Acute cardiopulmonary and metabolic responses to high intensity interval training

Journal of Strength and Conditioning Research Publish Ahead of PrintDOI: 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: [email protected]

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.


ACCEPTED

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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 ± 6.5 b c d 50.4 59.2

80/50 74.7 ± 8.7 a c d 68.8 80.5

100/0 65.4 ± 9.0 a b d 59.4 71.4

Average

100/50 82.8 ± 6.2 a b c 78.6 86.9

80/0 67.5 ± 6.3 b c d 63.2 71.7

80/50 79.8 ± 8.8 a d 73.9 85.7

100/0 80.9 ± 8.7 a d 75.0 86.7

Peak

100/50 90.2 ± 6.6 a b c 85.7 94.6

80/0 43.6 ± 7.5 b c d 38.5 48.6

80/50 72.6 ± 8.9 a c d 66.5 78.5

100/0 51.7 ± 9.9 a b d 45.0 58.3

Nadir

100/50 78.9 ± 7.3 a 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 ± 6.6 a c 81.7 90.6

100/0 83.2 ± 7.2 b d 78.3 88.1

Average

100/50 90.9 ± 3.8 a c 88.3 93.4

80/0 83.2 ± 5.5 b c d 79.5 87.0

80/50 88.8 ± 5.9 a 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 + 2.8 e 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).


ACCEPTED


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 ± 2.5 a 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

Sports

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