al. [9] showed significant ergogenic effects of acute CAF consumption on power output, as assessed by vertical jump height. However, the results of the current study are inconsistent with most of the previous findings, which indicated an improvement of power output in lower body ballistic tasks after acute CAF intake [8,9,34]. Although, acute CAF intake has been shown to improve CMJ performance, previous studies mostly involved male subjects [31,[35][36][37] or both sexes [8,38], thus indicating that the gender may have an impact on CAF ergogenicity during ballistic tasks, such as CMJ. ...
... Although, acute CAF intake has been shown to improve CMJ performance, previous studies mostly involved male subjects [31,[35][36][37] or both sexes [8,38], thus indicating that the gender may have an impact on CAF ergogenicity during ballistic tasks, such as CMJ. To the best of the authors' knowledge, only few studies examined CAF impact on CMJ performance in women [16,34,39]. Stojanović et al. [39] reported that CAF dose of 3 mg/kg/body mass provided small non-significant increases in CMJ performance in professional female basketball players. ...
... Stojanović et al. [39] reported that CAF dose of 3 mg/kg/body mass provided small non-significant increases in CMJ performance in professional female basketball players. By contrast, Norum et al. [34] demonstrated that CAF dose of 4 mg/kg/body mass significantly increased CMJ height in resistance-trained females during the early follicular phase. In should be pointed that the phase of the menstrual cycle is a factor that might influence strength and power performance; however, this issue has not been taken into account in other studies [34]. ...
Full-text available
Introduction. The main goal of this study was to examine the effect of acute intake of 3 mg/kg/body mass (b.m.) of caffeine (CAF) on countermovement jump (CMJ) performance in recreationally trained women habituated to CAF. Material and Methods. 17 healthy recreationally trained women habitually using CAF participated in the study. The experiment followed randomized, cross-over, double-blind design under three different conditions: control test (CONT) or consumed placebo (PLAC) or consumed 3 mg/kg/b.m. of CAF (CAF-3). Each participant performed 2 sets of 2 CMJ. The following variables were recorded: concentric peak velocity (PV), peak power (PP) and jump height (JH). Results. The two-way repeated measure ANOVA (substance × set) revealed no statistically significant interaction and main effects for all measured variables between conditions. In comparison to the CONT and PLAC, the intake of CAF-3 was not effective at increasing PV (p = 0.533), JH (p = 0.417) and PP (p = 0.871) during 2 sets of the CMJ. Conclusions. This study suggests that 3 mg/kg/b.m. of CAF did not improve CMJ height in recreationally trained women habituated to CAF. Furthermore, the level of athletic performance might be considered a factor in regard to CAF ergogenicity.
... An ergogenic effect of caffeine on muscular endurance has been commonly observed in the literature. For example, Norum et al. [39] provided strength trained female participants with 4 mg/kg of caffeine and observed an ergogenic effect on muscular endurance in the squat and bench press; results that echo ours. Filip-Stachnik et al. [40] used 6 mg/ kg and also reported that caffeine ingestion enhanced upperbody muscular endurance in the bench press exercise in female participants. ...
Full-text available
Purpose The aim of this study was to explore the isolated and combined effects of caffeine and citrulline malate (CitMal) on jumping performance, muscular strength, muscular endurance, and pain perception in resistance-trained participants. Methods Using a randomized and double-blind study design, 35 resistance-trained males (n = 18) and females (n = 17) completed four testing sessions following the ingestion of isolated caffeine (5 mg/kg), isolated CitMal (12 g), combined doses of caffeine and CitMal, and placebo. Supplements were ingested 60 min before performing a countermovement jump (CMJ) test (outcomes included jump height, rate of force development, peak force, and peak power), one-repetition maximum (1RM) squat and bench press, and repetitions to muscular failure in the squat and bench press with 60% of 1RM. Pain perception was evaluated following the repetitions to failure tests. The study was registered at ISRCTN (registration number: ISRCTN11694009). Results Compared to the placebo condition, isolated caffeine ingestion and co-ingestion of caffeine and CitMal significantly enhanced strength in 1RM bench press (Cohen’s d: 0.05–0.06; 2.5–2.7%), muscular endurance in the squat (d: 0.46–0.58; 18.6–18.7%) and bench press (d: 0.48–0.64; 9.3–9.5%). However, there was no significant difference between isolated caffeine ingestion and caffeine co-ingested with CitMal, and isolated CitMal supplementation did not have an ergogenic effect in any outcome. No main effect of condition was found in the analysis for CMJ-derived variables, 1RM squat and pain perception. Conclusion Caffeine ingestion appears to be ergogenic for muscular strength and muscular endurance, while adding CitMal does not seem to further enhance these effects.
... The men in the study possessed body mass-normalized bench press and back squat strength values of 1.27 ± 0.23 and 1.74 ± 0.27, respectively, which (relative to prior literature) is in line with men who have prior resistance training experience (Hoeger et al., 1990;Shimano et al., 2006). The women in the study possessed body mass-normalized bench press and back squat strength values of 0.64 ± 0.16 and 1.40 ± 0.32, respectively, which again aligns with women possessing prior resistance training experience (Hoeger et al., 1990;Norum et al., 2020). ...
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Abstract Limited research exists examining how resistance training to failure affects applied outcomes and single motor unit characteristics in previously trained individuals. Herein, resistance‐trained adults (24 ± 3 years old, self‐reported resistance training experience was 6 ± 4 years, 11 men and 8 women) were randomly assigned to either a low‐repetitions‐in‐reserve (RIR; i.e., training near failure, n = 10) or high‐RIR (i.e., not training near failure, n = 9) group. All participants implemented progressive overload during 5 weeks where low‐RIR performed squat, bench press, and deadlift twice weekly and were instructed to end each training set with 0–1 RIR. high‐RIR performed identical training except for being instructed to maintain 4–6 RIR after each set. During week 6, participants performed a reduced volume‐load. The following were assessed prior to and following the intervention: (i) vastus lateralis (VL) muscle cross‐sectional area (mCSA) at multiple sites; (ii) squat, bench press, and deadlift one‐repetition maximums (1RMs); and (iii) maximal isometric knee extensor torque and VL motor unit firing rates during an 80% maximal voluntary contraction. Although RIR was lower in the low‐ versus high‐RIR group during the intervention (p
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The maximal number of repetitions that can be completed at various percentages of the one repetition maximum (1RM) [REPS ~ %1RM relationship] is foundational knowledge in resistance exercise programming. The current REPS ~ %1RM relationship is based on few studies and has not incorporated uncertainty into estimations or accounted for between-individuals variation. Therefore, we conducted a meta-regression to estimate the mean and between-individuals standard deviation of the number of repetitions that can be completed at various percentages of 1RM. We also explored if the REPS ~ %1RM relationship is moderated by sex, age, training status, and/or exercise. A total of 952 repetitions-to-failure tests, completed by 7289 individuals in 452 groups from 269 studies, were identified. Study groups were predominantly male (66%), healthy (97%), < 59 years of age (92%), and resistance trained (60%). The bench press (42%) and leg press (14%) were the most commonly studied exercises. The REPS ~ %1RM relationship for mean repetitions and standard deviation of repetitions were best described using natural cubic splines and a linear model, respectively, with mean and standard deviation for repetitions decreasing with increasing %1RM. More repetitions were evident in the leg press than bench press across the loading spectrum , thus separate REPS ~ %1RM tables were developed for these two exercises. Analysis of moderators suggested little influences of sex, age, or training status on the REPS ~ %1RM relationship, thus the general main model REPS ~ %1RM table can be applied to all individuals and to all exercises other than the bench press and leg press. More data are needed to develop REPS ~ %1RM tables for other exercises.
The major changes that the COVID-19 pandemic has brought into the lives of the student population are also visible in the altered way in which students spend their free time, the growing trend of sedentary habits prompted by an increased time spent in front of the screen, i.e. playing online games. Over the last decade, numerous authors have tried to explore the frequency and time spent playing online games and their impact on the psychophysical condition of people by means of various questionnaires. After the American Psychiatric Association (APA) defned the term IGD (Internet Gaming Disorder) as a gaming disorder in 2013. and included it in the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders), in 2017 Király et al.24 constructed their Ten Item Internet Gaming Disorder Test (IGDT-10), a short psychometric instrument that showed in statistical analyses its validity and reliability for the evaluation of IGD as proposed in the DSM-5. The primary goal of this study was to investigate the association of time spent playing online games with kinesiological activities. The sample of the subjects in the study consisted of 1000 students of the University of Zagreb (M=480 and F=520). The data collection was carried out online by an anonymous questionnaire comprised of the Ten-item Internet Gaming Disorder Test (IGDT-10)24, International Physical Activity Questionnaire (IPAQ-SF)9 and Previous engagement in kinesiological activities Questionnaire (KINAKT)10. The SPSS software package (version 26.0, SPSS Inc., Chicago, IL, USA) was used to process the data. For all variables, descriptive parameters expressed through frequencies and percentages, arithmetic mean and standard deviations were calculated. Univariate and multivariate methods, correlation analysis and multiple linear regression were used. To check differences with regard to sociodemographic indicators, the Kruskal-Wallis H test was used, and to test differences in independent variables with two level (level of kinesiological activity, playing time, year of study), a nonparametric replacement for a t-test (MannWhitney U test) was used. The results showed that all the values indicating physical activity are negatively related to the time spent playing online games, so it can be concluded that with more frequent and longer playing of online games, values indicating physical activity decrease. Also, a statistically significant positive correlation was established between the total weekly sitting time and the frequency of playing online games, so it can be concluded that more frequent and extended playing of online games increases the time spent sitting and its frequency, thereby reducing physical activity. In conclusion, the results suggest that students are becoming less and less involved in any form of kinesiological activities, spending too much time sitting, on the internet or playing online games more frequently, which results in an increased risk of IGD. Keywords: online games, kinesiological activity, students, sedentary lifestyle
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Position Statement: The International Society of Sports Nutrition (ISSN) bases the following position stand on a critical analysis of the literature regarding the effects of energy drink (ED) or energy shot (ES) consumption on acute exercise performance, metabolism, and cognition, along with synergistic exercise-related performance outcomes and training adaptations. The following 13 points constitute the consensus of the Society and have been approved by the Research Committee of the Society: Energy drinks (ED) commonly contain caffeine, taurine, ginseng, guarana, carnitine, choline, B vitamins (vitamins B1, B2, B3, B5, B6, B9, and B12), vitamin C, vitamin A (beta carotene), vitamin D, electrolytes (sodium, potassium, magnesium, and calcium), sugars (nutritive and non-nutritive sweeteners), tyrosine, and L-theanine, with prevalence for each ingredient ranging from 1.3 to 100%. Energy drinks can enhance acute aerobic exercise performance, largely influenced by the amount of caffeine (> 200 mg or >3 mg∙kg bodyweight [BW⁻¹]) in the beverage. Although ED and ES contain several nutrients that are purported to affect mental and/or physical performance, the primary ergogenic nutrients in most ED and ES based on scientific evidence appear to be caffeine and/or the carbohydrate provision. The ergogenic value of caffeine on mental and physical performance has been well-established, but the potential additive benefits of other nutrients contained in ED and ES remains to be determined. Consuming ED and ES 10-60 minutes before exercise can improve mental focus, alertness, anaerobic performance, and/or endurance performance with doses >3 mg∙kg BW⁻¹. Consuming ED and ES containing at least 3 mg∙kg BW⁻¹ caffeine is most likely to benefit maximal lower-body power production. Consuming ED and ES can improve endurance, repeat sprint performance, and sport-specific tasks in the context of team sports. Many ED and ES contain numerous ingredients that either have not been studied or evaluated in combination with other nutrients contained in the ED or ES. For this reason, these products need to be studied to demonstrate efficacy of single- and multi-nutrient formulations for physical and cognitive performance as well as for safety. Limited evidence is available to suggest that consumption of low-calorie ED and ES during training and/or weight loss trials may provide ergogenic benefit and/or promote additional weight control, potentially through enhanced training capacity. However, ingestion of higher calorie ED may promote weight gain if the energy intake from consumption of ED is not carefully considered as part of the total daily energy intake. Individuals should consider the impact of regular coingestion of high glycemic index carbohydrates from ED and ES on metabolic health, blood glucose, and insulin levels. Adolescents (aged 12 through 18) should exercise caution and seek parental guidance when considering the consumption of ED and ES, particularly in excessive amounts (e.g. > 400 mg), as limited evidence is available regarding the safety of these products among this population. Additionally, ED and ES are not recommended for children (aged 2-12), those who are pregnant, trying to become pregnant, or breastfeeding and those who are sensitive to caffeine. Diabetics and individuals with preexisting cardiovascular, metabolic, hepatorenal, and/or neurologic disease who are taking medications that may be affected by high glycemic load foods, caffeine, and/or other stimulants should exercise caution and consult with their physician prior to consuming ED. The decision to consume ED or ES should be based upon the beverage’s content of carbohydrate, caffeine, and other nutrients and a thorough understanding of the potential side effects. Indiscriminate use of ED or ES, especially if multiple servings per day are consumed or when consumed with other caffeinated beverages and/or foods, may lead to adverse effects. The purpose of this review is to provide an update to the position stand of the International Society of Sports Nutrition (ISSN) integrating current literature on ED and ES in exercise, sport, and medicine. The effects of consuming these beverages on acute exercise performance, metabolism, markers of clinical health, and cognition are addressed, as well as more chronic effects when evaluating ED/ES use with exercise-related training adaptions.
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Recent literature confirms the ergogenic effect of acute caffeine intake to increase muscle strength and power in men. However, the information about the effect of caffeine on muscle performance in women is uncertain and it is unknown whether its ergogenicity is similar during the menstrual cycle. The goal of this investigation was to assess the effect of acute caffeine intake on mean and peak velocity of half-squat exercise during three different phases of the menstrual cycle. Thirteen trained eumenorrheic athletes (age = 31 ± 6 years; body mass = 58.6 ± 7.8 kg) participated in a double-blind, crossover and randomized experimental trial. In the early follicular (EFP), late follicular (LFP) and mid luteal phases (MLP), participants either ingested a placebo (cellulose) or 3 mg/kg/bm of caffeine in an opaque and unidentifiable capsule. In each trial, participants performed a half-squat exercise at maximal velocity with loads equivalent to 20%, 40% 60% and 80% of one repetition maximum (1RM). In each load, mean and peak velocity were measured during the concentric phase of the exercise using a rotatory encoder. In comparison to the placebo, a two-way ANOVA showed that the ingestion of 3 mg/kg/bm of caffeine increased mean velocity at 60% 1RM in EFP (Δ = 1.4 ± 2.7%, p = 0.04; ES: 0.2 ± 0.2) and LFP (Δ = 5.0 ± 10.4%, p = 0.04; ES: 0.3 ± 0.4). No other statistical differences were found for the caffeine-placebo comparison for mean velocity, but caffeine induced an ergogenic effect of small magnitude in all of the menstrual cycle phases. These results suggest that the acute intake of 3 mg/kg/bm of caffeine induces a small effect to increase movement velocity during resistance exercise in eumenorrheic female athletes. The positive effect of caffeine was of similar magnitude in all the three phases of the menstrual cycle.
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Background: The main goal of this study was to assess the acute effects of the intake of 9 and 11 mg/kg/ body mass (b.m.) of caffeine (CAF) on maximal strength and muscle endurance in athletes habituated to caffeine. Methods: The study included 16 healthy strength-trained male athletes (age = 24.2 ± 4.2 years, body mass = 79.5 ± 8.5 kg, body mass index (BMI) = 24.5 ± 1.9, bench press 1RM = 118.3 ± 14.5 kg). All participants were habitual caffeine consumers (4.9 ± 1.1 mg/kg/b.m., 411 ± 136 mg of caffeine per day). This study had a randomized, crossover, double-blind design, where each participant performed three experimental sessions after ingesting either a placebo (PLAC) or 9 mg/kg/b.m. (CAF-9) and 11 mg/kg/b.m. (CAF-11) of caffeine. In each experimental session, participants underwent a 1RM strength test and a muscle endurance test in the bench press exercise at 50% 1RM while power output and bar velocity were measured in each test. Results: A one-way repeated measures ANOVA revealed a significant difference between PLAC, CAF-9, and CAF-11 groups in peak velocity (PV) (p = 0.04). Post-hoc tests showed a significant decrease for PV (p = 0.04) in the CAF-11 compared to the PLAC group. No other changes were found in the 1RM or muscle endurance tests with the ingestion of caffeine. Conclusion: The results of the present study indicate that high acute doses of CAF (9 and 11 mg/kg/b.m.) did not improve muscle strength nor muscle endurance in athletes habituated to this substance.
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Caffeine is a widely utilized performance-enhancing supplement used by athletes and non-athletes alike. In recent years, a number of meta-analyses have demonstrated that caffeine’s ergogenic effects on exercise performance are well-established and well-replicated, appearing consistent across a broad range of exercise modalities. As such, it is clear that caffeine is an ergogenic aid—but can we further explore the context of this ergogenic aid in order to better inform practice? We propose that future research should aim to better understand the nuances of caffeine use within sport and exercise. Here, we propose a number of areas for exploration within future caffeine research. These include an understanding of the effects of training status, habitual caffeine use, time of day, age, and sex on caffeine ergogenicity, as well as further insight into the modifying effects of genotype. We also propose that a better understanding of the wider, non-direct effects of caffeine on exercise, such as how it modifies sleep, anxiety, and post-exercise recovery, will ensure athletes can maximize the performance benefits of caffeine supplementation during both training and competition. Whilst not exhaustive, we hope that the questions provided within this manuscript will prompt researchers to explore areas with the potential to have a large impact on caffeine use in the future.
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Objective To systematically review, summarise, and appraise findings of published meta-analyses that examined the effects of caffeine on exercise performance. Design Umbrella review. Data sources Twelve databases. Eligibility criteria for selecting studies Meta-analyses that examined the effects of caffeine ingestion on exercise performance. Results Eleven reviews (with a total of 21 meta-analyses) were included, all being of moderate or high methodological quality (assessed using the AMSTAR 2 checklist). In the meta-analyses, caffeine was ergogenic for aerobic endurance, muscle strength, muscle endurance, power, jumping performance, and exercise speed. However, not all analyses provided a definite direction for the effect of caffeine when considering the 95% prediction interval. Using the GRADE criteria the quality of evidence was generally categorised as moderate (with some low to very low quality of evidence). Most individual studies included in the published meta-analyses were conducted among young men. Summary/Conclusion Synthesis of the currently available meta-analyses suggest that caffeine ingestion improves exercise performance in a broad range of exercise tasks. Ergogenic effects of caffeine on muscle endurance, muscle strength, anaerobic power, and aerobic endurance were substantiated by moderate quality of evidence coming from moderate-to-high quality systematic reviews. For other outcomes, we found moderate quality reviews that presented evidence of very low or low quality. It seems that the magnitude of the effect of caffeine is generally greater for aerobic as compared with anaerobic exercise. More primary studies should be conducted among women, middle-aged and older adults to improve the generalisability of these findings.
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Sex hormone concentrations of eumenorrheic females typically fluctuate across the menstrual cycle and can affect neural function such that oestrogen has neuro-excitatory effects, and progesterone induces inhibition. However, the effects of these changes on corticospinal and intracortical circuitry, and the motor performance of the knee-extensors, are unknown. The present two-part investigation aimed to i) determine the measurement error of an exercise task, transcranial magnetic stimulation (TMS) and motor nerve stimulation (MNS) derived responses in females ingesting a monophasic oral contraceptive pill (hormonally-constant), and ii) investigate whether these measures were modulated by menstrual cycle phase (MCP), by examining them before and after an intermittent isometric fatiguing task (60% of maximal voluntary contraction, MVC) with the knee-extensors until task failure in eumenorrheic females on days 2, 14, and 21 of the menstrual cycle. The repeatability of neuromuscular measures at baseline and fatigability ranged between moderate-excellent in females taking the oral contraceptive pill. Maximal voluntary contraction was not affected by MCP (P=0.790). Voluntary activation (MNS and TMS) peaked on day 14 (P=0.007 and 0.008, respectively). Whilst corticospinal excitability was unchanged, short-interval intracortical inhibition was greatest on day 21 compared to days 14 and 2 (P=0.001). Additionally, time to task failure was longer on day 21 compared to both days 14 and 2 (24 and 36%, respectively; P=0.030). The observed changes were larger than the associated measurement errors. These data demonstrate that neuromuscular function and fatigability of the knee-extensors varies across the menstrual cycle, and may influence exercise performance involving locomotor muscles.
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Key points: We propose and validate a method for accurately identifying the activity of populations of motor neurons during contractions at maximal rate of force development in humans. The behaviour of the motor neuron pool during rapid voluntary contractions in humans is presented. We show with this approach that the motor neuron recruitment speed and maximal motor unit discharge rate largely explains the individual ability in generating rapid force contractions. The results also indicate that the synaptic inputs received by the motor neurons before force is generated dictate human potential to generate force rapidly. This is the first characterization of the discharge behaviour of a representative sample of human motor neurons during rapid contractions. Abstract: During rapid contractions, motor neurons are recruited in a short burst and begin to discharge at high frequencies (up to >200 Hz). In the present study, we investigated the behaviour of relatively large populations of motor neurons during rapid (explosive) contractions in humans, applying a new approach to accurately identify motor neuron activity simultaneous to measuring the rate of force development. The activity of spinal motor neurons was assessed by high-density electromyographic decomposition from the tibialis anterior muscle of 20 men during isometric explosive contractions. The speed of motor neuron recruitment and the instantaneous motor unit discharge rate were analysed as a function of the impulse (the time-force integral) and the maximal rate of force development. The peak of motor unit discharge rate occurred before force generation and discharge rates decreased thereafter. The maximal motor unit discharge rate was associated with the explosive force variables, at the whole population level (r2 = 0.71 ± 0.12; P < 0.001). Moreover, the peak motor unit discharge and maximal rate of force variables were correlated with an estimate of the supraspinal drive, which was measured as the speed of motor unit recruitment before the generation of afferent feedback (P < 0.05). We show for the first time the full association between the effective neural drive to the muscle and human maximal rate of force development. The results obtained in the present study indicate that the variability in the maximal contractile explosive force of the human tibialis anterior muscle is determined by the neural activation preceding force generation.
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Background Caffeine is commonly used as an ergogenic aid. Literature about the effects of caffeine ingestion on muscle strength and power is equivocal. The aim of this systematic review and meta-analysis was to summarize results from individual studies on the effects of caffeine intake on muscle strength and power. Methods A search through eight databases was performed to find studies on the effects of caffeine on: (i) maximal muscle strength measured using 1 repetition maximum tests; and (ii) muscle power assessed by tests of vertical jump. Meta-analyses of standardized mean differences (SMD) between placebo and caffeine trials from individual studies were conducted using the random effects model. Results Ten studies on the strength outcome and ten studies on the power outcome met the inclusion criteria for the meta-analyses. Caffeine ingestion improved both strength (SMD = 0.20; 95% confidence interval [CI]: 0.03, 0.36; p = 0.023) and power (SMD = 0.17; 95% CI: 0.00, 0.34; p = 0.047). A subgroup analysis indicated that caffeine significantly improves upper (SMD = 0.21; 95% CI: 0.02, 0.39; p = 0.026) but not lower body strength (SMD = 0.15; 95% CI: -0.05, 0.34; p = 0.147). Conclusion The meta-analyses showed significant ergogenic effects of caffeine ingestion on maximal muscle strength of upper body and muscle power. Future studies should more rigorously control the effectiveness of blinding. Due to the paucity of evidence, additional findings are needed in the female population and using different forms of caffeine, such as gum and gel.