Acute cardiopulmonary and metabolic responses to high intensity interval training

Journal of Strength and Conditioning Research Publish Ahead of Print
DOI: 10.1519/JSC.0000000000001414
Revisions to Manuscript # R371415
TITLE: ACUTE CARDIOPULMONARY AND METABOLIC RESPONSES TO HIGH-
INTENSITY INTERVAL TRAINING (HIIT) PROTOCOLS USING 60S OF WORK AND
60S RECOVERY
Running Head: Acute Responses to Various HIIT protocols
Laboratory: CSULB KIN Exercise Physiology Laboratory
Authors: Ralph Rozenek, James W. Salassi III, Nicole M. Mier, John Fleming
Department: Department of Kinesiology
Institution: California State University Long Beach
Address: 1250 Bellflower Blvd.
Long Beach, California, USA, 90840
Funding: None
Author Contact Information
Corresponding Author: Ralph Rozenek, Ph.D.
Email: rrozenek@csulb.edu
Copyright ª 2016 National Strength and Conditioning Association
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Phone: 1-562-985-4083
Fax: (562) 985-8067
Abstract
Low-volume, high-intensity interval training (HIIT) consisting of 60s work and 60s recovery
(60s/60s) repeated for 10 repetitions has previously been found to produce beneficial
cardiopulmonary, cellular, and metabolic adaptations in healthy and at-risk populations. There
is currently relatively little information pertaining to the acute changes that take place during
individual training sessions. The purpose of this study was to examine the acute physiological
responses to 60s/60s x 10 HIIT protocols using several combinations of work and recovery
intensities. Eleven healthy adults (mean age + s.d. = 26.0 ± 5.3 yrs) performed four HIIT trials on
separate days at varying percentages of peak power output (PPO) that consisted of the
following work/recovery intensities: a) 80% PPO/0% PPO (80/0); b) 80% PPO/50% PPO (80/50);
c) 100% PPO/0% PPO (100/0) and; d) 100% PPO/50% PPO (100/50). 100/50 produced higher (p
< 0.05) peak, average, and nadir %VGO2peak compared to the other protocols. Other than the
nadir values resulting from the 80/0 trial, all trials produced average, peak, and nadir %VGO2peak
and %HRpeak values that were within exercise intensity ranges (≈45-90% VGO2max; ≈65-90% HRmax)
recommended by the American College of Sports Medicine (ACSM) for improvement of
cardiopulmonary function. 80/50 and 100/0 produced similar average HR and peak HR, RPE,
blood lactate and %VGO2peak values. However, the average %VGO2peak was significantly higher
(~9.3% absolute) in 80/50. It appeared that use of the 80/0, 80/50, and 100/0 protocols would
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be appropriate for individuals who are at the low to moderate end of the cardiopulmonary
fitness spectrum.
Keywords: Intermittent Exercise, Oxygen Uptake, Power Output, Blood Lactate, Rating of
Perceived Exertion
INTRODUCTION
Low-volume high-intensity interval training (HIIT) has become an increasingly popular
form of training for both athletic (3,27) and clinical populations (20,22, 26). Individual training
sessions are comprised of scheduled repetitive periods of relatively high intensity work
followed by recovery periods consisting of lower intensity work or rest. Programs can be
tailored to emphasize either anaerobic or aerobic metabolic pathways or a combination of the
two depending upon how the various training variables are manipulated. Variables include the
durations and intensities of the work and recovery intervals, as well as the total number of
intervals to be completed. Two long-known advantages of using interval-type training over
continuous training have been that individuals can work at higher intensities than typically
encountered during continuous training and more total work can potentially be performed
(14,30). Regardless of the type of program, studies have shown HIIT to be effective and can
produce beneficial cardiopulmonary and metabolic adaptations equal to or greater than those
achieved with moderate intensity continuous training with considerably less time involvement
(11,19).
Since there are many ways the duration and intensity of an interval program can be
controlled, it has been a challenge to find an optimized HIIT program (4,27). Further
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complicating matters are how such factors as level of training, health status, age, and gender
influence an individual’s reaction to interval training (4,24,43). Programs incorporating low
volume, short duration work intervals (< 30s duration) performed at intensities described as “all
out” (11,19, 21) or “supramaximal” (28,40) have been found to produce significant central and
peripheral adaptations within a relatively short time period. However these types of programs
may not be suitable for all individuals because of the high training intensities involved (44). As a
consequence, Gibala et al. (20) have suggested using an interval protocol consisting of 60s of
work and 60s of recovery (60s/60s) repeated for 10 repetitions. A rationale for this type of
program has been that by using somewhat longer duration work intervals than those employed
in all-out or supramaximal studies, relative work intensities could be reduced, thus
accommodating HIIT to a wider range of individual capabilities. Several training studies
incorporating this 60s/60s protocol have found significant beneficial cardiopulmonary, cellular,
and metabolic adaptations in healthy individuals (25) as well as in those with coronary artery
disease (13) and type II diabetes (29). From a practical perspective, a 60s/60s program appears
to be time-efficient and well-tolerated even in populations where higher intensity training has
not been traditionally used to a large degree (25,29).
Although the 60s/60s x 10 repetition HIIT program appears to be effective, there is
relatively little information pertaining to the physiological responses during individual training
sessions. Work and recovery intensity combinations that would make the overall effect of a
training session too intense may be ineffective because, in some cases, individuals may not be
able to complete all prescribed intervals which would lead to a reduction in total training
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volume and/or possibly limit participation in such a program (35). On the other hand, there may
be combinations that can be completed but are not intense enough to produce the desired
adaptations because thresholds for training intensity, duration, and/or volume have not been
met. Since short-term responses will ultimately dictate the type and magnitude of training
adaptations, it would be beneficial to identify the acute cardiopulmonary and metabolic
responses to different types of HIIT training. Therefore the purpose of this study was twofold:
1) to characterize the acute responses of a 1:1 HIIT protocol consisting of 60s of work and 60s
of recovery and; 2) to compare the acute responses for several combinations of work and
recovery intensities when using this protocol with apparently healthy individuals. It was
hypothesized that the 60s/60s x 10 interval training protocols incorporating the 100% PPO work
interval intensities would produce the highest observed average, peak, and nadir
cardiopulmonary and metabolic responses.
METHODS
Experimental Approach to the Problem
Previous studies investigating 60s/60s x 10 repetition interval programs have primarily focused
on training effects (13,25,29) with only limited information provided regarding acute responses
to this type of activity (29). Although the studies generally found improvements in
cardiopulmonary and metabolic function, they used various methods to determine work and
recovery intensities for the training sessions. As an example, Hood et al. (25) had their subjects
perform the work intervals at 60% of peak power output (PPO) with recovery intervals
performed at 30 watts. By contrast subjects in the study by Currie et al. (13) performed the
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work intervals at 89% PPO while recovery intervals were performed at 10% PPO. It would be
expected that the subjects would respond differently to the individual training sessions and
thus ultimately produce different training outcomes. From a practical point of view, the
observed inconsistencies in the application of the work and recovery intensities would make
comparison of results among the studies difficult. How individuals may respond to various
combinations of work and recovery intensities is currently poorly characterized.
For individuals who are interested in designing interval-type programs based upon a
60s/60s x 10 repetition format, it would be beneficial to know the time-course of the response
as well as ranges of expected values for typical physiological measurements of interest. This
study attempted to quantify and compare the acute physiological responses to a 60s/60s x 10
repetition format by controlling the work and recovery intensities using four different
combinations of the two variables. The independent variable in this study was the
work/recovery intensity combination used in each trial. Oxygen uptake, heart rate, rating of
perceived exertion, and blood lactate concentrations were selected as the dependent variables
because they are commonly used indicators of cardiopulmonary function and physiological
stress. Work and recovery intensities were quantified as percentages of peak power output
(PPO). The selected work intensities used during the work interval were selected because they
represented intensities within an effort range generally considered moderate to heavy. The
recovery interval intensities were chosen because they span the low to moderate range of
exercise intensities and it is approximately within this range that lactate removal rates tend to
be at their highest (2). By using the selected work and recovery intensities, it was thought that
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this approach could be used to more clearly delineate the acute responses to a 60s/60s x 10
repetition interval training program.
Subjects
Eleven healthy, recreationally active adults (5 male, 6 females) ranging in age from 20 to
36 years (mean age + s.d. = 26.0 ± 5.3 yrs.) participated in the study. None of the subjects had
participated in a regular high-intensity interval training program for at least 3 months prior to
the start of the study. All subjects read and signed informed consent documents prior to
participation and all protocols were approved by a university institutional review board for the
protection of human subjects prior to the initiation of the study. Physical characteristics of all
subjects can be seen in Table 1.
[Insert Table 1 about here]
Procedures
Height, body mass, and body composition were determined using a stadiometer and a
Bod Pod (LMI, Concord, CA, USA) calibrated to manufacturer specifications before testing each
subject. An initial graded exercise test (GXT) was performed on a Lode Corival electronic cycle
ergometer (Lode BV, Groningen, The Netherlands) to determine peak oxygen uptake (VGO2peak),
peak heart rate (HRpeak), peak power output (PPO), and peak blood lactate concentrations [bLa].
The GXT began with an initial 5-minute warm-up period at 25 watts (W). Subjects were
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encouraged to find a self-selected cadence to use throughout the test, typically between 70-90
revolutions x min-1
. The ergometer was capable of maintaining a constant prescribed power
output when subjects pedaled at cadences between 50 and 120 revolutions x min-1
. Following
warm-up, subjects rode at increasing power outputs with increments of 25 W x min-1
. Once 250
W for males and 150 W for females were reached, power output was then incremented by 10
W x min-1
until test termination. Criteria for test termination included: a plateau in VGO2, a drop
in cadence below 50 rev x min-1
, signs and symptoms of exercise intolerance, or the subject
decided to stop. The highest VGO2 value obtained meeting the test criteria was termed VGO2peak.
A 5-minute active cool-down at 25 W was used during the recovery period. No signs or
symptoms of exercise intolerance were observed during or following testing. Heart rate (HR)
was monitored continuously with a Polar (Polar Electro, Oy, Finland) transmitter belt worn
around the chest that was capable of sending data to a metabolic measurement system
(Vacumed, Ventura, CA) used to measure gas exchange and ventilation for VGO2 determination.
The highest HR value obtained meeting the test criteria was termed HRpeak. The standard error
of measurement for repeated determination of HR expressed as a percentage (%SEM) was
4.1%. The gas analyzers were calibrated prior to every test using known gas concentrations
(16% O2/4% CO2) and ambient air. A 3-liter syringe was used to calibrate the turbine flow
meter. The %SEM for repeated VGO2 measurement s was 3.3%. Blood lactate concentrations
were measured using a finger stick method (41). Samples were obtained at rest and
immediately following the GXT. A Lactate Plus portable lactate analyzer (Nova Biomedical,
Waltham, MA, USA) calibrated using low and high controls prior to each GXT and interval trial
was used for the measurements. The %SEM for blood lactate concentration measurements
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was 4.0%. Ratings of perceived exertion (RPE) were measured each minute during the GXT
using the Borg 6-20 scale (8). The PPO obtained at VGO2peak was used to determine the relative
work and recovery intensities implemented in subsequent HIIT trials.
Four HIIT trials were conducted at varying percentages of PPO and consisted of the
following work/recovery intensities: a) 80% PPO/0% PPO (80/0); b) 80% PPO/50% PPO (80/50);
c) 100% PPO/0% PPO (100/0) and; d) 100% PPO/50% PPO (100/50). Each HIIT trial used a 1:1
work to recovery ratio and consisted of alternating 60s periods of work and recovery. This
work/recovery cycle was repeated 10 times for a total duration of 20 minutes. Each 20-minute
trial was preceded by a 5-minute warm-up period performed at 25 W and followed by a 5-
minute recovery period at 25 W. During the 0% PPO phase of the 80/0 and 100/0 trials,
resistance on the ergometer was reduced to 0W on the display (7W actual) while subjects
actively maintained the pedal cadence from the previous work interval. Oxygen uptake and HR
were recorded every 20s using the previously described methods. Blood lactate concentrations
were measured at rest just prior to the start of warm-up period, at the end of the fifth work
interval, and immediately following the last work interval. Ratings of perceived exertion were
recorded at the end of each minute of work and recovery. During all phases of testing, subjects
were requested to maintain their normal diets and hydration status. In addition subjects were
to refrain from consuming caffeinated beverages and/or exercising at least four hours prior to
the testing sessions. The four trials were performed in random order on different days at
approximately the same time of day with each trial separated by at least 48 hours.
Statistical Analyses
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An a priori calculation of sample size using Minitab version 15 (Minitab, Inc. State
College, PA) determined that a minimum of 10 subjects was sufficient to detect an absolute
difference of 10.0% between trials with 80% power, α = 0.05, and an assumed standard
deviation of 5.0% on the primary outcome measure of %VGO2peak. Collected data were initially
analyzed using a repeated measures analysis of variance with trial and sex used as between
subject factors and time as a within subject factor. When significance was observed, a single
factor repeated-measures ANOVA was used for further analyses with a Tukey post-hoc follow-
up. A Bonferroni correction for multiple comparisons was used where appropriate. Effect sizes
for the main effect of trial were reported as eta2
(η2
) with values of 0.01, 0.06, and 0.14
representing “small,” “medium, “ and “large” effects respectively, as suggested by Cohen (12).
The Statistical Package for Social Sciences version 20 (SPSS Statistics, SPSS, Chicago, IL) was
used for all analyses. Statistical significance was set at (p ≤ 0.05).
RESULTS
As expected males, on average, were taller, weighed more, and had lower body fat
percentages than females (p < 0.05). Additionally the males produced higher values for VGO2peak
and PPO than the females during the GXT (p < 0.05). However, no significant differences in the
response to the intervals were observed between males and females, nor were any significant
interactions (trial x time x sex) noted for any of the variables investigated (p > 0.05); hence data
were pooled for the remaining analyses. It should be noted that only 7 of the 11 subjects
completed all test trials. Four of the subjects were not able to complete the 100/50 trial. One
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subject stopped as early as the fifth work interval, whereas the others completed at least 7
work intervals before stopping exercise.
The response of VGO2 to the various interval trials is shown in Figure 1. Average, peak,
and nadir, VGO2 values can be found in Table 2. A single-factor repeated-measures ANOVA
showed a main effect of trial on the averaged %VGO2peak across intervals (F(3,30) = 98.39, p <
0.001, η2
= 0.669). Post-hoc analyses indicated that all four trials were different (p<0.05) from
one another. The 100/50 trial produced the highest average %VGO2peak with a value of 82.8 +
6.2%, while the 80/0 trial produced the lowest value, 54.8 + 2.0%. A main effect of trial was
observed for the peak %VGO2peak (F(3,30) = 38.23, p < 0.001, η2
= 0.547). The peak value for
%VGO2peak, was highest while performing the 100/50 trial reaching 90.2 + 8.6% and was different
(p < 0.05) from the other trials. The 80/0 trial elicited the lowest peak %VGO2peak value (67.5 +
6.3%) and was lower (p < 0.05) than all other trials. The 80/50 and 100/0 trials produced similar
peak %VGO2peak values, 79.8 + 8.8% and 80.9 + 8.7% respectively (p > 0.05). However, both trials
were different (p < 0.05) from either the 100/50 or 80/0 trials. In all trials the average and peak
%VGO2peak values increased as the number of intervals increased up to completion of the fifth
interval. With the exception of the 80/50 trial (p = 0.044), %VGO2peak did not increase beyond the
fifth interval (p > 0.05).
A main effect of trial for the nadir %VGO2peak values was also observed, F(3,30) = 195.60, p
< 0.001), η2
= 0.763. The 100/50 trial produced the highest nadir value (%VGO2peak = 78.9 + 7.3%)
while the lowest value resulted from the 80/0 trial (%VGO2peak = 43.6 + 7.5%). The 80/50 and
100/0 trials produced nadir %VGO2peak values of 72.6 + 8.9% and 51.7 + 9.9% respectively. Post-
hoc analyses revealed that values for all trials were different from each other (p < 0.05).
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With regard to the nadir %VGO2peak values, no differences (p > 0.05) were found from the
first through the last interval in the 80/0 trial. Nadir %VGO2peak values were higher (p < 0.05)
following the 5th interval compared to the first interval in the 80/50, 100/0, and 100/50 trials
with no differences (p > 0.05) found beyond that point.
[Insert Figure 1 about here]
Heart rate responses expressed as %HRpeak can be seen in Figure 2. A main effect of trial
on the average heart rate response (average %HRpeak) was observed (F(3,30) = 263.31, p <
0.001, η2
= 0.477). The 80/0 trial produced an average %HRpeak of 74.9 + 7.1% and was lower
(p<0.05) than the other trials. By contrast, the 100/50 trial produced the highest average
%HRpeak with a mean value of 90.9 + 3.8% which was higher (p < 0.05) than the others. There
was no difference (p > 0.05) between the average %HRpeak values obtained from 80/50 and
100/0 trials (Table 3).
The main effect of trial on Peak %HRpeak responses (F(3,30) = 32.88, p < 0.001, η2
=
0.403) showed a similar pattern to the average %HRpeak. The peak %HRpeak for the 80/0 and
100/50 trials were different (p < 0.05) than either the 80/50 or 100/0 trials with values of 83.2 +
5.5% and 94.6 + 3.0% respectively. The %HRpeak values of 88.8 + 5.9% for the 80/50 and 90.0 +
5.5% for the 100/0 trial were not different (p > 0.05) from one another. The main effect of trial
also influenced the nadir % HRpeak values (F(3,30) = 72.78, p < 0.001, η2
= 0. 518). However, a
somewhat different pattern of responses was observed with the average nadir HR responses.
The nadir %HRpeak values of 67.8 + 10.2% for the 80/0 trial and 77.5 + 9.9% for the 100/0 trial
were different (p < 0.05) than all other trials. The highest nadir %HRpeak value of 89.8 + 5.2 was
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obtained during the 100/50 trial and was different than (p = 0.054) the nadir value of 85.3 +
7.2% obtained during the 80/50 trial.
The number of intervals completed within a trial produced significant differences among
the first, fifth, and tenth intervals (p < 0.05). As the number of intervals completed progressed,
steady increases in the peak %HRpeak values were observed. By the end of the last interval in the
100/50 trials, peak HR values of 100.2 + 3.0% HRpeak were achieved. Nadir %HRpeak values also
increased (p < 0.05) from the first through the tenth interval within each of the trials. However
in the cases of the 100/0 and 100/50 trials no differences (p > 0.05) were observed for nadir
%HRpeak between the fifth and tenth interval.
No difference in [bLA] was observed among the trials under resting conditions (F(4,50) =
0.598, p = 0. 666, η2
= 0.046) (Figure 3, Table 4). However [bLA] rose from rest to the end of the
fifth work interval during each of the trials (p < 0.05). Significant differences were observed
among groups at the end of the fifth (F(3,40) = 10.20, p = 0.001, η2
= 0.433) and also at the end
of the final work interval (F(4,47) = 8.41, p < 0.001, η2
= 0.417). By the end of the fifth work
interval, mean [bLA] during the 100/50 trial was higher (10.5 + 3.0 mmol x l-1
) (p < 0.05) than
either the 80/0 (4.9 + 1.9 mmol x l-1
) or the 80/50 (6.8 + 2.6 mmol x l-1
) trial, but not considered
different (p > 0.05) from the 100/0 trial (8.5 + 2.3 mmol x l-1
). Blood lactate concentrations at
the end of the last work interval for the 80/0 trial were lower (5.0 + 2.4 mmol x l-1
)(p < 0.05)
than the others (7.4 + 2.8 mmol x l-1
, 9.6 + 4.0 mmol x l-1
, and 10.2 + 2.7 mmol x l-1
for 80/50,
100/0, and 100/50 respectively) with no differences (p > 0.05) observed among the other trials.
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Additionally [bLa] for the 100/0 and 100/50 trials at the end of the final interval were not
different (p > 0.05) than the peak values obtained from the GXT (Table 1). No differences (p >
0.05) in [bLa] were noted between the end of fifth and final intervals within any of the trials.
A main effect of trial on the reported average RPE was observed (F(3,30) = 24.79, p =
0.001, η2
= 0.448) (Figure 4, Table 5). The average RPE for 80/0 trial (10.5 + 1.3) was lower (p <
0.05) than all other trials. By contrast average RPE for 100/50 trial (14.3 + 1.8) was higher (p <
0.05) than the 100/0 trial (12.5 + 1.7) but not the 80/50 trial (13.4 + 1.7) (p > 0.05). Differences
in the peak RPE responses were also found (F(3,30) = 21.70, p = 0.001, η2
= 0.451). When
averaged across an entire 20 minute interval session, peak RPE was lower (p < 0.05) in the 80/0
trial (12.6 + 1.1) compared to all others. The 100/50 trial (15.7 + 1.2) produced an average peak
RPE response that was higher (p < 0.05) than all other trials. Similar peak RPE was observed in
the 80/50 (14.3 + 1.5) and 100/0 (14.9 + 1.4) trials (p > 0.05).
Differences were observed among the trials in the average nadir RPE response ((F(3,30)
= 21.23, p = 0.001, η2
= 0.413). The 80/0 trial produced the lowest average nadir RPE with a
value of 8.4 + 1.9. This was lower (p < 0.05) than either the 80/50 (12.4 + 2.2) or 100/50 (12.8 +
2.5) trial but not the 100/0 trial (10.0 + 2.3) (p > 0.05). Within each trial, Peak RPE increased
significantly (p < 0.05) from the first to the fifth interval. However no significant differences
were observed from the fifth interval onward in any of the trials. With the exception of the
80/50 trial where nadir RPE values increased (p < 0.05) from the first to the tenth interval, no
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significant changes (p > 0.05) occurred as a result of increasing interval number in any of the
other trials.
DISCUSSION
Results of this study characterize the acute effects of varying either the work or
recovery intensity during a 60s/60s interval program when intensity was based upon a
percentage of PPO. Manipulating either the work or recovery intensity affected the overall
response to an individual exercise session as exhibited by the significant differences of the
average %VGO2peak response among the four interval trials. Other than the nadir values resulting
from the 80/0 trial, all trials produced average, peak, and nadir %VGO2peak and %HRpeak values
that were within exercise intensity ranges (≈45-90% VGO2max; ≈65-90% HRmax) currently
recommended by the American College of Sports Medicine (ACSM) for the improvement of
cardiopulmonary function (18).
Altering the intensity of the recovery interval had a greater effect on the average
%VGO2peak than changing the intensity of the work interval. For example, increasing recovery
intensity from 0% PPO to 50% PPO resulted in a 19.8 + 4.7% absolute increase in the average
%VGO2peak when performing at 80% PPO and a 17.4 + 7.0% increase during the 100% PPO trial. By
contrast, increasing work interval intensity from 80% PPO to 100% PPO increased the average
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%VGO2peak by an absolute amount of 10.6 + 6.5% when 0% PPO was used during recovery and
only 8.1 + 5.0% when 50% PPO was used. Although the work interval intensity was lower, the
80/50 trial produced a significantly higher average %VGO2peak when compared to the 100/0 trial,
whereas peak values were very similar. The addition of an active recovery period resulted in a
smaller difference between the peak and nadir responses thus reducing amplitude, defined as
the ratio of the difference between the work and recovery intensities divided by the average
intensity (5), yielding higher overall average VGO2 and HR responses. From a metabolic
standpoint, increasing the intensity of work during the recovery period would reduce the rate
of phosphocreatine resynthesis (38,45), as well as alter blood lactate removal rates (31), the
recharging of myoglobin stores (16), and the restoration of pH (15). These changes could
potentially contribute to alterations to the oxygen uptake kinetics of subsequent work intervals
(32) and thus affect the overall exercise response.
Subjects had relatively little difficulty completing the 80/0 trial as demonstrated by the
relatively low peak and average RPE responses as well as [bLa] which remained at
approximately 5.0 mmol x l-1
from the end of the fifth work interval until completion of the
session. On the other hand, The 100/50 trial was the only one where all ten of the prescribed
intervals were not completed by all subjects. Only seven of the original eleven subjects were
able to complete the 100/50 trial indicating that this trial was likely too intense for some
subjects. Blood lactate concentrations were quite high, averaging above 10 mmol x l-1
from the
end of the fifth work interval onward and were not significantly different than those observed
at the end of the initial GXT. The leveling off of [bLa] during intermittent exercise has been
reported previously (36,39) and thought to be a result of better balance between lactate
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production and utilization as activity proceeds. The 100/0 trial resulted in [bLa] that was only
slightly lower than the 100/50 trial, yet all subjects were able to complete it. This would suggest
that elevated [bLa] may not have been responsible for fatigue in those who could not complete
the 100/50 trial. In a previous study, Brickley et al. (9) found a significant correlation (r = 0.779;
p < 0.05) between VGO2max and time to exhaustion at critical power. However in the current
study it did not appear that a subject’s level of cardiopulmonary fitness, as determined by
VGO2peak, had an effect on the number of intervals completed (r = 0.256; p > 0.05). Other factors
potentially contributing to the onset of fatigue may have included those associated with central
nervous system and/or metabolic function (1,17). The results suggest that using a recovery
intensity of 50% PPO in combination with a work interval intensity of 100% PPO may be
approaching the upper limits of what may be achievable in order to complete a 60s/60s x 10
repetition protocol for individuals who possess moderate levels of cardiopulmonary fitness.
Contrary to the original hypothesis, the interval training sessions using the highest work
intensities did not necessarily produce the largest average, peak, or nadir responses in the
measured variables. In two of the trials, 80/50 and 100/0, average and Peak HR, RPE, [bLa], and
peak %VGO2peak were all similar. However, the average %VGO2peak were significantly higher (~9.3%
absolute) in the 80/50 trial. Although there were similarities in the two trials, it is possible that
the observed subtle differences in responses to the 80/50 and 100/0 trials could ultimately
produce different long-term adaptations. Both interval trials resulted in increased contribution
of aerobic and anaerobic metabolism to overall energy demand during the activity. A potential
benefit of utilizing the 80/50 or 100/0 formats as part of a regular training program may be that
they would allow an individual to work at a relatively high percentage of VGO2peak while also
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maintaining high [bLa] and completing the prescribed number of intervals. Repeated exposure
to high lactate concentrations may stimulate adaptations leading to enhanced lactate removal
(10), improved buffering capacity (19,42) and increased mitochondrial enzyme content (23)
that could potentially improve an individual’s ability to tolerate high intensity exercise.
Ideally when planning an interval training session, it should be designed in such a way as
to meet a threshold for training intensity and allow for completion of the session so that
sufficient duration and volume has also been attained (35). It has been stated (34) that highly
trained individuals may need to exercise at intensities as high as 95-100% VGO2max to improve
VGO2max, while for those who are moderately trained, exercise intensities in the range of
approximately 70-80% VGO2max may be sufficient to produce aerobic adaptations. Furthermore,
it has been suggested that the time spent training at PPO or the velocity associated with VGO2max
may be an effective way to elicit improvements in performance (28,37). Billat et al. (6,7) have
attempted to develop interval programs that allow individuals to operate at relatively high
percentages of VGO2max for longer amounts of time when compared to endurance activity of
similar intensity. In a relatively short duration interval protocol such as the one used in this
study, only the 100/50 trial produced values that at their peak were just above 90% VGO2peak. The
actual amount of time spent at this intensity would be rather low due to the fluctuations in VGO2
resulting from repeated transitions from exercise to recovery. If the amount of training time
above 90% is important for athletic performance (33), then the 60s/60s protocol used in this
study may not be optimal since the total training time was only 20 minutes and the highest
average %VGO2peak and peak %VGO2peak observed during the 100/50 trial were 82.8 + 6.2% and
90.2 + 6.6% respectively. Although not measured in this study, it may be possible that the
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individual average VGO2 responses were above lactate threshold or critical power and it is this
effect repeated over a number of training sessions that eventually leads to adaptations rather
than the short periods of time spent at PPO. Since interval exercise is sinusoidal in nature with
the amplitude and frequency of the intervals being affected by the intensity and duration of the
work and recovery intervals as well as the total number of intervals completed, attention
should be given to both peak responses and the average response when considering long-term
adaptations. Previous training studies using the 60s/60s protocol (13,25,29) have used work
intensities ranging from 60% PPO up to 95% PPO. In the studies by Currie et al (13) and Little et
al. (29) work rates during the intervals were individualized in order to obtain heart rates of
approximately 73 + 10% of age-predicted maximum and 90% of maximum respectively.
Recovery intensities in the various studies were at 10% PPO (13), 30W (25), or a choice
between rest and 50W (29). Inconsistencies in the reporting of absolute and relative work rates
or exercise intensity derived from either predicted or measured heart rates along with
differences in recovery intensities have made it difficult to compare responses to these interval
protocols. Despite the differences, beneficial adaptations have been observed and
demonstrate the utility of low volume HIIT training. It has been suggested to use PPO or
vVGO2max as a standard to follow when designing interval programs (5,28). The current study
used this approach to more clearly delineate the acute responses to a 60s/60s interval program
when different protocols are used.
PRACTICAL APPLICATIONS
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Results from the present study quantify the effects of altering either the intensity of the
work or the recovery interval when performing interval sessions consisting of 60s of work and
60s of recovery for multiple repetitions. The information provided may aid those interested in
designing interval training sessions by providing ranges of values that could be expected for
individuals who possess moderate levels of cardiopulmonary fitness. Using a work intensity of
80% or 100% VGO2peak and a recovery intensity of 0% or 50% VGO2peak, subjects were able to
exercise within the ACSM recommended range for exercise intensity. Based upon the data it
would appear that a protocol such as the 80/0 may be appropriate for those individuals who
are just beginning a program or have little experience with interval-type activity. By contrast, a
100/50 protocol could not be completed by all of the subjects and therefore may be too intense
for some individuals. Although the 100/0 protocol produced high peak VGO2 values and [bLa],
the average VGO2 response was relatively low. On the other hand, a protocol such as 80/50 may
allow an individual to sustain a relatively high average %VGO2peak and [bLa] and based upon the
average and peak RPE responses most likely be able to complete the training session. From the
range of values obtained for VGO2, HR, RPE and [bLa] obtained for the 80/0, 80/50 and 100/0
protocols, it appeared that the use of these protocols would be appropriate for individuals who
are at the low to moderate end of the cardiopulmonary fitness spectrum.
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Acknowledgements
The authors wish to thank the dedicated group of participants who volunteered their time and
effort to allow this research to be made possible. The results of this present study do not
constitute endorsement of any product by the authors or the NSCA. The authors report no
conflict of interest with their research.
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Figure Legends
Figure 1: Time course for changes in VGO2 expressed as %VGO2peak for each of the four intervals
trials. * indicates a difference (p < 0.05) from the first interval. ♦
indicates a difference (p < 0.05)
from the fifth interval. (p < 0.05). Between-trial differences in average, peak, and nadir %VGO2peak
and 95% confidence intervals can be found in Table 2. Dotted horizontal lines represent ACSM
recommended range for exercise intensity. Data points are expressed as a mean + standard
deviation. File format: PPT.
Figure 2: Time course for changes in HR expressed as %HRpeak for each of the four intervals
trials. * indicates a difference (p < 0.05) from the first interval. ♦
indicates a difference (p < 0.05)
from the fifth interval. (p < 0.05). Between-trial differences in average, peak, and nadir %HRpeak
and 95% confidence intervals can be found in Table 3. Dotted horizontal lines represent ACSM
recommended range for exercise intensity. Data points are expressed as a mean + standard
deviation. File format: PPT.
Figure 3: Time course for changes in blood lactate concentrations [bLa] for each of the four
interval trials. * indicates a difference (p < 0.05) from rest. Between-trial [bLa] differences at
rest, at the end of the 5th
and 10th
work interval and 95% confidence intervals can be found in
Table 4. Bars are expressed as a mean + standard deviation. File format: PPT.
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Figure 4: Time course for changes in RPE for each of the four intervals trials. * indicates a
difference (p < 0.05) from the first interval. Between-trial differences in average, peak, and
nadir RPE and 95% confidence intervals can be found in Table 5. Dotted horizontal lines
represent ACSM recommended range for exercise intensity. Data points are expressed as a
mean + standard deviation. Data points are expressed as a mean + standard deviation. File
format: PPT.
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Tables
Table 1: Physical Characteristics of Subjects. Values are expressed as a mean + the standard deviation.
Subjects
(n)
Age
(yr)
Height
(cm)
Mass
(kg)
Body Fat
(%)
VGO2peak
(ml x min-1
x kg-1
)
HRpeak
(b x min-1
)
PPO
(W)
Pre-GXT
[bLa]
(mmol x l-1
)
Post-GXT
[bLa]
(mmol x l-1
)
5 Males 28.0 ± 5.8 178.0 ± 7.3* 75.6 ± 7.6* 13.6 ± 4.8* 51.8 ± 10.0 181.6 ± 6.9 278.0 ± 13.0* 1.4 ± 0.9 13.4 ± 3.8
6 Females 24.3 ± 4.6 164.7 ± 6.2 59.0 ± 6.7 24.5 ± 5.0 45.8 ± 8.7 182.5 ± 6.2 200.0 ± 34.6 1.2 ± 0.4 10.3 ± 2.2
11 Total 26.0 ± 5.3 170.7 ± 9.4 66.5 ± 11.0 19.5 ± 7.3 48.5 ± 9.4 182.1 ± 6.2 235.5 ± 48.2 1.3 ± 0.6 11.7 ± 3.1
* = males different than females (p < 0.05).
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Acute Responses to Various HIIT protocols 2
Table 2: Oxygen uptake expressed as a percentage of VGO2peak (%VGO2peak) obtained from the GXT.
Values represent averages across all intervals within a trial expressed as a mean + the standard
deviation.
95% Confidence Intervals
Trial %VGO2peak Lower Upper
80/0 54.8 ± 6.5b c d
50.4 59.2
80/50 74.7 ± 8.7a c d
68.8 80.5
100/0 65.4 ± 9.0a b d
59.4 71.4
Average
100/50 82.8 ± 6.2a b c
78.6 86.9
80/0 67.5 ± 6.3b c d
63.2 71.7
80/50 79.8 ± 8.8a d
73.9 85.7
100/0 80.9 ± 8.7a d
75.0 86.7
Peak
100/50 90.2 ± 6.6a b c
85.7 94.6
80/0 43.6 ± 7.5 b c d
38.5 48.6
80/50 72.6 ± 8.9a c d
66.5 78.5
100/0 51.7 ± 9.9a b d
45.0 58.3
Nadir
100/50 78.9 ± 7.3a b c
73.9 83.8
a = Trial was different from 80/0 (p < 0.05).
b = Trial was different from 80/50 (p < 0.05).
c = Trial was different from 100/0 (p < 0.05).
d = Trial was different from 100/50 (p < 0.05).
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Table 3: Heart rate expressed as a percentage of HRpeak (%HRpeak) obtained from the GXT.
Values represent averages across all intervals within a trial expressed as a mean + the standard
deviation.
Trial %HRpeak Lower Upper
80/0 74.9 ± 7.1 b c d
70.2 79.7
80/50 86.1 ± 6.6a c
81.7 90.6
100/0 83.2 ± 7.2 b d
78.3 88.1
Average
100/50 90.9 ± 3.8a c
88.3 93.4
80/0 83.2 ± 5.5 b c d
79.5 87.0
80/50 88.8 ± 5.9a d
84.9 92.8
100/0 90.0 ± 5.5 a d
86.3 93.7
Peak
100/50 94.6 ± 3.0 a b c
92.5 96.6
80/0 67.8 ± 10.2 b c d
60.9 74.7
80/50 85.3 ± 7.3 a c
80.4 90.3
100/0 77.5 ± 9.9 a b d
70.8 84.2
Nadir
100/50 89.8 ± 5.2 a c
86.3 93.3
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Table 4: Blood lactate concentration [bLa] obtained at rest at the end of the fifth and tenth
work intervals. Values expressed as a mean ± the standard deviation.
Trial [bLa] Lower Upper
GXT 1.3 + 0.8 0.7 1.8
80/0 1.4 + 0.7 0.9 1.9
80/50 1.5 + 0.5 1.2 1.8
100/0 1.2 + 0.5 0.8 1.5
Rest
100/50 1.2 + 0.3 1.0 1.4
80/0 4.9 + 1.9 c d
3.6 6.2
80/50 6.8 + 2.6 d
5.0 8.6
100/0 8.5 + 2.3 a
6.9 10.0
End of 5th
Work Interval
100/50 10.5 + 3.0 a b
8.5 12.5
GXT 11.8 + 3.1 a b
9.7 13.9
80/0 5.0 + 2.4 c d e
3.4 6.6
80/50 7.4 + 2.8e
5.5 9.3
100/0 9.6 + 3.6 a
7.2 12.0
End of 10th
Work Interval
100/50 10.2 + 2.7 a
7.9 12.5
e = Trial was different from GXT (p < 0.05).
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Table 5: RPE obtained from all trials. Values represent averages across all intervals within a trial
expressed as a mean + the standard deviation.
Trial RPE Lower Upper
80/0 10.5 ± 1.3 b c d
9.6 11.4
80/50 13.4 ± 1.7 a
12.3 14.6
100/0 12.5 ± 1.7 a d
11.3 13.6
Average
100/50 14.3 ± 1.8 a c
13.1 15.5
80/0 12.6 ± 1.1 b c d
11.8 13.4
80/50 14.3 ± 1.5 a d
13.4 15.3
100/0 14.9 ± 1.4 a d
13.9 15.8
Peak
100/50 15.7 ± 1.2 a b c
14.8 16.4
80/0 8.4 ± 1.9 b d
7.0 9.7
80/50 12.4 ± 2.2 a c
11.0 13.9
100/0 10.0 ± 2.3 b d
8.5 11.6
Nadir
100/50 12.8 ± 2.5a c
11.1 14.5
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Trial
1 2 3 4 5 6 7 8 9 10
*
¨
*
* **
* *
*
¨*
*
*
*
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Trial
1 2 3 4 5 6 7 8 9 10
¨
*
*
*
*
**
*
*
*
*
*
¨
¨
¨
¨
¨
¨
¨
*
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*
**
*
*
*
*
*
*
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Trial
1 2 3 4 5 6 7 8 9 10
*
*
*
*
*
*
*
*
*
*
*
*
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Acute cardiopulmonary and metabolic responses to high intensity interval training

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Acute cardiopulmonary and metabolic responses to high intensity interval training