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

Abstract

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.

Full text

2116
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Scand J Med Sci Sports. 2020;30:2116–2129.
wileyonlinelibrary.com/journal/sms
Received: 6 December 2019
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Revised: 28 June 2020
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Accepted: 9 July 2020
DOI: 10.1111/sms.13776
ORIGINAL ARTICLE
Caffeine increases strength and power performance in resistance-
trained females during early follicular phase
MartinNorum1
|
Linn ChristinRisvang1,2
|
ThomasBjørnsen3,4
|
LygeriDimitriou1,5
|
Per OlaRønning2
|
MortenBjørgen6
|
TrulsRaastad7
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,
Norway
Correspondence
Linn Christin Risvang, Department of
Mechanical, Electronics and Chemical
Engineering, Faculty of Technology, Art
and Design, Oslo Metropolitan University,
Oslo, Norway.
Email: linnrisv@oslomet.no
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.
KEYWORDS
caffeine supplementation, female athletes, muscular activation level, muscular endurance, strength
and power performance

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NORUM et al.
1
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INTRODUCTION
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.

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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.
2
|
METHODS
2.1
|
Participants
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.
2.2
|
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-
3497
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.

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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.
2.3
|
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.
2.4
|
Supplementation
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.
2.5
|
Measurements
2.5.1
|
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

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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.
2.5.2
|
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
force:
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.
2.5.3
|
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
(1)
Muscle activation %
=100−





D×

Mean forceMVCpre −stimulus
Peak forceMVC

Peak forceEvoked at rest





×
100
D
=
Peak forceEvoked MVC
−
Mean forceMVC pre
−
stimulus

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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.
2.5.4
|
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.
2.6
|
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
270minutes.
2.7
|
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

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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).
3
|
RESULTS
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.
3.1
|
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.
3.2
|
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
Effect
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.

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3.3
|
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).
3.4
|
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).
3.5
|
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
Analyte
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
theophylline.
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

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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).
3.6
|
Habituation
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).
4
|
DISCUSSION
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
interval

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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
caffeine.
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

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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
method.40
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,

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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.
4.1
|
Perspectives
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
warranted.
ACKNOWLEDGEMENT
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.
AUTHOR CONTRIBUTIONS
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.
ORCID
Linn Christin Risvang https://orcid.
org/0000-0003-4532-2129
Thomas Bjørnsen https://orcid.
org/0000-0002-4010-8038
Lygeri Dimitriou https://orcid.
org/0000-0002-5093-558X
Per Ola Rønning https://orcid.org/0000-0003-1341-3229
Truls Raastad https://orcid.org/0000-0002-2567-3004
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SUPPORTING INFORMATION
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–
2129. https://doi.org/10.1111/sms.13776