Vol.:(0123456789)
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European Journal of Applied Physiology (2020) 120:2397–2405 
https://doi.org/10.1007/s00421-020-04463-w
ORIGINAL ARTICLE
High‑intensity decreasing interval training (HIDIT) increases time 
above 90% ̇VO2peak
Filippo Vaccari1,2  · N. Giovanelli1,2 · S. Lazzer1,2
Received: 16 July 2019 / Accepted: 4 August 2020 / Published online: 11 August 2020 
© The Author(s) 2020
Abstract
Purpose Training near ̇VO2max is considered to be the most effective way to enhance ̇VO2max. High-intensity interval 
training (HIIT) is a well-known time-efficient training method for improving cardiorespiratory and metabolic function and 
̇VO2max. While long HIIT bouts allow ̇VO2max to be achieved quickly, short HIIT bouts improve time to exhaustion (Tlim). 
The aim of this study was to evaluate the time spent above 90% ̇VO2peak (T > 90% ̇VO2peak) during three different HIIT 
protocols.
Methods Twelve cyclists performed three HIIT sessions. Each protocol had the same work and recovery power and ratio 
of work·recovery−1. The protocols consisted of long-interval HIIT  (LIHIIT, 3 min work—2 min recovery), short-interval 
HIIT  (SIHIIT, 30 s work—20 s recovery), and high-intensity decreasing interval training (HIDIT, work from 3 min to 30 s 
and recovery from 2 min to 20 s). T > 90% ̇VO2peak, Tlim, blood lactate [La], and rate of perceived exertion (RPE) were 
measured at Tlim.
Results T > 90% ̇VO2peak was greater in HIDIT (312 ± 207 s) than in  SIHIIT (182 ± 225 s; P = 0.036) or  LIHIIT (179 ± 145 s; 
P = 0.027). Tlim was not significantly different (P > 0.05) between HIDIT (798 ± 185 s),  SIHIIT (714 ± 265 s), and  LIHIIT 
(664 ± 282). At Tlim, no differences in [La] and RPE were found between protocols (P > 0.05).
Conclusion HIDIT showed the highest T > 90% ̇VO2peak, suggesting that it may be a good strategy to increase time close 
to ̇VO2peak, despite similar Tlim, [La], and RPE at Tlim.
Keywords ̇VO2max · ̇VO2max training · Time at ̇VO2max · HIIT
Abbreviations
%CP-Load Peak 
 Percentage of critical power relative to 
load peak
% ̇VO2peak 
 Oxygen consumption in percentage 
relative to its peak
%HRpeak 
 Heart rate in percentage relative to its 
peak
[La] 
 Blood (capillary) lactate concentration
ANOVA 
 Analysis of variance
CP 
 Critical power
CR10 Scale 
 Validated scale of perceived exertion
ES 
 Effect size
HIDIT 
 Decreasing intervals HIIT (combining 
high phosphocreatine intensity from 
3′ to 30″ and low intensity from 2′ to 
20″)
HIIT 
 High-intensity interval training
ICP 
 Intermittent critical power
LIHIIT 
 Long intervals HIIT (3′ high—2′ 
low-intensity)
[Pcr] 
 Muscular concentration of 
phosphocreatine
QR 
 Gas-exchange ratio
RPE 
 Rate of perceived exertion
SIHIIT 
 Short intervals HIIT (30″ high—20″ 
low-intensity)
Tlim 
 (Time to exhaustion)
T > 90% ̇VO2peak 
 Time spent above 90% ̇VO2peak
̇VCO2 
 CO2 output
̇VO2 
 Pulmonary  O2 uptake
̇VO2max 
 Maximal theoretical aerobic power
Communicated by Håkan Westerblad.
 * Filippo Vaccari 
 
filippo.vaccari@live.com
1 
Department of Medicine, University of Udine, P.le Kolbe 4, 
33100 Udine, Italy
2 
School of Sport Sciences, University of Udine, Udine, Italy
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European Journal of Applied Physiology (2020) 120:2397–2405
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̇VO2peak 
 Maximal ̇VO2 achieved during incre-
mental exercise
W′ 
 Amount of work that can be done dur-
ing exercise above CP
Introduction
Maximal oxygen uptake ( ̇VO2max) refers to the oxygen con-
sumption attained during a maximal exercise. It is reached 
when the ̇VO2 does not increase any further despite further 
increases in workload, and it defines the limits of the car-
diorespiratory system (Hill and Lupton 1923). ̇VO2max is a 
relevant parameter of cardiorespiratory capacity, which is 
important for both endurance athletes (di Prampero 2003) 
and patients (Poole et al. 2012). It has been shown that, to 
improve ̇VO2max, a training protocol should prolong the 
time at which the oxygen uptake remains close to the maxi-
mum (within 5–10% of ̇VO2max) (Wenger and Bell 1986; 
Midgley and Mc Naughton 2006). High-intensity interval 
training (HIIT) is very effective at maintaining the metabolic 
rate near ̇VO2max (Buchheit and Laursen 2013a), better than 
continuous steady-state training (Midgley and Mc Naughton 
2006), and can be comprised of either short or long bouts 
of high intensity (work) alternated with recovery periods 
(recovery) at low intensity (or rest) (Buchheit and Laursen 
2013a).
The minimum intensity that allows one to reach ̇VO2max 
during a steady-state exercise is called critical power (CP). 
Theoretically, it is possible to maintain a metabolic steady 
state and prolong effort up to the CP threshold indefinitely. 
In contrast, above the CP, even if the external power out-
put remains constant, ̇VO2 increases up to ̇VO2max, leading 
to exhaustion within a few minutes (Jones and Vanhatalo 
2017).
HIIT can be set based on CP, setting the work intervals 
above CP and the recovery intervals below CP (Morton 
and Billat 2004). The CP is mathematically defined as the 
power asymptote of the hyperbolic relationship between 
power output and time to exhaustion (Jones et al. 2010). 
Physiologically, CP represents the boundary between steady-
state and non-steady-state exercise intensity domains (Jones 
et al. 2010; Jones and Vanhatalo 2017). Exercise above CP 
leads to reduced muscle phosphocreatine concentration [Pcr] 
and pH (Meyer 1988; Chidnok et al. 2013; Jones and Van-
hatalo 2017), making it difficult to prolong exercise (i.e., 
W′: amount of work that can be done during exercise above 
CP) (Ferguson et al. 2010; Skiba et al. 2012, 2014, 2015). 
Since muscle ̇VO2 is related to muscle reduction [Pcr] (di 
Prampero and Margaria 1968; Meyer 1988), the faster [Pcr] 
is depleted, the faster the ̇VO2 increases. Conversely, during 
the recovery phase (below CP), [Pcr] resynthesis and W′ 
recovery follow an exponential trend (Meyer 1988; Ferguson 
et al. 2010; Skiba et al. 2012, 2014; Jones and Vanhatalo 
2017; Vinetti et al. 2017). Indeed, when exercise generates 
a large depletion of [Pcr], the resynthesis rate is faster in the 
beginning of the recovery and it slows when approaching 
complete restoration.
Thus, an HIIT protocol that aims to stimulate ̇VO2max 
should start with long work intervals (2–4 min) to quickly 
increase ̇VO2. Subsequently, when the subject approaches 
exhaustion, short intervals can help to prolong the exer-
cise for longer: in this situation, the recovery ratio is fast 
and requires only few seconds to ensure sufficient recovery 
while simultaneously preventing the ̇VO2 from decreasing 
too much.
Therefore, the aim of this study was to compare the time 
above 90% of ̇VO2peak (T > 90% ̇VO2peak) in three different 
HIIT protocols. The proposed HIIT protocols had the same 
intensity and work/recovery ratio and were structured as 
follows: (1) constant long intervals  (LIHIIT); (2) decreasing 
interval duration (high-intensity decreasing interval training, 
HIDIT), and (3) constant short intervals  (SIHIIT). It has been 
hypothesized that the T > 90% ̇VO2peak should be longer 
in HIDIT. We hypothesized that the protocol with longer 
intervals followed by shorter intervals would elicit longer 
time above 90%.
Materials and methods
Subjects
Twelve middle-age amateur cyclists, all non-smokers, were 
enrolled in the study (41 ± 11 years; 76 ± 10 kg; ̇VO2peak 
4.32 ± 0.47 L min−1), Table 1. They reported at least three 
training sessions per week in the previous 6 months. None 
Table 1  Descriptive characteristics of the participants (n = 12)
All values are mean and standard deviation (SD)
HR heart rate, ̇VO2peak peak oxygen consumption, CP critical power, 
W′ total work sustainable above critical power, High and Low inten-
sity the average intensity sustained during HIIT tests
Mean ± SD
Min–Max
Age (year)
41 ± 11
29–62
Body mass (kg)
76 ± 10
66–95
HRpeak (b  min−1)
174 ± 10
155–193
̇VO2peak (L  min−1)
4.32 ± 0.47
3.66–5.10
Load peak (W)
356 ± 40
295–436
CP (W)
254 ± 30
212–320
W’ (kJ)
12.8 ± 4.1
8.5–22.7
High intensity (W)
297 ± 35
249–364
Low intensity (W)
212 ± 30
172–275
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of the subjects had evidence of significant diseases or took 
regular medications.
Study protocol
The Ethics Committee of the Friuli-Venezia-Giulia approved 
the study (protocol number 9626). During the first visit to 
the laboratory, an operator explained the purposes and 
objectives of the study to each subject and obtained writ-
ten informed consent. Then, participants underwent medical 
examinations and performed a maximal ramp-incremental 
exercise test on a cycle ergometer to measure the ̇VO2peak. 
Although the objectives were explained to all subjects, the 
study hypothesis was not revealed so as not to influence the 
results. After the first visit, the participants were examined 
three or four times to determine the critical power, and they 
performed the  SIHIIT, HIDIT, and  LIHIIT tests three times. 
Every visit was separated from the previous one by 2 days. 
Participants were instructed to avoid the consumption of 
caffeinated beverages for at least 8 h before each test and 
to abstain from vigorous physical activity in the 24 h pre-
ceding each testing session. Every subject concluded the 
entire protocol within 4 weeks from the first visit. The criti-
cal power parameters were used to program the HIIT tests. 
Subsequently, during the three HIIT tests, time to exhaustion 
(Tlim), T > 90% ̇VO2peak, blood lactate concentration [La], 
rate of perceived exertion using the Borg CR10 Scale (Borg 
et al. 2010), and ̇VO2 were measured at the 3rd minute and 
at the end of exercise.
Incremental exercise
The incremental exercise was performed under medical 
supervision, and standard safety procedures were followed. 
During the first visit, an operator instructed the subjects to 
correctly report the rate of perceived exertion on the CR10 
scale (Borg et al. 2010). The incremental exercise, critical 
power trials, and HIIT test protocols were performed uti-
lizing a cycle ergometer (CE) (Monark Ergomedic 839E). 
Every test was preceded by the same warm-up procedure: 
10 min cycling at 100 W followed by 2-min resting. During 
the first warm-up, subjects chose their preferred pedaling 
cadence (~ 90 rpm). The incremental exercise was a con-
stant incremental ramp test started at 100 W and gradually 
increased by 1 W every 2.4 s (25 W min−1) throughout the 
test until voluntary exhaustion. The exhaustion (during the 
incremental test and the HIITs) was defined as the inability 
to maintain the assigned cadence within 10 rpm longer than 
5 s despite strong encouragement from the operator.
̇VO2 and ̇VCO2 were measured breath-by-breath using a 
metabolic unit (Quark CPET, Cosmed, Italy). The ventila-
tion was measured by a turbine calibrated before each test 
with a 3-L syringe at three different flow rates. Calibration 
of  O2 and  CO2 analysers was performed before each test 
by utilizing calibration gas mixtures of known composition 
(16.00%  O2; 4.00%  CO2). ̇VO2peak corresponded to the 
highest mean ̇VO2 obtained in 30 s at the end of the incre-
mental exercise.
Power–duration relationship
The same warm-up and cadence from the incremental test 
were also used for the critical power (CP) test. CP and the 
amount of work that could be done during exercise above CP 
(W′) (Jones and Vanhatalo 2017; Burnley and Jones 2018) 
were estimated from three to four high-intensity trials at 
exhaustion from 80 to 100% of the peak power detected dur-
ing the incremental test and adopted to result in ‘exhaustion’ 
in a minimum of ~ 2 min and a maximum of ~ 15 min (Jones 
and Vanhatalo 2017). The work done in each of the separate 
exercise bouts has been plotted against Tlim. The follow-
ing work (W) − time (t) linear regression was then used to 
find CP and W′ (Moritani et al. 1981; Hill 1993; Jones and 
Vanhatalo 2017):
According to the equation, CP is given by the slope of the 
regression, and the W′ is the y-intercept.
HIIT tests
After the incremental test and the critical power trials, sub-
jects performed three HIIT tests in a randomized order. The 
power during the work and recovery bouts and the work/
recovery duration ratio were the same in each trial, although 
the duration of the intervals was changed (see Table 1 for 
mean values). The ratio work/recovery time was set at 3/2 for 
all the training tests. The power used for the high-intensity 
bouts was customized for each subject and corresponded to 
the power that was supposed to lead to exhaustion in 5 min 
(300 s) according to the following equation (Jones et al. 
2010):
and it corresponded to approximately 117% of CP. The 
power used for the low-intensity bout was mirrored below 
CP (approximately 83% of CP). Thus, the CP threshold was 
exactly in the middle between the high and low intensities.
The three tests were structured as follows (Fig. 1):
Short intervals  (SIHIIT): 30 s at high intensity and 20 s at 
low intensity repeated until volitional exhaustion of the 
subject.
(1)
W = CPt + W.
(2)
Power =
W
t = 300 s + CP,
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High-intensity decremental interval training (HIDIT): 
3 min at high intensity and 2 min at low intensity; 2 min 
at high intensity and 1 min and 20 s at low intensity; 
1 min at high intensity and 40 s at low intensity; 45 s at 
high intensity and 30 s at low intensity; and finally 30 s 
at high intensity and 20 s at low intensity, repeated until 
volitional exhaustion of the subject. The high–low ratio 
intensity duration was always 3/2.
Long intervals  (LIHIIT): 3 min at high intensity and 2 min 
at low intensity repeated until volitional exhaustion of 
the subject.
Throughout the HIIT protocols, the ventilatory param-
eters were measured using a breath-by-breath metabolic unit 
(CPET, Cosmed, Italy) and then averaged every 5 s. Before, 
after 3 min and at the end of exercise, ̇VO2, HR, [La], and 
RPE were measured, and the respiratory quotient (RQ) was 
calculated. An operator collected a capillary blood sample 
from the earlobe to measure the [La] with a dedicated device 
(Lactate Pro 2, Arkaray Inc., Japan), while the subjects 
reported RPE consulting the CR10 scale positioned in front 
of them. Finally, the total time spent above 90% of ̇VO2peak 
was determined as the sum of each averaged 5-s when the ̇V
O2 was equal to or higher than 90% of ̇VO2peak.
Statistical analyses
Statistical analysis was performed using SPSS 20.0 software 
(IBM, Chicago, USA) with significance set at P < 0.05. All 
results were expressed as the means and standard deviations 
(SD). The differences between HIIT training protocols in 
Tlim; T > 90% ̇VO2peak; T > 90% ̇VO2peak—Tlim−1; work 
above CP (calculated as the total time in seconds above CP 
multiply by the difference between the high-intensity power 
and CP, in Watts); average ̇VO2; and, finally, the values at 
the third minute and at Tlim ( ̇VO2, HR, [La], CR10-scale 
and RQ) were investigated. All parameters were analyzed by 
one-way repeated-measures analysis of variance (ANOVA). 
Where the analysis found a significant difference, planned 
contrast between HIDIT and  SIHIIT and between HIDIT 
and  LIHIIT were used with Bonferroni correction to deter-
mine the origin of such effects. The confidence intervals 
(CIs) of the differences and the effect size (ES) were calcu-
lated using Cohen’s d (0 < d < 0.20, small; 0.20 < d < 0.50, 
medium; d > 0.50, large) (Cohen 1988). The precision of 
Cp and W′ estimation was calculated comparing the param-
eter estimates with the work-time model and with the  time−1 
model through a t test. For our purposes, a sample size of 12 
subjects was calculated to have a statistical power of 80% to 
refute the null hypothesis and to obtain an ES of 0.88 with an 
alpha error of 0.05 and a beta error of 0.20 using a one-way 
ANOVA with Bonferroni correction, according to a previous 
Fig. 1  HIIT protocols for a representative subject.  SIHIIT: short-inter-
val HIIT (30″ high—20″ low-intensity); HIDIT: decreasing intervals 
HIIT (combining high intensity from 3′ to 30″ and low intensity from 
2′ to 20″);  LIHIIT: long-interval HIIT (3′ high—2′ low-intensity); the 
dotted lines represent the breath-by-breath ̇VO2 data averaged every 
5 s; the dashed lines represent the threshold of 90% of ̇VO2peak; the 
solid lines represent the actual power
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1 3
study (De Aguiar et al. 2013) that implemented a procedure 
similar to that of our study.
Results
Incremental test and CP trials
Peak values attained during the incremental test, CP, total 
work above CP (W′), and the power imposed for the high- 
and low-intensity bouts are shown in Table 1. Although the 
attainment of ̇VO2peak was not set as a priori criteria for the 
constant work rate tests of the power–duration relationship, 
it was always reached by the subjects. The parameter esti-
mates through the “work-time model” used for our purposes 
have been compared with the parameter estimates through 
the “1·time−1” model, and the results were comparable, as 
shown in Table 2.
HIIT tests
The power corresponding to high-intensity intervals was 
117 ± 6% of CP, and the low-intensity power was 83 ± 6% 
of the CP (Table 3).
T > 90% ̇VO2peak was significantly longer in HIDIT 
compared with  SIHIIT (P = 0.036; ES: 0.62) and  LIHIIT 
(P = 0.027; ES: 0.64) (Table 3, Fig. 2), and the ratio 
T > 90% ̇VO2peak—Tlim−1 tended to be higher in HIDIT 
than in  SIHIIT and  LIHIIT (Table 3). However, there were no 
differences in Tlim and in work > CP (P = 0.136) between 
the three protocols (Table 3). Finally, the average ̇VO2 
maintained during the HIDIT test was significantly higher 
than in  LIHIIT (P = 0.022; ES: 0.17) but not significantly 
different than in  SIHIIT (P = 0.106; ES: 0.10).
% ̇VO2peak after 3 min was similar between HIDIT and 
 LIHIIT (P = 0.339; ES: 0.18), but it was significantly higher 
in HIDIT than  SIHIIT (P = 0.006; ES: 0.83) (Table 3). Addi-
tionally, %HRpeak after 3 min was similar between HIDIT 
and  LIHIIT (P = 0.160; ES: 0.37), but it was significantly 
higher in HIDIT compared with  SIHIIT (P = 0.019; ES: 
0.61). Similarly, the CR10-scale after 3 min was similar 
in HIDIT and  LIHIIT (P = 0.824; ES: 0.05) but significantly 
higher than  SIHIIT (P = 0.031; ES: 0.55). Finally, RQ after 
3 min was not significantly different in HIDIT and  LIHIIT 
(P = 0.410; ES: 0.05), but it was significantly higher than 
in  SIHIIT (P = 0.031; ES: 0.25) (Table 3).
There was no significant difference in [La] at rest before 
the three tests  (SIHIIT, HIDIT, and  LIHIIT) (1.13 ± 0.20; 
1.19 ± 0.26; and 1.17 ± 0.27  mmol  L−1, respectively; 
P > 0.05), and after 3 min, [La] was similar in HIDIT and 
 LIHIIT (P = 0.007; ES: 0.05), but lower in  SIHIIT (P = 0.003; 
ES: 0.78) (Table. 3). At Tlim, neither [La] nor ̇VO2, HR 
nor RPE were significantly different between the three 
tests (see Table 3).
Table 2  Comparison of the power–duration relationship derived from 1/time model CP and work-time model CP
R2 coefficient of determination of the linear regression, CP critical power, W′ total work sustainable above the critical power
Student paired t test: no significant differences between the parameters of the power–duration relationship derived from the two different CP 
models were observed
Subject
Critical power estimates
W’ estimates
R2
1/Time model 
CP (W)
Work-time model 
CP (W)
1/Time model 
W′ (kJ)
Work-time model 
W′ (kJ)
1/Time model
Work-time model
1
212
217
11.9
11.2
0.966
0.997
2
259
262
9.9
9.5
0.999
0.994
3
221
225
8.5
7.8
0.999
0.942
4
254
252
12.8
13.3
1.000
0.997
5
278
278
9.9
9.9
1.000
1.000
6
248
240
12.5
14.2
0.996
0.956
7
256
255
8.0
8.1
0.999
1.000
8
320
317
13.3
14.0
0.999
0.993
9
258
258
13.7
13.8
1.000
1.000
10
280
275
22.7
24.9
0.999
0.981
11
223
223
18.1
18.2
0.999
1.000
12
243
240
12.2
13.0
0.997
0.984
Mean
254
254
12.8
13.2
0.996
0.987
Standard deviation
30
28
4.1
4.7
0.010
0.019
t test
0.456
0.178
0.183
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Discussion
The results of the present study show that a new HIDIT 
protocol maintains the ̇VO2 above 90% of ̇VO2peak for a 
longer period compared with two classical HIIT protocols 
with short and long intervals. Nevertheless, the Tlim, [La], 
HR, RPE, and ̇VO2 were similar among the protocols. This 
is the first study that has demonstrated that it is possible to 
increase the time close to ̇VO2peak solely through decreasing 
the duration of the intervals and, therefore, avoiding reduc-
ing the power/intensity as previously shown (De Aguiar 
et al. 2013; Lisbôa et al. 2015; Rønnestad and Hansen 2016).
In HIDIT (and  LIHIIT), the protocol begins with 3 min 
at high intensity, as opposed to just 30 s in  SIHIIT, and this 
resulted in a greater ̇VO2, HR, [La], CR10 scale, and RQ 
after 3 min of exercise. This is consistent with the studies 
by Millet et al. (2003) and Turner et al. (2006), in which 
during long-interval HIIT, a faster metabolic stimulation 
occurred at the beginning of the cycling exercise. However, 
in our study, there were no differences at Tlim in any of the 
parameters mentioned above, suggesting that the participants 
reached their personal maximal performances, regardless of 
Table 3  Main results of 
the HIIT tests and selected 
physiological variable at 3rd 
minute and at the end of the 
tests
All values are mean and standard deviation (SD)
SIHIIT short-interval HIIT, HIDIT high-intensity decremental intervals training, LIHIIT long-interval HIIT, 
Tlim time to exhaustion, T > 90% ̇VO2peak time spent above 90% ̇VO2peak, % ̇VO2peak oxygen uptake in 
percentage relative to its peak, mean% ̇VO2peak mean % ̇VO2peak maintained during HIIT tests, %HRpeak 
heart rate in percentage relative to its peak, [La] blood lactate concentration, CR10-scale perceived exer-
tion, RQ respiratory quotient
Significance by one-way repeated-measure ANOVA. When P < 0.05, planned contrasts with Bonferroni 
correction
a P < 0.05 in post hoc HIDIT vs  SIHIIT
b P < 0.05 in post hoc HIDIT vs  LIHIIT
SIHIIT
HIDIT
LIHIIT
P
Tlim (s)
714 ± 265
798 ± 185
664 ± 282
0.144
T > 90% ̇VO2peak (s)
183 ± 225
312 ± 207a,b
179 ± 145
0.029
T > 90% ̇VO2peak × Tlim−1
0.25 ± 0.29
0.39 ± 0.24
0.26 ± 0.21
0.070
Work > CP (KJ)
18.74 ± 8.95
22.01 ± 10.40
19.28 ± 11.06
0.136
Mean % ̇VO2peak
81.50 ± 6.61
84.16 ± 4.00b
79.58 ± 7.08
0.044
Values at 3rd minute
 % ̇VO2peak
85.33 ± 7.11
90.75 ± 5.94a
89.58 ± 6.52
0.004
 %HRpeak
89.00 ± 4.00
91.00 ± 3.91a
92.60 ± 3.60
0.003
 [La] (mmol L−1)
5.69 ± 1.62
8.03 ± 2.69a
7.85 ± 3.01
0.007
 CR10-scale
5.29 ± 1.57
6.67 ± 2.12a
6.52 ± 2.03
0.008
 RQ
1.04 ± 0.06
1.10 ± 0.09a
1.11 ± 0.08
> 0.001
Tlim
 % ̇VO2peak
99.75 ± 8.62
100.17 ± 5.27
99.83 ± 8.36
0.981
 %HRpeak
97.80 ± 3.99
97.40 ± 2.99
97.50 ± 3.98
0.802
 [La] (mmol L−1)
10.75 ± 2.04
10.71 ± 4.72
10.83 ± 3.58
0.991
 CR10-scale
9.48 ± 0.70
9.25 ± 1.78
9.56 ± 1.08
0.701
 RQ
0.97 ± 0.05
0.95 ± 0.05
1.00 ± 0.10
0.113
Fig. 2  Time above 90% of ̇VO2 peak in seconds. *Significance by 
one-way repeated-measures ANOVA and planned contrast with Bon-
ferroni correction between HIDIT and  SIHIIT and between HIDIT and 
 LIHIIT were used post hoc comparison, P < 0.05
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the protocol adopted. Indeed, ̇VO2 and HR were close to the 
peak values (100% and 97%, respectively), while Borg scale 
was near 10 and [La] was above 10 mmol L−1. It is worth 
noting that HIDIT led to longer T > 90% ̇VO2peak despite the 
same RPE at the end of the exercise. In other words, HIDIT 
has potentially better training benefits, despite the same 
perceived effort. On the other hand, even though Tlim in 
HIDIT (798 s) was longer than in  LIHIIT (664 s) and, similar 
to  SIHIIT, (714 s), the ANOVA did not show any significant 
difference (P = 0.144). Our results seem to contradict results 
from the previous studies (Millet et al. 2003; Turner et al. 
2006; Rønnestad and Hansen 2016). Millet et al. (2003) 
showed that when comparing some matched work HIIT pro-
tocols, those with shorter intervals elicited lower ̇VO2, HR, 
and RPE at the end of the exercise, suggesting that the dura-
tion may be longer when shorter intervals are used. Simi-
larly, Turner et al. (2006) compared four HIIT protocols with 
the same intensity (work and recovery) and work/recovery 
ratio, reporting that in HIIT with shorter intervals, the [La] 
was lower after 30 min of exercise compared with longer 
intervals. In particular, in the HIIT protocol with shorter 
intervals (work 10 s/recovery 20 s), the [La] reached steady 
state after 30 min of exercise, whereas the one with longer 
intervals (work 90 s/recovery 180 s), the subjects lasted less 
than 10 min before exhaustion.
Surprisingly, there are a few studies in which the authors 
analyze the effects of interval duration at a fixed work/recov-
ery ratio and a fixed intensity (Millet et al. 2003; Turner 
et al. 2006; Rønnestad and Hansen 2016). It is known that 
increasing work interval durations prolongs the time close to 
̇VO2max (Rozenek et al. 2007; Wakefield and Glaister 2009). 
Conversely, longer recovery interval duration decreases the 
time close to ̇VO2max (Smilios et al. 2017). However, to 
our knowledge, the only study that measured the time close 
to ̇VO2max and Tlim in HIIT matching work rate and work/
recovery ratio and isolating the interval duration variable 
was performed by Rønnestad and Hansen (Rønnestad and 
Hansen 2016). They compared three cycling HIIT protocols 
in which the intensity of the work bouts was set at maximal 
aerobic power ( ̇VO2max power), the recovery at 50% of the 
̇VO2max power, and the work/recovery ratio was 2/1. They 
concluded that HIIT with shorter interval durations (30 s) led 
to a longer Tlim (~ 1400 s), a longer Time > 90% ̇VO2peak 
(~ 680 s) and a higher ratio of Time > 90% ̇VO2peak·Tlim−1 
(0.55) (Rønnestad and Hansen 2016). Tlim, Time > 90% 
̇VO2peak, and their ratio were lower in our study. This 
discrepancy may be attributed to the different protocols 
used and to the higher fitness level of the participants ( ̇V
O2peak = 66 mL kg−1 min−1 compared to 57 mL kg−1 min−1) 
(Rønnestad and Hansen 2016). Another possible explana-
tion might be the relative intensity at which our protocol 
was set (on average ~ 83% of load peak). This relative inten-
sity refers to the load peak attained during a ramp protocol, 
which is reported to be 10–15% higher than the load peak 
reached with a step modality (Revill et al. 2002; Bentley and 
McNaughton 2003; Zuniga et al. 2012). Therefore, it can 
be assumed that the relative power would have been above 
90% of the load peak if the incremental test was performed 
using steps. Nevertheless, the incremental ramp test was 
used alone in the present study only to determine ̇VO2peak, 
while the intensity of HIIT was set exclusively considering 
CP, as described above.
In an attempt to benefit from faster ̇VO2 kinetics at the 
beginning of exercise, we imposed long first intervals. Alter-
nately, other authors proposed a fast start strategy (De Agu-
iar et al. 2013; Lisbôa et al. 2015; Rønnestad et al. 2019). 
Fast start strategy HIIT protocol (starting from 125% of 
the intermittent critical power, ICP, and decreasing it until 
105%) enhanced the time above 95% of ̇VO2max compared 
to other protocols with a constant work rate at 125% ICP and 
a constant work rate at 105% ICP (De Aguiar et al. 2013). 
Nevertheless, the protocol that used lower intensity (105% 
ICP) increased Tlim, and the protocol that adopted higher 
intensity bouts (125% ICP) showed a greater ratio of Tlim/
time above 95% of ̇VO2max−1. Lisbôa et al (2015) decreased 
the intensity within every single interval, but attained simi-
lar results. In addition, the recent work of Rønnestad et al. 
(2019) confirmed that the fast start pacing strategy can be a 
good strategy to increase the average ̇VO2, but the time close 
to ̇VO2max was not longer compared to traditional HIIT. 
Therefore, the fast start strategy is a useful tool to improve 
time near/at ̇VO2max and could be successfully applied to 
HIIT, although it impairs Tlim in comparison with protocols 
with the same final exercise work rate and the ratio T > 90% 
̇VO2peak − Tlim−1 in comparison with protocols with the 
same initial intensity (De Aguiar et al. 2013). Compared 
to fast start protocols, HIDIT has the advantage of quickly 
stimulating oxygen uptake at the beginning without affecting 
Tlim. Moreover, fast start strategy HIIT reduces the ratio 
T > 90% ̇VO2peak—Tlim−1, while HIDIT tends to increase 
it (not significantly). Therefore, the HIDIT protocol that this 
study proposed combines the advantages of different previ-
ously studied protocols and can be used during training ses-
sions that aim to accumulate time close to ̇VO2max.
Nonetheless, it is interesting that several participants 
were able to drastically increase the T > 90%VO2peak 
in the HIDIT protocol, whereas others performed much 
worse. In addition, as discussed above, the ANOVA failed 
to find differences in Tlim between the three HIIT proto-
cols, which could be due to the heterogeneity of the sub-
jects, despite our efforts to minimize differences by set-
ting up HIIT reliant on CP and W′. In fact, high intensity 
was set as the percentage of CP that allowed each subject 
to last for 5 min before exhaustion according to equation 
[2]. While the intensity of HIIT is often set relying on 
% ̇VO2max, relying exclusively on ̇VO2max does not take 
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1 3
into account the anaerobic characteristics of the subjects, 
which are very important in HIIT. For instance, whether 
two athletes present a similar ̇VO2max intensity but differ-
ent W′ (and CP) when exercising with similar % ̇VO2max 
intensity during HIIT, the exercise will actually involve 
a different proportion of their W′, which results in a dif-
ferent exercise tolerance (Blondel et al. 2001). Therefore, 
expressing intensity as a percentage of CP for high-inten-
sity exercises allows individual differences in W′ to be 
taken into account and eased as much as possible. Indeed, 
W′ was not correlated with Tlim of any HIIT test, since 
it has been used to adjust the intensity with equation [2]. 
Furthermore, there was no correlation among age/HRpeak, 
the ̇VO2 kinetics during the first 3 min of HIDIT and  LIHIIT 
(unpublished), and the other main outcomes. Additionally, 
there were no relationships between ̇VO2peak or CP and 
the main outcomes as well. The lack of relationship among 
age and other variables suggests that age did not influ-
ence our main results. In fact, our data may even support 
the idea that HIDIT could be applied in well-trained male 
adults over a wide range of age. Another major physiologi-
cal determinant that may explain the variability between 
subjects in Tlim during interval and continuous exercises 
is the differences between lactate threshold intensity and 
̇VO2max intensity (Midgley et al. 2007). Midgley et al. 
suggested that athletes with larger differences will replete 
their anaerobic capacity to a greater extent during each 
relief interval, increasing the time to exhaustion. Similarly, 
the relationship between the CP-load peak difference and 
Tlim during HIIT has been verified in this study to deter-
mine whether it can affect the Tlim of HIIT. As a result, 
only 59% of the variance in Tlim in  SIHIIT was explained 
by the difference between CP and load peak in percentage, 
while in the other two protocols, there were no relation-
ships. Therefore, future research that aims to investigate 
Tlim in HIIT may benefit by selecting subjects with homo-
geneous difference %CP-load peak, although Tlim in HIIT 
with longer intervals does not seem to correlate with it. It 
is, therefore, tempting to suggest that individuals with a 
wide gap between the CP and the load peak could benefit 
more from short-interval HIIT to prolong Tlim.
Further research is needed to verify whether T > 90% ̇V
O2peak may be enhanced with HIDIT in different HIIT pro-
tocols (i.e., at different intensities) and in different popu-
lations. However, HIDIT might be useful in sport training 
when the aim is to maintain a high ̇VO2max and/or maintain 
a specific power or velocity as long as possible, such as in 
training for track cycling races. If the aim is to allow the 
athlete to finish the race at a given time, the most specific 
training is to ride at that velocity for that race time for a 
distance as near as possible to the distance of the race. After 
the recovery, repeat for a shorter distance and so on. Starting 
with short intervals would not be sufficiently specific, and 
continuing with the first interval distance would not be pos-
sible for the fatigued athlete.
Furthermore, HIDIT could be useful for patients or for 
wellness purposes, setting a lower percentage of ̇VO2max or 
other physiological parameters. For example, if an exercise 
is intended to avoid exceeding a given [La] cut-off, it can 
start with a longer interval to save time and then decrease 
the length of the interval to avoid exceeding the [La] cut-off. 
However, we suggest adopting this protocol in athletes and 
patients who aim to train and improve their ̇VO2max.
Conclusions
In conclusion, HIDIT applied to cycling exercise in well-
trained amateur cyclists can enhance T > 90% ̇VO2peak with-
out reducing Tlim, the ratio of T > 90% ̇VO2peak and  Tlim−1, 
or the average ̇VO2. In fact, the average ̇VO2 was even higher 
in HIDIT than in  LIHIIT. Finally, despite the higher stimu-
lation of ̇VO2, the rate of perceived exertion and the other 
physiological parameters at the end of the exercise were 
not different compared with long- or short-interval HIIT, 
suggesting that HIDIT was not more demanding. In light 
of the favorable or similar physiological and/or perceptual 
responses to HIDIT compared to the other protocols and 
given the improved capability to prolong the time close to 
̇VO2peak, it could be used as a preferable method to elicit 
similar or greater physiological adaptations.
Acknowledgements Open access funding provided by Università degli 
Studi di Udine within the CRUI-CARE Agreement. We would like to 
thank the participants in the study for their time and effort to ensure the 
success of the project, in particular the “Pedale Gemonese” associa-
tion (Gemona del Friuli, Udine, Italia). The study was supported by 
Fondazione Pietro Pittini (Italy).
Author contributions All authors conceived and designed the research. 
FV and NG conducted experiments. FV analyzed the data. FV wrote 
the manuscript, NG and SL the manuscript. All authors read and 
approved the manuscript.
Compliance with ethical standards 
Conflict of interest The authors report no conflict of interest.
Open Access This article is licensed under a Creative Commons Attri-
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tion, distribution and reproduction in any medium or format, as long 
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provide a link to the Creative Commons licence, and indicate if changes 
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copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.
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High-intensity decreasing interval training (HIDIT) increases time above 90% [Formula: see text]O<sub>2</sub>peak.
Vaccari, Filippo,Giovanelli, N,Lazzer, S