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RESISTANCE TRAINING FREQUENCIES OF 3AND 6
TIMES PER WEEK PRODUCE SIMILAR MUSCULAR
ADAPTATIONS IN RESISTANCE-TRAINED MEN
JURAJ SARIC,
1
DOMAGOJ LISICA,
1
IVAN ORLIC,
1
JOZO GRGIC,
2
JAMES W. KRIEGER,
3
SASA VUK,
1
AND
BRAD J. SCHOENFELD
4
1
Faculty of Kinesiology, University of Zagreb, Zagreb, Croatia;
2
Institute for Health and Sport (IHES), Victoria University,
Melbourne, Australia;
3
Weightology, LLC, Redmond, Washington; and
4
Department of Health Sciences, Lehman College,
Bronx, New York
ABSTRACT
Saric, J, Lisica, D, Orlic, I, Grgic, J, Krieger, JW, Vuk, S, and
Schoenfeld, BJ. Resistance training frequencies of 3 and 6
times per week produce similar muscular adaptations in
resistance-trained men. J Strength Cond Res XX(X): 000–
000, 2018—We examined the effects of resistance training
(RT) frequency performed 3 times per week (RT3) vs. RT per-
formed 6 times per week (RT6) under volume-equated condi-
tions in resistance-trained men. Twenty-seven men were
randomly allocated to RT3 (n= 14) or RT6 (n= 13). The
supervised training intervention lasted for 6 weeks. Upper-
and lower-body strength were assessed using the 1 repetition
maximum test. Also, muscular endurance (60% 1 repetition
maximum performed to momentary failure) and muscle thick-
ness (elbow flexors, elbow extensors, rectus femoris, and
vastus intermedius) were measured before and after interven-
tion. Pre-to-post intervention, both groups increased upper-
body strength (RT3: +4%; RT6: +6%) and lower-body
strength (RT3: +22%; RT6: +18%) with no significant
between-group differences. No significant pre-to-post interven-
tion increases in muscular endurance were seen in either of the
training groups. Both groups increased elbow extensor thick-
ness (RT3: +14%; RT6: +11%), rectus femoris thickness
(RT3: +5%; RT6: +6%), and vastus intermedius thickness
(RT3: +10%; RT6: +11%) with no significant between-group
differences. Only the RT3 group significantly increased elbow
flexor thickness from pre-to-post intervention (+7%). When
training volume is equated, it seems that RT performed either
3 or 6 times per week can result in similar strength gains over
a 6-week training period. Furthermore, under volume-equated
conditions, comparable hypertrophy results may also be ex-
pected with both RT frequencies. Finally, no changes were
seen in muscular endurance possibly because of the consider-
able interindividual variability in responses. The findings pre-
sented herein might be of interest to coaches, exercise
practitioners, athletes, and recreational trainees.
KEY WORDS skeletal muscle, ultrasound, growth, muscle size,
strength
INTRODUCTION
Participation in resistance training (RT) programs
can bring about significant increases in muscle size
and strength (2). Both of these muscular compo-
nents are important for health, activities of daily
living, and athletic performance (29,30). Muscle hypertrophy
occurs as cumulative result of transient increases in muscle
protein synthesis (MPS) above that of muscle protein break-
down (17). Muscle protein synthesis can be significantly
stimulated by the ingestion of dietary protein as well as with
RT (18). This RT-induced increase in MPS can overcome
rates of muscle protein breakdown and thus promote a net
protein accretion (17). In untrained individuals, the MPS
increase after RT is elevated for ;48 hours (18), and likely
contributes to the American College of Sports Medicine (2)
recommendation for a training frequency of 2–3 times per
week when the training goal is muscular hypertrophy. How-
ever, there is evidence that in resistance-trained individuals,
this response is blunted whereby the postexercise MPS
response lasts only for ;24 hours (7). Based on these find-
ings and considering that only a few sets per training session
are sufficient to increase MPS (5), some researchers have
hypothesized that trained individuals may benefit from
working a muscle group with a higher weekly training fre-
quency and a lower per session volume (8). These research-
ers suggested that training a muscle group up to 6 times per
week might be a beneficial strategy for increasing muscle
mass in this population through the frequent elevations in
MPS (8). Two recent studies explored this topic in trained
Address correspondence to Brad J. Schoenfeld, bradschoenfeldphd@gmail.com.
00(00)/1–8
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individuals and compared training frequencies of 1 vs. 5
times per week, and 3 vs. 6 times per weeks (Ref. 9, Ref. 6
respectively). However, these studies assessed changes in
lean body mass and did not use site-specific measures of
muscular hypertrophy such as B-mode ultrasound, thus leav-
ing a gap in the literature.
A recent meta-analysis suggested that a dose-response
relationship exists between weekly RT frequency and gains
in muscular strength (10). However, this analysis also
showed that under volume-equated conditions, there was
no significant effect of RT frequency on strength gains. Still,
the authors noted the analysis was limited by the small
number of studies (i.e., 3) conducted in trained individuals.
Furthermore, almost all the studies examined training fre-
quencies of 4 times per week or less; none of the included
studies investigated very high training frequencies such as
RT performed 6 times per week. Data presented at the
2012 European College of Sport Science conference showed
that trained powerlifters increased muscular strength to
a greater extent when training 6 times per week in compar-
ison with the group training only 3 times per week (19).
Interestingly, this effect was observed even under volume-
equated conditions. However, the results of this study have
not been published, and thus, the findings cannot be
adequately scrutinized. In contrast to these preliminary
findings, a recent study by Colquhoun et al. (6) compared
training frequencies of 3 vs. 6 times per week and reported
similar increases in strength in both groups. Given the
current limited and contrasting findings, it is evident that
further work exploring this topic is warranted.
For muscle hypertrophy, the current body of evidence
indicates that training a muscle group 2 times per week may
be more effective than training a muscle group once per
week (26). However, the hypertrophy responses to very high
weekly training frequencies remain unclear (26). Further-
more, for gains in strength, the current findings indicate that
similar strength gains can be attained using vastly different
training frequencies, provided that total volume is equated
(10). That said, again, the responses to very high frequencies
such as training 6 times per week are still underinvestigated
(10).
Therefore, taking into account the evident lack of similar
studies conducted in this area, the purpose of this study was
to investigate the influence of volume-equated RT frequen-
cies of 3 vs. 6 times per week in trained men on muscular
strength, endurance, and hypertrophy. Based on the volume-
equated study design (10,21,25), we hypothesized that there
would be no significant differences between the training
groups for any of the evaluated outcomes.
METHODS
Experimental Approach to the Problem
Thirty resistance-trained men were randomly assigned to
training 3 times per week (RT3; n= 15) or 6 times per week
(RT6; n= 15) for 6 weeks. The RT3 group trained each
muscle group 3 times per week, whereas the RT6 group
trained each muscle group 6 times per week using a full-
body routine. The training protocol included a mixture of
single-joint and multi-joint exercises. The weekly set training
volume was equated between the groups. All exercises were
performed for 6–12 repetitions to muscular failure. Testing of
muscular strength was performed using the 1 repetition max-
imum (1RM) for the barbell bench press exercise and barbell
back squat exercise. Changes in muscle thickness (MT) of
the elbow flexors, elbow extensors, rectus femoris, and vastus
lateralis were measured using B-mode ultrasound.
Subjects
Based on an a priori power analysis using the G*Power
software (Germany, Du¨sseldorf, version 3) with an expected
effect size of 0.60 (for vastus lateralis MT as the outcome
measure (27)), the alpha error level of 0.05, and the statistical
power of 80%, the required sample size for this study was 20
participants. We recruited 30 resistance-trained men (mean
6SD; age = 22.6 62.1 years [all subjects were over 18. The
youngest subject in this study was 20 years of age], stature =
183.1 66.0 cm, body mass = 87.2 611.6 kg) for the study.
The criterion for defining participants as resistance-trained
was that they were training minimally 2 times per week for
the past 6 months before the start of the study, using exer-
cises for both upper- and lower-body musculature. This was
set as an inclusion criterion for the current study. The par-
ticipants were randomly assigned to training 3 times per
week (RT3; n= 15) or 6 times per week (RT6; n= 15).
An independent t-test showed no significant difference
between the groups in age, body mass, or height at baseline.
Three participants dropped out of the study (one participant
due to personal reasons and 2 because of non–training-
related injuries). Therefore, 14 and 13 participants from the
RT3 and RT6 per week groups, respectively, were included
in the final analysis. Training adherence was 100% in both
groups, and no adverse effects occurred from the interven-
tion. The experimental protocol, risks, and benefits were
explained to the participants before the training protocol
began, and all the participants gave a written informed
consent. Ethical approval was granted by the Committee for
Scientific Research and Ethics of the Faculty of Kinesiology
at the University of Zagreb, Croatia.
Procedures
Training Protocol. The RT3 group trained each muscle group
3 times per week, whereas the RT6 group trained each
muscle group 6 times per week using a full-body routine.
The training protocol included a mixture of single-joint and
multi-joint exercises. A summary of the RT programs can be
found in Table 1. The weekly set training volume was
equated between the groups. All exercises were performed
for 6–12 repetitions to muscular failure. When necessary,
loads were adjusted to maintain the prescribed number of
repetitions. Concentric and eccentric actions were
performed with a cadence of 1–2 seconds. Rest interval
Frequency of 3 vs. 6 Times Per Week
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duration between sets was 60–90 seconds and between ex-
ercises 2–3 minutes (11). The training intervention lasted for
6 weeks. Volume load was calculated as load 3repetitions
3sets and was analyzed per muscle group (i.e., chest, back,
shoulders, elbow extensors, elbow flexors, anterior thigh,
and posterior thigh). All training sessions were supervised
by personal trainers.
Strength Assessment. Testing of muscular strength was
performed 48 hours after an RT session using the 1RM
test, defined as the maximum load lifted with a proper form
through a full range of motion. Upper-body strength was
assessed first, using the barbell bench press exercise; lower-
body strength was assessed second, using the barbell back
squat exercise. A detailed explanation of the protocols can
be found elsewhere (27). Briefly, after a 5–10-minute warm-
up, the participants performed 3 sets, progressively increas-
ing the load from 50%, 60–80%, to 90% of their estimated
1RM while performing 5, 2–3, and 1 repetition, respectively.
Then, the participants performed the 1RM test. The load
for the subsequent attempts was increased or decreased
based on whether the participant lifted the load or not.
All assessments were performed within 5 attempts, using
a 3–5-minute rest interval between attempts. After the final
1RM attempt, the participants rested for 5 minutes, after
which muscular endurance was assessed. The muscular
endurance test consisted of one “all-out” set with a load
of 60% 1RM performed to muscular failure on the above-
mentioned upper- and lower-body exercises. A cadence of
1–2 seconds was used both for the concentric and eccentric
portions of the lift. When the participants were unable to do
the whole range of motion of the exercise or maintain the
prescribed cadence, the test was terminated.
Muscle Thickness. Before and after the RT intervention MT of
the elbow flexors, elbow extensors, rectus femoris, and
vastus lateralis were measured by a skilled technician using
B-mode ultrasound (Sonoscape S40; Sonoscape Co. Ltd.,
Beijing, China). The MT assessment was performed 48
hours after the last training session. To prevent confounding
factors for the assessment of MT, the measurement sessions
were performed at the same time of day for each
participant. Participants were instructed to maintain their
usual level of hydration, and the measurements were
performed a minimum of 2 days after an RT session to
prevent any swelling effects. A detailed explanation of the
protocol can be found elsewhere (1). In brief, a water-
soluble transmission gel was applied to the muscle being
assessed. Thereafter, a 5–13-MHz ultrasound probe was
placed perpendicular to the tissue interface. Caution was
taken not to depress the skin. When the quality of the image
was satisfactory, it was saved on a hard drive. Two images
were taken and the average values were used for the anal-
ysis. The MT dimensions were measured as described by
Abe et al. (1). For the elbow flexors and extensors, the
TABLE 1. Resistance training protocols.*
Group Monday Tuesday Wednesday Thursday Friday Saturday Sunday
RT3 Cross cable fly 34 Off Dumbbell fly 34 Off Pec-dec fly 34 Off Off
Bent-over barbell row 34 Seated cable row 34 Lat-pulldown 34
Lateral raises 34 Face pulls 34 Dumbbell shoulder press 34
Overhead dumbbell extensions 34 Lying triceps press 34 Triceps extension 34
Machine biceps curl 34 Dumbbell biceps curl 34 Barbell biceps curl 34
Leg press 34 Leg extension 34 Squat 34
Lying leg curl 34 Stiff-leg deadlift 34 Lying leg curl 34
RT6 Cross cable fly 32 Dumbbell fly 32 Pec-dec fly 32 Cross cable fly 32 Dumbbell fly 32 Pec-dec fly 32Off
Bent-over barbell row 32 Seated cable row 32 Lat-pulldown 32 Bent-over barbell
row 32
Seated cable row 32 Lat-pulldown 32
Lateral raises 32 Face pulls 32 Dumbbell shoulder press 32 Lateral raises 32 Face pulls 32 Dumbbell shoulder press 32
Overhead dumbbell extensions 32 Lying triceps press 32 Triceps extension 32 Overhead dumbbell
extensions 32
Lying triceps press 32 Triceps extension 32
Machine biceps curl 32 Dumbbell biceps curl 3
2
Barbell biceps curl 32 Machine biceps curl
32
Dumbbell biceps curl 32 Barbell biceps curl 32
Leg press 32 Leg extension 32 Squat 32 Leg press 32 Leg extension 32 Squat 32
Lying leg curl 32 Stiff-leg deadlift 32 Lying leg curl 32 Lying leg curl 32 Stiff-leg deadlift 32 Lying leg curl 32
*RT3 = resistance training 3 times per week; RT6 = resistance training 6 times per week.
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measurements were taken 60% distal from acromion to lat-
eral epicondyle of the humerus. The vastus lateralis and
rectus femoris measures were taken at a distance of 50%
between the lateral epicondyle of the femur and the greater
trochanter. The coefficients of variations for repeated meas-
ures for the elbow flexors, elbow extensors, rectus femoris,
and vastus lateralis MT were 2.4%, 3.2%, 2.5%, and 2.1%,
respectively. Ultrasound imaging has been used in similar
previous research and is highly correlated with magnetic
resonance imaging (3).
Statistical Analyses
Data were modeled using both a frequentist and Bayesian
approach. The frequentist approach involved 2 32 linear
mixed models with repeated measures, estimated by
a restricted maximum likelihood algorithm, and the
Bayesian approach involves JZS Bayes factor repeated-
measures analysis of variance with default prior scales
(22). Training frequency (RT3 or RT6) was included as
the between-subject factor, time (pre and post) was
included as the repeated within-subjects factor, and group
3time was included as the interaction. The subject was
included as a random effect in the linear mixed models,
and repeated covariance structures were specified as
compound symmetry. When significant interactions were
identified in the linear mixed models, post hoc
comparisons between groups over time were compared
using F-tests with a Bonferroni correction. Effect sizes
were calculated as the mean pre-post change divided by
the pooled pretest SD (14). In addition, percent changes
were calculated. Total volume load between the groups
was compared using an independent t-test. Baseline differ-
ences in the dependent variables between the groups were
compared using an independent t-test. No significant
baseline differences were observed for the dependent var-
iables (p.0.05 for all dependent variables). Analyses
were performed using NCSS Version 12 (Kaysville, UT,
USA) and JASP 0.8.5 (Amsterdam, the Netherlands). Ef-
fects were considered significant at p#0.05. Bayes factors
for effects were interpreted as “weak,” “positive,” “strong,”
or “very strong” according to Raftery (20). Data are re-
ported as 6SD unless otherwise specified.
RESULTS
Volume Load
There were no differences in volume load between the RT3
and RT6 groups for chest (RT3 = 25,264 66,866;
RT6 = 29,585 66,884; p= 0.155), back (RT3 = 53,872 6
13,343; RT6 = 58,703 68,357; p= 0.268), shoulders (RT3
= 20,122 65,067; RT6 = 22,815 64,401; p= 0.152), elbow
extensors (RT3 = 23,609 66,261; RT6 = 26,846 63,640; p
= 0.112), elbow flexors (RT3 = 20,706 64,840; RT6 =
23,946 64,561; p= 0.085), and anterior thigh
(RT3 = 107,404 623,659; RT6 = 121,660 631,664;
p= 0.201). A significant difference in volume load was seen
for the posterior thigh favoring RT6 (RT3 = 55,286 6
15,675; RT6 = 66,285 69,741; p= 0.038).
Muscle Size
There was a significant interaction for elbow flexor MT (p,
0.001). Elbow flexor MT significantly increased in the RT3
group (p= 0.005), but not in the RT6 group (p= 1.0). There
was positive evidence (3 #BF
10
#20) in favor of the inter-
action compared with the null model, and positive evidence
(BF
10
= 4.6) in favor of the interaction over main effects.
There was a significant improvement (p,0.001) from pre
to post in elbow extensor MT, with no significant group by
time interaction. There was very strong evidence (BF
10
.
150) in favor of a time effect for elbow extensor MT com-
pared with the null model, and positive evidence (BF
10
=
3.9) in favor of a main time effect over an interaction.
There was a significant improvement (p= 0.008) from pre
to post in rectus femoris MT, with no significant group by
time interaction. There was positive evidence (3 #BF
10
#
20) in favor of a time effect for rectus femoris MT compared
with the null model, and positive evidence (BF
10
= 3.7) in
favor of a main time effect over an interaction.
There was a significant improvement (p,0.001) from
pre to post in vastus lateralis MT, with no significant
group by time interaction. There was strong evidence
(20 #BF
10
#150) in favor of a time effect for vastus
lateralis MT compared with the null model, and positive
evidence (BF
10
= 5.2) in favor of a main time effect over an
interaction.
Strength and Endurance
There was a significant improvement (p,0.001) from pre to
post in squat 1RM, with no significant group by time inter-
action. There was very strong evidence (BF
10
.150) in
favor of a time effect for squat 1RM compared with the null
model, and positive evidence (BF
10
= 3.2) in favor of a main
time effect over an interaction (Table 2). There was a signif-
icant improvement (p,0.001) from pre to post in bench
1RM, with no significant group by time interaction. There
was very strong evidence (BF
10
.150) in favor of a time
effect for bench 1RM compared with the null model, and
weak evidence (BF
10
= 1.9) in favor of a main time effect
over an interaction.
There were no significant main effects or interactions for
lower-body muscular endurance. Evidence favored the null
hypothesis (BF
10
,1) of no group or time effects. There
were no significant main effects or interactions for upper-
body muscular endurance. Evidence favored the null
hypothesis (BF
10
,1) of no group or time effects.
DISCUSSION
This study aimed to investigate the effects of RT fre-
quency performed 3 vs. 6 times per week on muscular
adaptations in trained men. Our hypothesis was that there
would be no significant differences between the training
Frequency of 3 vs. 6 Times Per Week
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TABLE 2. Summary of study results.*
Outcome Group Pre mean 6SD Post mean 6SD p (Group) p(Time)
p
(group
3time)
BF
10
(group)
BF
10
(time)
BF
10
(group + time)
BF
10
(group + time
+ group 3
time) ES
Percent
changes
(%)
Squat 1RM (kg) 3 122.9 629.6 149.8 622.7 0.72 ,0.001†0.35 0.44 3.79 310
8
z2.30 310
8
1.18 310
8
1.01 +22
6 128.5 624.1 151.2 625.3 0.85 +18
Bench 1RM (kg) 3 91.3 619.4 94.5 619.5 0.35 ,0.001†0.14 0.71 260.39z168.45 136.17 0.17 +4
6 96.7 618.4 102.9 619.2 0.33 +6
Squat with 60%
1RM
(repetitions)
3 35.5 615.1 41.4 611.1 0.21 0.16 0.17 0.76 0.67 0.50 0.36 0.49 +17
6 33.5 68.0 33.5 68.7 0.01 0
Bench press with
60% 1RM
(repetitions)
3 23.7 63.0 24.9 64.6 0.34 0.63 0.34 0.51 0.70 0.16 0.09 0.44 +5
6 23.4 62.1 23.0 63.8 20.15 22
Elbow flexor
thickness (mm)
3 38.2 63.8 40.9 64.3 0.28 0.05 ,0.001†0.67 1.17 0.81 3.70z0.66 +7
6 41.3 63.9 40.9 63.3 20.09 21
Elbow extensor
thickness (mm)
3 39.5 67.4 45.1 64.9 0.31 ,0.001†0.62 0.57 18,580.99z12,531.92 4,814.80 0.83 +14
6 42.1 66.1 46.9 65.0 0.71 +11
Rectus femoris
thickness (mm)
3 25.6 64.7 26.8 64.6 0.28 0.008†0.76 0.72 6.98z4.91 1.89 0.30 +5
6 23.9 62.6 25.3 62.7 0.37 +6
Vastus intermedius
thickness (mm)
3 21.0 64.1 23.1 63.1 0.61 ,0.001†0.89 0.49 82.72z44.46 15.91 0.57 +10
6 21.6 63.5 23.9 63.8 0.61 +11
*1RM = 1 repetition maximum; ES = effect size.
†Significant at p,0.05.
zPreferred model based on highest BF
10
$1.
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groups for any of the evaluated outcomes. Herein, we
show that both training frequencies were equally effective
for increasing muscular strength. For muscle size, training
a muscle group either 3 or 6 times per week resulted in
similar gains for most, but not all, of the measured sites.
Finally, neither of the training groups increased muscular
endurance from pre-to-post training intervention. Taken
together, these results only partially confirmed our initial
hypothesis.
This study is the first that compared training frequencies
of 3 and 6 times per week in trained men while using site-
specific measures of muscular growth. In most of the
measured sites, both groups showed significant pre-to-post
training intervention increases in MT with no between-
group differences. This would suggest that gains in muscle
mass were similar between the groups, despite speculation
that the group training 6 times per week stimulated MPS
more often than the group training 3 times per week. These
results contradict the hypothesis put forth by Dankel et al.
(8). As acknowledged by those authors, their assumption
was solely based on the MPS response after RT, which
may not reflect long-term hypertrophy adaptations to regi-
mented RT. It is interesting to note that only the RT3 group
experienced significant pre-to-post increases in MT of the
elbow flexors. Although unexpected, these findings might
be explained by the specificity of the training protocol. As
compared to the elbow extensors, training for the elbow
flexors included more indirect activation through exercises
that were used to target the back musculature, whereas the
elbow extensors were mostly trained through single-joint,
isolation exercises. Therefore, it is plausible that the elbow
flexors experienced more per session fatigue in the RT6
group, and thus were not completely recovered before the
next training session. Taken together, it might be that this
constant fatigue in the elbow flexors blunted significant
growth in the RT6 group; although, given that we did not
assess fatigue directly in this study, this idea remains
speculative.
Upper- and lower-body strength increased from pre-to-
post intervention in both groups, with no significant
between-group differences. A comparison of the present
results with the findings from the literature is difficult,
considering the paucity of similar studies. To the best of
our knowledge, only one previously published study used
a comparable study design. Colquhoun et al. (6) included 28
trained men in their trial, in which one group trained 3 times
per week (n= 16), whereas the other group trained 6 times
per week (n= 12). After a 6-week intervention, both training
groups increased upper- and lower-body muscular strength
with no significant between-group differences. Our results
lend support to these findings, indicating that strength gains
are very similar between these training frequencies. Other
studies in trained men that compared training frequencies
of (a) 1 vs. 2 times per week, (b) 1 vs. 3 times per week,
and (c) 2 vs. 4 times per week also showed statistically sim-
ilar increases in strength regardless of training frequency
(4,28,31). Furthermore, a recent study by Gomes et al. (9)
compared training frequencies of 1 vs. 5 times per week and
also reported no significant differences in strength gains
between the groups. In summary, coupled with the meta-
analytic findings (10), it seems that under volume-equated
conditions, training frequency does not play a large role in
strength gain as other training variables such as load (24). It
is possible that training frequency would have a more sub-
stantial impact over longer time courses, as the Norwegian
Frequency Project lasted 16-weeks; however, this remains
speculative and future research is warranted. Furthermore,
it should be made clear that our program had more of
Figure 1. Individual responses in upper- and lower-body muscular endurance from the groups training 3 and 6 times per week.
Frequency of 3 vs. 6 Times Per Week
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a hypertrophy focus, whereas the Norwegian Frequency
Project was more directed toward improvements in strength,
given their sample (i.e., powerlifters).
To explore the importance of training specificity for gains
in strength, we used testing of strength both in an exercise
that was used in the training program (i.e., squat) and an
exercise that was not included in the RT program (i.e., bench
press). Although the participants increased both upper- and
lower-body strength, the effect sizes and percent changes
were larger for lower-body strength gains (as compared to
the upper-body changes). These results do highlight that RT
may result with strength gains even when tested in an “unac-
customed” exercise; however, these gains are mostly specific
to the movement that is trained (23).
The exercise protocol used in this study was not
sufficient to elicit significant increases in muscular
endurance in either of the training groups. These results
are somewhat surprising, considering that improvements
in muscular endurance with regimented RT in trained
individuals have been noted by others (27). It is relevant to
highlight that the RT3 group did show moderate effect
sizes both for upper-body (effect size = 0.44; +5%) and
lower-body (effect size = 0.49; +17%) muscular endur-
ance. No effects were observed in the RT6 group for upper
and lower-body muscular endurance. It might be that sig-
nificance was not observed in this test due to the large
variability in responses among the participants in both
groups (Figure 1).
Although this study had several strengths, including the
use of a direct measure of muscle growth, full RT supervi-
sion, and 100% adherence to the training sessions, there are
several limitations that warrant consideration. First, the
training protocol was somewhat short because it lasted only
6 weeks. It is conceivable that results would have differed
with a longer-duration training intervention. Second, we did
not control for nutritional status. The importance of dietary
protein for muscular growth is well established (15) and this
may have confounded results. That said, randomization
should theoretically have reduced the effects of any
between-group differences in nutrient consumption. Third,
our results are specific to young, resistance-trained men.
Men and women have different physiological characteristics
such as different muscle fiber distribution, differences in mus-
cle perfusion, and marked differences in fatigability (12).
Women seem to experience faster recovery of muscular
strength as compared to men (13) and, therefore, might
respond better to higher training frequencies (10). Similar
study designs are warranted in women to examine whether
there is a sex-specific response to RT frequency. Further-
more, our results are specific to young individuals. Therefore,
they cannot be generalizable to older adults, given that
recovery from RT is altered in this age group (16), which
may warrant different RT frequency prescription. Finally, we
did not consider the RT frequencies usually used by the
participants before their enrollment into the study. There-
fore, for some, RT frequencies used in the present training
program might have provided a novel stimulus and for
others, they might have been comparable with their usual
training practices. Future studies performed in trained indi-
viduals should circumvent this limitation by asking the par-
ticipants to indicate their usual RT frequency per muscle
group before starting the training program.
In conclusion, this study indicates that under volume-
equated conditions, there are no additional strength benefits
to training muscle groups 6 days per week in resistance-
trained men, and that very high training frequencies may, in
some cases, hinder muscle hypertrophy.
PRACTICAL APPLICATIONS
Both RT frequencies of 3 and 6 times per week can be
effective for increases in muscular strength in trained men.
Although training 6 times per week can result in significant
hypertrophy, it seems that training 3 times per week under
volume-equated conditions can, in some cases, be even more
effective.
ACKNOWLEDGMENTS
The authors thank all the participants for their participation
in this study. The authors also thank Dr. Franka Jelavic Kojic,
Luka Krmpotic, Ana Marija Manduric, and Ninoslav Rud-
man for their help in the data collection.
J. Saric, D. Lisica, I. Orlic, and J. Grgic contributed equally to
this work.
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