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ORIGINAL RESEARCH
published: 05 January 2022
doi: 10.3389/fphys.2021.789403
Edited by:
Helmi Chaabene,
University of Potsdam, Germany
Reviewed by:
Brad Schoenfeld,
Lehman College, United States
Matt S. Stock,
University of Central Florida,
United States
*Correspondence:
Håvard Hamarsland
Havard.hamarsland@inn.no
Specialty section:
This article was submitted to
Exercise Physiology,
a section of the journal
Frontiers in Physiology
Received: 04 October 2021
Accepted: 03 December 2021
Published: 05 January 2022
Citation:
Hamarsland H, Moen H,
Skaar OJ, Jorang PW, Rødahl HS and
Rønnestad BR (2022) Equal-Volume
Strength Training With Different
Training Frequencies Induces Similar
Muscle Hypertrophy and Strength
Improvement in Trained Participants.
Front. Physiol. 12:789403.
doi: 10.3389/fphys.2021.789403
Equal-Volume Strength Training With
Different Training Frequencies
Induces Similar Muscle Hypertrophy
and Strength Improvement in Trained
Participants
Håvard Hamarsland*, Hermann Moen, Ole Johannes Skaar, Preben Wahlstrøm Jorang,
Håvard Saeterøy Rødahl and Bent R. Rønnestad
Section for Health and Exercise Physiology, Inland Norway University of Applied Sciences, Lillehammer, Norway
The main goal of the current study was to compare the effects of volume-equated
training frequency on gains in muscle mass and strength. In addition, we aimed to
investigate whether the effect of training frequency was affected by the complexity,
concerning the degrees of freedom, of an exercise. Participants were randomized to
a moderate training frequency group (two weekly sessions) or high training frequency
group (four weekly sessions). Twenty-one participants (male: 11, female: 10, age:
25.9 ±4.0) completed the 9-week whole-body progressive heavy resistance training
intervention with moderate (n= 13) or high (n= 8) training frequency. Whole-body and
regional changes in lean mass were measured using dual-energy x-ray absorptiometry,
while the vastus lateralis thickness was measured by ultrasound. Changes in muscle
strength were measured as one repetition maximum for squat, hack squat, bench press,
and chest press. No differences between groups were observed for any of the measures
of muscle growth or muscle strength. Muscle strength increased to a greater extent in
hack squat and chest press than squat and bench press for both moderate (50 and 21%
vs. 19 and 14%, respectively) and high-frequency groups (63 and 31% vs. 19 and 16%,
respectively), with no differences between groups. These results suggest that training
frequency is less decisive when weekly training volume is equated. Further, familiarity
with an exercise seems to be of greater importance for strength adaptations than the
complexity of the exercise.
Keywords: skeletal muscle, resistance training, training frequency, hypertrophy, strength, trained individuals
INTRODUCTION
Resistance training is an essential tool in the pursuit of athletic performance (McGuigan et al.,
2012) and for improving health (El-Kotob et al., 2020). To optimize the effects of resistance
training, the manipulation of several factors, primarily training volume, load, and frequency,
is central (American College of Sports Medicine, 2009). The current recommendations on
training frequency, defined as the number of sets or training sessions on a given muscle group
performed within a given timeframe, have been criticized for being based on limited evidence
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Hamarsland et al. Training Frequency in Trained Individuals
(Grgic et al., 2018;Ralston et al., 2018;Schoenfeld et al., 2019).
The latest meta-analyses suggest a limited role for training
frequency, given that the weekly training volume is kept identical
between groups (Grgic et al., 2018;Ralston et al., 2018;Schoenfeld
et al., 2019). However, in the current literature, studies isolating
frequency by keeping total weekly volume matched between
groups are limited; most include untrained participants and
compare training frequencies of one to three sessions per week.
As trained individuals are more likely to have greater training
frequencies than less trained individuals, and the frequencies
used are likely to be greater than those investigated in most
studies (Strömbäck et al., 2018), a need for more research on this
group has been expressed (Grgic et al., 2018;Ralston et al., 2018).
To our knowledge, there are ten published studies on volume
equated resistance training frequency in trained individuals
published to date (McLester et al., 2000;Schoenfeld et al., 2015;
Brigatto et al., 2018;Colquhoun et al., 2018;Gentil et al., 2018;
Gomes et al., 2019;Lasevicius et al., 2019;Saric et al., 2019;
Zaroni et al., 2019;Johnsen and van den Tillaar, 2021). Five of
these studies have focused on higher frequencies than 3 days per
week (Colquhoun et al., 2018;Gomes et al., 2019;Saric et al.,
2019;Zaroni et al., 2019;Johnsen and van den Tillaar, 2021).
Zaroni et al. (2019) reported greater muscle hypertrophy but no
differences in strength gain when comparing one (lower body)
and two (upper body) weekly sessions with five sessions per week
in young, trained men. Apart from this study, there seems to be
no difference in muscular adaptations when comparing one and
five (Gomes et al., 2019), two and four (Johnsen and van den
Tillaar, 2021), or three and six (Colquhoun et al., 2018;Saric et al.,
2019) weekly training sessions in young trained men.
Although the current evidence for a beneficial effect of higher
training frequencies in trained individuals is weak, it is based on
small studies with limited power to detect small but meaningful
differences between protocols. Thus, there is a need for more data
to be able to make precise and evidence-based recommendations.
In addition, there are several theoretical advantages to increased
training frequency. The protein synthetic response to a training
stimulus is considered to last for at least 24–48 h after a
bout of resistance exercise in untrained individuals (MacDougall
et al., 1995;Phillips et al., 1997;Burd et al., 2011). A higher
training frequency may therefore allow for more time with a
net positive protein balance and greater muscular adaptations to
resistance training. In contrast to untrained individuals, exercise-
induced elevations of protein synthesis appear to last only about
24 h after resistance exercise in resistance-trained individuals
(Damas et al., 2015). Thus, the advantages of higher training
frequencies may increase with training status. Further, it has
been suggested that distributing training volume across several
days may reduce fatigue during the sessions (Dankel et al., 2016)
and reduce recovery time between sessions (Pareja-Blanco et al.,
2020). This may allow for greater training loads, potentially
resulting in superior muscular adaptations to resistance training.
Lastly, more frequent neuromuscular stimuli may optimize
motor learning, increasing strength through neurological factors
(Shea:2000dt). This may be more pronounced in complex multi-
joint, free weight lifts, with greater degrees of freedom, compared
with simpler single joint or machine-based exercises. Although
neurological adaptations are primarily expected to occur at the
beginning of training, antagonist inhibition has been shown to
improve also after years of exercise (Balshaw et al., 2019). To the
best of our knowledge, no study has yet explored whether exercise
complexity influences the potential benefits of training frequency
in trained individuals.
Therefore, the current study aimed to compare the effects of
two and four weekly volume equated heavy resistance training
sessions on gains in muscle mass and -strength in resistance-
trained men and women. Further, we investigated whether the
effect of training frequency was influenced by the complexity of
the exercises. We hypothesized that a volume equated weekly
training frequency of four sessions per week would result
in greater muscular adaptations and strength gains than two
sessions per week. We further hypothesized that the benefits of
a higher training frequency would be greater in more complex
exercises (squat and bench press) compared to less complex
exercises (hack squat and chest press).
MATERIALS AND METHODS
Ethical Approval
The study was performed according to the ethical standards
established by the Declaration of Helsinki 2013 and was
approved by the Local Ethical Committee at the Inland Norway
University of Applied Sciences (20/03749) and pre-registered in
a Norwegian public database (Norwegian Center for Research,
project number 300667). All participants signed an informed
consent form before participation.
Participants
Thirty-four moderately resistance-trained men and women
volunteered to participate in the study. To be included in the
study, participants had to be between 18 and 35 years of age,
free of injury, performed one resistance-training workout per
week on average over the last 6 months, and be familiar with the
powerlifting exercises squat and bench press. Participants were
randomized into a high-frequency group (HF) (n= 17) and a
low-frequency group (LF) (n= 17) stratified by sex, age, years of
resistance training experience, and 1 repetition maximum (RM)
results from the first test. Participants who had more than 10 days
without a workout or were not able to complete 95% of the
planned sets were excluded (n= 3). Due to the ongoing sars-
CoV-2 pandemic, the number of participants who were allowed
to exercise simultaneously was controlled and training times were
strict. Participants who were unable to attend these times, due to
quarantine (n= 3) or sars-CoV-2 infection (n= 1), were excluded
(n= 3). Three participants were unable to complete the study due
to injuries and three participants dropped out of the study due
to personal reasons. Thus, 21 participants were included in the
analysis (HF: n= 8, LF:n= 13).
Experimental Design
The intervention consisted of a 9-week training period (see
Table 1). While the total amounts of weekly sets and training
load (RM) were identical, the HF and LF groups performed
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TABLE 1 | Training protocol.
Group Day 1 exercise: sets Day 2 exercise: sets Day 3 exercise: sets Day 4 exercise: sets Total weekly sets Repetitions and load
High
frequency
Squat: 2 Hack squat: 2 Squat: 2 Hack squat: 2 First workout: 70%
1RM Week 1–3: 12RM
Week 4–6: 10RM Week
7–9: 8RM
Hack squat: 1 Squat: 1 Hack squat: 1 Squat: 1
Bench press: 2 Chest press: 2 Bench press: 2 Chest press: 2
Chest press: 1 Bench press: 1 Chest press: 1 Bench press: 1
Lat pulldowns: 2 Seated row: 2 Lat pulldowns: 2 Seated row: 2
Total workout
sets
8 8 8 8 32
Low
frequency
Squat: 4 Hack squat: 4
Hack squat: 2 Squat: 2
Bench press: 4 Chest press: 4
Chest press: 2 Bench press: 2
Lat pulldowns: 4 Seated row: 4
Total workout
sets
16 16 32
4 and 2 sessions per week, respectively. Thus, HF performed
half the training volume of the LF per workout. The training
period was divided into three blocks based on RM number: week
1–3: 12RM, week. If failure was reached before the intended
repetition range, the resistance was quickly reduced to allow
for the remaining repetitions to be completed. If participants
were able to complete more than the intended repetitions the
set was performed to failure and the resistance was increased
at the next set. Each week, the exercise order was rotated to
balance the number of workouts, starting with a more complex
exercise (squat and bench press) and a less complex exercise
(hack squat and chest press in a machine). Four minutes of rest
was given between each set. To equate the warm-up sets across
each week, LF performed two sets (12 reps of 30% 1RM and 12
reps of 50% 1RM), whereas HF performed four sets (2×12 reps
of 30% 1RM and 2×12 reps of 50% 1RM). There was a strong
focus on completing all sets in every workout. All sessions were
supervised by trained personnel. To counteract a potential effect
of differences in protein intake participants were provided 20 g of
whey protein mixed in water after each workout. To balance the
protein supplementation, the protein was also ingested the day
after workouts in the low-frequency group. Before and after the
training period, participants performed a set of 1RM tests (squat,
hack squat, bench press, and chest press) and underwent a dual-
energy x-ray absorptiometry scan (DXA) and an ultrasound scan
of the vastus lateralis muscle from both legs.
Testing Procedures
The participants were instructed to avoid exercise and strenuous
physical activity for 48 h before all tests. Instructional videos
explaining the 1RM testing procedures were emailed to and
watched by all participants before testing. Before the 1RM test,
participants warmed up for 5 min on a rowing ergometer
(Concept 2 inc., Vermont, VT, United States) with an intensity
of 9–11 on the 6–20 Borg-scale (Borg, 1982), followed by two
sets of 20 walking lunges and two sets of 10 shoulder rotations
using a wooden dowel. On the first 1RM test, individual settings
were established for each participant, and identical settings were
used on subsequent testing. The 1RM tests were performed by
completing single repetitions with increasing load and 4 min
of rest between each repetition. The goal was to reach 1RM
on the tenth repetition. After attaining 1RM for squat and
bench press, participants had a 30-min break with a small meal
before completing the 1RM tests for hack squat and chest press.
The meal was identical at all 1RM tests within an individual
participant but differed between participants. The squat and
bench press were performed in a Tteka BN-02 combo (TTEKA
company, Montreal, QC, Canada). The execution of the squat
and bench press was performed according to the standardization
of the International Powerlifting Federation (2021) technical
rules, except the grip width in the bench press, which was set
to be 81 cm from the fifth finger to the fifth finger. The hack
squat was performed in a Cybex International Hack Squat (Cybex
International, Inc., Massachusetts, MA, United States). Joint
angles were measured using a goniometer. The correct depth was
set as a hip joint angle of ≤90◦and a knee joint angle smaller
than 90◦. Foot placement was adjusted to 22.5, 27.5 (middle of
the plate), or 32.5 cm to achieve the intended hip and knee
angles. The correct depth for each participant was noted on a
vertical measuring band attached to the hack squat. Chest press
was performed in a Cybex Converging Plate Loaded Chest Press
(Cybex International, Inc., Massachusetts, MA, United States).
The chest press started with arms in the extended position and
was finished with arms returned to the extended position. A band
was attached between the handles, and an accepted 1RM required
the band to touch the chest at proc. xiphoideus. During the lift,
the whole foot remained in contact with the floor, the buttocks
in contact with the seat and the back, shoulder blades, and
head in contact with the back support. A test leader controlled
all 1RM attempts.
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FIGURE 1 | Weekly training volume (A) and average load (B) with 2 (LF) and 4 (HF) weekly sessions during a 9-week resistance training intervention.
Body composition was determined using a dual-energy
x-ray absorptiometry (DXA) (Lunar Prodigy, GE Healthcare,
Madison, WI, United States) after an overnight fast and was
analyzed following the protocol of the manufacturer, using the
software of the manufacturer. The participants were placed in
the supine position with a neutral neck position. Hands were
semi-pronated and arms were placed close to the border of
the measurement area to simplify the post-scan measurements.
The foot position was standardized in a neutral position using
a foam rubber cast (10 cm between heels) with negligible
absorbance. Regions of interest were used to analyze different
body regions. Dividing lines between arms and thorax were
drawn through the glenohumeral joint. Dividing lines between
the thorax and legs were drawn midway through the femoral neck
at a 90-degree angle to the femoral neck. All DXA scans were
performed by a researcher blinded to subject randomization.
The intraclass correlation (ICC) for lean body mass in our lab
is 0.99. The muscle thickness of m. vastus lateralis of both legs
were measured using B-mode ultrasonography (SmartUs EXT-1
M, Telemed, Vilnius, Lithuania) with a 39 mm 12 MHz, linear
array probe. Image depth was set to 8 cm, frequency to 12 Hz,
the dynamic range was set to 72 dB, and the gain was set to
43%. Longitudinal images were obtained ∼50% distally from the
trochanter major toward the femoral lateral epicondyle. Three
images were captured before and after the training intervention.
The probe’s position was marked on the skin and subsequently
marked on a soft transparent plastic sheet superimposed on the
thigh. Landmarks such as moles and scars were also marked on
the plastic sheet for relocation of the probe during post-training
measurements. Furthermore, pre-test images were used to locate
anatomical landmarks on the post-test images to ensure the
same measuring location. During analysis, pre and post-images
from the same participant were analyzed consecutively using the
Fiji software macro tool “Simple Muscle Architecture Analysis”
(Seynnes and Cronin, 2020). The average muscle thickness of the
three images from each leg was averaged, and the average of both
legs at the given time point was used for further analyses. All
ultrasound measures were performed by a researcher blinded to
subject randomization. ICC for ultrasound measures of vastus
lateralis in our lab is 0.96. After the training period, the DXA
and ultrasound were measured between 48 and 96 h after the last
training session.
Statistical Analysis
Statistical analyses were performed using jamovi (version 1.6.23.0
for mac; the jamovi project, retrieved from www.jamovi.org)
and GraphPad Prism (version 9.2.0 for mac; GraphPad Software,
La Jolla, CA, United States). The effect of HF vs. LF on
study variables was analyzed using ANCOVA, with post-
intervention outcomes as dependent variables and baseline
values and sex as covariates. Pearson’s correlation coefficients
(r) were calculated for changes in 1RM and lean mass.
The effect of HF vs. LF in simple and complex exercises
and training volume and load were analyzed by a two-
way ANOVA with repeated measures. Furthermore, the effect
size (ES) was calculated as Cohen’s dusing the mean pre-
post change in HF minus the mean pre-post change in LF,
divided by the pooled pre-test standard deviation (Morris,
2008). Outcomes are reported with standard deviation unless
otherwise specified. A two-tailed P-value less than 0.05 was
considered significant.
RESULTS
Muscle Strength
The weekly training volume and load are displayed in Figure 1.
Both groups improved 1RM in squat (HF: 16.3 ±5.2 kg,
LF: 14.8 ±4.2 kg), hack squat (HF: 33.4 ±13.9 kg, LF:
34.4 ±9.7), bench press (HF: 7.5 ±3.5, LF: 7.7 ±3.0 kg),
and chest press (HF: 15.9 ±13.8 kg, LF: 15.0 ±5.0 kg)
during the training intervention. The improvements in
1RM did not differ between groups (see Table 2 and
Figure 2).
Lean Mass and Muscle Thickness
Lean mass (HF: 1.14 ±2.0 kg, LF: 1.46 ±1.38 kg), lean
leg mass (HF:0.35 ±0.79 kg, LF:0.64 ±0.63 kg), lean trunk
mass (HF:0.41 ±1.17 kg, LF:0.66 ±0.93 kg), lean arm mass
(HF:0.38 ±0.40 kg, LF:0.18 ±0.24 kg), and vastus lateralis
thickness (HF:0.51 ±0.22 cm, LF:0.48 ±0.18) improved in
both groups. There were no differences between groups for
any of the measures of muscle growth (see Table 2 and
Figure 1).
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TABLE 2 | Comparisons between groups are based on estimated marginal
means.
ANCOVA HF vs. LF
Mean effect PES 95% CI
Lean mass total −0.1 kg 0.897 −0.07 −1.09 to 0.97
Lean mass legs −0.4 kg 0.296 −0.54 −1.61to 0.54
Lean mass trunk 0.0 kg 0.969 −0.02 −0.99 to 1.04
Lean mass arms 0.2 kg 0.210 0.61 −0.41 to 1.63
VL thickness −0.00 cm 0.950 −0.03 −1.01 to 0.95
1RM total 5.5 kg 0.637 0.3 −0.79 to 1.25
1RM squat 4.2 kg 0.192 0.69 −0.41 to 1.79
1RM hack squat −1.27 kg 0.824 −0.11 −1.11 to 0.89
1RM bench press 0.7 kg 0.676 0.20 −0.80 to 1.21
1RM chest press 2.7 kg 0.554 0.26 −0.74 to 1.26
FIGURE 2 | Percent changes in one repetition maximum for squat, hack
squat, bench press, and chest press. Horizontal lines are averages with error
bars representing SD. #Difference between simple and complex exercises,
p<0.05.
Complex vs. Simpler Exercises
The hack squat 1RM (HF: 62.5 ±48.8%, LF: 50.2 ±23.6%)
increased more in relative terms than the squat 1RM (HF:
19.1 ±10.3%, LF: 18.7 ±8.6%) in both groups (P<0.01 for both).
Chest press 1RM increased more in relative terms than bench
press 1RM in HF (31.2 ±27.9% and 15.8 ±11.5%, respectively,
P= 0.011) but not in LF (13.8 ±7.4% and 21.3 ±7.8%,
respectively, P= 0.171). Combining both groups 1RM in the less
complex exercises increased more than more complex exercises
(Lower body: 18.9 ±10.7% vs. 54.9 ±36.3%, P<0.00; Upper
body: 14.6 ±9.9% vs. 25.0 ±20.2%, P= 0.001). The differences
between squat and hack squat, and bench press and chest press
did not differ between groups. There was a significant correlation
between changes in 1RM for all exercises (squat – hack squat:
r= 0.64, squat – bench press: r= 0.83, squat – chest press: r= 0.69,
hack squat – bench press: r= 0.74, hack squat – chest press:
r= 0.73, bench press – chest press: r= 0.78, p>0.001 for all).
There were no significant correlations between changes in muscle
strength and changes in muscle mass.
DISCUSSION
The current study found that in moderately resistance-trained
individuals distributing weekly resistance training volume into
two or four workouts did not result in different outcomes for
1RM, lean mass, or vastus lateralis thickness. Further, muscle
strength in complex exercises does not benefit more from a higher
training frequency than in simpler exercises.
In line with most previous studies, weekly training frequency
seems to be subordinate to training volume in terms of increasing
muscle mass, given training volume is kept identical within each
week (Colquhoun et al., 2018;Gomes et al., 2019). Our finding
goes against the theoretical benefit of multiple upregulations
of stimulating protein synthesis and thus increased anabolic
stimulus with an increased frequency. This could result from
a suboptimal training stimulus when the training volume was
distributed across 4 days. However, previous studies suggest that
relatively small training volumes can produce a robust muscle
protein synthetic response in resistance-trained and active
individuals (Burd et al., 2010, 2011). Several recent contributions
may help shed more light on this discrepancy. Evidence suggests
that volume-sensitive long-term adaptations of translational
capacity, rather than repeated acute changes in translational
efficiency, are associated with hypertrophy (Figueiredo, 2019;
Hammarström et al., 2020). This is further supported by the lack
of measurable differences in myofibrillar protein synthesis over
7 days with a matched training volume distributed across one or
five workouts (Shad et al., 2021). Alternatively, the manipulation
of resistance training variables may be secondary to other
intrinsic factors. At least when a sufficient stimulus is provided,
as suggested by Damas et al. (2019) who elegantly displayed
remarkable stability within individuals despite manipulating
resistance training variables, in contrast to a substantial between-
individual variability.
It has been suggested that an increased training frequency may
enhance neural adaptations to a greater extent in more complex
exercises compared with simple exercises (Shea et al., 2000).
Given a more complex movement pattern, we expected strength
in squat and bench press to increase more compared with hack
squat and chest press in the HF group when compared with the
LF group. To the best of our knowledge, this is the first study
to investigate this effect from a resistance training perspective.
Contrary to our hypothesis, 1RM increased more in the less
complex exercises. The difference was surprisingly large given
the exercises had similar movement patterns and were trained
with the same weekly volume. This is possibly explained by the
participants having considerably more experience with squat and
bench press than hack squat and chest press (see Table 3). Thus,
our data suggest familiarity with an exercise to be of greater
importance than the complexity of the exercise when it comes to
improvements in muscle strength.
Although the relative changes in 1RM differed between
similar exercises, we observed moderate to strong correlations
between these changes in all measured exercises. This may
suggest that changes in e.g., both squat and hack squat 1RM
to a large extent, should represent changes in leg strength as a
phenomenon. However, as these correlations exist between all
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TABLE 3 | Descriptive data of participants before and after 9 weeks of heavy
resistance exercise.
Characteristic HF pre HF post LF pre LF post
N (♂/♀) 3/5 3/5 8/5 8/5
Age (years) 26.8 ±3.9 25.5 ±4.3
Body mass (kg) 80.7 ±15.7 74.6 ±12.8
Lean mass (kg) 53.7 ±12.4 54.8 ±12.4 53.7 ±9.4 55.2 ±12.1
Lean mass legs (kg) 18.5 ±4.7 18.9 ±4.4 18.2 ±3.1 18.8 ±4.4
Lean mass trunk (kg) 25.9 ±5.5 26.3 ±5.5 25.9 ±4.8 26.6 ±4.7
Lean mass arms (kg) 6.1 ±2.2 6.4 ±2.2 6.4 ±1.6 6.6 ±1.7
Body fat (%) 31.9 ±4.9 31.6 ±4.3 25.1 ±6.9 24.4 ±6.0
Vastus lateralis
thickness (cm)
2.4 ±0.5 2.9 ±0.4 2.6 ±0.4 3.1 ±0.4
Squat 1RM (kg) 106 ±56 122 ±56 96 ±37 110 ±34
Hack squat 1RM (kg) 88 ±61 122 ±57 89 ±52 123 ±53
Bench press 1RM (kg) 70 ±42 78 ±40 70 ±31 78 ±30
Chest press 1RM (kg) 93 ±67 108 ±60 97 ±52 112 ±49
Weekly sessions squat 1.5 ±0.9 1.3 ±0.8
Weekly sessions hack
squat
0.3 ±0.9 0 ±0
Weekly sessions bench
press
1.2 ±1.1 1.2 ±0.9
Weekly sessions chest
press
0.5 ±0.9 0.6 ±0.6
Resistance training age
(years)
3.6 ±2.2 3.5 ±1.3
Weekly sessions are an estimate of the weekly average number of sessions
including the exercise over the last 6 months. Values are average ±SD.
measured exercises, they may well be driven by the training state
of the participants. It is expected that the weaker participants will
improve more for all exercises during the training intervention.
Accordingly, the baseline 1RM performance did correlate
negatively with changes in 1RM during the intervention (from
−0.83 to −0.94, p<0.001 for all measures). Strength is often
considered a relatively universal trait. However, the present and
previous data (Harris et al., 2007) suggest that a significant part
of improvements in strength are specific to the given exercise
and may differ significantly between similar exercises trained
identically over a training period.
The current study has several strengths, among these a full
supervision of all workouts, a 100% attendance, and a direct
measure of muscle growth. However, it shares the limitation of
a relatively short intervention period with the rest of the current
training frequency literature in trained individuals. Even with a
3-week cutback, due to corona restrictions, the current 9-week
study is longer than the previous 6- to 8-week interventions
(Colquhoun et al., 2018;Gomes et al., 2019;Zaroni et al.,
2019;Johnsen and van den Tillaar, 2021). Training protocols
with equated training volume, but different training frequencies
are not expected to result in large differences in adaptations.
Consequently, longer interventions may be needed to discern
differences between them. Measures of muscle thickness were
only obtained at one point along the vastus lateralis. Previous
studies have shown non-uniform hypertrophy in the quadriceps
in response to resistance exercise (Narici et al., 1996). Recently,
drop sets were reported to induce greater gains in hypertrophy
for the rectus femoris, but not vastus lateralis, compared with
traditional resistance exercise training (Varovi´
c et al., 2021).
Consequently, we cannot exclude the possibility of regional
differences in hypertrophy within or between muscles. Still,
any potential regional differences were not large enough to be
observed with the DXA measures. Given the large intraindividual
variability and limited resources, a unilateral approach to further
investigate the effects of training frequency seems sensible, at
least for muscle growth where cross-learning is unlikely to occur
(MacInnis et al., 2017). Furthermore, if two exercise regimens
are compared in a contralateral manner in resistance-trained
individuals, it is our belief that potential cross-learning effects on
strength too will be minimal.
CONCLUSION
In conclusion, the current study did not show an effect of
resistance training frequency on changes in muscle strength and
muscle growth when weekly resistance training volume was kept
identical in moderately resistance-trained individuals. Further,
higher resistance training frequency did not result in greater
improvements in strength for complex exercises compared to
simpler exercises.
DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be
made available by the authors, without undue reservation.
ETHICS STATEMENT
The studies involving human participants were reviewed
and approved by the Local Ethical Committee at Inland
Norway University of Applied Sciences (20/03749). The
patients/participants provided their written informed consent to
participate in this study.
AUTHOR CONTRIBUTIONS
HH, HM, OS, PJ, HR, and BR contributed to the conception and
design of the study. HM, OS, PJ, and HR conducted the training
intervention. HH, HM, OS, PJ, and HR performed the testing.
HH wrote the first draft of the manuscript. HM, OS, PJ, HR, and
BR wrote sections of the manuscript. All authors contributed to
manuscript revision, read, and approved the submitted version.
ACKNOWLEDGMENTS
The authors would like to thank Berit Hauge Aakvik for help
with the ultrasound and DXA scans during testing. Big thanks is
also extended to Spenst Arena Lillehammer for training facilities,
access to members, and being flexible and accommodating covid-
restrictions allowing the completion of the study.
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Conflict of Interest: The authors declare that the research was conducted in the
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