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Journal of Human Kinetics volume 91/2024, 87–103 DOI: 10.5114/jhk/185637 87

Modern strategies to support health, fitness and sports training

Muscle Physiology Laboratory, Department of Exercise Science and Health Promotion, Florida Atlantic University, Boca Raton,

FL, USA.

James J. Peters VA Medical Center, Spinal Cord Injury Research, Bronx, NY, USA.

Department of Neurology, Icahn School of Medicine at Mount Sinai, New York, USA.

Department of Dietetics and Nutrition, Florida International University, Miami, FL, USA.

Correspondence: [email protected]

The Effect of Time-Equated Concurrent Training Programs

in Resistance-Trained Men

Chad Dolan

, Justin M. Quiles

, Jacob A. Goldsmith

2,3

, Kristin M. Mendez

Alex Klemp

, Zac P. Robinson

, Joshua C. Pelland

, Catherine Coccia

Michael C. Zourdos

The purpose of this investigation was to compare the effects of three different concurrent training (CT) programs

and a resistance training (RT) program. Twenty-three resistance trained men (age: 24 ± 3 years) were randomized into

four groups: concurrent RT and high intensity interval cycling (CTH, n = 6), concurrent RT and moderate intensity

continuous cycling (CTM, n = 5), RT and barbell circuit training (RTC, n = 6), or RT only (RT, n = 6). Back squat and

bench press strength, quadriceps, and pectoralis muscle thickness, VO2peak, and maximum workload (Wmax, Watts) were

assessed. Squat strength gains were meaningful in all groups and comparable among CTH (16.88 kg [95% CrI: 11.15,

22.63]), CTM (25.54 kg [95% CrI: 19.24, 31.96]), RTC (17.5 kg [95% CrI: 11.66, 23.39]), and RT (20.36 kg [95% CrI:

15.29, 25.33]) groups. Bench press strength gains were meaningful in all groups and comparable among CTH (11.86 kg

[95% CrI: 8.28, 15.47]), CTM (10.3 kg [95% CrI: 6.49, 14.13]), RTC (4.84 kg [95% CrI: 1.31, 8.47]), and RT (10.16 kg

[95% CrI: 7.02, 13.22]) groups. Quadriceps hypertrophy was meaningful in all groups and comparable among CTH

(2.29 mm [95% CrI: 0.84, 3.76]), CTM (3.41 mm [95% CrI: 1.88, 4.91]), RTC (2.6 mm [95% CrI: 1.17, 4.05]), and RT

(2.83 mm [95% CrI: 1.55, 4.12]) groups. Pectoralis hypertrophy was meaningful in CTH (2.29 mm [95% CrI: −0.52,

5.1]), CTM (5.14 mm [95% CrI: 2.1, 8.15]), and RTC (7.19 mm [95% CrI: 4.26, 10.02]) groups, but not in the RT group

(1 mm [95% CrI: −1.59, 3.59]); further, between-group contrasts indicated less pectoralis growth in the RT compared to

the RTC group. Regarding cardiovascular outcomes, only the RTH and RTM groups experienced meaningful

improvements in either measure (VO

2peak or Wmax). These data suggest that the interference effect on maximal strength

and hypertrophy can be avoided when the aerobic training is moderate intensity cycling, high intensity cycling, or a novel

barbell circuit for ~one hour per week and on non-RT days. However, the barbell circuit failed to elicit meaningful

cardiovascular adaptations.

Keywords: resistance exercise; aerobic training; strength; hypertrophy

Introduction

Concurrent training (CT) is the inclusion

of both resistance (RT) and aerobic training (AT)

within the same program (Hickson, 1980).

Commonly, CT is used for improving body

composition by weight class or physique sport

athletes. Although CT improves body

composition, previous data (Bell et al., 1997;

Hickson, 1980; Shaw et al., 2009) have

demonstrated that the inclusion of AT in a RT

program can attenuate muscle strength,

hypertrophy, and power adaptations, known as

the interference effect (Hickson, 1980).

The interference effect can manifest both

acutely and chronically during CT. Acutely, the

additional fatigue from AT can decrease RT work

88 The effect of time-equated concurrent training programs in resistance-trained men

Journal of Human Kinetics, volume 91, March 2024 http://www.johk.pl

capacity (total volume) when performed on the

same day (Abernethy, 1993). Given the positive

relationship between RT volume and adaptations

(Ralston et al., 2017; Schoenfeld et al., 2017a),

decreasing RT work capacity could impair RT

benefits. Chronically, excessive AT during a RT

program may compromise recovery by increasing

training sessions and total stress (Goto et al., 2004;

Hickson, 1980; Rhea et al., 2002). Additionally, CT

promotes divergent signaling pathways (AT:

ubiquitin proteasome system; RT: mammalian

target of rapamycin-mTOR) (Coffey and Hawley,

2007) and neuromuscular adaptations (Bell et al.,

1997; Hakkinen et al., 2003; Hickson, 1980).

Specifically, AT causes fiber-type interconversions

toward type I, while RT facilitates interconversions

toward type II (Wilson et al. 2012a). In general, if

RT adaptations are the primary goal, performing

AT violates the foundational principles of

specificity.

Despite the potential negative effects of the

interference effect on RT adaptations, previous

data have demonstrated that carefully designed

programming (Wilson et al., 2012b) (i.e., intensity,

duration of AT) and sufficient calorie intake

(Murach and Bagley, 2016) can minimize or avoid

the interference effect. For example, RT

adaptations are significantly less hindered with

shorter duration (i.e., 30–40 min) AT compared to

longer (50–60+ min) bouts (Wilson et al., 2012b),

and when separating AT bouts by at least 3 h from

RT bouts (Schumann et al., 2022). Additionally, the

modality of AT during CT may influence the

magnitude of the interference effect, with some

analyses reporting less of an interference effect

from cycling compared to running (Lundberg et

al., 2022; Wilson et al., 2012b) on lower body

strength and hypertrophy, possibly due to less

muscle damage, a lower session rating of perceived

exertion (RPE), and reduced muscle soreness

(Krzysztofik et al., 2023; Mathieu et al., 2022;

Wilson et al., 2012b). However, other analyses

report no difference between running and cycling

(Sabag et al., 2018; Schumann et al., 2022). Further,

high intensity interval training may aid in the

attenuation of the interference effect when used as

AT (Balabinis et al., 2003; Chen et al., 2024; Lee et

al., 2020) due to the similarities to RT regarding

cellular and neuromuscular adaptations and being

consistent with the principles of specificity.

Therefore, the current evidence suggests that

shorter duration AT, especially high intensity

interval training, performed on a separate day

from lower body RT will most likely diminish

interference with hypertrophy and strength during

CT.

However, for athletes focused on maximizing

RT adaptations during CT, there may be more

optimal approaches than performing traditional

AT. For example, circuit RT is effective to enhance

both muscular performance and body composition

(Alcaraz et al., 2008, 2011) and adheres more

closely to the principle of specificity than AT.

Further, if circuit RT replaced the traditionally

used modes of AT during CT, total RT volume

would increase, which has a positive relationship

with both hypertrophy (Schoenfeld et al., 2017a)

and strength (Ralston et al., 2017). Thus, it is

possible that circuit RT could not only diminish the

interference effect when used as a mode of AT, but

could potentially enhance hypertrophy and

strength adaptations to a greater degree than RT

alone via an increase in total training volume.

Therefore, the primary aim of this study was

to compare the effects of four different eight-week

interventions in resistance trained males: (1) RT

only [RT]; (2) concurrent RT and high intensity

interval cycling [CTH]; (3) concurrent RT and

moderate intensity continuous cycling [CTM]; (4)

RT and barbell circuit training [RTC], on muscular

hypertrophy and strength. It was hypothesized

that RTC would elicit the greatest muscular

improvements followed by RT, that CTH would

have similar adaptations to RT, and CTM would

produce the lowest degree of muscular

improvements.

Methods

Participants reported to the laboratory 42

times over eight and a half consecutive weeks. All

CT groups (CTH, CTM, RTC) trained five days per

week, while RT trained three times per week. All

groups performed the same daily undulating

programming RT protocol on non-consecutive

days (i.e., Monday, Wednesday, Friday). The RT

program featured the back squat and the bench

press as main exercises, and the barbell overhead

press, the barbell bent-over row, and the barbell

biceps curl as accessory exercises. CT groups

performed the group-specific AT modality on the

days between RT sessions (i.e., Tuesday,

Thursday), which was controlled for time (30 min

by Chad Dolan et al. 89

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each group). The protocol design is displayed in

Table 1A.

At pre- and post-study, one-repetition

maximum (1RM) strength on the squat and the

bench press, muscle thickness of the quadriceps

and the chest, peak oxygen uptake (VO

2peak), and

the maximum workload (Wmax) were assessed.

Week one served as an introductory training week,

weeks two through seven were the main training

program, while week eight served as a taper and

post-study testing. Thirty minutes prior to each

session (RT and AT), participants ingested

branched chain amino acids (Xtend, Scivation,

Burlington, N.C., USA) containing 3.5 g of leucine.

Then, immediately after each training session, 30 g

of whey protein (Scivation Whey, Scivation,

Burlington, N.C., USA) were ingested. The

branched chain amino acids and whey protein

were provided to control nutrient timing. Both

supplements were ingested in a powdered form

mixed with water. Participants were asked to

maintain regular use of non-ergogenic

supplements and halt use of ergogenic

supplements during the study.

Participants

Twenty-five college-aged resistance

trained males were recruited for this study.

Participants were randomly assigned to one of the

four mentioned groups: RT (n = 6), CTH (n = 6),

CTM (n = 5), and RTC (n = 6). Two individuals were

removed from participation, one due to minor

injury (RTM group) and one because of non-

compliance (RTH group). Therefore, data from 23

participants (age: 24 ± 3 years, body mass: 80.5 ±

10.2 kg, body fat content: 11.2 ± 4.0%) were

included. Inclusion criteria were as follows: (1) at

least two years of resistance training experience; (2)

a minimum training frequency of the squat and the

bench press of once per week for the previous six

months immediately preceding participation; (3) a

1RM squat of ≥1.5 times body mass and a

minimum bench press of ≥1.25 times body mass; (4)

semi-regular consumption of whey protein during

the previous six months. These criteria were

confirmed via a physical activity questionnaire

(Zourdos et al., 2016a). Additionally, a health

history questionnaire was completed and

participants were excluded if any

contraindications to exercise were reported.

Finally, all participants signed an informed

consent form that was approved by the Florida

Atlantic University Institutional Review Board

(protocol code: 680161-3; approval date: 18

November 2014).

Measures

1RM Testing. Testing for 1RM was

performed in accordance with previously

validated procedures (Zourdos et al., 2016b)

following a five-minute dynamic warm-up. To find

the most accurate 1RM, investigators used the

average velocity (m

−1

) via a Tendo Weightlifting

Analyzer (TENDO Sports Machines, Trencin,

Slovak Republic) and participants reported their

RPE (Zourdos et al., 2016b) to determine the

following attempt. Each participant was given five

to seven minutes of rest between 1RM attempts. A

1RM was accepted as valid if one of three

conditions was met: (i) the participant reported a

‘10’ on the RPE scale and the investigator

determined a subsequent attempt with increased

weight would not be successfully completed, (ii)

the participant reported a ‘9.5’ RPE and failed the

subsequent attempt with a load increase of 2.5 kg

or less, (iii) the participant reported an RPE of 9

and failed the subsequent attempt with a load

increase of 5 kg or less. The squat and the bench

press were performed under the rules and

regulations of the United States of America

Powerlifting (USAPL and Administrators, 2001).

Wilks Score. The Wilks score is a validated

measure of relative strength (Vanderburgh and

Batterham, 1999). This calculation compares

strength levels of individuals with various body

masses by multiplying the amount of weight lifted

(i.e., 1RM) by a standardized body weight

coefficient.

Anthropometric and Relative Body

Composition. Body height (cm) was measured using

a wall-mounted stadiometer and body mass (kg)

was assessed via a calibrated digital scale. Body fat

content or relative body composition was assessed

with the BodyMetrix BX-2000 A-mode ultrasound

(BodyMetrix, IntelaMetrix, Livermore, CA) and

lean body mass was then calculated (Campbell et

al., 2018). To assess subcutaneous fat thickness, the

ultrasound probe emits a single beam with a

standardized frequency of 2.5 MHz. The probe was

connected by USB to a laptop loaded with the

manufacturer software (BodyView Professional

Software). Measurements were taken at the thigh,

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the chest, and the abdomen from the right side of

the body, while the participant was standing.

During sampling, the probe was held

perpendicular to the participant with minimal

movement across the skin (+/− 5 mm) and enough

pressure to maintain surface contact between the

device and the participant, but not depressing the

participant’s subcutaneous fat tissue.

Manufacturer directions were followed, and the

average of two scans was used for assessment. The

average represented the final site-specific

subcutaneous adipose tissue thickness

measurement. The software calculated body

composition via Jackson and Pollock 3-site formula

(Jackson and Pollock, 1978).

Muscle Thickness. Muscle thickness was

assessed via ultrasonography (BodyMetrix Pro

System, IntelaMetrix, Inc., Livermore, CA., USA).

The ultrasound settings (frequency: 2.5 MHz,

depth: 60 mm) were kept constant to standardize

the measurements of the targeted muscles. All

scans were performed on the right side of the body

prior to 1RM assessment on pre- and post-testing

days. The muscle at each site was scanned laterally

to medially with the transducer positioned

perpendicularly to the skin. Two scans were

completed at each site with the average used for

analysis; however, if there was a difference of >2

mm between scans, a third scan was taken. In the

event of a 3

scan, the average of the two

measurements within 2 mm was used. Participants

were positioned supine on a massage table in an

anatomical position for at least 10 min to allow for

fluid compartment shifts to occur prior to the

initiation of scans. The chest site was determined

as half the distance between the anterior axillary

line and the nipple. Three lower body sites: lateral

quadriceps mid (LQM), lateral quadriceps distal

(LQD), and anterior quadriceps (AQ), were

identified. The LQM and LQD sites were measured

at 50% and 70%, respectively, of the distance from

the greater trochanter to the lateral epicondyle of

the femur (Klemp et al., 2016), while AQ was

assessed at 70% of the distance from the greater

trochanter to the medial epicondyle of the femur.

The same investigator performed palpations and

scans throughout the study.

2peak & Wmax Cycle Test. Pre- and post-

study VO2peak testing was performed using

previously validated procedures (Leveritt et al.,

2003). Each participant was outfitted with a heart

rate monitor (FT1 Heart Rate Monitor, Polar,

Kempele, Finland), and an electronically braked

cycle ergometer (Excalibur Sport, Lode,

Netherlands) was used for the incremental exercise

test. After a three-minute warm-up at 25 Watts

(W), one-minute stages were employed, starting at

50 W, and increasing in the workload by 25 W each

stage, until test termination. Participants pedaled

at a fixed cadence of 80 revolutions per minute

(RPM). During the test, respiratory gases were

monitored and continuously analyzed by open-

circuit spirometry (True One 2400+ Metabolic

Measurement System, Parvo-Medics Inc., Provo,

UT). The metabolic system measured minute

ventilation, the oxygen consumption rate, the

carbon Dioxide expiration rate, and the respiratory

exchange ratio (RER). Data were averaged over 30-

s intervals. The metabolic cart was calibrated prior

to each test with room air for the flow rate and

gases (i.e., O

, CO

) of known volume and

concentration. The heart rate (HR), the workload

(W), and the RPE (Borg 20-point scale) were

measured and recorded at the end of every stage

(last five seconds). Tests were terminated when the

pedal cadence of 80 RPM could not be maintained

for > 10 s or due to volitional fatigue. Tests were

accepted as peak tests if participants met any two

of the following criteria: plateau in VO

despite an

increase in the workload (<150 ml/min); RPE ≥ 17;

RER > 1.15; HR ≥ 95% of age-predicted maximum

(220 − age). Wmax was calculated from the formula,

max

= W

+ (t/180) ∙ 25, where W

= the value of the

last completed workload (W); t = the time the last

workload was maintained (s), and 25 = the W

output difference between the last two workloads

(W).

Design and Procedures

The exact details of the training program

including sets, repetitions, loading progressions,

and adjustments for all exercises during the study

are presented in Table 1A–D.

Squat and Bench Press. Loads were pre-

planned for the introductory microcycle (week 1)

and the first week of the main training cycle (week

2; Table 1A). For weeks 3–7, load progression was

individualized based upon weekly performance

assessment or “plus set”, which is known as

autoregulatory progressive resistance exercise

(Mann et al., 2010), and this load progression can

be seen in Table 1B. Further, if a participant failed

by Chad Dolan et al. 91

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to complete the prescribed repetitions for any main

lift set, there was a 2.5-kg reduction in the load per

repetition failed on subsequent sets (Table 1C), and

a 2.5-kg load reduction in that exercise for the

remainder of the week. Finally, when a repetition

was missed, the plus set based load progression

was reduced by 50% for that exercise the following

week. Load progression was resumed as planned

when an entire training week was completed as

prescribed. Investigator-administered rest

intervals were 5–7 min for main exercises (Zourdos

et al., 2016b). In the final week of training (taper

microcycle; week 8), participants performed pre-

planned sets, repetitions, and loads (reduced

volume, but similar loads) the first two sessions of

the week to prepare for their post-testing session.

Accessory Exercises. For accessory exercises,

participants were asked to perform the repetitions

at a predetermined load corresponding to a RIR-

based RPE (Zourdos et al., 2016b). For the first set

during week 1, participants were instructed to

choose a load that would elicit an RPE of 8 (RIR =

2). In all other weeks, the final set load used in the

previous week was used as the starting load of the

next week. In each session, the load was increased

or decreased for the subsequent set if the target

RPE was not met. The details of load changes can

be seen in Table 1D. Rest intervals of 1–3 min were

used.

Concurrent Training Protocols. All CT

interventions were performed on off days from RT.

During weeks 1 and 8, the CT interventions were

performed once per week for 18 min, and during

weeks 2–7 the CT interventions were performed

twice per week for 30 min.

Concurrent Training High Intensity (CTH

Group). The intervals consisted of 60 s of work

followed by 120 s of active recovery (1:2

work:recovery). The intensity was set to 90%Wmax

for week 1, 100%Wmax for weeks 2 and 3, 105%Wmax

for weeks 4 and 5, and 110%Wmax for weeks 6 and

7, and 95%Wmax for week 8. Participants were

instructed to cycle as fast as possible during each

work period, and to maintain slow cycling without

any resistance during each recovery period.

Concurrent Training Moderate Intensity

(CTM Group). The continuous cycling intensity was

set to the workload (W) at 30% VO2peak during week

1, 40%VO2peak during weeks 2 and 3, 45%VO2peak

during weeks 4 and 5, 50%VO2peak during weeks 6

and 7, and 35% VO2peak during week 8. All

participants were instructed to pedal at a

maintainable pace with minimal variation in RPM

for the duration of each exercise session.

Resistance Training Circuit (RTC Group).

The resistance training circuit consisted of the

same exercises from the RT program performed in

a series to a prescribed number of repetitions on

each exercise. Completion of one set of all exercises

was considered “one round”, resulting in an

average of 4.2 rounds per session. The objective

was to complete as many rounds as possible in 30

min. Squat and bench press exercises were

performed at 40% of 1RM and accessory exercises

at 75% of the load used on the first day of each

week of the RT program (i.e., Monday). The

exercises were organized in the following series: a

back squat, a barbell overhead press, a bench press,

a barbell bent-over row, and a barbell biceps curl.

All repetitions were required to be completed for

each exercise before progressing to the subsequent

exercise in the series. During week 1, eight

repetitions were performed for all exercises, and

one repetition was added bi-weekly during the

main RT program (i.e., weeks 1, 2, and 3: 8

repetitions, weeks 4 and 5: 9 repetitions, weeks 6

and 7: 10 repetitions). During the taper, the load

was reduced (i.e., main exercises: 35% of 1RM;

accessory: 70% of the Monday’s load) and 10

repetitions were performed of each exercise.

Dietary Recalls. To inform dietary intake,

investigators performed 24-h dietary recalls three

times during the first and the final week of the

study. Investigators were directly trained by a

registered dietitian to perform the recalls. This was

performed to educate participants regarding their

nutritional habits in an effort to ensure

maintenance of these habits throughout the study.

Statistical Analysis

All analyses were conducted in R language

and environment for statistical computing (v 4.3.0;

R Core Team, https://www.r-project.org/). All data

and code can be accessed at <https://osf.io/9t7ny/>.

In addressing our research questions, we avoided

dichotomizing the findings and did not employ

traditional null hypothesis significance testing,

which has been extensively critiqued (Amrhein et

al., 2019). Instead, we took an estimation-based

approach within a Bayesian framework in which

all outcomes compatible with the data were

considered, with the greatest emphasis placed on

92 The effect of time-equated concurrent training programs in resistance-trained men

Journal of Human Kinetics, volume 91, March 2024 http://www.johk.pl

the point estimates using the “brms” and

“marginaleffects” packages (Kruschke and Liddell,

2018; Mengersen et al., 2016).

To incorporate our expectations given

previous data and to improve the precision of our

estimates given a small sample size, weakly

informative prior distributions were used.

Specifically, the data from our lab using three

similarly designed training studies (Helms et al.,

2018; Klemp et al., 2016; Robinson, 2021) was used

to inform the expected distributions of changes in

strength and muscle size. Additionally, we

consulted the best available evidence to determine

the expected differences between RT and CT

conditions (Schumann et al., 2022). Because our lab

did not have necessary data to inform the expected

distributions for changes in cardiovascular

outcomes (VO

2peak and Wmax), these models were

only fit with the default uninformed priors.

Each model used four Monte Carlo

Markov Chains with 1000 warm-up and 8000

sampling iterations. Before extracting any

estimates, each model was visually examined via

trace plots to inspect chain convergence and

posterior predictive checks to examine model

validity. For the variables of interest from each

model (i.e., marginal effects for condition), draws

(n = 8000) were taken from the posterior

distribution to construct a probability density

function (i.e., mean and quartile intervals) that was

used to make probabilistic inferences. The

probability density functions related to the

primary research questions were also compared to

a region of practical equivalence (ROPE). For

hypertrophy outcomes, the ROPE was defined by

the typical error of measurement (Swinton et al.,

2018; Weir, 2005); however, for strength and

cardiovascular outcomes, the ROPE was defined as

the raw-unit equivalent of a ± 0.25 standardized

mean difference (Swinton et al., 2022).

To compare changes in 1RM strength

(back squat and bench press exercises), pectoralis

major hypertrophy, and cardiovascular fitness

(VO

2peak and Wmax) linear regression models were

constructed to mimic an analysis of covariance (i.e.,

ANCOVA) with an adjustment for the baseline

value of the dependent variable. Specifically,

change from baseline was considered the response

variable while condition (4-level categorical) and

the pretest value of the dependent variable

(continuous) were included as population-level

effects. However, for changes in quadriceps muscle

size linear mixed effect models were used.

Specifically, change from baseline was considered

the response variable while condition (4-level

categorical), site (3-level categorical), and the

pretest value of the dependent variable

(continuous) were included as population-level

effects. As the model contained multiple

observations per participant, group-level

intercepts were included. After initially fitting the

model with a maximal group-level slope structure

(Barr et al., 2013; Oberauer, 2022), the model was

reduced to include group-level slopes for the site

and the pretest value of the dependent variable at

the participant level.

Results

Descriptive summaries (i.e., mean ±

standard deviation) of participants’ characteristics

can be seen in Table 2. All model output and the

unadjusted values of the primary outcomes can be

found in the supplementary materials

<https://osf.io/9t7ny/>.

Back Squat 1RM

The mean values of the marginal posterior

distributions suggest that all conditions

demonstrated meaningful increases in back squat

1RM strength. Specifically, the RT condition

presented an increase of 20.36 kg [95% CrI: 15.29,

25.33] with a 100% probability of the change being

greater than the ROPE. The RTC condition

presented an increase of 17.5 kg [95% CrI: 11.66,

23.39] with a 100% probability of the change being

greater than the ROPE. The CTM group observed

an increase of 25.54 kg [95% CrI: 19.24, 31.96] with

a 100% probability of the change being greater than

the ROPE. Finally, the CTH group observed an

increase of 16.88 kg [95% CrI: 11.15, 22.63] with a

100% probability of the change being greater than

the ROPE. The credible intervals of all contrasts

among conditions were compatible with the ROPE

(Table 3). These results are visualized in Figure

1A&C.

Bench Press 1RM

The mean values of the marginal posterior

distributions suggest that most conditions

demonstrated significant increases in bench press

1RM strength. Specifically, the RT group observed

by Chad Dolan et al. 93

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an increase of 10.16 kg [95% CrI: 7.02, 13.22] with a

99.98% probability of the change being greater than

the ROPE. The RTC group observed an increase of

4.84 kg [95% CrI: 1.31, 8.47] with a 62% probability

of the change being greater than the ROPE. The

CTM group observed an increase of 10.3 kg [95%

CrI: 6.49, 14.13] with a 99.92% probability of the

change being greater than the ROPE. Finally, the

CTH group observed an increase of 11.86 kg [95%

CrI: 8.28, 15.47] with a 100% probability of the

change being greater than the ROPE. The credible

intervals of all contrasts among conditions were

compatible with the ROPE (Table 3). These results

are visualized in Figure 1B&D.

Quadriceps Hypertrophy

The mean values of the marginal posterior

distributions suggest that all conditions induced

meaningful increases in quadriceps muscle

thickness. Specifically, the RT group observed an

increase of 2.83 mm [95% CrI: 1.55, 4.12] with a

99.88% probability of the change being greater than

the ROPE. The RTC condition resulted in an

increase of 2.6 mm [95% CrI: 1.17, 4.05] with a

99.41% probability of the change being greater than

the ROPE. The CTM group presented an increase

of 3.41 mm [95% CrI: 1.88, 4.91] with a 99.94%

probability of the change being greater than the

ROPE. Finally, the CTH group observed an

increase of 2.29 mm [95% CrI: 0.84, 3.76] with a

97.86% probability of the change being greater than

the ROPE. The credible intervals of all contrasts

among conditions were compatible with the ROPE

(Table 3). These results are visualized in Figure

2A&C.

Pectoralis Major Hypertrophy

The mean values of the marginal posterior

distributions suggest that some conditions

demonstrated significant increases in quadriceps

muscle thickness. Specifically, the RT group

observed an increase of 1 mm [95% CrI: −1.59, 3.59]

with a 23.84% probability of the change being

greater than the ROPE. The RTC group presented

an increase of 7.19 mm [95% CrI: 4.26, 10.02] with a

99.98% probability of the change being greater than

the ROPE. The CTM group observed an increase of

5.14 mm [95% CrI: 2.1, 8.15] with a 98.09%

probability of the change being greater than the

ROPE. Finally, the CTH group presented an

increase of 2.29 mm [95% CrI: −0.52, 5.1] with a

59.58% probability of the change being greater than

the ROPE. The credible intervals of all contrasts

among conditions but one (i.e., RT-RTC) were

compatible with the ROPE (Table 3). These results

are visualized in Figure 2B&D.

2peak

The mean values of the marginal posterior

distributions suggest that some conditions

demonstrated meaningful changes in VO2.

Specifically, the RT condition resulted in a change

of −2.72 (ml∙kg∙min

−1

) [95% CrI: −6.41, 1.04] with a

77.28% probability of the change being greater than

the ROPE. The RTC condition observed a change

of −4.21 (ml∙kg∙min

−1

) [95% CrI: −7.78, −0.6] with a

94.36% probability of the change being greater than

the ROPE. The CTM condition presented a change

of −0.18 (ml∙kg∙min

−1

) [95% CrI: −3.9, 3.69] with a

26.27% probability of the change being greater than

the ROPE. Finally, the CTH condition resulted in a

change of 1.95 (ml∙kg∙min

−1

) [95% CrI: −1.6, 5.6]

with a 63.02% probability of the change being

greater than the ROPE. The credible intervals of all

contrasts among conditions were compatible with

the ROPE (Table 3). These results are visualized in

Figure 3AC.

Maximum Workload

The mean values of the marginal posterior

distributions suggest that some conditions

demonstrated significant changes in W

max.

Specifically, the RT condition resulted in a change

of −19.8 (W) [95% CrI: −39.79, 0.06] with a 86.12%

probability of the change being greater than the

ROPE. The RTC condition presented a change of

−11.45 (W) [95% CrI: −29.62, 6.45] with a 60.55%

probability of the change being greater than the

ROPE. The CTM condition observed a change of

14.29 (W) [95% CrI: −6.6, 34.65] with a 69.47%

probability of the change being greater than the

ROPE. Finally, the CTH condition resulted in a

change of 21.59 (W) [95% CrI: 3.34, 40.35] with a

91.26% probability of the change being greater than

the ROPE. The credible intervals of all contrasts but

one (i.e., RT-CTH) among conditions were

compatible with the ROPE (Table 3). These results

are visualized in Figure 3BD.

94 The effect of time-equated concurrent training programs in resistance-trained men

Journal of Human Kinetics, volume 91, March 2024 http://www.johk.pl

Table 1A. Weekly schedule of the main training intervention.

Daily training session details

Groups Monday Tuesday Wednesday Thursday Friday

Resistance

Training

Aerobic Training Resistance

Training

Aerobic Training Resistance

Training

RT (n = 6)

Main: 4 x 8 at

70%1RM

Acc: 3 x 10 at

8RPE

n/a

Main: 4 x 6 at

75%1RM

Acc: 3 x 8 at

8RPE

n/a

Main: 5 x 4+ at

80%1RM

Acc: 3 x 6 at

8RPE

RTH (n = 6) 10 intervals cycling,

1:2 work:rest

10 intervals cycling,

1:2 work:rest

RTM (n = 5) 30-min steady state

cycling

30-min steady state

cycling

RTC (n = 6)

30-min barbell circuit 30-min barbell circuit

Table 1B. Summary of weekly progression based on Friday “plus set.”

Week Weekly Load Weekly Repetition

Target

Repetitions

Performed

Weekly Load

Adjustment

1 70–75%1RM

2 −2.5 kg

2 80% 3 +0.0 kg

3 APRE 4 +1.0 kg

4 APRE 5 +2.5 kg

5 APRE 6 or 7 +5.0 kg

6 APRE 8 or more +7.5 kg

7 APRE

Table 1C. Summary of load adjustments due to incomplete or failed repetitions on main exercises.

Failed Repetitions Load Adjustment

1 −2.5 kg

2 −5.0 kg

3 −7.5 kg

4 −10.0 kg

by Chad Dolan et al. 95

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Table 1D. Summary of RPE/RIR scale-based load adjustments to accessory exercises every set.

Target RPE(RIR) Reported RPE(RIR) Load Adjustment

8(2)

5–6(4–6) +5.0 kg

7(3) +2.5 kg

8(2) +0.0 kg

9(1) −2.5 kg

10(0) −5.0 kg

Week 1 (introductory microcycle) consisted of 1 less set for all exercises and 5–10% lower training

loads on main exercises; Main = back squat and bench press; ACC = barbell overhead press, barbell

bent-over row, and barbell biceps curl; training prescription = sets x repetitions; no training occurred

on Saturday and Sunday; RT = resistance training control group; RTH = high intensity interval

cycling group; RTM = moderate intensity steady state cycling group; RTC = barbell circuit training

group; %1RM = percentage of one repetition maximum strength; RPE = rating of perceived exertion;

plus set = performance set for squat and bench press where the last set taken to volitional repetition

maximum, denoted by 5 x 4+; APRE = autoregulated progressive resistance exercise; RPE/RIR =

resistance training specific rating of perceived exertion scale (RPE) based on repetitions in reserve

(RIR), denoted by RPE value (RIR value)

Table 2. Descriptive Summaries.

Characteristic RT (n = 6) RTC (n = 6) CTM (n = 5) CTH (n = 6)

Age (years) 23.67 ± 4.27 22.33 ± 1.75 24.80 ± 2.28 24.33 ± 2.94

Height (cm) 174.09 ± 5.47 175.90 ± 6.09 176.25 ± 8.14 175.13 ± 8.88

Pre Body Mass (kg) 79.07 ± 5.81 78.60 ± 8.71 78.29 ± 15.68 85.53 ± 9.79

Post Body Mass (kg) 80.72 ± 6.66 80.71 ± 6.80 79.10 ± 16.37 86.99 ± 10.43

Δ Body Mass (kg) 1.66 ± 3.22 2.11 ± 2.85 0.81 ± 2.38 1.47 ± 1.28

Pre Estimated Body Fat (%) 10.90 ± 2.66 11.30 ± 2.83 9.38 ± 2.22 13.10 ± 6.50

Post Estimated Body Fat (%) 11.63 ± 4.18 12.80 ± 3.15 11.08 ± 3.83 14.02 ± 5.72

Δ Estimated Body Fat (%) 0.73 ± 4.63 1.50 ± 2.25 1.70 ± 2.15 0.92 ± 2.77

Pre SQ + BP Wilks (a.u.) 176.85 ± 21.59 175.83 ± 22.99 166.13 ± 18.39 169.79 ± 26.14

Post SQ + BP Wilks (a.u.) 196.61 ± 21.14 186.61 ± 21.52 191.87 ± 19.30 184.92 ± 24.57

Δ SQ + BP Wilks (a.u.) 19.76 ± 9.36 10.78 ± 5.23 25.73 ± 3.84 15.13 ± 8.19

SQ = Squat; BP = Bench Press

96 The effect of time-equated concurrent training programs in resistance-trained men

Journal of Human Kinetics, volume 91, March 2024 http://www.johk.pl

Table 3. Contrasts of Marginal Effects.

95% Credible Interval 90% Credible Interval

Contrast Mean Lower Bound Upper Bound Lower Bound Upper Bound Probability > ROPE

Back Squat 1RM (kg)

RT-RTC 2.8609334 −4.0846545 9.8445959 −2.9900589 8.6733361 19.09

RT-CTM −5.1817953 −12.3641477 1.9650008 −11.2418077 0.8447211 41.65

RT-CTH 3.4811883 −3.4673020 10.2776647 −2.3851331 9.1622511 24.33

RTC-CTM −8.0427287 −16.5812854 0.4734758 −15.2711040 −0.8034491 68.94

RTC-CTH 0.6202549 −7.2826118 8.5589916 −5.9941269 7.3057987 9.79

CTM-CTH 8.6629836 0.1481244 16.9454710 1.5896963 15.6504887 73.94

Bench Press 1RM (kg)

RT-RTC 5.3158512 0.8971960 9.6453147 1.6036098 8.9430146 68.79

RT-CTM −0.1416484 −4.7170248 4.2473153 −3.9035598 3.5351813 3.67

RT-CTH −1.7025270 −6.0350349 2.6154737 −5.3529359 1.9390903 12.30

RTC-CTM −5.4574996 −10.5828831 −0.3042842 −9.7186019 −1.1019246 67.45

RTC-CTH −7.0183782 −12.0280629 −1.9001775 −11.2486451 −2.7617697 85.88

CTM-CTH −1.5608787 −6.6396648 3.6380486 −5.8887099 2.7970995 15.08

Quadriceps MT (mm)

RT-RTC 0.2331128 −1.0231289 1.4824821 −0.8196912 1.2758891 18.77

RT-CTM −0.5715763 −1.8668436 0.7520933 −1.6625213 0.5313926 37.55

RT-CTH 0.5477428 −0.7351115 1.7857279 −0.5153661 1.5828387 35.30

RTC-CTM −0.8046890 −2.4362611 0.8760444 −2.1592544 0.5925250 51.15

RTC-CTH 0.3146300 −1.2972764 1.8950979 −1.0328927 1.6396616 27.81

CTM-CTH 1.1193190 −0.6436698 2.7594095 −0.3361734 2.4979418 65.63

Pectoralis Major MT (mm)

RT-RTC −6.1938923 −9.7729415 −2.4186233 −9.2605426 −3.0545362 98.55

RT-CTM −4.1446791 −7.8649111 −0.3213127 −7.2288202 −0.9459705 87.24

RT-CTH −1.2934901 −4.9269436 2.3404023 −4.2923877 1.7241561 36.23

RTC-CTM 2.0492132 −2.1260678 6.1317030 −1.4460896 5.5172007 52.23

RTC-CTH 4.9004022 0.8176847 8.8656350 1.4804937 8.2529682 92.28

CTM-CTH 2.8511890 −1.3134206 6.9280953 −0.5855341 6.2845286 67.12

VO₂peak (ml·kg·min⁻¹)

RT-RTC 1.4886938 −3.5743158 6.5461344 −2.6898642 5.7195229 51.69

RT-CTM −2.5416357 −7.7878821 2.7533342 −6.8804397 1.8936094 67.83

RT-CTH −4.6669771 −9.8563058 0.5577217 −8.9575569 −0.4198616 89.89

RTC-CTM −4.0303295 −9.3033274 1.2196273 −8.3330765 0.3219204 84.79

RTC-CTH −6.1556709 −11.5342907 −0.8106426 −10.5421846 −1.7344040 96.28

CTM-CTH −2.1253413 −7.3168312 3.1365400 −6.4149551 2.2027414 61.84

Maximum Workload (W)

RT-RTC −8.3497351 −36.0545846 19.0359073 −31.2202500 13.9357210 47.31

RT-CTM −34.0883236 −63.3308357 −4.3644052 −58.1133701 −9.7243947 95.30

RT-CTH −41.3920198 −68.4819356 −14.2133650 −63.7696172 −18.8117681 98.84

RTC-CTM −25.7385884 −53.4337318 1.7631891 −48.6445671 −2.7409799 88.92

RTC-CTH −33.0422846 −59.5051235 −6.8155657 −54.5621033 −11.3338846 96.33

CTM-CTH −7.3036962 −35.0153318 20.8236505 −30.1572542 15.8893172 44.49

1RM = one-repetition maximum; RT = resistance training; MT = muscle thickness; RTC = resistance

training circuit; CTM = concurrent training moderate intensity; CTH = concurrent training high

intensity; VO2peak = peak oxygen uptake

by Chad Dolan et al. 97

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license.

Figure 1. Strength Outcomes.

Marginal posterior distributions for changes in the back squat 1RM (A) and the bench press 1RM

(B) and differences among conditions for the back squat 1RM (C) and the bench press 1RM (D).

Vertical dashed lines represent the region of practical equivalence (i.e., ROPE) defined by the raw-

unit equivalent of a ± 0.25 standardized mean difference. Colored dots and intervals represent the

mean and quartile intervals (90 and 95%) from the posterior distribution. Finally, individual data

are visualized below with solid circles. The marginal effects are adjusted for the pretest scores of the

dependent variable.

Figure 2. Hypertrophy Outcomes.

Marginal posterior distributions for changes in quadriceps (A) and pectoralis major muscle thickness

(B) and differences among conditions for quadriceps (C) and pectoralis major muscle thickness (D).

Vertical dashed lines represent the region of practical equivalence (i.e., ROPE) defined by the typical

error of measurement. Colored dots and intervals represent the mean and quartile intervals (90 and

95%) from the posterior distribution. Finally, individual data are visualized below with solid circles.

The marginal effects are adjusted for the pretest scores of the dependent variable, and the measurement

site.

CTH

CTM

RTC

Condition

0 102030

∆ Back Squat 1RM (kg)

−5 0 5 10 15 20

∆ Bench Press 1RM (kg)

CTM−CTH

RTC−CTH

RTC−CTM

RT−CTH

RT−CTM

RT−RTC

Contrast

−20 −10 0 10 20

Difference in ∆ Back Squat 1RM (kg)

−10 0 10

Difference in ∆ Bench Press 1RM (kg)

CTH

CTM

RTC

Condition

−5 0 5 10

∆ Quadriceps MT (mm)

−5 0 5 10 15

∆ Pectoralis Major MT (mm)

CTM−CTH

RTC−CTH

RTC−CTM

RT−CTH

RT−CTM

RT−RTC

Contrast

−2.5 0.0 2.5 5.0

Difference in ∆ Quadriceps MT (mm)

−15 −10 −5 0 5 10

Difference in ∆ Pectoralis Major MT (mm)

98 The effect of time-equated concurrent training programs in resistance-trained men

Journal of Human Kinetics, volume 91, March 2024 http://www.johk.pl

Figure 3. Cardiovascular Outcomes.

Marginal posterior distributions for changes in VO2peak (A) and Wmax (B) and differences among

conditions for VO2peak (A) and Wmax (B). Vertical dashed lines represent the region of practical

equivalence (i.e., ROPE) defined by the raw-unit equivalent of a ± 0.25 standardized mean difference.

Colored dots and intervals represent the mean and quartile intervals (90 and 95%) from the posterior

distribution. Finally, individual data are visualized below with solid circles. The marginal effects are

adjusted for the pretest scores of the dependent variable.

Discussion

The main findings of this study do not align

with our hypothesis and are as follows: 1) all

groups experienced meaningful increases in squat

and bench press 1RM strength, with no significant

between-group contrasts; 2) all groups experienced

meaningful increases in quadriceps muscle

thickness, with no significant between-group

contrasts; 3) all groups, with the exception of the

RT group, experienced meaningful increases in

pectoralis major muscle thickness, with a

significant between-group contrast indicating RTC

> RT; 4) only the CTH group significantly increased

2peak while RT and RTC groups experienced a

meaningful decrease in VO2peak, but no between-

group contrasts were significant; 5) Wmax

meaningfully increased in the CTM and CTH

groups while the RT and RTC groups experienced

meaningful decreases, with a significant between-

group contrast indicating CTH > RT. Overall, these

results suggest that the interference effect can be

avoided when the duration of AT is limited to 30

min and separated from RT by 24 h. Further, RTC

does not enhance strength adaptations of RT, but

may provide a slight hypertrophic benefit in the

upper body.

The present study did not find evidence of

the interference effect on lower body strength gains

in either cycling condition (CTM or CTH). This

conflicts with a recent meta-analysis (Chen et al.,

2024) that reported a meaningful interference effect

on lower body strength gains when AT is moderate

intensity continuous cycling (SMD = −0.38; 95% CI

= −0.62 to −0.14) or, to a lesser degree, high intensity

interval cycling training (SMD = −0.18; 95% CI =

−0.49 to 0.13). However, those authors noted a

CTH

CTM

RTC

Condition

−10−50 510

∆ Peak VO

(ml·kg·min

−1

)

−40 0 40

∆ Maximum Workload (W)

CTM−CTH

RTC−CTH

RTC−CTM

RT−CTH

RT−CTM

RT−RTC

Contrast

−10 0 10

Difference in ∆ Peak VO

(ml·kg·min

−1

)

−100 −50 0 50

Difference in ∆ Maximum Workload (W)

by Chad Dolan et al. 99

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license.

limitation of their analysis which was that 67.5% of

the included studies implemented AT and RT in

the same session. Indeed, another meta-analysis

(Petré et al., 2021) reported the interference effect

on strength gains was present in trained

individuals, but not in untrained individuals;

however, this was only the case when AT was

performed in the same session (SMD = −0.66; −1.08

to −0.25), but not when the sessions were separated

(SMD = −0.10; 95% CI = −0.43 to −0.23).

Similarly, the present study did not find

evidence of the interference effect for either cycling

intensity (CTH or CTM) on upper body strength

gains, which aligns with a previous meta-analysis

reporting no influence of lower body AT on upper

body strength gains (Sabag et al., 2018). However,

the RTC group experienced the smallest nominal

bench press 1RM gain (4.84 kg) and lowest

probability of exceeding the ROPE (62%), while all

other groups experienced gains > 10 kg and a > 99%

probability. While our low statistical power due to

a small sample size must be considered, the present

study is novel in its use of upper body AT via the

barbell circuit. Thus, it is possible that the

interference effect attenuated gains due to upper

body AT, especially given the exercises in the

barbell circuit were primarily upper body (a bench

press, a barbell overhead press, a barbell bent-over

row, a barbell biceps curl). Despite the low loads

used in the barbell circuit, it is also possible that the

minimal rest used and multiple rounds (average of

4.2 per session) led to repetitions closer to failure,

potentially contributing to upper body fatigue for

the subsequent RT sessions. Indeed, appropriately

managing fatigue has previously been reported to

enhance training performance (i.e., training loads)

and subsequently 1RM strength throughout a RT

program (Zourdos et al., 2016a); thus, the potential

additional upper body fatigue in the RTC group

may have compromised progression given the

current study utilized a performance-based

progression. However, caution is warranted given

the low sample size, and future research is

warranted to explore this question.

While the RTC group experienced the

smallest increase in bench press 1RM, this group

simultaneously experienced the largest nominal

increase in pectoralis muscle thickness of 7.19 mm

[95% CrI: 4.26, 10.02]. Thus, the additional bench

press repetitions performed in the barbell circuit

sessions may have provided a minor hypertrophic

stimulus. Although circuit training was low load,

participants subjectively indicated the protocol to

be difficult, and it is plausible that acute fatigue

may have led to a meaningful hypertrophic

stimulus. This aligns with research indicating a

dose-response relationship between volume and

hypertrophy (Baz-Valle et al., 2022; Schoenfeld et

al., 2017a) and that hypertrophy can be achieved

with a wide loading range (Schoenfeld et al.,

2017b). On the other hand, strength gains appear to

have a dose-response relationship with loads

(Lopez et al., 2021); thus, fatigue from the barbell

circuit in the RTC group may have compromised

performance and thus loads used. However,

caution is once again warranted given the small

sample and lack of significant between-group

contrasts between RTC and all other groups.

The upper body findings are slightly

opposed to the findings for quadriceps

hypertrophy, in which all groups experienced

relatively similar increases in muscle thickness

(2.29 to 3.41 mm), with no meaningful between-

group contrasts. This aligns with multiple meta-

analyses that either report that lower body

hypertrophy does not suffer from the interference

effect (Chen et al., 2024; Sabag et al., 2018;

Schumann et al., 2022), or if it does, it is diminished

if AT is performed in a separate session (Petré et

al., 2021) and when the duration and frequency are

lower (Wilson et al., 2012b). Thus, our data provide

additional evidence that hypertrophy has low

likelihood of being interfered as a result of CT with

certain program design choices.

However, it should be noted that while CT

groups (CTM, CTH, RTC) were time-matched, the

RT group had two less sessions per week. Thus,

while the addition of AT generally did not lead to

a meaningful net effect on strength and

hypertrophy, it should be considered that AT can

contribute to hypertrophy and strength (Ozaki et

al., 2015); thus, it may be that the combined effect

of RT + additional AT counteracted the interference

effect in the present study.

While the barbell circuit did not introduce

a clear interference effect on strength and

hypertrophy adaptations, it did fail to promote

cardiovascular adaptations as measured by VO

2peak

and Wmax. Thus, the present study suggests that

VO2peak and Wmax are better enhanced with

traditional cardiovascular training compared to a

barbell circuit in resistance trained participants.

100 The effect of time-equated concurrent training programs in resistance-trained men

Journal of Human Kinetics, volume 91, March 2024 http://www.johk.pl

This is notable because subjective participant

feedback indicated the barbell circuit was

challenging. However, caution is once again

warranted given the small sample size and lack of

meaningful between-group contrasts.

It is suggested that CT program design

decisions be made based on the desired

physiological outcomes, time available to train,

and sound fatigue management. It appears that

some cardiovascular adaptations can occur from

CTM and CTH, but the effects in the present study

were modest. While ~one hour per week of AT can

promote some cardiovascular adaptations and

generally avoid the interference effect, a greater

dosage of AT may be required to maximize

cardiovascular adaptations.

The chief limitation of this study is the low

sample size. Additionally, as noted, the

interference effect appears to be modulated by

various factors such as AT proximity to RT, AT

modality, AT duration, AT frequency, training

status, and nutritional energy balance. Thus, the

present findings cannot be extrapolated to other

configurations of program design variables or

trainee nutritional energy balance. In the present

study, each group experienced an average increase

in body mass, indicating a positive energy balance.

Indeed, a positive energy balance seems to

compensate for the increased training demands of

CT and has been shown to counteract the

interference effect (Murach and Bagley, 2016).

Therefore, since individuals in this study were not

in a negative energy balance, our results cannot be

extrapolated to athletes who may be in a negative

energy balance, such as weight class or physique

sport athletes. In other words, those in a negative

energy balance should still be mindful of the

potential interference effect associated with CT.

Despite these limitations, the present study

employed an ecologically valid CT protocol for

individuals interested in maximizing RT

adaptations in periods of positive energy balance.

Conclusions

In summary, our data indicate that the

interference effect on maximal strength and

hypertrophy can be avoided when AT is moderate

or high intensity cycling for ~one hour per week

and on non-RT days. Further, the novel AT barbell

circuit utilized did not promote robust

cardiovascular or strength adaptations, but may be

sufficient to provide a small additional upper body

hypertrophic stimulus.

Author Contributions: Conceptualization: C.D., J.M.Q., A.K., C.C. and M.C.Z.; methodology: C.D., J.M.Q.,

A.K., C.C. and M.C.Z.; software: C.D., J.M.Q., J.A.G., K.M.M., A.K., Z.P.R., J.C.P., C.C. and M.C.Z.; formal

analysis: C.D., Z.P.R., J.C.P. and M.C.Z.; investigation: C.D., J.M.Q., J.A.G., K.M.M., A.K., C.C. and M.C.Z.;

resources: C.C. and M.C.Z.; data curation: C.D., J.M.Q., J.A.G., K.M.M., A.K., Z.P.R., J.C.P. and M.C.Z.;

writing—original draft preparation: C.D., Z.P.R., J.C.P. and M.C.Z.; writing—review & editing: C.D., J.M.Q.,

J.A.G., K.M.M., A.K., Z.P.R., J.C.P., C.C. and M.C.Z. All authors have read and agreed to the published version

of the manuscript.

Funding Information: No funding was received for this project.

Institutional Review Board Statement: This study was conducted following the principles of the Declaration

of Helsinki, and approved by the Florida Atlantic University Institutional Review Board (protocol code:

680161-3; approval date: 18 November 2014).

Informed Consent: Informed consent was obtained from all participants included in the study.

Conflicts of Interest: Z.P.R., J.C.P., and M.C.Z. are all coaches and writers within the fitness industry. No other

authors of a conflict of interest.

Received: 31 January 2024

Accepted: 04 March 2024

by Chad Dolan et al. 101

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license.

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