Multi-modal High-Intensity Interval Training Increases Muscle Function and Metabolic Performance in Females

Abstract

High-intensity interval training (HIIT) is a time-efficient method of improving aerobic and anaerobic power and capacity. In most individuals, however, HIIT using modalities such as cycling, running, and rowing does not typically result in increased muscle strength, power, or endurance. The purpose of this study is to compare the physiological outcomes of traditional HIIT using rowing (Row-HIIT) with a novel multimodal HIIT (MM-HIIT) circuit incorporating multiple modalities, including strength exercises, within an interval. Twenty-eight recreationally active women (age 24.7 ± 5.4 years) completed 6 weeks of either Row-HIIT or MM-HIIT and were tested on multiple fitness parameters. MM-HIIT and Row-HIIT resulted in similar improvements (p < 0.05 for post hoc pre- vs. post-training increases for each group) in maximal aerobic power (7% vs. 5%), anaerobic threshold (13% vs. 12%), respiratory compensation threshold (7% vs. 5%), anaerobic power (15% vs. 12%), and anaerobic capacity (18% vs. 14%). The MM-HIIT group had significant (p < 0.01 for all) increases in squat (39%), press (27%), and deadlift (18%) strength, broad jump distance (6%), and squat endurance (280%), whereas the Row-HIIT group had no increase in any muscle performance variable (p values 0.33-0.90). Post-training, 1-repetition maximum (1RM) squat (64.2 ± 13.6 vs. 45.8 ± 16.2 kg, p = 0.02), 1RM press (33.2 ± 3.8 vs. 26.0 ± 9.6 kg, p = 0.01), and squat endurance (23.9 ± 12.3 vs. 10.2 ± 5.6 reps, p < 0.01) were greater in the MM-HIIT group than in the Row-HIIT group. MM-HIIT resulted in similar aerobic and anaerobic adaptations but greater muscle performance increases than Row-HIIT in recreationally active women.

Full text

ARTICLE
Multimodal high-intensity interval training increases muscle
function and metabolic performance in females
Stephanie Buckley, Kelly Knapp, Amy Lackie, Colin Lewry, Karla Horvey, Chad Benko, Jason Trinh,
and Scotty Butcher
Abstract: High-intensity interval training (HIIT) is a time-efficient method of improving aerobic and anaerobic power and
capacity. In most individuals, however, HIIT using modalities such as cycling, running, and rowing does not typically result in
increased muscle strength, power, or endurance. The purpose of this study is to compare the physiological outcomes of
traditional HIIT using rowing (Row-HIIT) with a novel multimodal HIIT (MM-HIIT) circuit incorporating multiple modalities,
including strength exercises, within an interval. Twenty-eight recreationally active women (age 24.7 ± 5.4 years) completed
6 weeks of either Row-HIIT or MM-HIIT and were tested on multiple fitness parameters. MM-HIIT and Row-HIIT resulted in similar
improvements (p< 0.05 for post hoc pre- vs. post-training increases for each group) in maximal aerobic power (7% vs. 5%),
anaerobic threshold (13% vs. 12%), respiratory compensation threshold (7% vs. 5%), anaerobic power (15% vs. 12%), and anaerobic
capacity (18% vs. 14%). The MM-HIIT group had significant (p< 0.01 for all) increases in squat (39%), press (27%), and deadlift (18%)
strength, broad jump distance (6%), and squat endurance (280%), whereas the Row-HIIT group had no increase in any muscle
performance variable (pvalues 0.33–0.90). Post-training, 1-repetition maximum (1RM) squat (64.2 ± 13.6 vs. 45.8 ± 16.2 kg, p= 0.02),
1RM press (33.2 ± 3.8 vs. 26.0 ± 9.6 kg, p= 0.01), and squat endurance (23.9 ± 12.3 vs. 10.2 ± 5.6 reps, p< 0.01) were greater in the
MM-HIIT group than in the Row-HIIT group. MM-HIIT resulted in similar aerobic and anaerobic adaptations but greater muscle
performance increases than Row-HIIT in recreationally active women.
Key words: sprint interval training, intermittent training, strength training, aerobic power, HIIT, multimodal, rowing.
Résumé : L’entraînement par intervalle d’intensité élevée (« HIIT ») est une méthode efficace pour gagner du temps afin d’améliorer
la puissance et les capacités anaérobie et aérobie. Toutefois, chez la plupart des individus, l’HIIT soit a
`vélo, a
`la course et a
`l’aviron ne
suscite pas systématiquement une augmentation de la force musculaire, de la puissance et de l’endurance. Cette étude se propose de
comparer les adaptations physiologiques enregistrées a
`la suite de l’HIIT classique a
`l’aviron (« Row-HIIT ») a
`celles d’une nouvelle
approche multimodale (« MM-HIIT ») comprenant des exercices de force au sein d’un intervalle. Vingt-huit femmes (24,7 ± 5,4 ans)
actives par loisir participent a
`six semaines de Row-HIIT ou de MM-HIIT et sont évaluées sur plusieurs aspects de la condition physique.
MM-HIIT suscite des améliorations semblables a
`Row-HIIT (p< 0,05, pré et post entraînement) dans les deux groupes en ce qui
concerne les variables suivantes : puissance maximale aérobie (7 vs 5 %), seuil anaérobie (13 vs 12 %), seuil de compensation ventilatoire
(7 vs 5 %), puissance anaérobie (15 vs 12 %) et capacité anaérobie (18 vs 14 %, respectivement). Le groupe MM-HIIT présente des différences
significatives (p< 0,01 dans tous les cas) en ce qui concerne l’amélioration de la force a
`l’accroupissement (39 %), au développé (27 %)
et au soulevé de terre (18 %), au saut en longueur (6 %) et a
`l’endurance a
`l’accroupissement (280 %); le groupe Row-HIIT ne présente
aucune amélioration de toutes les variables de performance musculaire (pvariant de 0,33 a
`0,90). Après la période d’entraînement,
MM-HIIT suscite par rapport a
`Row-HIIT des plus hautes valeurs de 1RM a
`l’accroupissement (64,2 ± 13,6 vs 45,8 ± 16,2 kg, p= 0,02), au
développé (33,2 ± 3,8 vs 26,0 ± 9,6 kg, p= 0,01) et a
`l’endurance a
`l’accroupissement (23,9 ± 12,3 vs 10,2 ± 5,6 répétitions, p< 0,01,
respectivement). MM-HIIT suscite chez des femmes actives par loisir des adaptations similaires en aérobie et en anaérobie, mais de
meilleures performances musculaires par rapport a
`Row-HIIT. [Traduit par la Rédaction]
Mots-clés : entraînement par intervalle de sprint, entraînement intermittent, entraînement a
`la force, puissance aérobie, HIIT,
multimodal, aviron.
Introduction
High-intensity interval training (HIIT) consists of repeated exe-
cution of high-intensity exercise interspersed with recovery peri-
ods of low-intensity exercise or complete rest (Laursen 2010). HIIT
has been shown to increase aerobic and anaerobic performance
within a short time period (Bayati et al. 2011;Hazell et al. 2010;
Ziemann et al. 2011). HIIT results in physiological benefits includ-
ing improvements in aerobic capacity, cardiorespiratory fitness,
glucose tolerance, exercise endurance, skeletal muscle oxidative
capacity, and glycogen content and reductions in the rate of lac-
tate production and glycogen utilization (Gibala et al. 2012;Nybo
et al. 2010;Weston et al. 2014). Nybo et al. (2010) reported that HIIT
resulted in a significant reduction in arterial blood pressure and
superior improvements in cardiorespiratory fitness, as indicated
by increases in maximal aerobic power (V
˙O
2max
), compared with
traditional prolonged training.
Received 12 May 2015. Accepted 19 July 2015.
S. Buckley, K. Knapp, A. Lackie, C. Lewry, and K. Horvey. School of Physical Therapy, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
C. Benko. Synergy Strength and Conditioning, Saskatoon, Saskatchewan, Canada; BOSS Strength Institute, Saskatoon, Saskatchewan, Canada.
J. Trinh. Synergy Strength and Conditioning, Saskatoon, Saskatchewan, Canada.
S. Butcher. School of Physical Therapy, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; BOSS Strength Institute, Saskatoon, Saskatchewan,
Canada.
Corresponding author: Scotty Butcher (e-mail: scotty.butcher@usask.ca).
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Despite the evidence supporting metabolic adaptations with
HIIT, there is less evidence regarding the muscle strength benefits
of HIIT. It is well known that strength training induces muscle
hypertrophy and strength increases (Nybo et al. 2010). Skeletal
muscle strength has been shown to be an important determinant
of functional capacity (Verdijk et al. 2009). Typically, HIIT does not
have a significant long-term impact on muscle mass or indicators
of skeletal health (Astorino et al. 2012;Nybo et al. 2010). Thus, the
addition of a strength training program to a HIIT program would
be required to optimize muscle function, but excessive time
would be needed to complete both aspects of training (Astorino
et al. 2012).
One potential solution that has emerged to reduce the time
required for multiple adaptations is multimodal (MM) training.
MM training uses a variety of resistance, body-weight, and (or)
traditional conditioning modalities (e.g., sprinting, rowing, kettle-
bell swings, battle ropes) performed in either a continuous circuit
(Myers et al. 2015) or an interval (McRae et al. 2012) format. While
the continuous circuit methodology uses multiple modalities
within a single session, previous work with interval-style MM
training has limited individual sessions to a single body-weight
modality (McRae et al. 2012) and rotated modalities between ses-
sions. While this format is effective for inducing aerobic, anaero-
bic, and muscle endurance adaptations (McRae et al. 2012), the
lack of a whole-body strength component within each interval
may limit the muscle performance adaptations.
In this study, we examined a novel multimodal HIIT (MM-HIIT)
protocol that incorporates resistance and conditioning modalities
within the work interval to combine multiple training effects
while minimizing training time. This form of training is common
in many fitness facilities but has not undergone scientific investi-
gation. The purpose of this study was to document the physiolog-
ical benefits of an MM-HIIT program compared with HIIT using a
more traditional aerobic modality (rowing). First, we hypothe-
sized that MM-HIIT and traditional HIIT using rowing (Row-HIIT)
would lead to similar improvements in aerobic and anaerobic energy
systems. Our second hypothesis was that the MM-HIIT group
would improve to a greater degree in strength, muscle power, and
muscle endurance compared with the Row-HIIT group.
Materials and methods
Study design
A prospective longitudinal group-randomized trial was conducted.
Participants were grouped using convenience assignment based
on their availability for training, and each group was then ran-
domly assigned to complete either the Row-HIIT or the MM-HIIT
training program. On day 1, all participants underwent screening
and initial testing by a physical therapist, which consisted of a
medical history, baseline blood pressure, height and weight, and
a treadmill V
˙O
2max
test. Following screening, participants en-
gaged in 3 familiarization, practice, and adaptation sessions for
the movements to be used during training and testing. Familiar-
ization was followed by 3 field testing sessions conducted during
week 2. Participants then underwent 6 weeks of group training
3 times a week. Post-training testing was then performed on the
same measures as the pre-training tests. For the full testing, famil-
iarization, and training program schedules, see Tables S1 and S2
1
.
Individual participant attendance was recorded and data were
accepted if participants achieved greater than 80% attendance.
Test evaluators were blinded to the group randomization.
Participants
Thirty-two recreationally active women between the ages of
18 and 35 were recruited and gave consent to participate in the
study, which was approved by the local institutional ethics review
board. Recreationally active was defined as engaging in periodic
physical activity or exercise between 1 and 3 h weekly for 1 month
or longer. Participants were not engaging in systematic endur-
ance or weight training. Study participants were instructed to stay
consistent with their levels of exercise activity prior to the study
and to not progress these activities. Participants were excluded if
they reported any acute or chronic cardiovascular, metabolic, re-
spiratory, or other conditions that would have prevented them
from participating in heavy exercise. In addition, if adverse findings
were recorded during screening that would have been associated
with an increased risk of injury during exercise, participants were
excluded from the study. Sixteen participants were assigned to each
training group at baseline. Three participants (2 from the Row-HIIT
group and 1 from the MM-HIIT group) withdrew consent prior to
initiation of the training phase. One participant in the MM-HIIT
group dropped out of the study owing to an injury obtained outside
of the study. There were no other dropouts for any reason. Thus, 14
participants in each group completed the study (Table 1).
Tests and measures
V
˙O
2max
test
V
˙O
2max
was assessed using a customized graded treadmill pro-
tocol. Participants completed a 3-min warm-up at 3.5 km × h
−1
and
1% grade on a treadmill (TrackMaster TMX425C, Full Vision, New-
ton, Kans., USA), which was followed by incremental 1-min stages.
In the initial stages, speed was progressed by 1 km × h
−1
every
minute and the grade was held constant at 1% until participants
indicated they had reached a pace slightly greater than their sub-
jectively determined sustainable running speed. At this point, the
speed was held constant and the grade was increased by 2% per
minute until voluntary exhaustion. Heart rate was measured con-
tinuously with a chest strap (Polar Electro Canada, Lachine, Que.,
Canada). Ventilatory gases were measured and analysed with a
commercial metabolic measurement system (TrueOne 2400, Par-
voMedics, East Sandy, Utah, USA). Rating of perceived exertion
(RPE) was recorded every 2 min and at peak exercise using the
10-point Borg scale (Noble et al. 1983). V
˙O
2max
was considered to be
obtained if 2 of the following criteria were met: (1) heart rate ≥ 90%
of the predicted maximum (220 – age), (2) respiratory exchange
ratio (V
˙CO
2
/V
˙O
2
) ≥ 1.10, and (3) RPE ≥ 9/10. Data were reported in
20-s averages.
In addition to aerobic power, the anaerobic threshold (AT) and
respiratory compensation threshold (RCT) were determined by
analysis of ventilatory gases. AT was defined as the breakpoint in
the ventilatory equivalent for oxygen (V
˙
E
/V
˙O
2
) while V
˙
E
/V
˙CO
2
had
plateaued and the respiratory exchange ratio was between 0.98
and 1.02. RCT was defined as the subsequent breakpoint in V
˙
E
/
V
˙CO
2
(Simon et al. 1983).
Wingate Anaerobic Test
Anaerobic power and capacity were determined by a 30-s Win-
gate Anaerobic Test as per standard protocol (Bar-Or 1987). A
Monark 894E cycle ergometer was used and the resistance was set
at 0.075 kg × kg
−1
body mass. The test was preceded by a practice
drop for3sat0.03 kg × kg
−1
body mass and a 1-min rest period.
Power was calculated using standard Monark Anaerobic Test soft-
ware. Anaerobic power was determined to be the highest 5-s mean
power. Anaerobic capacity was determined to be the mean power
over the entire 30 s.
Muscle strength
On separate days, muscle strength was assessed using 1-repetition
maximum (1RM) testing for back squat, press, and deadlift using
standardized protocols (Baechle and Earle 2008). The back squat
1
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was performed to below parallel (hip crease below the apex of the
patella), the press was initiated with the bar on the clavicles, and
the deadlift was performed in the conventional stance.
Muscle endurance and power
Muscle endurance was assessed by repetitive back squats to
voluntary exhaustion. The load was set at 70% of the pre-training
1RM back squat for both pre- and post-training tests. Movement
standards were the same as those for the 1RM test, and the total
number of successful repetitions achieved without a rest was re-
corded. Muscle power was assessed using a static broad jump. The
best horizontal distance achieved out of 3 trials was recorded.
Training program
During the 6-week training program, participants attended
group training sessions for 60 min 3 times a week. The 60-min
training period was broken down into 20 min of active warm-up,
24 min of training, and 16 min of cooldown. Both the Row-HIIT
and MM-HIIT groups were given the same instructions to perform
at all-out intensity every interval (i.e., achieve an RPE of 9/10 or
10/10 for each interval).
Rowing (Row-HIIT)
Participants randomized into the Row-HIIT group performed
60 s of all-out intensity rowing followed by 3 min of rest (passive
recovery) for a total of 6 rounds. Participants were instructed to
engage in all-out intensity during the work period to achieve max-
imum distance on each interval.
MM-HIIT
Participants randomized into the MM-HIIT group also per-
formed 60 s of all-out intensity work followed by a 3-min rest for
6 rounds. Each MM-HIIT interval was broken down into 3 parts: a
strength exercise for 4–6 repetitions, an accessory movement for
8–10 repetitions, and a metabolic component conducted all out
for the remainder of the 60 s. For the strength exercise and acces-
sory movement, participants were instructed to load “heavy”,
with the purpose of fatiguing by the end of the intervals. For
example, for the strength exercise, a “heavy” load was defined as
the 4–6 repetition maximum; if participants were able to sustain
6 repetitions across all 6 intervals, the load would be progressed
in the next session using the same exercise. Participants were
instructed to engage in all-out intensity during every interval.
Please see Table S2
1
for more information.
Statistical analysis
Statistica V8.0 (StatSoft Inc., Tulsa, Okla., USA) was used to anal-
yse the data. Means and standard deviations were reported for
each descriptive variable. A 2-way, repeated-measures factorial
ANOVA was used to analyse the main effects for each of the main
outcomes (V
˙O
2max
test, Wingate test, and muscle strength/power/
endurance tests). Where significant main effects were observed,
Tukey’s post hoc analysis was performed to determine specific
differences. Percent attendance was compared between groups
using an unpaired ttest. Alpha was set a priori to 0.05.
Results
Attendance
All participants who completed the study performed greater
than 80% of the training sessions. There was no difference in
attendance between groups (89% ± 5% and 91% ± 6% for MM-HIIT
and Row-HIIT, respectively; p= 0.69).
V
˙O
2max
test
All participants achieved the criteria for attaining V
˙O
2max
and
reached tolerance at 8:13 ± 0:44 min:sec for the pre-training test
and 10:36 ± 1:21 min:sec for the post-training test. There were no
interaction effects for the V
˙O
2
values at maximum, AT, or RCT.
There was a significant main effect of time, but not group, for
each of V
˙O
2max
(p< 0.01), V
˙O
2
at AT (p< 0.01), and V
˙O
2
at RCT
(p< 0.01). Table 2 shows the individual group data and indicates
that there were increases in V
˙O
2max
and V
˙O
2
at AT for both groups,
whereas V
˙O
2
at RCT increased only in the MM-HIIT group. There
were no significant post-training differences between groups for
V
˙O
2max
(p= 0.99), V
˙O
2
at AT (p= 0.09), or V
˙O
2
at RCT (p= 0.08).
Wingate test
There were no interaction effects for anaerobic power or capac-
ity. There were significant main effects for time, but not group, for
both anaerobic power (p< 0.01) and capacity (p< 0.01). As shown in
Table 2, there were significant increases in anaerobic power and
capacity for both groups, but there were no post-training differ-
ences between the Row-HIIT and MM-HIIT groups for anaerobic
power (p= 0.643) or capacity (p= 0.825).
Strength
There were significant interaction effects for 1RM squat (p< 0.01)
and 1RM press (p< 0.01). There were also significant main effects
for time in squat, press, and deadlift (p< 0.01) and for group in
squat and press (p< 0.01). Post hoc analysis revealed that the
MM-HIIT group increased strength in the squat (p< 0.01), press
(p< 0.01), and deadlift (p< 0.01), whereas the Row-HIIT group did
not (Fig. 1;p= 0.69, 0.82, and 0.33, respectively). There were sig-
nificant post-training differences between groups for the 1RM
squat (p= 0.02) and the 1RM press (p= 0.01) but not the 1RM deadlift
(p= 0.99).
Muscle power and endurance
There was a significant interaction for squat endurance (p< 0.01).
There were significant main effects for time for both the broad
jump test (p< 0.01) and the squat endurance test (p< 0.01). Figure 2
shows that the MM-HIIT group increased distance achieved in the
broad jump (Fig. 2A;p< 0.01) and the number of reps completed
on the squat endurance test (Fig. 2B;p< 0.01), whereas the Row-
HIIT group did not (p= 0.90 and 0.88, respectively). Post hoc test-
ing revealed there was a significant difference in post-training
values between groups for the squat endurance test (p< 0.01) but
not the broad jump (p= 0.99).
Table 1. Participant characteristics.
Row-HIIT MM-HIIT
Age (years) 24.3±5.2 25.1±5.6
Gender Female (n= 14) Female (n= 14)
Height (cm) 171.6±7.6 166±2.4
Weight (kg)
Pre 73.2±11.9 66.6±9.8
Post 73.6±11.9 67.6±10.2
Note: Row-HIIT, high-intensity interval train-
ing using rowing; MM-HIIT, multimodal high-
intensity interval training.
Table 2. Maximal aerobic power (V
˙O
2
max) and Wingate anaerobic
test data.
Row-HIIT MM-HIIT
Pre Post Pre Post
V
˙O
2
max (mL·kg
−1
·min
−1
) 36.7±4.7 38.3±4.6* 36.2±5.7 38.5±5.4*
V
˙O
2
at RCT (mL·kg
−1
·min
−1
) 33.9±4.7 35.5±4.7 31.1±5.7 33.2±5.6*
V
˙O
2
at AT (mL·kg
−1
·min
−1
) 28.7±5.0 31.7±3.8* 27.2±6.7 29.7±5.7*
Anaerobic power (W) 481±135 536±133* 424±93 484±94*
Anaerobic capacity (W) 371±104 417±100* 333±62 389±69*
Note: RCT, respiratory compensation threshold; AT, anaerobic threshold.
*p< 0.05 vs. pre-training value. There were no between-group differences.
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Fig. 1. Muscle strength test data for (A) 1-repetition maximum (1RM) squat, (B) 1RM press, and (c) 1RM deadlift for the multimodal high-intensity
interval training (MM-HIIT) and traditional HIIT with rowing (Row-HIIT) groups. *Significantly different than the corresponding pre-training value,
p< 0.05; †significantly different than post-training for Row-HIIT, p< 0.05.
Fig. 2. Muscle power (A: broad jump) and muscle endurance (B: squat endurance at 70% 1RM pre-training squat) for the MM-HIIT and Row-HIIT
groups. *Significantly different than the corresponding pre-training value, p< 0.05; †significantly different than post-training for Row-HIIT, p< 0.05.
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Discussion
We examined the effects of a novel MM-HIIT protocol compared
with Row-HIIT on aerobic and anaerobic performance and muscle
strength, power, and endurance. During the 6-week training pe-
riod, both MM-HIIT and Row-HIIT increased anaerobic threshold
(13% vs. 12%), V
˙O
2max
(7% vs. 5%), and anaerobic power (15% vs. 12%)
and capacity (18% vs. 14%). Only the MM-HIIT group had significant
increases in RCT (7%). These findings support our first hypothesis
that Row-HIIT and MM-HIIT would produce similar adaptations in
aerobic and anaerobic energy systems.
Only MM-HIIT resulted in significant improvements in squat,
deadlift, and overhead press strength. Similarly, significant in-
creases in muscle power (broad jump) and endurance (70% 1RM
back squat) were observed only in the MM-HIIT group. These find-
ings support our second hypothesis that the MM-HIIT group
would improve muscle performance to a greater degree than the
Row-HIIT group.
Traditional HIIT has consistently been shown to increase aero-
bic and anaerobic performance compared with continuous endur-
ance exercise (Gist et al. 2013;Hazell et al. 2010;Tabata et al. 1996).
Although our study did not directly compare HIIT with continu-
ous exercise, our findings are similar in regard to aerobic and
anaerobic performance adaptations for both methods of HIIT
training. Increases due to HIIT of 4%–13% for V
˙O
2max
and 4%–12%
for anaerobic performance have been observed previously (Cook
et al. 2010;Driller et al. 2009;Gist et al. 2013;Hazell et al. 2010;
Rønnestad et al. 2015;Sloth et al. 2013). These changes are similar
to the degree of adaptation we observed with both protocols,
indicating that our novel MM-HIIT and the rowing HIIT are simi-
larly effective for inducing metabolic system changes. In contrast,
traditional HIIT does not typically increase muscle strength (Astorino
et al. 2012;Nybo et al. 2010), and this is supported by the present
findings. However, we observed increases in muscle strength,
power, and endurance with our MM-HIIT protocol that are not
typically observed with traditional HIIT.
There are a limited number of studies reporting the effects of
multimodal training on multiple aspects of performance. McRae
et al. (2012) studied the effects of HIIT using whole-body move-
ments (burpees, jumping jacks, mountain climbers, or squat thrusts)
rotated across a 4-week training period but did not incorporate
higher resistance activities or different movements within each
interval. They demonstrated similar improvements in aerobic
capacity but greater benefits to muscular endurance (40%–200%
increase) with their HIIT protocol compared with continuous
treadmill endurance training. Myers et al. (2015) studied the ef-
fects of 5 weeks of training using a continuous MM circuit proto-
col, which rotated exercises and modalities within and between
sessions, compared with a combined strength and aerobic train-
ing protocol. They demonstrated superior aerobic changes with
their circuit protocol (11% in V
˙O
2max
) but similar changes in anaer-
obic power and capacity (5% and 3%, respectively) compared with
the combined program. In terms of muscle strength, while the
combined group increased all measures, the MM circuit group
increased only vertical chest press (21%) and hamstring curl (8%)
strength and not lat pull down or knee extension strength. In
addition, the circuit group was inferior in measures of muscle
endurance; however, Myers et al. compared the number of repe-
titions completed at 60% of 1RM on each strength exercise at the
corresponding time point rather than using the pre-training 1RM
for both tests. In comparison, MM-HIIT in our study resulted in
increases in strength of 39% for the 1RM back squat, 27% for the
1RM overhead press, and 18% for the 1RM deadlift, an increase in
power of 6% for the broad jump distance, and an increase in
muscle endurance of 280% for the squat, demonstrating a large
improvement in multiple measures of muscle performance with
our protocol, in addition to the aerobic and anaerobic adapta-
tions. While direct comparison between our protocol and that of
Myers and colleagues (2015) and McRae and colleagues (2012) can-
not be conclusive owing to methodological differences between
studies, our protocol appears to result in at least similar, or in
some cases greater, adaptations across multiple measures of fit-
ness. Future work should examine the adaptations achieved with
differing MM protocols.
Combining HIIT with strength training is typically achieved by
participants performing 2 simultaneous training protocols (con-
current training) and requires increased time to complete. Wong
et al. (2010) demonstrated that professional soccer players concur-
rently performing HIIT and muscular strength training had sig-
nificantly greater improvements in 10-m and 30-m sprint times,
vertical jump height, and maximal muscle strength (back half
squat and bench press) compared with soccer training alone, but
they required increased training time. Concurrent training in-
volving sprint interval and strength training has been shown to
result in similar upper and lower body strength gains compared
with strength training alone, with the addition of greater in-
creases in V
˙O
2max
. The findings of Wong et al. (2010) and others
(Cantrell et al. 2014;de Souza et al. 2013;McNamara and Stearne
2013) suggest that concurrent training with HIIT does not attenu-
ate the strength response. In comparison with traditional concur-
rent training, our MM-HIIT protocol resulted in improvements in
aerobic and anaerobic performance, along with improvements in
muscle strength, power, and endurance, without the addition of
excess training time. Based on these findings, MM-HIIT is a time-
efficient training method to achieve metabolic and muscular ad-
aptations; however, further study is required to determine whether
an MM-HIIT protocol can maintain the same degree of strength
adaptations as more traditional concurrent protocols or strength
training alone. In addition, because the generalizability of our
study is limited to young, healthy, active females without specific
training experience, future studies should examine the effects of
MM-HIIT in sedentary individuals, males, and older adults or
those with chronic conditions.
Exercise protocols that result in the greatest level of physiolog-
ical and health adaptations in a limited time frame are of interest
to rehabilitation, health, and exercise professionals. Traditional
HIIT methods are time-efficient protocols resulting in multiple
health and aerobic/anaerobic performance benefits (Gibala et al.
2012), but they usually fail to produce adaptations in muscle strength,
power, and (or) endurance. Our novel MM-HIIT protocol results in
aerobic and anaerobic performance adaptations similar to those
resulting from traditional HIIT protocols but has the additional
benefit of improving muscle strength, power, and endurance in
recreationally active females.
Conflict of interest statement
Mr. Benko is co-owner of, and Dr. Butcher is an unpaid consul-
tant with, Synergy Strength and Conditioning, where the research
was conducted.
Acknowledgements
The authors acknowledge the technical assistance of Evelyn
Pata.
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Table S1. Outline of testing and training
Baseline
Screening Assessment (completed 2-14 days prior to Week 1)
History, blood pressure, anthopometry, VO2max assessment
Week 1
Skill Acquisition Week
Monday – Warm-up, squats, back squats, front squats, press
Wednesday – Deadlift, rowing
Friday – Ring rows, push-ups
Week 2
Testing
Monday – 1RM back squat and 70% 1RM back squat for max effort reps unbroken
Wednesday – 1RM overhead press and Wingate testing
Friday – 1RM deadlift and max broad jump
Weeks 3-8
Training
See Table A2
Week 9
Testing
Monday – 1RM back squat and 70% 1RM back squat for max effort reps unbroken
Wednesday – 1RM overhead press and Wingate testing
Friday – 1RM deadlift and max broad jump
Follow-up
Post-training Assessment (completed 4-7 days following Week 9)
Anthopometry, VO2max assessment

Table S2. Multi-modal HIIT Training.
*60sec Q4min x 6 rounds – indicates each work interval is 60 seconds every 4 minutes (ie. 3 minutes rest interval) for 6 rounds or sets
per session. DB – dumbbell; KBS – kettlebell swings; RDL – Romanian deadlift.
Monday
Wednesday
Friday
Week 1
A. In 60sec complete;
4-6 Back squats
8-10 ring rows
Box jumps for the remainder of 60sec
Q4min x 6 rounds*
A. In 60sec complete;
4-6 Fat bar bench press
8-10 DB walking lunges
D-ball slams for the remainder of 60sec
Q4min x 6 rounds
A. In 60sec complete;
4-6 Trap bar deadlift
8-10 DB push press
Double rope undulations for the
remainder of 60sec Q4min x 6 rounds
Week 2
A. In 60sec complete;
4-6 Press
8-10 DB bent over rows
Top end burpees for the remainder of
60sec Q4min x 6 rounds
A. In 60sec complete;
4-6 Front squat
8-10 push-ups
Lateral hurdle hops for the remainder of
60sec Q4min x 6 rounds
A. In 60sec complete;
4-6 Incline bench press
8-10 ring rows
KBS for the remainder of 60sec Q4min x
6 rounds
Week 3
A. In 60sec complete;
4-6 Clean grip RDL
8-10 DB bench press
D-ball slams for the remainder of 60sec
Q4min x 6 rounds
A. In 60sec complete;
4-6 Push press
8-10 DB RDL
Box jumps for the remainder of 60sec
Q4min x 6 rounds
A. In 60sec complete;
4-6 Back squat
8-10 DB Push press
Alternating rope undulations for the
remainder of 60sec Q4min x 6 rounds
Week 4
A. In 60sec complete;
4-6 Front squat
8-10 push-ups
Lateral hurdle hops for the remainder of
60sec Q4min x 6 rounds
A. In 60sec complete;
4-6 Fat bar bench press
8-12 DB walking lunges
D-ball slams for the remainder of 60sec
Q4min x 6 rounds
A. In 60sec complete;
4-6 Back squats
8-12 ring rows
Box jumps for the remainder of 60sec
Q4min x 6 rounds
Week 5
A. In 60sec complete;
4-6 Press
8-12 DB bent over rows
Top end burpees for the remainder of
60sec Q4min x 6 rounds
A. In 60sec complete;
4-6 Back squats
8-12 ring rows
Box jumps for the remainder of 60sec
Q4min x 6 rounds
A. In 60sec complete;
4-6 Trap bar deadlift
8-12 DB push press
Double rope undulations for the
remainder of 60sec
Q4min x 6 rounds
Week 6
A. In 60sec complete;
4-6 Clean grip RDL
8-12 DB bench press
Alternating rope undulations for the
remainder of 60sec Q4min x 6 rounds
A. In 60sec complete;
4-6 Push press
8-12 DB bent over rows
Box jumps for the remainder of 60sec
Q4min x 6 rounds
A. In 60sec complete;
4-6 Back squat
8-12 push-ups
D-ball slams for the remainder of 60sec
Q4min x 6 rounds