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Application of the Speed-Duration Relationship to
Normalize the Intensity of High-Intensity Interval
Training
Carrie Ferguson
1,2
*, John Wilson
2
, Karen M. Birch
1
, Ole J. Kemi
2
1School of Biomedical Sciences, Faculty of Biological Sciences, University of Leeds, Leeds, United Kingdom, 2Institute of Cardiovascular and Medical Sciences, College of
Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, United Kingdom
Abstract
The tolerable duration of continuous high-intensity exercise is determined by the hyperbolic Speed-tolerable duration (S-
t
LIM
) relationship. However, application of the S-t
LIM
relationship to normalize the intensity of High-Intensity Interval Training
(HIIT) has yet to be considered, with this the aim of present study. Subjects completed a ramp-incremental test, and series of
4 constant-speed tests to determine the S-t
LIM
relationship. A sub-group of subjects (n = 8) then repeated 4 min bouts of
exercise at the speeds predicted to induce intolerance at 4 min (WR
4
), 6 min (WR
6
) and 8 min (WR
8
), interspersed with bouts
of 4 min recovery, to the point of exercise intolerance (fixed WR HIIT) on different days, with the aim of establishing the
work rate that could be sustained for 960 s (i.e. 464 min). A sub-group of subjects (n = 6) also completed 4 bouts of exercise
interspersed with 4 min recovery, with each bout continued to the point of exercise intolerance (maximal HIIT) to determine
the appropriate protocol for maximizing the amount of high-intensity work that can be completed during 464 min HIIT. For
fixed WR HIIT t
LIM
of HIIT sessions was 399681 s for WR
4
, 8926181 s for WR
6
and 15176346 s for WR
8
, with total exercise
durations all significantly different from each other (P,0.050). For maximal HIIT, there was no difference in t
LIM
of each of
the 4 bouts (Bout 1: 229627 s; Bout 2: 262637 s; Bout 3: 235649 s; Bout 4: 235653 s; P.0.050). However, there was
significantly less high-intensity work completed during bouts 2 (153.5640. 9 m), 3 (136.9638.9 m), and 4 (136.7639.3 m),
compared with bout 1 (264.9658.7 m; P.0.050). These data establish that WR
6
provides the appropriate work rate to
normalize the intensity of HIIT between subjects. Maximal HIIT provides a protocol which allows the relative contribution of
the work rate profile to physiological adaptations to be considered during alternative intensity-matched HIIT protocols.
Citation: Ferguson C, Wilson J, Birch KM, Kemi OJ (2013) Application of the Speed-Duration Relationship to Normalize the Intensity of High-Intensity Interval
Training. PLoS ONE 8(11): e76420. doi:10.1371/journal.pone.0076420
Editor: Franc¸ois Hug, The University of Queensland, Australia
Received March 22, 2013; Accepted August 27, 2013; Published November 14, 2013
Copyright: ß2013 Ferguson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The authors have no support or funding to report.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: C.Ferguson@leeds.ac.uk
Introduction
In classic epidemiological data it is well established that there
are significant health benefits associated with leading a physically
active lifestyle (e.g. [1,2,3]). This assertion is further strengthened
by the demonstration that training interventions can increase the
maximal rate of pulmonary oxygen uptake ( _
VVO2
max
) (a primary
measure of physical fitness/exercise capacity and performance,
and a strong predictor of all-cause mortality [4,5]) (e.g. [6,7]); and
improve both metabolic and cardiovascular function when
integrated as part of a lifestyle intervention or rehabilitation
program (e.g. [8,9,10,11]). Hence, exercise training has the
capacity to improve both performance/exercise tolerance and
reduce risk factors for both metabolic and cardiovascular disease.
Therefore, given the implications of training for improving
exercise performance, and in the prevention/rehabilitation of
chronic disease, establishing optimal training strategies – not only
to maximize training adaptations and associated health-related
benefits, but also to improve participation and adherence in the
general population – is of critical importance.
Key in this regard is the intensity of the exercise. It has been
suggested that improvements in physiological functioning resulting
from exercise training exist on a continuum [12,13,14], such that
continuous higher-intensity exercise leads to greater benefits than
that of a moderate-intensity [6,15,16,17]. However, accumulation
of high volumes of continuous, progressively higher intensity
exercise is limited by the mechanisms that result in rapid exercise
intolerance – i.e. tolerable duration is intensity dependent [18,19].
This has led to significant interest in High-Intensity Interval
Training (HIIT). Repeated short-duration (i.e. ,30 s) all-out
Wingate-style HIIT; i.e. Sprint Interval Training (SIT) is popular,
and has been demonstrated to effectively improve endurance
capacity and time-trial performance [20,21,22,23], muscle oxida-
tive enzyme activity [20,21,22,23] and aerobic capacity ( _
VVO2
max
)
([21,24]), as well as specific health-related parameters such as
insulin sensitivity [24,25], blood pressure [24] and vascular
function [26] in a time-efficient manner (compared with current
moderate-intensity physical activity guidelines; i.e. 150 min/week;
[27]).
Despite the significant evidence demonstrating benefits in both
health and performance related parameters with short-duration
(i.e. ,30 s) SIT, there is evidence to suggest there may be similar,
or even greater benefits attained from lowering the absolute work
rate, prolonging the duration of the high-intensity interval (i.e.
PLOS ONE | www.plosone.org 1 November 2013 | Volume 8 | Issue 11 | e76420
,4 min) and performing this as either an all-out sprint (matched
for total work with a SIT session; [28]), or at a constant WR, in
both health and disease (e.g. [10,11,12,29,30]). However, when
high-intensity constant-load exercise bouts are extended beyond
,2 min, exercise tolerance is shaped by the hyperbolic Power-
tolerable duration (P-t
LIM
) relationship (analogous to the Speed-
tolerable duration (S-t
LIM
) relationship in treadmill exercise)
[18,31]. The P-t
LIM
relationship is therefore of critical significance
when trying to identify the correct WR for an HIIT protocol in
which the exercise bouts are prolonged.
In the P-t
LIM
model, once a critical threshold (i.e. the critical
power (CP) or critical speed (CS)) is exceeded – with this the
asymptote of the P-t
LIM
relationship which represents the upper
limit for which a steady-state in _
VVO2, arterial blood acid-base
status and intramuscular phosphocreatine and inorganic phos-
phate can be attained [18,32] – tolerable duration is predictably
determined by the rate at which a fixed quantity of work above the
CP asymptote is performed. This fixed quantity of supra-CP work
is termed W9(cycle ergometry) or D9(treadmill exercise), with this
hypothesized to reflect either a fixed energy store associated with
O
2
deficit-related mechanisms (i.e. muscle phosphocreatine, stored
O
2
, glycolysis/glycogenolysis) or the accumulation of related
fatigue metabolites (e.g. intramuscular inorganic phosphate and
H
+
, interstitial K
+
) to a fixed critical limit [18,32,33]. As the
asymptote (CP) of the hyperbolic P-t
LIM
relationship does not
change with prior exercise [34,35], subsequent high-intensity
(supra-CP) exercise tolerance is therefore determined by the
balance between the extent of W9depletion in the preceding bout
and subsequent W9repletion during the intervening recovery
period [34,35,36].
Despite this, there has been little consideration of the P-t
LIM
relationship when determining the ‘intensity’ (or more correctly,
the work rate) for HIIT that is comprised of exercise bouts longer
than ,2 min, with studies typically defining the work rate used
based on % HR
max
(,95% HR
max
; [10,11,12,30]) or % _
VVO2
max
(,90% _
VVO2
max
; [29,37,38]). However, as CP does not occur at a
fixed % of HR
max
or _
VVO2
max
[19] and W9does not represent the
same volume of supra-CP exercise in all individuals (e.g. [39])
these approaches are sub-optimal. The consequence is that the
metabolic stress and thus the exercise intensity experienced during
the HIIT program will be variable between participants unless the
P-t
LIM
is accounted for. However, given the proposed relationship
between intensity and both health- and performance-related
fitness benefits [12,13,14], and the potential for the duration of
the high-intensity exercise bout to have an impact on the training
adaptations [28], the P-t
LIM
relationship should be taken into
account when normalizing the intensity of HIIT to appropriately
investigate these assertions.
HIIT protocols comprising 464 min bouts are commonly used
in both health and disease as a viable, more effective training
protocol than traditional moderate-intensity interventions (e.g.
[10,11,12,29,30]). Hence, the purpose of this investigation was to
determine the appropriate constant-WR for a 464 min HIIT that
would allow for the completion of the desired 464 min bouts,
normalizing the intensity of HIIT between individuals, and then
consider how the P-t
LIM
relationship can be applied to maximize
the volume of high-intensity work that can be completed in
464 min bouts in a HIIT program, thus making longer duration
HIIT analogous to SIT (i.e. all-out effort in each bout) and
providing a method to consider the relative importance of the
work rate profile during intensity-matched training to the
physiological adaptations. We hypothesized that for constant-
WR HIIT the P-t
LIM
relationship can be used to identify the WR
that normalizes the intensity of a 464 min HIIT protocol. In
addition, we hypothesized that W9depletion and subsequent W9
repletion occurs at a fixed rate, allowing the P-t
LIM
relationship to
be used to maximize the volume of work that can be completed in
a464 min HIIT program.
Methods
Subjects
A total of 11 healthy, recreationally active males (mean 6
standard deviation (SD); age 2364 yr; height 17865 cm; mass
7265 kg) who met the inclusion criteria (i.e. recreationally active
males, aged 18–35 yr who were free from illness or any medical
condition) volunteered, and provided written informed consent to
participate in the study (as approved by the Faculty of Biomedical
and Life Sciences Ethical Committee for non-clinical research,
University of Glasgow, in accordance with the Declaration of
Helsinki). All subjects were well accustomed to high-intensity
exercise. Although none of the subjects were participating in
competitive training at the time of the study 2 subjects had a
running background, with the others involved in recreational
running training. Following familiarisation with all equipment,
protocols and procedures, subjects visited the laboratory on at least
6 separate occasions, each at a similar time of day, with at least
24 hr between each test. Each individual participated in no more
than 3 experimental sessions in any given week. For each test,
subjects were instructed to arrive rested (no strenuous exercise in
the previous 24 hr), and having abstained from alcohol (24 hr),
food (2 hr minimum) and caffeine ingestion (4 hr) prior to each
test. Throughout the study participants were asked to consume
their normal diet, and prior to all testing, arrive at least 2 hr
postprandial having consumed a normal, healthy meal.
Equipment and measurements
All exercise tests were conducted on a motor driven program-
mable treadmill (PPS Med, Woodway, Weil am Rhein, Germany)
set at a gradient of 1% to take into account the lack of air
resistance with indoor treadmill running, and thus match the
energetic cost of the treadmill exercise with that of outdoor
running [40]. During all tests subjects breathed through a
mouthpiece connected to a large 2-way non-rebreathing valve
(2700 series, Hans Rudolph, Shawnee, KS, USA), allowing
collection of the respired gas (via a 1.5 m length of 3.5 cm
diameter tubing) in a Douglas bag. This allowed measurement of
the expired gas concentrations (Paramagnetic (O
2
) and Infrared
(CO
2
) analyzers; Servopro 4100 gas analyzer, Servomex, Crow-
borough, UK) and gas volume (Dry gas meter; Harvard
Apparatus, Edenbridge, UK), thus allowing calculation of gas
exchange variables (specifically _
VVO2). Prior to each test the gas
analyzers were calibrated in accordance with manufacturers
guidelines using precision analyzed gases which spanned the
physiological range of inspired and expired gas concentrations,
with gas mixtures re-sampled post-test to confirm stability in
relation to the initial gas calibration.
Throughout all tests heart rate (HR) was measured and
recorded every 5 s using a short-range telemetry HR monitor
(S610i, Polar Electro Oy, Kempele, Finland). At specific time
points in all protocols a small sample (approximately 25 ml) of
capillary blood was obtained from the fingertip of the heated hand
and analyzed immediately post-test for whole-blood [lactate]
([L
2
]) using an automated analyzer (GM7, Analox Instruments,
London, UK). The analyzer was calibrated using an 8 mM
standard L
2
solution, the concentration of which was also checked
post-test to confirm the validity of the measurements obtained.
Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 2 November 2013 | Volume 8 | Issue 11 | e76420
Exercise protocols
All exercise tests were preceded by a period of at least 6 min
brisk walking at a speed of 5.5 km?h
21
(with the exception of the
incremental-ramp test; see below for details), and concluded with a
period of 6 min walking at a speed of 4.0 km?h
21
. For each test,
subjects were instructed to run as long as possible (i.e. to the point
of exercise intolerance), and at the point at which they could not
longer maintain the set treadmill speed – despite strong verbal
encouragement – they were instructed to support their weight on
the handrails and straddle the treadmill. At this point (i.e. exercise
intolerance) the speed of the treadmill was immediately reduced,
and the 6 min cool-down at 4.0 km?h
21
commenced. For a
schematic of the exercise protocols please refer to Figure 1.
Incremental-ramp test. This test, to exercise intolerance,
was performed to determine peak _
VVO2(_
VVO2
peak
), and establish an
appropriate starting speed to characterize the S-t
LIM
relationship
(see below). In the incremental-ramp test, following a period of
6 min running at 8 km?h
21
, speed was increased at a rate of
1km?h
21
?min
21
, until the point of exercise intolerance (Figure 1).
Once the subject was considered to be close to the point of exercise
intolerance, serial expired gas samples of a 60 s duration were
collected in Douglas bags to ensure _
VVO2
peak
was captured. In the
event that the limit of tolerance was obtained less than ,20 s into
the gas collection, the value obtained from the previous 60 s gas
collection was assumed to be _
VVO2
peak
.
Characterization of the Speed-tolerable duration (S-tLIM)
relationship. A randomized series of four separate constant-
speed tests were conducted across a range of speeds selected to
induce intolerance within a duration of ,3–20 min [18]. During
these tests the treadmill speed was rapidly increased to that
required (treadmill acceleration 0.72 km?h
21
?s
21
, 0.200 m?s
21
?
s
21
) from the 5.5 km?h
21
baseline, with subjects instructed to
continue running at this speed until the point of exercise
intolerance (Figure 1). From these tests the S-t
LIM
relationship
Figure 1. Schematic of the treadmill speed profiles performed during the Incremental-ramp test, the constant-speed tests for
characterization of the Speed-tolerable duration (S-t
LIM
) relationship – dotted line (top left and right respectively), the fixed HIIT
protocols in which 4 min bouts at WR
4
,WR
6
or WR
8
were alternated with 4 min recovery bouts until the limit of tolerance was
attained (middle row), and the maximal HIIT protocol in which each bout was performed to the limit of tolerance, with a fixed
4 min recovery between each bout (bottom). Nrepresents the limit of exercise tolerance in all protocols.
doi:10.1371/journal.pone.0076420.g001
Interval Training and Exercise Intensity
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was characterized, with CS (intercept) and D9(slope), the
parameters of this relationship, estimated using least-squares
linear regression of the linear S-t
LIM
21
relationship (i.e. S = (D9/
t
LIM
)+CS) [18]. Following estimation of the parameters of the S-
t
LIM
relationship within acceptable limits (defined as the standard
error (SE) of the estimate being less than 2% for CS and 10% for
D9; requiring additional tests at a different speed in 2 subjects), the
speeds predicted to induce exercise intolerance at 4 min (WR
4
),
6 min (WR
6
) and 8 min (WR
8
) were derived by interpolation of
the S-t
LIM
relationship, and used as the work rates for the ‘‘ON’’
bouts for the interval training sessions.
_
VVO2in these tests was measured in the final minute of the
5.5 km?h
21
warm-up, thus establishing the baseline _
VVO2._
VVO2
peak
was established by serial sampling of the expired gas (60 s
collections) once the subject was considered to be close to the point
of exercise intolerance (see Incremental-ramp test protocol above
for further details). This _
VVO2
peak
was confirmed as _
VVO2
max
for
each subject by establishing no difference in the _
VVO2
peak
attained
with increases in constant-speed. Capillary blood samples were
taken for lactate concentration ([L
2
]) analysis at rest, 30 s prior to
the end of the 5.5 km?h
21
warm-up and immediately following
the attainment of the limit of tolerance.
Fixed WR HIIT. A sub-group of 8 subjects completed a series
of 3 HIIT sessions, one at WR
4
,WR
6
and WR
8
, in a random
order. Following the completion of the 5.5 km?h
21
warm-up, the
work rate alternated between 4 min of the appropriate ON work
rate (i.e. WR
4
,WR
6
,orWR
8
) and 4 min brisk walking at
5.5 km?h
21
. This was repeated until the point of exercise
intolerance, or until a maximum of 8 ON bouts were completed
(Figure 1), allowing the total ON time, % of the target 16 min ON
duration (i.e. 4 ON bouts of 4 min) to be calculated for each of the
work rates performed.
Maximal HIIT. Given that effort is not maximal until the
final bout in the fixed-WR HIIT protocol, a sub-group of 6
subjects completed a HIIT session in which the aim was to
maximize effort in each of the 4 ON bouts, thus maximizing the
amount of high-intensity work that can be completed with this
format of training (i.e. analogous to SIT), with an anticipated
duration of 4 min for each bout. The first ON bout was conducted
at WR
4
until the point of exercise intolerance was attained (at
which point D9is theorized to be fully ‘depleted’; [18,31,35]). The
remaining 3 ON bouts were conducted at WR
8
and continued
until the point of exercise intolerance, with this theorized to result
in a t
LIM
of ,4 min (based on evidence suggesting a D9recovery
of ,50% with an intervening recovery of 4 min [35]; Figure 1). In
each ON bout t
LIM
was recorded and used to calculate the extent
of D9recovery in the preceding recovery period, and the amount
of supra-CS work done for each bout.
During both HIIT protocols, ‘‘baseline’’ _
VVO2was measured in
the final 60 s of the initial 5.5 km?h
21
warm-up, and in the final
60 s of each 4 min recovery between each ON bout. _
VVO2
peak
was
also measured in the final 60 s of each ON bout, with serial
sampling conducted when the subject was considered to be close to
their tolerable limit (see above) to ensure _
VVO2
peak
was captured at
the point of intolerance. Similarly, capillary blood samples were
taken for [L
2
] analysis at rest, 30 s prior to the end of the
5.5 km?h
21
warm-up and 30 s prior to the onset of the next ON
bout (‘‘baseline’’), immediately following the completion of each
ON bout and immediately at the point of exercise intolerance.
Subjects were informed during the HIIT protocols that if access to
water was required this could be provided during the fixed 4 min
recovery periods.
Analysis
Normal data distribution was confirmed using Kolmogorov-
Smirnov test. A one-way ANOVA for repeated measures, with
post hoc analysis (bonferroni) where appropriate, was used to
compare _
VVO2
peak
and peak [L
2
] values obtained in all protocols
and baseline _
VVO2and [L
2
] values obtained during the maximal
HIIT protocol. Similarly this test was used to compare the ON
duration sustained during HIIT at WR
4
,WR
6
and WR
8
, and the
amount of supra-CS work performed during each interval during
the maximal HIIT protocol. In addition, where appropriate,
Cohen’s d was used of provide a measure of the Effect size. The a
was set at 0.050. Values are expressed as mean 6SD unless
otherwise stated.
Results
Incremental-ramp test
_
VVO2
peak
(4.1260.42 l?min
21
; 57.664.3 ml?kg
21
?min
21
; Range
50.9–65.0 ml?kg
21
?min
21
) was attained at an average speed of
18.961.8 km?h
21
during the incremental-ramp test. Peak [L
2
]
was 8.961.4 mM, and peak HR was 19268 beats?min
21
.
Characterization of the S-t
LIM
relationship
The individual values for _
VVO2
peak
were not influenced by
treadmill speed (P.0.050), hence the mean of these values was
taken as _
VVO2
max
(4.1360.39 l?min
21
). Similarly, there was no
difference in peak [L
2
](P.0.050; mean 8.561.3 mM) or peak
HR (P.0.050; mean 18868 beats?min
21
) with work rate at the
point of exercise intolerance. Tolerable duration was well
described by a hyperbolic function of the external treadmill speed,
with the SE of the CS and D9estimates of this relationship
,0.06 m?s
21
(,2%; Range 0.3–1.8%) and ,18 m (,10%;
Range 1.6–7.9%), respectively, in all instances (Figure 2). CS
and D9averaged 3.85360.429 m?s
21
(equivalent to
13.961.5 km?h
21
) and 269.1673.2 m, respectively. WR
4
,WR
6
and WR
8
interpolated from this S-t
LIM
relationship were
4.97460.527 m?s
21
(17.961.9 km?h
21
), 4.60060.475 m?s
21
(16.661.7 km?h
21
) and 4.41360.455 m?s
21
(15.961.6 km?h
21
),
respectively.
Fixed WR HIIT
In the sub-group of 8 subjects who completed the 3 fixed work
rate HIIT sessions at WR
4
,WR
6
and WR
8
the tolerable duration
of the HIIT sessions were 399681 s (95% CI; 331–467 s),
8926181 s (95% CI; 741–1044 s), and 15176346 s (95% CI;
1228–1807 s), respectively with total ON durations all significantly
different from each other (P,0.050) (Figure 3A). This was
equivalent to 41.668.4% (95% CI; 34.5–48.6%), 93618.9%
(95% CI; 77.2–108.8%) and 158.1636.1% (95% CI; 127.9–
188.2%) of the target 960 s (i.e. 464 min) ON duration. There
was, however, no difference in the _
VVO2attained at the limit of
tolerance of WR
4
,WR
6
or WR
8
protocols, with this _
VVO2not
different from _
VVO2
max
in this cohort of 8 subject (P.0.050), thus
confirming _
VVO2
max
was attained in all protocols. However, there
was a tendency for the _
VVO2attained during WR
8
to be lower than
_
VVO2
max
(Cohen’s d = 0.55) due to some subjects being able to
complete the maximum 8 ON bouts, hence these subjects did not
attain the point of exercise intolerance before the protocol was
terminated (Figure 3B; Table 1). Similarly, there was no difference
in peak HR at the point of exercise intolerance in all protocols
(P.0.050; Table 1). In addition, peak [L
2
] was not significantly
different from that attained during the constant-speed tests used to
Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 4 November 2013 | Volume 8 | Issue 11 | e76420
characterize the S-t
LIM
relationship at the point of exercise
intolerance in WR
6
and WR
8
(P.0.050); however, peak [L
2
] was
significantly higher in WR
4
at the point of exercise intolerance
than that attained in all other protocols (P,0.050; Table 1). Thus
WR
6
provides the appropriate work rate to normalize the intensity
of HIIT to the very-heavy intensity domain, with this speed
equivalent to 8863% (Range 83–93%) of the speed attained at
_
VVO2
max
in the incremental-ramp test.
Maximal HIIT
In the sub-group of 6 subjects who completed this protocol,
there was no significant difference in tolerable duration for each of
the 4 ON bouts (ON Bout 1: 229627 s; Bout 2: 262637 s; Bout 3:
235649 s; Bout 4: 235653 s; P.0.050); with _
VVO2
max
attained in
each of the 4 ON bouts (Table 2; Figure 4). Although there was a
statistical difference in the _
VVO2attained at the point of exercise
intolerance between ON bouts 2 and 3 (P = 0.047), neither of these
was different from _
VVO2
max
determined during the constant-speed
tests (P.0.050), and there was less than a 0.20 l?min
21
difference
Figure 2. The relationship between speed and tolerable duration for 4 constant-speed tests (continued to exercise intolerance) in a
representative subject (N). A hyperbolic relationship has been fitted to these data (solid line) allowing estimation of critical speed and D9. Also
plotted is the _
VVO2for each of these constant-speed tests (#), demonstrating _
VVO2
max
was attained in each test.
doi:10.1371/journal.pone.0076420.g002
Figure 3. Left panel: The tolerable duration of HIIT at WR
4
,WR
6
and WR
8
(i.e. the work rate interpolated from the Speed-tolerable
duration relationship to induced exercise intolerance in 4 min, 6 min and 8 min if performed at a constant speed), relative to the
target HIIT duration of 960 s (i.e. 464 min; dotted vertical line). Right panel: The corresponding _
VVO2attained at exercise intolerance during
HIIT at WR
4
,WR
6
and WR
8
, relative to _
VVO2
max
(dotted line), and that attained in the constant-speed (CL) tests. Note _
VVO2
max
was attained in all
protocols, although _
VVO2during HIIT at WR
8
was slightly (insignificantly) lower due to some subjects being able to complete the maximum 8 bouts.
doi:10.1371/journal.pone.0076420.g003
Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 5 November 2013 | Volume 8 | Issue 11 | e76420
between all measurements of _
VVO2at the point of exercise
intolerance in all subjects and _
VVO2
max
. Hence, _
VVO2remained well
within the expected test-retest variability of 10% [41]. Similarly,
there was no difference in HR at the point of exercise intolerance
between each of the ON bouts, or that attained during the
constant-speed tests (P.0.050; Table 2). In addition, although
there was a significant difference in peak [L
2
] between ON bouts
1 and 2 (P,0.050), there was no difference in peak [L
2
] between
all other bouts, and that attained in the constant-speed tests
(P.0.050; Table 2).
_
VVO2prior to each ON bout was elevated compared to the pre-
exercise baseline value (P,0.050), however this was not signifi-
cantly different between recovery (REC) bouts (P.0.050; Table 3,
Figure 4). Similarly HR was elevated compared to the pre-exercise
baseline value (P.0.050); however, there was no significant
difference between the HR attained during the recovery bouts
(Table 3). In addition, [L
2
] was significantly elevated prior to the
pre-exercise baseline value in all recovery bouts (P.0.050);
however, there was no significant difference in [L
2
] attained
between each intervening recovery bout (P.0.050; Table 3).
These results are consistent with the finding that there was no
difference in supra-CS work in Bout 1 compared with D9
(264.9658.7 m vs. 283.7679.5 m; P.0.050), but that there was
significantly less work accomplished in ON bouts 2, 3 and 4
(P,0.050), with the amount of supra-CS work not different
between these bouts (Bout 2: 153.5640. 9 m; Bout 3:
136.9638.9 m; Bout 4: 136.7639.3 m; Figure 5; P.0.050)
suggesting a constant rate of D9‘recovery’ – thus fixed quantity of
D9recovered in 4 min – between bouts. D9recovery averaged
54.767.8%, 48.9610.2% and 48.9611.2% for bouts 2, 3 and 4
respectively, with this recovery not significantly different between
bouts (P,0.050).
Discussion
This is the first study to apply the S-t
LIM
relationship to identify
the appropriate work rate for HIIT to normalize the relative
intensity between subjects to the very-heavy intensity domain,
identifying that WR
6
for a 464 min HIIT session provides the
appropriate balance between D9depletion during the ON bouts,
and repletion in the intervening 4 min recovery period that
allowed for the completion of the required ,4 (3.760.7; i.e. 93%)
ON bouts. Hence, this protocol allows for the appropriate
consideration of the role of exercise intensity in determining
training adaptations, normalizing this between individuals. Fur-
thermore, this study establishes a protocol that, with knowledge of
the extent of D9recovery between bouts, maximizes the amount of
high-intensity work that can be completed in a 464 min HIIT
protocol, precisely normalizing the intensity of both the overall
session, and each ON bout (i.e. each ON bout resulted in the
attainment of _
VVO2
max
). Hence, this protocol provides a means of
differentiating the relative importance of the work rate profile (c.f.
SIT) and exercise intensity to promote physiological adaptations.
Exercise intensity
While a specific work rate can be of a high absolute intensity
(e.g. 100% _
VVO2
max
) when performed as a continuous bout, this
same specific work rate can be undertaken during HIIT in a
manner which means the overall intensity of the training session
can be either moderate, (metabolic rate,Lactate threshold (LT),
no sustained metabolic acidosis), heavy (metabolic rate.LT,CS/
CP, sustained metabolic acidosis which eventually attains a steady-
state) or very-heavy/severe (progressive increase in _
VVO2, resulting
in the attainment of _
VVO2
max
if continued to t
LIM
, progressive
metabolic acidosis which continues throughout the exercise until
t
LIM
) [19,42]. While the specific work rate performed in relation to
the overall intensity of training is not a consideration in short-
duration SIT, as the ,30 s sprints are an all-out effort (e.g.
[20,21,22,23,24]) with this long enough to result in the attainment
(or very near attainment) of HR
max
and _
VVO2
max
in each sprint (i.e.
very-heavy/severe intensity), the specific work rate used during
HIIT is an essential consideration with respect to exercise intensity
(and normalizing this between participants) when the duration of
the ON exercise bout is extended.
Exercise bouts of 4 min are frequently used in HIIT both for
health and performance benefits (e.g. [9,10,12,29,30]) in the
format of a 464 min training session, with an intervening recovery
of 3–4 min. Typically work rate is determined from % HR
max
or
%_
VVO2
max
; however, this fails to account for the variability of the
derived work rate with respect to the parameters of the high-
intensity relationship (i.e. CS and D9) between individuals (e.g.
[19,39]). This is highlighted by the result in this study that WR
6
exists at 87% of the speed attained at _
VVO2
max
in the incremental-
ramp test, but with a range of 83–93%. In addition, any specific
prescribed % _
VVO2or % HR during HIIT is only attained
fleetingly as a steady-state is never achieved, with these variables
continuing to increase towards their respective maxima through-
out each bout [19]. However, by accounting for the S-t
LIM
relationship during treadmill running to normalize exercise
intensity we were able to demonstrate that WR
6
(i.e. the work
rate derived from the S-t
LIM
relationship that leads to the limit of
tolerance in 6 min) was optimal, providing the required balance
between D9‘‘depletion’’ during ON bouts and ‘‘repletion’’ during
the intervening recovery that allowed for the completion of the
required ,4 ON bouts. As this resulted in the attainment of
_
VVO2
max
and peak lactate in the final bout this, by definition
[19,42], puts the overall intensity of training for all subjects within
the very-heavy intensity domain.
Maximal HIIT
While WR
6
defines a work rate for HIIT that normalizes
intensity to the very-heavy intensity domain for 464 min HIIT,
with this resulting in a high _
VVO2and HR (with these variables
continuing to increase towards their respective maxima through-
out each bout until the limit of tolerance is attained), maximal
effort is not required until the final bout (c.f SIT; [43]). Therefore,
while WR
6
normalizes exercise intensity it does not maximize the
Table 1. _
VVO2
max
, HR and [L
2
] attained at the limit of
tolerance during Control, and fixed WR HIIT protocols
performed at WR
4
,WR
6
and WR
8
.
Control WR
4
WR
6
WR
8
_
VVO2
max
(l?min
21
)3.9560.26 3.9460.23 3.9260.21 3.8160.25
HR (beats?min
21
)188643 19063 1896618864
[L
2
] (mM) 8.461.1 9.761.1* 8.561.5 8.061.0
Values are means 6SD. _
VVO2
max
(maximal rate of pulmonary oxygen uptake);
HR (heart rate) and [L
2
] (Lactate concentration) measured at the limit of
tolerance during the constant-speed tests used to characterize the Speed-
tolerable duration (S-t
LIM
) relationship (Control) and during HIIT performed at
WR
4
,WR
6
and WR
8
(work rates predicted to induce exhaustion at 4, 6 and 8 min
respectively).
*Significantly higher [L
2
] than that attained in Control, WR
6
and WR
8
protocols.
doi:10.1371/journal.pone.0076420.t001
Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 6 November 2013 | Volume 8 | Issue 11 | e76420
amount of supra-CS work that can be completed during 464 min
HIIT. For high-intensity, supra-CS exercise of ,2–30 min
tolerable duration is dependent on the rate of D9depletion, with
this rate of depletion increasing proportionally with work rate.
Therefore, interpolating WR
4
from the S-t
LIM
relationship
maximizes supra-CS work on Bout 1 of 464 min HIIT (Bout 1
t
LIM
: 229627 s; Range 117–248 s), with this leading to the
attainment of HR
max
and _
VVO2
max
. As CS is unchanged following
fatiguing exercise, subsequent exercise tolerance is dependent
exclusively on the extent of D9recovery, with this demonstrated to
be ,50% in 4 min recovery [35]. Hence, in Bout 2 WR
8
should
be sustainable for ,4 min, thus providing the necessary work rate
to maximize supra-CS work in 4 min. In this study as D9recovery
averaged ,50%, and confirms the assumption that the extent of
D9recovery does not differ between repeated bouts [44], WR
8
is
then the appropriate work rate for bouts 2, 3 and 4 to maximize
the amount of supra-CS work that can be accumulated in 4 min
(t
LIM
: Bout 2: 262637 s; Bout 3: 235649 s; Bout 4: 235653 s),
and provides a protocol to maximize the amount of supra-CS
work that can be accumulated in 464 min HIIT, resulting in the
attainment of HR
max
and _
VVO2
max
in each bout. This makes this
464 min HIIT analogous to SIT, allowing the relative contribu-
tion of the work rate profile, when matched for exercise intensity,
to be investigated. These data also confirm the assumption that
during this 464 min HIIT protocol performance is determined by
the S-t
LIM
relationship, with the profile of D9depletion and
recovery alone ‘‘shaping’’ supra-CS exercise tolerance [44].
Consideration of the work rate profile of HIIT and
practical applications
By correctly defining and normalizing the intensity of HIIT
between participants to maximize the amount of supra-CS work
that can be accumulated in a 464 min HIIT protocol, thus
ensuring the same metabolic stress throughout training, this allows
appropriate comparison of different interval training strategies
(e.g. short vs. long duration ON bouts). Hence, the relative
Figure 4. The _
VVO2response during the maximal HIIT protocol. Although there was some (insignificant) variability in ON duration at WR
4
(bout 1) and WR
8
(bouts 2, 3 and 4) (horizontal error bars), note the constancy of the _
VVO2attained, with this indistinguishable from _
VVO2
max
. Similarly,
although _
VVO2did not recover to baseline (BASE) following 4 min recovery (REC; P,0.050, w), there was no difference in the _
VVO2attained in REC
following WR
4
(bout 1) or WR
8
(bouts 2 and 3).
doi:10.1371/journal.pone.0076420.g004
Table 2. _
VVO2
max
, HR and [L
2
] attained at the limit of
tolerance during the control and maximal HIIT protocols.
Control
ON
Bout 1
ON
Bout 2
ON
Bout 3
ON
Bout 4
_
VVO2
max
(l?min
21
)4.2260.51 4.2860.47 4.2960.53 4.1860.48 4.2060.53
HR (beats?min
21
) 1906318866 18569 1896518866
peak [L
2
] (mM) 8.261.1 7.261.5* 9.761.6 9.361.2 9.161.4
Values are means 6SD. _
VVO2
max
(maximal rate of pulmonary oxygen uptake);
HR (heart rate) and [L
2
] (Lactate concentration) measured at the limit of
tolerance during the constant-speed tests used to characterize the Speed-
tolerable duration (S-t
LIM
) relationship (Control) and ON bouts 1, 2, 3 and 4 of
the maximal HIIT protocol.
*Significantly lower [L
2
] than that achieved in Bout 2.
doi:10.1371/journal.pone.0076420.t002
Table 3. _
VVO2
max
, HR and [L
2
] attained at the pre-exercise
baseline, and in the 4 min recovery bouts during the maximal
HIIT protocol.
Baseline REC 1 REC 2 REC 3
_
VVO2
max
(l?min
21
)1.3760.13 1.6460.18* 1.6760.24* 1.6160.20*
HR (beats?min
21
)10866 13267* 13367* 13768*
peak [L
2
](mM) 0.960.2 9.261.4* 8.861.3* 8.661.9*
Values are means 6SD. _
VVO2
max
, maximal rate of pulmonary oxygen uptake;
HR, heart rate; [L
2
], Lactate concentration and; REC, recovery bout.
*Significantly higher than the pre-exercise baseline.
doi:10.1371/journal.pone.0076420.t003
Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 7 November 2013 | Volume 8 | Issue 11 | e76420
contributions of both exercise intensity and the intermittent/
interval work rate profile to any training induced physiological
adaptations can be appropriately deconvoluted, with this having
important implications when investigating the mechanistic basis
for training adaptations.
While exercise intensity is an essential consideration with
regards to training adaptations [16,30,45,46,47] there is evidence
emerging that the actual work rate profile is also important
[12,28]. Even when appropriately matching for exercise intensity
and total work the physiological changes during HIIT (in terms of,
for example, the dynamics and proportional contribution of the
different energy systems to the energy demand, and blood flow
dynamics) will be significantly different with short, compared with
long ON bouts. That is, when the overall intensity of the exercise
session is controlled, but the duration of the ON bout is extended,
there is a proportionally greater aerobic contribution to the overall
energy requirement when matched for energy expenditure. Hence
with longer ON bouts (i.e. ,4 min), given the response dynamics
of _
VVO2, HR and cardiac output, there will be a greater time
accumulated at a relatively high proportion of these respective
maxima, compared with short (i.e. 30 s) ON bouts. Although this
requires further systematic investigation, this is likely to have
significant consequences with regards to the specific physiological
adaptations seen (e.g. [28]) following a training program.
For example, it has been suggested that there may be an
intensity threshold over which exercise has to be performed to
promote cardiovascular benefits [13,14], although this suggestion
is not universal (e.g. [17]). Hence, it is likely that generating a high
relative HR and cardiac output (with respect to their maxima) is
important for inducing intrinsic cardiac benefits and promoting
improvements in vascular function [9,13,45]. Thus, as there is a
greater accumulation of time under these ‘‘conditions’’ i.e. high
HR and cardiac output in long vs. short bouts for the same overall
training session intensity and total training session time commit-
ment (i.e. ,30 min per session), the relevance of the work rate
profile is likely an essential consideration with regards to
developing optimal training strategies to maximize training
adaptations.
In addition, the physiological differences between different
HIIT protocols, even when matched for exercise intensity may be
of particular importance when considering adaptations relating to
metabolic and cardiovascular risk factors such as insulin sensitivity
and aerobic capacity. For example, increased mitochondrial
energy flux is associated with greater improvements in insulin
sensitivity [48]. In addition, while PCG-1a(a critical regulator of
mitochondrial biogenesis; [49]) has been demonstrated to be
activated following both short [50,51] and long [11] HIIT, given
the bioenergetic differences between the different interval training
strategies it is unclear which work rate profile will have the greatest
impact on, for example, mitochondrial capacity, insulin sensitivity
and aerobic capacity when exercise intensity is controlled.
Therefore, while the work rate profile likely contributes to training
adaptations, the specific work rate profile which maximizes specific
adaptations to subsequently improve physiological function (and
the interaction of this with the exercise intensity) has yet to be
resolved. However, the results from this study enable the correct
work rates to be identified, thus allowing intensity to be removed
as a confounding variable in order to investigate the relative
importance of the work rate profile in training strategies.
While it has been postulated that there is a dose-response
relationship between exercise intensity (quantified in terms of
_
VVO2
max
) and training adaptations [14], the full nature of this dose-
response has yet to be established. Therefore, whether it is
necessary to provide an all-out effort to maximize any physiolog-
ical adaptations from training has yet to be resolved. Hence, it is
possible that, similar to the proposal that there is a minimum
intensity for some specific training induced adaptations [13,14],
there may also be an upper limit/optimal exercise intensity above
which the magnitude of any training induced adaptations is
diminished. However, calculating appropriate work rates based on
the S-t
LIM
relationship and the extent of D9recovery, provides a
method of normalizing, and then titrating the overall training
intensity for longer duration ON bouts to identify the existence of
Figure 5. The quantity (with units of meters, m) of supra-CS work (i.e. D9) performed during the maximal HIIT protocol. Bars represent
the group mean (6SD), with #representing the individual data. In bout 1, the amount of supra-CS work performed is indistinguishable from D9
determined from the Speed-tolerable duration relationship. However, in bouts 2, 3 and 4 significantly less supra-CS work is performed (,50%) with
this significantly less than D9(P,0.050, w).
doi:10.1371/journal.pone.0076420.g005
Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 8 November 2013 | Volume 8 | Issue 11 | e76420
any intensity (comparing this with appropriate proposed ‘‘practi-
cal’’ short duration HIIT strategies; e.g. [52,53]), and thus effort
related threshold with regards to training adaptations. This
however, requires systematic investigation.
Of course underpinning all considerations with regards to
training strategies and physiological adaptations must be the
potential to translate effective lab-based training strategies in to the
home environment. There is evidence that interval training in
general may be a more enjoyable training strategy than continuous
moderate-intensity interventions [37], with this possibly related to
the challenge of undertaking the more challenging aspect of the
exercise intervals, rather than the monotony of continuous
moderate-intensity exercise. Hence, research into optimizing
HIIT has the potential to have a significant impact with regards
to establishing a range of effective training strategies as alternatives
to traditional continuous moderate-intensity exercise for the
improvement of exercise tolerance/performance and risk factors
for chronic illness, allowing individuals to adhere to training
strategies which fit with their individual training preferences and
lifestyle [54].
Limitations
While this study has identified that WR
6
normalizes the
intensity of HIIT to the very heavy-intensity domain, it must be
acknowledged that the subjects in this study were from a relatively
homogenous group; therefore, whether these findings can be
extended to other populations remains unclear. In addition,
although these results highlight the importance of the S-t
LIM
relationship to normalize the intensity of both continuous and
HIIT one of the primary limitations when considering the
translational application of these findings is the number of tests
required to characterize this relationship and identify the correct
work rates. Therefore, developing a strategy that allows quick and
accurate identification of the appropriate work rates from the S-
t
LIM
relationship for use in HIIT remains an important goal.
Conclusion
In conclusion, WR
6
derived from the S-t
LIM
relationship
provides the appropriate work rate to normalize the intensity of
464 min HIIT to the very heavy-intensity domain. In addition, as
there is no difference in the extent of D9recovery between fatigue
bouts, this study establishes an approach in which supra-CS work
can be maximized and exercise intensity can be normalized
precisely for each subject during 464 min HIIT. This strategy
therefore allows the relative contributions of exercise intensity and
the work rate profile to any training induced adaptations to be
appropriately quantified. This has important implications for
establishing HIIT strategies to maximize improvements in
physiological functioning.
Acknowledgments
The authors would like to express thanks to all subjects who volunteered to
participate, and to Kerry Griffiths Jamie Hitchmough, David Knox,
Andrew Leishman, Nandini Shah, Kerry Steel and Sun Weizhe for their
assistance during data collection.
Author Contributions
Conceived and designed the experiments: CF JW OJK. Performed the
experiments: CF JW OJK. Analyzed the data: CF JW KMB OJK. Wrote
the paper: CF JW KMB OJK.
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Interval Training and Exercise Intensity
PLOS ONE | www.plosone.org 10 November 2013 | Volume 8 | Issue 11 | e76420