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nutrients
Article
Effects of Creatine Supplementation during
Resistance Training Sessions in Physically Active
Young Adults
Scotty Mills 1, Darren G. Candow 1, *, Scott C. Forbes 2, J. Patrick Neary 1,
Michael J. Ormsbee 3,4 and Jose Antonio 5
1Faculty of Kinesiology and Health Studies, University of Regina, Regina, SK S4S0A2, Canada;
scottymills4@gmail.com (S.M.); Patrick.Neary@uregina.ca (J.P.N.)
2Department of Physical Education, Faculty of Education, Brandon University,
Brandon, MB R7A6A9, Canada; forbess@brandonu.ca
3Institute of Sports Sciences & Medicine, Department of Nutrition, Food, & Exercise Sciences,
Florida State University, Tallahassee, FL 32313, USA; mormsbee@fsu.edu
4Discipline of Biokinetics, Exercise and Leisure Sciences, University of KwaZulu-Natal,
Durban 4041, South Africa
5Department of Health and Human Performance, Nova Southeastern University, Davie, FL 33314, USA;
exphys@aol.com
*Correspondence: Darren.Candow@uregina.ca; Tel.: +1-306-209-0280
Received: 20 May 2020; Accepted: 22 June 2020; Published: 24 June 2020
Abstract:
The purpose was to examine the effects of creatine supplementation during resistance
training sessions on skeletal muscle mass and exercise performance in physically active young adults.
Twenty-two participants were randomized to supplement with creatine (CR: n=13, 26
±
4 yrs;
0.0055 g·kg−1
post training set) or placebo (PLA: n=9, 26
±
5 yrs; 0.0055 g
·
kg
−1
post training set)
during six weeks of resistance training (18 sets per training session; five days per week). Prior to and
following training and supplementation, measurements were made for muscle thickness (elbow and
knee flexors/extensors, ankle plantarflexors), power (vertical jump and medicine ball throw), strength
(leg press and chest press one-repetition maximum (1-RM)) and muscular endurance (one set of
repetitions to volitional fatigue using 50% baseline 1-RM for leg press and chest press). The creatine
group experienced a significant increase (p<0.05) in leg press, chest press and total body strength
and leg press endurance with no significant changes in the PLA group. Both groups improved total
body endurance over time (p<0.05), with greater gains observed in the creatine group. In conclusion,
creatine ingestion during resistance training sessions is a viable strategy for improving muscle
strength and some indices of muscle endurance in physically active young adults.
Keywords: intra-workout; muscle mass; strength; endurance; power
1. Introduction
Creatine is an organic compound naturally produced in the body from reactions involving the
amino acids arginine, glycine and methionine in the kidneys and liver or consumed in the diet
primarily from red meat, poultry, seafood [
1
] or supplementation practices. There is substantial
evidence that creatine supplementation and resistance training increases muscle mass and performance
(i.e., strength) more than placebo and resistance training, possibly by influencing phosphate metabolism,
cellular hydration status, calcium and protein kinetics, glycogen content, satellite cells, growth factors,
inflammation and oxidative stress (for reviews see [2–6]).
Nutrients 2020,12, 1880; doi:10.3390/nu12061880 www.mdpi.com/journal/nutrients
Nutrients 2020,12, 1880 2 of 11
Harris et al. [
7
] showed that prior muscle contractions augmented total intramuscular creatine
uptake from creatine supplementation, possibly through an upregulation in creatine transport kinetics,
an increase in sodium–potassium pump function during exercise, or by an increase in blood flow
delivery of creatine to exercising muscles (for review see [
8
]). Ingesting creatine immediately following
each set of resistance training may therefore increase creatine uptake into skeletal muscle, which over
time could lead to greater gains in muscle mass and performance compared to placebo. Therefore,
the purpose of this study was to examine the effects of creatine ingestion during resistance training
sessions in physically active young adults. It was hypothesized that the repeated ingestion of creatine
following each set of resistance training would lead to greater gains in muscle mass and performance
compared to placebo immediately following each set of resistance training.
2. Materials and Methods
2.1. Participants
Physically active males and females (19–35 years of age) who had been performing structured
resistance training (>3
×
/week for
≥
6 weeks) prior to the start of the study were recruited. An a priori
power analysis (G*Power v. 3.1.5.1) indicated that 34 participants were required. This calculation
was based on a moderate effect size (Cohen’s d =0.25), an alpha level of 0.05, a
β
-value of 0.8 for
a repeated measures within or between an analysis of variance (ANOVA) design with two groups
and a correlation among repeated measures value of 0.5 [
9
]. Participants were excluded from the
study if they were taking medications that could affect muscle biology (i.e., corticosteroids), had
ingested creatine monohydrate or dietary supplements containing creatine
≤
4 weeks prior to the start
of resistance training and supplementation, if they were vegetarian or if they had pre-existing kidney
or liver abnormalities.
Participants were instructed not to change their habitual diet or engage in additional physical
activity that was not part of their normal daily routine or consume non-steroidal anti-inflammatory
drugs during resistance training and supplementation, as these interventions can affect muscle protein
turnover [
10
]. The study was approved by the Research Ethics Board at the University of Regina.
Participants were informed of the risks, potential benefits and purposes of the study before written
consent was obtained.
2.2. Research Design
The study used a double-blind, placebo-controlled, repeated measures design. In order to minimize
group differences, participants were matched according to age, sex and body mass. After exclusion
criteria were applied, participants were randomized on a 1:1 basis to supplement with creatine
monohydrate (CR) or placebo (PLA). Prior to testing, participants were instructed to refrain from
alcohol and intense physical activity for 24 h and food and drink for 3 h (water was permitted ad libitum).
The primary dependent variables assessed prior to and following training and supplementation were
(1) muscle thickness (elbow and knee flexors/extensors, ankle plantarflexors; ultrasonography),
(2) power (vertical jump and medicine ball throw), (3) strength (1-RM leg press and chest press), and (4)
endurance (maximum number of repetitions performed for one set using 50% of baseline 1-RM for leg
press and chest press). In addition, participants filled out a three-day food diary during the first and
final week of training and supplementation to determine whether total energy (kcal) and macronutrient
intake changed over time.
2.3. Supplementation
Creatine (Creapure
®
AlzChem, Trostberg GmbH, Germany) and placebo (Globe
®
Plus 10 DE
Maltodextrin, Univar Canada) were in powder form. Both products were similar in taste, color (white),
texture and appearance. The purity of Creapure
®
was established at >99.9% by independent laboratory
testing (The Cary Company, Addison, IL., USA). Two individuals not involved in any other aspect of
Nutrients 2020,12, 1880 3 of 11
the study were responsible for randomizing participants into groups and for preparing participant
study kits which included their supplement for the duration of the study, detailed supplementation
instructions, measuring spoons and a water bottle. The creatine supplementation dosage was
0.1 g·kg−1·d−1
as this dosage has been shown to be effective, when combined with resistance training,
for increasing muscle mass and muscle performance [
11
]. On training days (five days/week), creatine
and placebo were mixed with water (900 mL) and participants consumed 50 mL of the solution
containing 0.0055 g
·
kg
−1
of creatine or placebo immediately after each set of resistance training (18 sets
per training day). Creatine and placebo were consumed in a plastic shaker bottle with gradations
(mL) on the side to ensure that 50 mL of the solution was consumed after each set. Participants were
instructed to refrain from food or drink (water was permitted ad libitum) for 1 h before and after
each training session so that a valid estimate of the effects of intra-workout creatine supplementation
could be made. On the non-training days (two days per week), participants refrained from consuming
creatine or placebo as the purpose of the study was to investigate the effects of creatine ingestion
during resistance training sessions. Adherence with the supplementation protocol was assessed by a
compliance log. Upon completion of the study, participants were asked whether they thought they
were administered creatine, placebo, or unsure about what supplement they consumed.
2.4. Resistance Training Program
Participants followed the same periodized resistance training program for six weeks. Resistance
training started on the first day of supplementation and consisted of a four-day split routine
involving three sets to volitional fatigue per exercise (Set 1: ~6 repetitions, Set 2: ~8 repetitions,
Set 3:
~10 repetitions
), with 2 min rest between sets. The load was adjusted (if necessary) following
each set to achieve volitional fatigue at the desired number of repetitions. We have used a similar
training program successfully to increase muscle mass and performance in physically active young
adults [
12
]. Day 1 involved leg and core musculature and included the following exercises in order:
barbell back squat, walking dumbbell lunge, leg extension, leg curl, calf raise and weight abdominal
crunch. Day 2 involved upper-body and core and included the following exercises in order: flat barbell
bench press, flat bench dumbbell chest fly, overhead cable triceps extension with rope, dumbbell curl,
dumbbell concentration curls and pallof press. Day 3 was a rest day from training. Day 4 involved
leg and core musculature and included the following exercises in order: leg press, dumbbell goblet
squat, dumbbell reverse lunge, leg curl, calf raise, and cable crunches. Day 5 involved upper-body
musculature and included the following exercises in order: flat barbell chest press, barbell row,
dumbbell shoulder press, dumbbell skull crushers, dumbbell hammer curls, and triceps extensions
with rope. Day 6 served as a rest day from training. This cycle was repeated for six weeks. Participants
filled out resistance training logs after each training session to determine adherence and compliance to
the study and to determine total training volume (sets ×repetitions ×load; kg) performed over time.
2.5. Muscle Thickness
Muscle thickness (right side) of the elbow and knee flexors and extensors and ankle plantarflexors
was measured using B-mode ultrasound (LOGIQ e, GE Medical Systems, China) as previously
described [
13
]. To help ensure that exercise induced muscle swelling (edema) did not influence the
results, participants were instructed not to perform resistance training for 48 h prior to baseline
measurements. Post-testing measurements were obtained 48 h after the last training session of the
study. The reproducibility of muscle thickness measurements was determined by assessing eight
participants on two separate days (24 h apart). The coefficients of variation (CV) and intraclass
correlation coefficients (ICC) were: elbow flexors (CV: 4.4%, ICC: 0.993), elbow flexors (CV: 7.1%,
ICC: 0.878), knee flexors (CV: 5.4%, ICC: 0.936), knee extensors (CV: 2.9%, ICC: 0.991), and ankle
plantarflexors (CV: 2.8%, ICC: 0.976). The same researcher performed all measurements.
Nutrients 2020,12, 1880 4 of 11
2.6. Muscle Performance
Power, strength, and endurance were assessed in the following order: (a) vertical jump, (b) medicine
ball throw, (c) leg press 1-RM, (d) chest press 1-RM, (e) leg press endurance, and (f) chest press endurance.
Each test was separated by 5 min of rest. Prior to the start of testing, participants performed a 5 min
warm-up on a stationary cycle ergometer (Ergomedic 828 E, Monark, GIH Sweden) at a self-selected
intensity and completed light stretching. The same researcher demonstrated how to properly perform
each test. To determine lower-body power, a vertical jump test was used. Participants standing reach
height was measured followed by three vertical jump tests to displace the Vertec vanes. Rest time
between jump tests was 30 s. Participants peak power was calculated from the highest of the three
vertical jump trials using Sayers Peak power equation [
14
] (Peak power [Watts; W] =[51.9
×
VJ (vertical
jump) height (cm)] +[48.9
×
Body mass (kg)]
−
2007). To measure upper-body power, a medicine ball
throw test was used. Participants stood behind a line marked on the floor in a standing position and
were instructed to throw the medicine ball (13.6 kg) using both hands with fingers pointed in from
chest level, similar to a chest pass in basketball, as far as they could horizontally. Participants were
further instructed not to use their lower body for power generation and to not step over the line after
the medicine ball was released. Participants performed three trials, with their longest throw being
used for analysis. Rest time between trials was 30 s.
Detailed procedures for determining leg press and chest press 1-RM strength are previously
described [
12
]. These two exercises were chosen as a measurement of strength because they involve
the major muscle groups in the lower and upper body [
15
]. The CV’s and ICC’s were 3.8% and 0.99 for
leg press 1-RM and 3.1% and 0.99 for chest press 1-RM [
16
]. To determine leg press and chest press
endurance, participants performed one set of repetitions to volitional fatigue (defined as the inability
to perform the concentric phase of a muscle contraction) using 50% baseline 1-RM for the leg press and
chest press.
2.7. Diet
Average total energy (kcal) and macronutrient intake for three days (two weekdays and one
weekend day) was determined during the first and final week of supplementation and resistance
training. Participants used a three-day food booklet to record all food items, including portion sizes
consumed, for the three designated days. MyFitnessPal, which shows good validity compared to
paper-based food records [17], was used to analyze three-day food records.
2.8. Adverse Events Assessment
In the case of an adverse event, participants were required to complete an adverse event form in
order to provide details on the type of adverse event, the severity (i.e., mild, moderate, severe, or life
threatening), the frequency, and the relationship to the intervention (i.e., not related, unlikely, possible,
probable, or definite).
2.9. Statistical Analyses
The primary analysis performed was a 2 (groups: creatine vs. placebo)
×
2 (time: pre-training
vs. post-training) repeated measures ANOVA to determine differences between groups over time for
changes in muscle thickness, power, strength, endurance, and diet. As a secondary analysis, a 2 (groups:
creatine vs. placebo)
×
2 (sex: males vs. females)
×
2 (time: pre-training vs. post-training) repeated
measures ANOVA was performed on all variables to determine differences between males and females.
If significant interactions were detected using ANOVA testing, file splitting and paired sample t-tests
were performed to determine where differences occurred between means. A one-factor ANOVA was
used to assess baseline data, training volume, and absolute change scores. Significance was set a priori
at an alpha level of p<0.05. Cohen’s deffect size (ES) was calculated as post-training mean minus
pre-training mean/pooled pre-training standard deviation mean [
18
]. An ES of 0.00–0.19 was considered
Nutrients 2020,12, 1880 5 of 11
trivial, 0.20–0.49 was considered small, 0.50–0.79 was considered moderate, and
≥
0.80 was considered
large. Statistical analyses were performed using IBM®SPSS®Statistics, v. 26 (Chicago, IL, USA).
3. Results
3.1. Participants
Based on the sample size calculation, our recruitment goal was 34 participants. However,
we were only able to enroll and randomize 26 eligible participants (13 male, 13 female) into the study,
which started in August 2019 and ended in December 2019, because the vast majority of participants
could only commit to the study during this time frame (see Figure 1for a summary of recruitment,
allocation and analysis). Following randomization, four females (all from the PLA group) withdrew
due to time constraints (n=2), doctor recommendations (n=1) and personal injury (n=1), all unrelated
to the study. Therefore, 22 participants (CR =13, (7 male, 6 female); PLA =9 (6 male, 3 female))
completed the study. One female in the creatine group reported gastrointestinal irritation during
the first week of creatine supplementation but this did not result in her withdrawing from the study.
Seventeen participants (CR =9, (6 male, 3 female); PLA =8 (5 male, 3 female)) were able to provide
three-day food records (first and final week of training and supplementation).
Nutrients 2020, 12, x FOR PEER REVIEW 6 of 12
Figure 1. Summary of recruitment, allocation and analyses.
Table 1. Baseline characteristics.
Creatine (n = 13)
Placebo (n = 9)
p-Value
Age (yrs)
26.15 (4.66)
26.44 (5.10)
0.891
Mass (kg)
80.5 (18.07)
79.88 (19.97)
0.936
Height (cm)
174.50 (10.53)
175.05 (12.09)
0.911
Muscle thickness (cm)
Elbow flexors
2.97 (0.88)
3.03 (0.93)
0.87
Elbow extensors
2.76 (0.78)
2.84 (0.65)
0.805
Knee extensors
3.92 (0.93)
3.82 (0.45)
0.783
Knee flexors
3.36 (0.57)
3.34 (0.53)
0.959
Ankle plantarflexors
3.53 (0.41)
3.12 (0.50)
0.051
Total muscle thickness
16.55 (2.50)
16.18 (2.28)
0.726
Muscle strength (kg)
Chest press
120.89 (54.8)
143.38 (65.10)
0.391
Figure 1. Summary of recruitment, allocation and analyses.
Nutrients 2020,12, 1880 6 of 11
Following the intervention, participants were asked whether they thought they were administered
creatine, placebo, or unsure about what supplement they consumed. In the CR group, 10 participants
correctly guessed they were consuming creatine and three did not know. In the PLA group,
five participants correctly guessed they were consuming placebo and four did not know.
Training compliance (CR: 25/28 sessions completed or 90.9%; PLA: 25/28 sessions completed or
89.3%) and supplementation compliance (CR: 27/28 sessions or 97.58%; PLA: 28/28 sessions or 100%)
were similar between groups over time (p>0.05).
Baseline data are presented in Table 1. Both groups experienced similar increases in body mass over
time (CR: pre 80.55 ±18.07 kg, post 81.72 ±17.44 kg; PLA: pre 79.88 ±19.97 kg, post 80.14 ±18.95 kg;
p=0.05; observed power =0.51). There was a group x time interaction (p=0.016) for total energy
intake (d=0.72; observed power =0.72; Table 2). The creatine group significantly decreased total
energy intake over time with no change in the placebo group. There were no changes over time for
carbohydrate, fat, protein or relative protein intake. There were no significant differences between
groups for total training volume performed over time (CR: 315,376 kg [95% CI: 222,936, 407,816]; PLA:
407,549 kg [95% CI: 326,615, 488,483]; p=0.134). There was a sex main effect (p<0.05) for body mass
and all measures of muscle thickness (except ankle plantarflexors, p=0.056), strength, and power,
with males having higher values compared to females. There were no differences between males and
females for measures of muscle endurance (p>0.05).
Table 1. Baseline characteristics.
Creatine (n=13) Placebo (n=9) p-Value
Age (yrs) 26.15 (4.66) 26.44 (5.10) 0.891
Mass (kg) 80.5 (18.07) 79.88 (19.97) 0.936
Height (cm) 174.50 (10.53) 175.05 (12.09) 0.911
Muscle thickness (cm)
Elbow flexors 2.97 (0.88) 3.03 (0.93) 0.87
Elbow extensors 2.76 (0.78) 2.84 (0.65) 0.805
Knee extensors 3.92 (0.93) 3.82 (0.45) 0.783
Knee flexors 3.36 (0.57) 3.34 (0.53) 0.959
Ankle plantarflexors 3.53 (0.41) 3.12 (0.50) 0.051
Total muscle thickness 16.55 (2.50) 16.18 (2.28) 0.726
Muscle strength (kg)
Chest press 120.89 (54.8) 143.38 (65.10) 0.391
Leg press 188.06 (88.68) 188.06 (107.96) 0.987
Total strength 308.96 (135.32) 326.87 (169.64) 0.792
Muscle endurance (repetitions)
Chest press endurance 22.76 (4.96) 24.22 (3.96) 0.474
Leg press endurance 17.53 (8.96) 19.25 (9.63) 0.684
Total endurance 40.30 (10.52) 43.75 (12.04) 0.499
Muscle power
Vertical jump (Watts) 4387.08 (1126.50) 4561.87 (1367.64) 0.746
Medicine ball throw (cm) 386.15 (103.58) 386.76 (103.12) 0.989
Total power 4539.12 (1161.86) 4714.15 (1404.32) 0.753
Diet
Total calories (kcal/day) 2348.31 (736.06) 2148.30 (625.60) 0.558
Carbohydrate (g/day) 254.23 (94.85) 239.26 (80.61) 0.57
Fat (g/day) 93.46 (46.86) 72.45 (29.42) 0.293
Protein (g/day) 112.52 (54.28) 134.78 (60.59) 0.437
Relative protein (g/kg) 1.34 (0.68) 1.60 (0.50) 0.404
Values are means (standard deviation).
Nutrients 2020,12, 1880 7 of 11
Table 2.
Mean absolute changes (95% confidence intervals) from baseline to six weeks for total calories
(kcal/day), macronutrients (carbohydrate, fat, protein; grams/day) and relative protein(
g/kg body mass
).
Creatine (n=9) Placebo (n=8) Time Group Interaction d
p-Value p-Value p-Value
Total calories −515.6 (−838.1, −193.1) * −17.1 (−285.4, 251.1) 0.011 0.861 0.016 0.72
Carbohydrate −39.3 (−81.5, 2.9) −4.6 (−29.5, 20.4) 0.063 0.84 0.134 0.39
Fat −33.4 (−69.0, 2.2) 1.7 (−21.4, 24.8) 0.113 0.786 0.082 0.84
Protein −14.5 (−33.1, 4.2) −3.5 (−18.3, 11.3) 0.105 0.82 0.311 0.19
Relative protein −0.18 (−0.40, 0.03) −0.03 (−0.22, 0.14) 0.104 0.255 0.265 0.32
* Creatine significantly different than placebo (p<0.05). d=effect size.
3.2. Muscle Thickness
There was a time main effect (p<0.05) for the elbow flexors, elbow extensors, knee extensors,
knee flexors, and all muscle groups combined (Table 3), with no significant differences between groups.
There was no change over time for the ankle plantarflexors (p=0.471).
Table 3.
Mean absolute changes (95% confidence intervals) from baseline to six weeks for muscle
thickness (cm).
Creatine
(n=13)
Placebo
(n=9)
Time Group Interaction d
p-Value p-Value p-Value
Elbow flexors 0.49 (0.28, 0.70) 0.36 (−0.17, 0.90) 0.001 0.992 0.59 0.14
Elbow extensors 0.29 (0.06, 0.52) 0.21 (0.02, 0.41) 0.002 0.892 0.591 0.1
Knee extensors 0.43 (0.22, 0.65) 0.33 (0.05, 0.61) <0.001 0.686 0.513 0.12
Knee flexors 0.27 (−0.11, 0.66) 0.25 (−0.02, 0.52) 0.034 0.908 0.919 0.05
Ankle plantarflexors −0.05 (−0.31, 0.19) 0.18 (−0.07, 0.43) 0.471 0.101 0.168 0.11
Total body 1.44 (0.63, 2.28) 1.35 (0.41, 2.28) <0.001 0.684 0.874 0.37
Total body =all muscle groups combined. d=effect size.
3.3. Muscle Performance
There was a group
×
time interaction for leg press (p=0.025, d=0.33, observed power =0.63;
Figure 2A), chest press (p=0.012, d=0.36, observed power =0.75; Figure 2B) and total body strength
(leg press and chest press combined: p=0.03; d=0.18, observed power =0.89; Figure 2C). Post hoc
analyses showed that the creatine group experienced a significant increase in strength over time,
with no significant changes in the placebo group.
Nutrients 2020, 12, x FOR PEER REVIEW 8 of 12
analyses showed that the creatine group experienced a significant increase in strength over time, with
no significant changes in the placebo group.
Figure 2. (A) Leg press (B) Chest press (C) Total body strength (pre and post training) for CR (n = 13)
and PLA (n = 8) groups. Each dot represents an individual participant. There was a significant group
x time interaction. Post hoc analyses showed that the CR group increased over time with no changes
in the PLA group. * Significantly different than baseline.
There was a group × sex x time interaction (p = 0.039) for chest press strength. Males on creatine
experienced a significant increase over time (CR: pre 166.53 ± 22.82 kg, post 187.59 ± 18.64 kg; d = 0.47,
observed power = 0.55) with no change for males on placebo (pre 181.05 ± 38.71 kg, post 180.30 ± 43.07
kg). There was no significant change over time for females on creatine (pre 67.66 ± 16.87 kg, post 72.19
± 14.65 kg, p = 0.203) or placebo (pre 68.03 ± 20.78 kg, post 69.55 ± 23.27 kg, p = 0.203).
There was a group × time interaction for leg press (p = 0.013, d = 0.95, observed power = 0.74;
Figure 3A) and total body endurance (leg press and chest press combined: p = 0.04, d = 0.96, observed
power = 0.87; Figure 3C). Post hoc analyses indicated that the creatine group significantly increased
the number of repetitions performed over time for the leg press with no significant change in the
placebo group (Figure 3A). Both groups increased total body endurance over time, but the
improvement was greater in the creatine group (Figure 3C). Both groups experienced a similar
change (p < 0.05) in the number of repetitions performed over time for the chest press (Figure 3B).
Figure 3. (A) Leg press (B) Chest press (C) Total body endurance (leg press and chest press combined;
pre and post training) for CR (n = 13) and PLA (n = 8) groups. Each dot represents an individual
participant. There was a significant group × time interaction for (A,C) and a significant main effect of
time for (B). (A) Post hoc analyses showed that the CR group increased over time with no changes in
the PLA group. (B) Both groups increased over time with no differences between groups. (C) Both
groups increase over time, however the increase was greater in the CR group compared to PLA. *
Significant change over time; # CR significantly greater than PLA.
Regarding muscle power, there was a significant time main effect (p < 0.05) for vertical jump
(CR: pre 4387.08 ± 1126.50 W, post 4629.80 ± 1161.66 W, PLA: pre 4561.87 ± 1367.64 W, post 4813.28 ±
1390.57 W), medicine ball throw (CR: pre 386.15 ± 103.58 cm, post 408.94 ± 109.19 cm; PLA: pre 386.76
± 103.12 cm, post 396.51 ± 96.90 cm) and total body power (vertical jump and medicine ball throw
Figure 2.
(
A
) Leg press (
B
) Chest press (
C
) Total body strength (pre and post training) for CR (n=13)
and PLA (n=8) groups. Each dot represents an individual participant. There was a significant group x
time interaction. Post hoc analyses showed that the CR group increased over time with no changes in
the PLA group. * Significantly different than baseline.
There was a group
×
sex x time interaction (p=0.039) for chest press strength. Males on
creatine experienced a significant increase over time (CR: pre 166.53
±
22.82 kg, post
187.59 ±18.64 kg
;
d=0.47
, observed power =0.55) with no change for males on placebo (pre 181.05
±
38.71 kg,
post
180.30 ±43.07 kg
). There was no significant change over time for females on creatine (pre
Nutrients 2020,12, 1880 8 of 11
67.66 ±16.87 kg
, post 72.19
±
14.65 kg, p=0.203) or placebo (pre 68.03
±
20.78 kg, post
69.55 ±23.27 kg
,
p=0.203).
There was a group
×
time interaction for leg press (p=0.013, d=0.95, observed power =0.74;
Figure 3A) and total body endurance (leg press and chest press combined: p=0.04, d=0.96, observed
power =0.87; Figure 3C). Post hoc analyses indicated that the creatine group significantly increased the
number of repetitions performed over time for the leg press with no significant change in the placebo
group (Figure 3A). Both groups increased total body endurance over time, but the improvement was
greater in the creatine group (Figure 3C). Both groups experienced a similar change (p<0.05) in the
number of repetitions performed over time for the chest press (Figure 3B).
Nutrients 2020, 12, x FOR PEER REVIEW 8 of 12
analyses showed that the creatine group experienced a significant increase in strength over time, with
no significant changes in the placebo group.
Figure 2. (A) Leg press (B) Chest press (C) Total body strength (pre and post training) for CR (n = 13)
and PLA (n = 8) groups. Each dot represents an individual participant. There was a significant group
x time interaction. Post hoc analyses showed that the CR group increased over time with no changes
in the PLA group. * Significantly different than baseline.
There was a group × sex x time interaction (p = 0.039) for chest press strength. Males on creatine
experienced a significant increase over time (CR: pre 166.53 ± 22.82 kg, post 187.59 ± 18.64 kg; d = 0.47,
observed power = 0.55) with no change for males on placebo (pre 181.05 ± 38.71 kg, post 180.30 ± 43.07
kg). There was no significant change over time for females on creatine (pre 67.66 ± 16.87 kg, post 72.19
± 14.65 kg, p = 0.203) or placebo (pre 68.03 ± 20.78 kg, post 69.55 ± 23.27 kg, p = 0.203).
There was a group × time interaction for leg press (p = 0.013, d = 0.95, observed power = 0.74;
Figure 3A) and total body endurance (leg press and chest press combined: p = 0.04, d = 0.96, observed
power = 0.87; Figure 3C). Post hoc analyses indicated that the creatine group significantly increased
the number of repetitions performed over time for the leg press with no significant change in the
placebo group (Figure 3A). Both groups increased total body endurance over time, but the
improvement was greater in the creatine group (Figure 3C). Both groups experienced a similar
change (p < 0.05) in the number of repetitions performed over time for the chest press (Figure 3B).
Figure 3. (A) Leg press (B) Chest press (C) Total body endurance (leg press and chest press combined;
pre and post training) for CR (n = 13) and PLA (n = 8) groups. Each dot represents an individual
participant. There was a significant group × time interaction for (A,C) and a significant main effect of
time for (B). (A) Post hoc analyses showed that the CR group increased over time with no changes in
the PLA group. (B) Both groups increased over time with no differences between groups. (C) Both
groups increase over time, however the increase was greater in the CR group compared to PLA. *
Significant change over time; # CR significantly greater than PLA.
Regarding muscle power, there was a significant time main effect (p < 0.05) for vertical jump
(CR: pre 4387.08 ± 1126.50 W, post 4629.80 ± 1161.66 W, PLA: pre 4561.87 ± 1367.64 W, post 4813.28 ±
1390.57 W), medicine ball throw (CR: pre 386.15 ± 103.58 cm, post 408.94 ± 109.19 cm; PLA: pre 386.76
± 103.12 cm, post 396.51 ± 96.90 cm) and total body power (vertical jump and medicine ball throw
Figure 3.
(
A
) Leg press (
B
) Chest press (
C
) Total body endurance (leg press and chest press combined;
pre and post training) for CR (n=13) and PLA (n=8) groups. Each dot represents an individual
participant. There was a significant group
×
time interaction for (
A
,
C
) and a significant main effect of
time for (
B
). (
A
) Post hoc analyses showed that the CR group increased over time with no changes in
the PLA group. (
B
) Both groups increased over time with no differences between groups. (
C
) Both
groups increase over time, however the increase was greater in the CR group compared to PLA. *
Significant change over time; # CR significantly greater than PLA.
Regarding muscle power, there was a significant time main effect (p<0.05) for vertical jump (CR: pre
4387.08
±
1126.50 W, post 4629.80
±
1161.66 W, PLA: pre 4561.87
±
1367.64 W, post
4813.28 ±1390.57 W
),
medicine ball throw (CR: pre
386.15 ±103.58 cm
, post 408.94
±
109.19 cm; PLA: pre
386.76 ±103.12 cm
,
post 396.51
±
96.90 cm) and total body power (vertical jump and medicine ball throw combined) (CR:
pre 4539.12
±
1161.86, post 4790.80
±
1198.50; PLA: pre 4714.15
±
1404.32, post 4969.39
±
1421.15;
p<0.05), with no significant differences between groups.
4. Discussion
This was the first study to examine the effects of creatine supplementation during resistance
training sessions on muscle accretion and performance. Results showed that creatine ingestion only on
training days produced greater gains in muscle strength and endurance (except chest press) compared
to placebo in a very small cohort of physically active young adults (Figures 2and 3). Males on creatine
improved chest press strength over time with no change for females on creatine. One participant
reported gastrointestinal irritation during the first week of creatine supplementation. No other
participant reported an adverse event. Overall, creatine ingestion during resistance training sessions is
a well-tolerated, viable strategy for improving muscle strength and endurance.
The significant increase in muscle strength (leg press:
∆
43
±
32 kg, chest press
∆
13
±
11 kg;
Figure 2) from creatine supplementation during resistance training sessions is comparable to our
previous studies showing significant improvements in muscle strength from creatine supplementation
immediately before and immediately after resistance training sessions in young (creatine before:
chest press
∆
7
±
8 kg; creatine after: chest press
∆
8
±
6 kg; [
19
]) and older adults (creatine before:
leg press
∆
36
±
26 kg, chest press
∆
15
±
13 kg; creatine after: leg press
∆
40
±
38 kg, chest press
∆15 ±12 kg; [11]
). Collectively, results across studies indicate that creatine supplementation prior to,
Nutrients 2020,12, 1880 9 of 11
during and following resistance training sessions are effective ingestion strategies to improve muscle
strength. However, it remains unknown which pattern of creatine ingestion (pre-exercise vs. during
exercise vs. post-exercise) would produce the greatest muscle benefits in young and older adults.
The greater increase in muscle strength (Figure 2) and endurance (Figure 3) from creatine
supplementation supports the findings of several meta-analyses and review articles [
4
,
20
–
23
]. While the
mechanistic actions of creatine were not measured in this study, creatine supplementation has been
shown to increase intramuscular PCr levels which may have accelerated ATP resynthesis and/or PCr
recovery following each set. Over time, this may have contributed to the greater gains in strength
and endurance. It is also possible that creatine supplementation augmented calcium reuptake into
the sarcoplasmic reticulum which would result in faster actin-myosin cross-bridge cycling during
repeated muscle contractions [
24
] leading to improvements in muscle strength and endurance over
time. Furthermore, creatine may have influenced muscle glycogen stores. Glycogen increases ATP
resynthesis during resistance training sessions [
4
] and, unfortunately, glycogen depletion occurs with
resistance training [
25
]. Creatine supplementation has been shown to increase the translocation and
content of GLUT-4 transport proteins in adults performing resistance training compared to placebo [
26
].
Potentially, an increase in GLUT-4 content would increase glucose disposal and attenuate glycogen
depletion during training sessions. It is somewhat puzzling that muscle strength and leg press
endurance did not increase over six weeks of training in the placebo group. However, with only nine
participants, we likely had insufficient power to detect significant changes over time.
The greater increase in chest press strength in males compared to females on creatine may be
related to initial PCr levels and muscle protein catabolism. Some research suggests that females may
have higher initial resting creatine levels than males [
27
]. A main variable which dictates an individual’s
responsiveness to creatine supplementation in initial levels of intramuscular creatine [
2
]. In addition,
females do not appear to experience a reduction in whole-body or muscle protein breakdown from
creatine supplementation [
28
,
29
]. These potential sex differences in creatine metabolism may have
influenced chest press results over time. However, no measure of intramuscular creatine or muscle
protein catabolism was performed so we can only speculate regarding the sex difference in chest
press strength. It is important to note that females on creatine experienced a ~5 kg increase in chest
press strength over time. Our small sample size may have decreased our ability to detect significant
differences with training.
Creatine supplementation had no greater effect on muscle accretion (Table 3) compared to placebo.
The lack of significant findings may be related to the short duration of training and supplementation (six
weeks), intermittent creatine dosing protocol, decrease in total energy intake over time and low sample
size. In the largest and most comprehensive meta-analysis performed to date, creatine supplementation
resulted in greater gains in muscle accretion when the study period was
≥
10 weeks in duration [
4
].
Furthermore, due to the objectives of the study, participants only ingested creatine on training days
(five days/week). Perhaps daily creatine ingestion during a resistance training program is required to
produce statistically significant greater gains in muscle accretion compared to placebo. The reduction
in total energy intake in the creatine group may have also masked any effects of creatine on muscle
accretion. Finally, our small sample size likely reduced our ability to detect significant differences
between groups for muscle thickness. Future research should investigate the mechanistic actions of
daily creatine supplementation (includes intra-workout and non-training days) and a longer training
period (>6 weeks) on muscle accretion in physically active young trained adults.
In addition to our small sample size, there were other limitations to the study which may have
influenced our findings. Three-day food records do not measure habitual dietary intake of creatine.
A high intake of red meat, seafood or poultry may have influenced the responsiveness to creatine
supplementation. Furthermore, food records typically have high variability due to the participant’s
memory and accuracy in recording and reporting correct portion sizes and frequency of food intake [
30
].
Finally, there was no measure of neuromuscular activation, muscle fiber morphology or recruitment,
muscle protein kinetics, satellite cells, growth factors, hormones, oxidative stress or inflammation.
Nutrients 2020,12, 1880 10 of 11
5. Conclusions
Creatine ingestion during resistance training sessions is a safe and effective strategy to increase
muscle strength and endurance in physically active young adults. However, it is unknown whether
intra-workout creatine supplementation is more beneficial than consuming creatine at other times
of the day during a resistance training program. Future research should determine the effects of the
timing of creatine supplementation on muscle mass and performance in a large cohort of physically
active young adults.
Author Contributions:
Conceptualization and methodology, S.M., D.G.C., S.C.F. and J.P.N.; formal analysis, S.M.,
D.G.C. and S.C.F.; investigation, S.M. and D.G.C., writing—original draft preparation, S.M., D.G.C., S.C.F., J.P.N.,
M.J.O., and J.A.; writing—review & editing, S.M., D.G.C., S.C.F., J.P.N., M.J.O., and J.A.; project administration,
S.M. and D.G.C. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest:
MJO serves on the Dymatize Scientific Advisory Board, a company that sells creatine in its
supplement portfolio but was not used in this study. All other authors declare no conflict of interest.
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