Comparison of Cardiorespiratory and Metabolic Responses in Kettlebell High-Intensity Interval Training versus Sprint Interval Cycling

Abstract

Williams, BM and Kraemer, RR. Comparison of cardiorespi-ratory and metabolic responses in kettlebell high-intensity interval training versus sprint interval cycling. J Strength Cond Res 29(12): 3317–3325, 2015—The purpose of this study was to determine the effectiveness of a novel exercise protocol we developed for kettlebell high-intensity interval training (KB-HIIT) by comparing the cardiorespiratory and metabolic responses to a standard sprint interval cycling (SIC) exercise protocol. Eight men volunteered for the study and completed 2 preliminary sessions, followed by two 12-minute sessions of KB-HIIT and SIC in a counterbalanced fashion. In the KB-HITT session, 3 circuits of 4 exercises were performed using a Tabata regimen. In the SIC session, three 30-second sprints were performed, with 4 minutes of recovery in between the first 2 sprints and 2.5 minutes of recovery after the last sprint. A within-subjects' design over multiple time points was used to compare oxygen consumption (V _ O 2), respiratory exchange ratio (RER), tidal volume (TV), breathing frequency (f), minute ventilation (V E), caloric expenditure rate (kcal$min 21), and heart rate (HR) between the exercise protocols. Additionally, total caloric expenditure was compared. A significant group effect, time effect, and group 3 time interaction were found for V _ O 2 , RER, and TV, with V _ O 2 being higher and TV and RER being lower in the KB-HIIT compared with the SIC. Only a significant time effect and group 3 time interaction were found for f, V E , kcal$min 21 , and HR. Additionally, total caloric expenditure was found to be significantly higher during the KB-HIIT. The results of this study suggest that KB-HIIT may be more attractive and sustainable than SIC and can be effective in stimulating cardiorespiratory and metabolic responses that could improve health and aerobic performance.

Full text

COMPARISON OF CARDIORESPIRATORY AND METABOLIC
RESPONSES IN KETTLEBELL HIGH-INTENSITY
INTERVAL TRAINING VERSUS SPRINT INTERVAL
CYCLING
BRIAN M. WILLIAMS AND ROBERT R. KRAEMER
Department of Kinesiology and Health Studies, Southeastern Louisiana University, Hammond, Louisiana
ABSTRACT
Williams, BM and Kraemer, RR. Comparison of cardiorespi-
ratory and metabolic responses in kettlebell high-intensity
interval training versus sprint interval cycling. JStrength
Cond Res 29(12): 3317–3325, 2015—The purpose of this
study was to determine the effectiveness of a novel exercise
protocol we developed for kettlebell high-intensity interval
training (KB-HIIT) by comparing the cardiorespiratory and
metabolic responses to a standard sprint interval cycling
(SIC) exercise protocol. Eight men volunteered for the study
and completed 2 preliminary sessions, followed by two
12-minute sessions of KB-HIIT and SIC in a counterbalanced
fashion. In the KB-HITT session, 3 circuits of 4 exercises
were performed using a Tabata regimen. In the SIC session,
three 30-second sprints were performed, with 4 minutes of
recovery in between the first 2 sprints and 2.5 minutes of
recovery after the last sprint. A within-subjects’ design over
multiple time points was used to compare oxygen consump-
tion (V
_
O
2
), respiratory exchange ratio (RER), tidal volume
(TV), breathing frequency (f), minute ventilation (V
E
), caloric
expenditure rate (kcal$min
21
), and heart rate (HR) between
theexerciseprotocols.Additionally, total caloric expenditure
was compared. A significant group effect, time effect, and
group 3time interaction were found for V
_
O
2
,RER,andTV,
with V
_
O
2
being higher and TV and RER being lower in the
KB-HIIT compared with the SIC. Only a significant time effect
and group 3time interaction were found for f, V
E
,
kcal$min
21
, and HR. Additionally, total caloric expenditure
was found to be significantly higher during the KB-HIIT.
The results of this study suggest that KB-HIIT may be more
attractive and sustainable than SIC and can be effective in
stimulating cardiorespiratory and metabolic responses that
could improve health and aerobic performance.
KEY WORDS oxygen consumption, caloric expenditure,
respiratory exchange ratio, heart rate, ventilatory response,
Tabata
INTRODUCTION
In recent years, high-intensity interval training (HIIT)
has become a popular modality for exercise training.
According to the American College of Sports Medicine
(ACSM), HIIT was reported to be the top fitness trend
worldwide for 2014 (27) and the second top fitness trend for
2015 (28). Importantly, HIIT has been shown to be effective
in producing musculoskeletal (19), metabolic (1–3,16), and
cardiorespiratory adaptations (4,5,8,9,17,18,29,30). High-
intensity interval training has generated much interest because
of the tissue adaptations it can produce that are similar to
traditional aerobic training and the time efficiency it provides
(4,17,18). The most common reason for nonadherence to an
exercise program is due to lack of time commitment for exer-
cise training (20). Because HIIT is a form of training that can
be used in a time-efficient manner (e.g., 3 plus bouts of high-
intensity exercise across 20 minutes) and has a high impact on
health benefits, it follows that HIIT should be an important
option for improvement of aerobic fitness. Even for the “vet-
eran” exerciser, changing regimen (such as incorporating
HIIT more often) could be an effective strategy to help main-
tain exercise adherence (20).
A modality often used by HI IT exercisers is sprint interval
cycling (SIC). In a study by Freese et al. (2013), cardiore-
spiratory responsiveness was examined during SIC with
participants performing 4 succeeding 30 second sprints, each
followed by a 4 minute active recovery. Oxygen consump-
tion (V
_
O
2
) increased from the first sprint to the second, but
was similar in the succeeding sprints and was above 80% of
the estimated maximal value. Respiratory exchange ratio
(RER) also was observed to decline from the first to the last
sprint. The findings of this previous study demonstrate the
extent to which aerobic metabolism is used during this type
Address correspondence to Brian M. Williams, brian.williams-2@selu.
edu.
29(12)/3317–3325
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VOLUME 29 | NUMBER 12 | DECEMBER 2015 | 3317
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of HIIT and suggests that metabolic and cardiorespiratory
adaptations can occur through SIC (8).
Modes of resistance training are often integrated with
interval-based exercise training. One popular mode is the use
of kettlebells (KBs). Kettlebells not only have been shown to
be effective in improving muscular strength (12,15,21,31),
but also have been seen to elicit strong cardiorespiratory
responses and can be adjusted to be performed continu-
ously (5,6,26). In a study by Farrar et al. (2010), continuous
KB swings for a total of 12 minutes were observed to elicit
an average of 65.3% of maximal oxygen consumption
(V
_
O
2
max) and an average of 86.8% of maximal heart rate
(HR
max
) responses, which meet ACSM recommendations
for optimal intensity for improving cardiorespiratory fitness
(6). In a recent study by Falatic et al. (2015), 20 minutes of
KB snatching (with a 15:15 work-to-rest ratio) performed 3
days per week for 4 weeks was found to significantly
increase aerobic capacity more (+2.3 ml$kg
21
$min
21
or
;6%) than circuit-weight training of the same training
duration and frequency (5). Therefore, existing research
suggests that KB training may influence a greater response
by the cardiorespiratory system than traditional resistance
exercise. Thus, this form of training may be used as an
effective mode of training for maintaining or improving
aerobic conditioning (5).
The cadence of the movements with the added resistance
of the KB makes it possible for specific KB exercises to elicit
greater responsiveness of the cardiorespiratory system. It is
even possible for KB training to be adjusted to be performed
continuously, with rest periods and alternation of exercises
integrated within the protocol to reduce fatigue. In a study
TABLE 1. Baseline anthropometric and
cardiorespiratory fitness level data of subjects
(men = 8).
Mean 6SD
Age (y) 21.5 60.86
Height (cm) 176 65.29
Weight (kg) 82.95 611.62
Body fat (%) 18.52 63.04
Fat-free mass (kg) 67.44 68.55
HR
max
(b$min
21
) 189.75 67.98
V
_
O
2
max (ml$kg
21
$min
21
) 52.16 66.55
HR = heart rate.
Figure 1. A) Sprint interval cycling (SIC) protocol, no repeat. B) Kettlebell high-intensity interval training (KB-HIIT) protocol, performed 3 times.
Comparison of KB-HIIT and SIC Responses
3318
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by Thomas et al. (2014), a KB protocol at an intensity level
of ;60% of V
_
O
2
max was compared with graded-treadmill
walking at roughly the same intensity. The protocol involved
completing 10 repetitions of KB swings followed by 10 rep-
etitions of KB sumo deadlifts at a set metronome cadence.
This was performed continuously for 3 circuits of 10-minute
duration with 3 minutes of rest in between. It was found that
not only was the KB protocol comparable in intensity level
to graded-treadmill walking, but also the KB protocol elicited
a higher average HR and rate of perceived exertion (RPE)
(26). These higher responses of HR and RPE likely were due
to the upper body working harder in comparison to the lower
extremities. Collectively, the studies by Falatic et al. (2015),
Farrar et al. (2010), and Thomas et al. (2014) support the
contention that KB training may be modified to elicit the
same benefits as more traditional aerobic exercise training
(i.e., treadmill walking). However, to date, there are no studies
that have investigated the cardiorespiratory and metabolic
responses of a specific KB protocol appropriate for HIIT.
A Tabata regimen is commonly used in HIIT and is
a regimen that could be modified for use with KBs. This
routine involves performing exercise intervals of 20 second
durations with 10 seconds of recovery in between and is
based on research by Tabata et al., who examined the acute
cardiorespiratory and metabolic
responses of high-intensity inter-
mittent cycling and its training
adaptations (24,25). These stud-
ies revealed that a Tabata routine
in comparison to moderate-
intensity aerobic exercise elicited
a greater percentage of V
_
O
2
max
(170% vs. 70%) and produced
a 28% increase in anaerobic per-
formance (25). Thus, the pur-
pose of this study was to
determine whether a kettlebell
high-intensity interval training (KB-HIIT) protocol that we de-
signed using Tabata training principles would be as effective as
a standard SIC protocol used for HIIT. We did this by compar-
ing the cardiorespiratory and metabolic responses of the respec-
tive protocols. It was hypothesized that the KB-HIIT protocol
and SIC protocol would elicit similar cardiorespiratory and
metabolic responses. If verified, the study could lead to the
useofanewformofHIITusingKBsbyathletesandrecrea-
tional exercisers that would provide an inexpensive and time-
saving form of training to improve health status and aerobic
performance.
METHODS
Experimental Approach to the Problem
Subjects first completed 2 preliminary sessions: an informa-
tional and familiarization session and a session to perform
assessment of cardiorespiratory fitness and anthropometric
measures. This was followed by completion of 2 experimen-
tal sessions in a counterbalanced, randomized fashion: a KB-
HITT session and a SIC session. There were 5–7 days
between these sessions. V
_
O
2
, RER, tidal volume (TV),
breathing frequency (f), minute ventilation (V
E
), caloric
expenditure rate (kcal$min
21
), HR, and total caloric expen-
diture were compared between the KB-HIIT and SIC
protocols. A within-subjects’
design over multiple time
points within the exercise pro-
tocols was used with most
of the statistical analyses to
reduce the variance of the car-
diorespiratory and metabolic
measures. The outcomes of
this study should provide
insight into the acute responses
of the novel KB-HIIT protocol
and allow its effectiveness for
training to be determined.
Subjects
Eight men between 20 and 23
years in age completed the
study. Descriptive characteristics
TABLE 2. Intraclass correlation coefficients.
V
_
O
2
RER TV f V
E
kcals$min
21
HR
KB-HIIT 0.963 0.941 0.963 0.972 0.972 0.955 0.991
SIC 0.731 0.765 0.97 0.858 0.972 0.934 0.767
RER = respiratory exchange ratio; TV = tidal volume; f = breathing frequency; V
E
= minute
ventilation; HR = heart rate; KB-HIIT = kettlebell high-intensity interval training; SIC = sprint
interval cycling.
Figure 2. Oxygen consumption across the 12 minutes of exercise between the protocols. Values are expressed
as mean 6SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
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of the subjects’ baseline anthropometric and cardiorespira-
tory fitness data are shown in Table 1. All subjects were
volunteers, and before their participation, they provided
informed written consent after they were given a descrip-
tion of the study procedures andassociatedrisks.Thesub-
jects had current or previous experience in some sports
(e.g., football, pole vault, shot put, cross country, cross-fit,
olympic lifting, and power lifting) and were considered
“very active” (based on the average V
_
O
2
max of 52.16 6
6.55 ml$kg
21
$min
21
; range = 41.2–60.5 ml$kg
21
$min
21
).
Most of the subjects had some previous experience with KB
exercise and cycling; however, they were not considered
trained KB athletes or cyclists.
Each perspective subject completed a medical history
questionnaire and excluded if they had a current upper body
or lower body musculoskeletal injury, musculoskeletal
impairment, or had cardiovascular, pulmonary, or metabolic
disease. The subjects were asked to (a) refrain from vigorous
physical activity within 24
hours before each session, (b)
maintain their normal dietary
habits, (c) stay hydrated the
night before and the day of
each session, and (d) sleep an
adequate amount of time (;8
hours) the night before each
session. The study was
approved by the Institutional
Review Board of Southeastern
Louisiana University.
Procedures
This study was conducted in
the spring time of the year. All
sessions were performed in the
afternoon and each session
lasted approximately 1 hour. A certified exercise physiologist
and a certified athletic trainer were present during each
session to ensure safety and procedural effectiveness.
In the first preliminary session, a certified KB instructor
familiarized the subjects with proper form and lifting
techniques of the KB exercises that they would perform.
The 4 KB exercises used and the order of the exercises in the
KB-HIIT protocol were (a) sumo squat (single-handed or
2-handed), (b) 2-handed swings, (c) clean and press (using
the dominant arm), and (d) sumo deadlift (single-handed or
2-handed). Subjects practiced each KB exercise in a group
setting for ;1 hour while the certified KB instructor evalu-
ated their individual performance. During this session, the
appropriate weight for each subject was determined for each
KB exercise that would allow them to maintain a lifting rate
without reaching volitional exhaustion quickly. The subjects
informed the researchers of the weight that felt most appro-
priate for each KB exercise. Weight ranges for the KBs used
in each exercise were sumo
squat = 18–22 kg, 2-handed
swing = 16–22 kg, clean and
press = 10–22 kg, and sumo
deadlift = 16–22 kg. Addition-
ally, before or after the KB
exercise familiarization, the
subjects were familiarized with
the specialized leg cycle
ergometer used for the SIC
protocol. Each subject per-
formed 1 SIC interval to
become accustomed to the
intensity of the SIC protocol.
In the second preliminary
session, body fat percentage
was calculated by the 7-site
skinfold measurement procedure
and corresponding equation
Figure 3. Respiratory exchange ratio across the 12 minutes of exercise between the protocols. Values are
expressed as mean 6SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 4. Tidal volume across the 12 minutes of exercise between the protocols. Values are expressed as mean
6SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Comparison of KB-HIIT and SIC Responses
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(11). Height (cm) and weight (kg) were measured using
a stadiometer scale. V
_
O
2
max and HR
max
were determined
from a graded-exercise test to exhaustion on a treadmill
(Cardiac Science Quinton Q-Stress TM65 treadmill; Mor-
tara Instrument, Inc., Milwaukee, WI, USA) using the
Kraemer protocol (14). The participants first warmed up
for 5 minutes on the treadmill at a speed of 1.5 mph and no
grade. The protocol then was initiated and initially started
at 2.5 mph and 4% grade. The speed was increased 1 mph
every 2 minutes until the participant reached volitional
exhaustion. Ventilatory and metabolic analyses were per-
formed concurrently throughout the test using a metabolic
analyzer (Parvo Medics’ TrueOne 2400; Parvo Medics,
Sandy, UT, USA). Heart rate was recorded every minute
of the test by a HR monitor (Polar WearLink and FT7
watch; Polar Electro, Inc., Lake Success, NY, USA). All
baseline anthropometric measures and graded-exercise
testing were conducted by a certified exercise physiologist.
During the SIC experimental
session, the subjects completed
the protocol on a specialized leg
cycle ergometer for sprint
cycling (Monark Ergomedic
894E Peak Bike; Monark Exer-
cise AB, Vansbro, Sweden).
Resistance (by a plate-loaded
weight basket) was applied to
the flywheel of the leg cycle
ergometer during each sprint
interval. This resistance was
calculated using 8.8% of the
participants’ fat-free body mass,
a method used by Freese et al.
(2013) to determine appropriate
flywheel resistance during SIC
(8). After a 5-minute passive
rest, the participants completed a 10-minute warm-up on the
bike at a speed of 50 revolutions per minute (rpm) with no
resistance. During the last 15 seconds of the warm-up, the
participants were instructed to cycle as fast as possible (“sprint”)
until they reached their peak rpm. At this point, the weight
basket on the cycle ergometer was dropped, activating the load
on the flywheel and the participants continued to sprint for 30
seconds against the resistance. Resistance was removed imme-
diately after the 30-second sprint. After each sprint, the partic-
ipants continued to pedal at 50 rpm with no resistance applied
during the recovery period. The durations of the recovery peri-
ods were 4 minutes for the first and second recoveries and
2.5 minutes for the third. The second and third sprints began
during the last 15-seconds of the first and second recovery
periods. See Figure 1A for the specifics of the SIC protocol.
Before the KB-HIIT experimental session, the subjects
were again familiarized with the KB exercises being used.
For the protocol, participants passively rested for 5 minutes
to record resting HR. After the
resting period, the participants
walked on the treadmill for
3 minutes at a speed of 1.5
mph and no grade, and then
practiced the 4 exercises with
the KBs for 7 minutes. Heart
rate was measured during the
last minute of the treadmill walk
and after practicing the KB ex-
ercises. After warming up, the
participants began the KB-HIIT
12-minute session. The pattern
of 20-seconds of exercise fol-
lowed by 10-seconds of rest
was repeated for each exercise
(Figure1B).Ametronomewas
used to standardize the cadence
of the lifting and lowering of the
Figure 5. Breaths per minute across the 12 minutes of exercise between the protocols. Values are expressed as
mean 6SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 6. Minute ventilation across the 12 minutes of exercise between the protocols bouts. Values are
expressed as mean 6SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
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KBs. The metronome was set at 40–44 b$min
21
. For each of
the exercises, except for the KB swings, the participants were
instructed to move 2 beats for the lift phase and then 2 beats for
the lowering phase of each exercise. For the KB swings, the
cadence was 1 beat for the lift phase and 1 beat for the lowering
phase of the exercise. Once the participant completed all 4 KB
exercises, the whole process was repeated 2 more times for
a total of 3 circuits of the 4 KB exercises.
During each experimental session, V
_
O
2
, RER, TV, f, V
E
,
and kcal$min
21
were measured continuously by the meta-
bolic analyzer. During the KB-HIIT session, HR was re-
corded immediately after the 20 second exercise and then
at the end of the 10 second recovery period. During the SIC
session, HR was recorded at the beginning and immediately
after each sprint and also every minute of recovery.
Statistical Analyses
Intraclass correlation coefficients were calculated to assess
the reliability of the V
_
O
2
,RER,TV,f,V
E
, kcal$min
21
,
and HR measures in each
exercise protocol (Table 2).
To examine the main effects
of V
_
O
2
,RER,TV,f,V
E
,
kcal$min
21
, and HR across
12 minutes of exercise
between the protocols, a 2
(Group) 312 (Time point)
repeated-measures analysis
of variance was used. To
examine the total caloric
expenditure difference between
the exercise protocols, an
independent t-test was used.
All statistical analyses were
performed using the SPSS
for Windows, version 20.0
(IBM Corp., Somers, NY,
USA), statistical software program using an alpha level
of p#0.05.
RESULTS
A significant (p#0.05) group effect, time effect, and group
3time interaction were found for V
_
O
2
, RER, and TV (Fig-
ures 2–4). However, only a significant (p#0.05) time effect
and group 3time interaction were found for f, V
E
,
kcal$min
21
, and HR (Figures 5–8). The group main effects
for V
_
O
2
[F(1,14) = 4.886, p= 0.044, partial eta-squared =
0.259,
h
2
= 0.539], RER [F(1,14) = 133.151, p,0.001,
partial eta-squared = 0.905,
h
2
= 1.00], and TV [(F(1,14) =
10.416, p= 0.006, partial eta-squared = 0.427,
h
2
= 0.851]
revealed that there were significant differences between the
KB-HIIT and SIC exercise protocols.
In the KB-HIIT protocol across time, there was a higher
(mean 6SE)V
_
O
2
(22.6 61.48 vs. 19.9 61.01
ml$kg
21
$min
21
) and lower RER (0.93 60.02 vs. 1.22 6
0.04) and TV (1.7 60.07 vs. 2.2 60.15 L) than for the
SIC protocol. V
_
O
2
was highest
at the peak of each SIC interval
in comparison to the KB-HIIT,
but it was the lowest during the
recovery periods (Figure 2).
During the SIC, the average
of the 3 highest V
_
O
2
values
(38.4 60.87 ml$kg
21
$min
21
)
equated to 73.6% of V
_
O
2
max,
in comparison to the
KB-HIIT (29.1 60.09
ml$kg
21
$min
21
) which was
55.7% of V
_
O
2
max. With the
KB-HIIT, V
_
O
2
fluctuated in
a specific pattern throughout
the 3 circuits of this bout, but
remained higher in comparison
to the SIC throughout most of
Figure 7. Caloric expenditure per minute across the 12 minutes of exercise between the protocols. Values are
expressed as mean 6SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 8. Heart rate across the 12 minutes of exercise between the protocols. Values are expressed as mean 6
SE. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Comparison of KB-HIIT and SIC Responses
3322
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the 12 minutes of exercise. Throughout a majority of the
12 minutes of exercise, RER was higher during the SIC than
during the KB-HIIT, especially during the initial minutes
(Figure 3). As time progressed during the SIC, RER gradu-
ally declined. During the KB-HIIT, RER remained fairly
constant and fluctuated little. With TV, this measure also
was highest during the SIC and declined during the recovery
periods of this exercise protocol; however, even during the
recovery periods, it remained higher in comparison to the
KB-HIIT (Figure 4). During the KB-HIIT, TV was lower and
it fluctuated less throughout the duration of the protocol in
comparison with the SIC.
Average HR (6SE) was higher during the KB-HIIT pro-
tocol than the SIC protocol (149.16 67.4 vs. 139.69 6
7.85 b$min
21
). Peak HR however was higher at the end of
each SIC interval (Figure 8). During the SIC, the average of
the 3 highest HR values (168 62.21 b$min
21
) equated
to 88.5% of HR
max
, in comparison to the KB-HIIT (166 6
1.41 b$min
21
) which was 87.5% of HR
max
.
Analysis of the resulting total caloric expenditure during
each exercise protocol using an independent t-test revealed
a significant difference between the protocols [t(14) = 2.323,
p= 0.036]. There was a greater average in total caloric
expenditure during the KB-HIIT (144.87 66.56 kcals) in
comparison to the SIC (122 67.34 kcals) (Figure 9).
DISCUSSION
Findings from this study revealed that the hypothesis was
partially supported. This is the first study to demonstrate
that a KB-HIIT protocol can produce similar kcal$min
21
,
V
E
, f, and HR responses over time as a standard SIC pro-
tocol, which is a well-known form of HIIT. Different from
what was hypothesized, the KB-HIIT produced higher V
_
O
2
across time compared with the SIC, and the SIC was shown
to elicit greater RER and TV across time. A comparison of
each of the cardiorespiratory and metabolic responses to the
KB-HIIT and SIC protocols
relative to findings from previ-
ous studies suggests that
KB-HIIT is an effective form
of HIIT.
Although the SIC produced
peak values for V
_
O
2
at 3 time
points that were higher than
those produced in the
KB-HIIT, the KB-H IIT still eli-
cited greater V
_
O
2
across time.
This further suggests that KB-
HIIT can be used effectively
to promote cardiorespiratory
and metabolic adaptations.
The average V
_
O
2
(22.6 6
1.48 ml$kg
21
$min
21
) for the
KB-HIIT across time in this
study only averaged 43.3% of
V
_
O
2
max, but this measure was affected by 10 seconds of
passive recovery in between each exercise interval and
including exercises that may not have been as metabolically
demanding compared with the other exercises (i.e., sumo
squat vs. swing). Thomas et al. (2014) reported an average
value of ;60% of V
_
O
2
max throughout 3 circuits of continuous
10 minute KB swinging and sumo lifting (26). In this study,
the 3 highest V
_
O
2
values averaged 55.7% of V
_
O
2
max. This was
a lower peak average %V
_
O
2
max than reported by Thomas
et al. (2014), which is likely due to greater contribution of
the phosphagen system and anaerobic glycolysis from the
greater intensity of the protocol (i.e., more repetitions
per minute) used in this study. This was indicated by higher
and sustained RER values across time in this study than in the
study by Thomas et al. (2014). RER, and also f, TV, and V
E
,
are important as well to note from this study because these
ventilatory measures provide perspective of the greater sus-
tainability of the KB-HIIT protocol compared with the SIC.
Respiratory exchange ratio was significantly different
between the KB-HIIT and SIC protocols. This was likely
due to the difference in degree of glycolytic activity and
buffering of hydrogen ions (23) across time. Sprint interval
cycling was associated with an erroneous RER (averaging at
;1.22 across the bout and peaking at 1.57), which presum-
ably was due to excessive bicarbonate buffering (23) and
greater TV from the elevated glycolytic activity during this
particular exercise protocol. Respiratory exchange ratio
peaked at the beginning of each SIC interval and declined
after each SIC interval. Erroneous RER values and a trend
for RER decline across the SIC bout were similar to what
was observed by Freese et al. (8), whose study design used
the same SIC protocol as in this study. The trend in decline
of RER across the SIC bout may have been due to increased
lipolysis over time from change in circulating catecholamine
and insulin concentrations (13). Conversely, for the
KB-HIIT protocol, there was less variability in RER with
Figure 9. Total caloric expenditure between the protocols. Values are expressed as mean 6SE. *Denotes significant
difference between the protocols, p#0.05. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
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an average of ;0.9. This is similar to RER responses in
previous KB studies. Hulsey et al. (2012) observed an aver-
age RER of ;0.95 (along with an average V
_
O
2
of ;34.1
ml$kg
21
$min
21
) during a 10 minute KB swinging bout,
which included 35 second intervals of KB swings and 25
seconds of rest in between the intervals (10). V
E
remained
relatively stable during the KB-HI IT compared with the S IC
(Figure 6). This was probably due to greater stability of TV
and less fluctuation in f (Figures 4 and 5). Stable but elevated
expired carbon dioxide (CO
2
) associated with elevated RER
would contribute to increased but stable V
E
because of the
relationship of arterial partial pressure of CO
2
with V
E
(22).
As a result, KB-HIIT may be easier to complete because of
less hyperpnea and tachypnea than SIC.
In addition to the ventilatory responses being more
sustained during the KB-HIIT than the SIC, so was HR
(Figure 8). There are few studies that have examined HR
responses across time with KB exercise. Hulsey et al. (2010)
reported that HR across 10 minutes of KB swinging averaged
at 89.1% of age-predicted HR
max
(10). Farrar et al. (2012)
examined HR responses across 12 minutes of KB swinging
and observed an average HR of 86.8% HR
max
(HR
max
value
was determined through grade-exercise testing) (6). The find-
ings of these previous studies are similar to what was observed
in this study. The average of the 3 highest values of HR during
the KB-HIIT in this study was observed at 87.5% of HR
max
,
and KB swings were performed during the time points of
these higher HR values. The percentage of HR
max
during
the KB-HIIT was similar to what was observed for the aver-
age of the 3 highest values of HR during the SIC (88.5% of
HR
max
). The average HR for SIC in this study was compara-
ble with previously reported HR responses to SIC. Freese
et al. (2013) found the peak value for HR at each SIC interval
to average around 83–89% of age-predicted HR
max
(8). A
comparison of the average highest HR values to HR
max
in
this study is important to note because unlike previous studies
examining KB or SIC effects on cardiorespiratory response
(i.e., Hulsey et al. [2012] and Freese et al. [2013]), in this study,
HR
max
was measured during preliminary graded-exercise test-
ing and used as a basis for comparison. %HR
max
provides
a more accurate measure of the degree of aerobic exercise
stress. Elevated HR, as expected, was concomitant with
increased metabolic rate and caloric expenditure.
The kcal$min
21
elicited by the KB protocol from the
study by Thomas et al. (2014) was found to be similar to
graded-treadmill walking set at the same intensity level (26).
In this study, the work rate of the SIC and KB-HIIT proto-
cols were different; however, there was no significant group
effect for kcal$min
21
. The average values across time were
9.51 kcal$min
21
during the KB-HIIT protocol and 8.6
kcal$min
21
during the SIC protocol. The average of the
peak kcal$min
21
of the SIC intervals was 15.78 kcal$min
21
.
During the 3 highest peaks in the KB-HIIT (when KB
swings were occurring), there was a 12.35 kcal$min
21
aver-
age. This is similar to the ;12.5 kcal$min
21
average in the
10 minutes of KB swings observed by Hulsey et al. (10).
Furthermore, although the average kcal$min
21
of the KB-
HIIT and SIC protocols were not significantly different
(although the work rates were different), the total
expenditure elicited by the KB-HIIT was significantly greater
than the SIC (144.87 66.56 vs. 122 67.34 kcals, p= 0.036).
The reason for the difference in total caloric expenditure, but
no group difference in kcal$min
21
, seems to be due to the 3
extended periods in which the KB-H IIT responses were ;5–
6 kcal$min
21
higher than the SIC (specifically during the
SIC recovery periods), whereas there were only brief time
periods in which the SIC responses were ;7–8 kcal$min
21
greater than the KB-HIIT.
In conclusion, all of these physiological comparisons
indicate that KB-HIIT can be an effective protocol for HIIT
and may even be more attractive and sustainable than SIC.
This study is 1 of 2 studies to date that has examined the use of
a Tabata regimen with KB exercise. In a study by Fortner et al.
(2014), a Tabata regimen with KB swinging was compared
with a traditional regimen of KB swinging (included fewer sets
and 90 seconds of rest between sets). It was found that not
only could individuals complete the Tabata regimen signifi-
cantly quicker (240.0 60.0 vs. 521.5 63.3 seconds), but also
the routine elicited significantly higher average V
_
O
2
(33.1 61.5
vs. 27.2 61.6 ml$kg
21.
min
21
), percentage of V
_
O
2
peak (71.0 6
0.3 vs. 58.4 60.3%), and HR response (162.4 64.6 vs. 145.6 6
4.8 b$min
21
) than the traditional KB swing routine (7). In
addition to this previous study, this study took into consider-
ation of muscular fatigue that could occur during this type of
HIIT. An onset of muscular fatigue, especially in large muscles
and joints, can reduce a KB exerciser’s performance and
increase their risk of injury. When performing intermittent
bouts of KB exercise, not only should the KB exerciser be
familiarized with proper technique to reduce injury risk, but
also localize fatigue needs to be considered with protocol pro-
gramming so that it can be reduced (6,7,26). To help reduce
fatigue, an alternation of more stressful and less stressful KB
exercises, such as what was used in this study, should be con-
sidered. Accordingly, the effectiveness of the KB-HIIT proto-
col from this study offers additional exercise programming
benefit, in addition to the time efficiency it can provide and
influence on acute responses that can effectively augment car-
diorespiratory and metabolic adaptations. In future studies,
a comparison of the metabolic demand of different KB exer-
cises would be beneficial, mostly to offer further insight into
refining appropriate HIIT protocols such as the one used in
this study. Additionally, the broader impact of this KB-HIIT
protocol should be studied. Recent evidence by Madsen et al.
(2015) indicates that HIIT can be used for effective glycemic
control and improve pancreatic beta-cell function in type 2
diabetic patients (16).
PRACTICAL APPLICATIONS
We developed a new KB-HIIT protocol that used Tabata
training principles and was designed to elicit cardiorespiratory
Comparison of KB-HIIT and SIC Responses
3324
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and metabolic responses similar to those induced by SIC. Our
data revealed that KB-HIIT can be used as a form of high-
intensity interval training and also provided further insight
into the sustainability of the KB-HIIT protocol compared
with SIC. This is a new and time-efficient exercise protocol
that can be used safely with less physical strain than SIC and
can produce physiological responses that could improve
cardiorespiratory fitness and metabolic function. This is
a protocol that should be considered by exercise practitioners
(i.e., personal trainers, strength and conditioning specialists) to
incorporate into their training programs for athletes or fitness
clients.
ACKNOWLEDGMENTS
The authors are grateful for the technical assistance and
safety assurance of certified athletic trainers Holly Parker,
Matthew Reynolds, and Elizabeth Verplank, and also the
kettlebell instruction of Casey Scalese.
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