Vol.:(0123456789)
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European Journal of Applied Physiology (2021) 121:425–434 
https://doi.org/10.1007/s00421-020-04535-x
ORIGINAL ARTICLE
The acute physiological and perceptual effects of recovery interval 
intensity during cycling‑based high‑intensity interval training
Christopher R. J. Fennell1 · James G. Hopker1 
Received: 4 May 2020 / Accepted: 13 October 2020 / Published online: 23 October 2020 
© The Author(s) 2020
Abstract
Purpose The current study sought to investigate the role of recovery intensity on the physiological and perceptual responses 
during cycling-based aerobic high-intensity interval training.
Methods Fourteen well-trained cyclists ( ̇VO2peak : 62 ± 9 mL kg−1 min−1) completed seven laboratory visits. At visit 1, the 
participants’ peak oxygen consumption ( ̇VO2peak ) and lactate thresholds were determined. At visits 2–7, participants com-
pleted either a 6 × 4 min or 3 × 8 min high-intensity interval training (HIIT) protocol with one of three recovery intensity 
prescriptions: passive (PA) recovery, active recovery at 80% of lactate threshold (80A) or active recovery at 110% of lactate 
threshold (110A).
Results The time spent at > 80%, > 90% and > 95% of maximal minute power during the work intervals was significantly 
increased with PA recovery, when compared to both 80A and 110A, during both HIIT protocols (all P ≤ 0.001). However, 
recovery intensity had no effect on the time spent at > 90% ̇VO2peak (P = 0.11) or > 95% ̇VO2peak (P = 0.50) during the work 
intervals of both HIIT protocols. Session RPE was significantly higher following the 110A recovery, when compared to the 
PA and 80A recovery during both HIIT protocols (P < 0.001).
Conclusion Passive recovery facilitates a higher work interval PO and similar internal stress for a lower sRPE when compared 
to active recovery and therefore may be the efficacious recovery intensity prescription.
Keywords Recovery components · Recovery interval intensity · High-intensity interval training · Near-infrared 
spectroscopy
Abbreviations
ANOVA  Analysis of variance
ACT 
 Active
AIT 
 Aerobic interval training
B[La] 
 Blood lactate concentration
HR 
 Heart rate
HRmax 
 Maximal minute heart rate
HIIT 
 High-intensity interval training
HHb 
 Deoxyhaemoglobin
LT 
 Lactate threshold
NIRS 
 Near-infrared spectroscopy
O2Hb 
 Oxyhaemoglobin
O2 
 Oxygen
PA 
 Passive
PO 
 Power output
RPE 
 Rating of perceived exertion
sRPE 
 Session RPE
TSI% 
 Tissue saturation index
VL 
 Vastus lateralis muscle
̇VO2 
 Pulmonary oxygen uptake
̇VO2peak 
 Peak oxygen consumption
̇VO2max 
 Maximal oxygen consumption
MMP 
 Maximal minute power
80A 
 80% Power output at lactate threshold
110A 
 110% Power output at lactate threshold
Introduction
High-intensity interval training (HIIT) is an intermittent 
mode of endurance training, characterised by short high-
intensity work intervals (4 s to ≥ 10 min). Its discontinuous 
Communicated by Jean-René Lacour.
 * James G. Hopker 
 
J.G.Hopker@kent.ac.uk
1 
School of Sport and Exercise Sciences, University of Kent 
at Medway, Medway Building, Kent, Chatham ME4 4AG, 
England, UK
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nature, by design, allows for the accumulation of a greater 
amount of time exercising in the ‘red zone’ (i.e. above criti-
cal power, the lactate steady state or ≥ 90% of maximal oxy-
gen consumption [ ̇VO2max ]; Buchheit and Laursen 2013), 
than could be tolerated during a single bout of continu-
ous intensity exercise (MacDougall and Sale 1981). This 
is important because there is strong evidence that the per-
formance of exercise at higher intensities elicits a greater 
activation of signalling pathways, associated with specific 
molecular responses which lead to an enhancement of the 
adaptive phenotype (Coffey and Hawley 2007). The perfor-
mance benefits of HIIT alone are particularly powerful in 
untrained and recreationally active individuals (Milanovic 
et al. 2016), whilst highly trained athletes can also further 
enhance endurance performance by undertaking relatively 
short periods of HIIT (Hawley et al. 1997; Iaia and Bangsbo 
2010; Laursen 2010).
The multivariate equation of HIIT programming contains 
five main components: work interval intensity, work interval 
duration, number of work intervals, recovery interval inten-
sity and recovery interval duration (Tschakert and Hofmann 
2013). Researchers have sought to optimise HIIT protocols, 
placing particular focus on the work interval components 
as this is where the training stimulus is primarily gener-
ated (Buchheit and Laursen 2013; Tschakert and Hofmann 
2013). Nevertheless, optimal work interval performance 
(accumulating time at effective training intensities i.e. ≥ 90% 
̇VO2max ), can only be achieved if separated by a correctly 
programmed recovery interval (Schoenmakers et al. 2019). 
Therefore, understanding the effects of altering the recov-
ery interval components on subsequent work interval per-
formance is key when looking to programme an effective 
HIIT session.
There has been a sizeable amount of research focusing 
specifically on understanding the acute effects of recovery 
interval intensity during cycling-based aerobic interval train-
ing (AIT; long work intervals ≥ 1 min; Barbosa et al. 2016; 
Coso et al. 2010; Dorado et al. 2004; Monedero and Donne 
2000; McAinch et al. 2004; Siegler et al. 2006; Stanley and 
Buchheit 2014). Researchers investigating recovery inten-
sity during cycling-based AIT have tended to use time to 
exhaustion work intervals (Barbosa et al. 2016; Siegler et al. 
2006; Dorado et al. 2004) and fixed intensity work intervals 
(Stanley and Buchheit 2014; Coso et al. 2010). Whilst only 
two have utilised self-paced fixed duration work interval 
prescriptions (McAinch et al. 2004; Monedero and Donne 
2000), which have been suggested to be an athlete’s typical 
approach to HIIT training (Seiler et al. 2011). McAinch et al. 
(2004), required participants to complete 2 × 20-min self-
paced maximal effort work intervals (i.e. isoeffort) separated 
by a 15-min passive (PA) recovery or active (ACT) recovery 
at 40% of ̇VO2peak . They found no difference in work per-
formed during intervals between the ACT and PA protocols. 
Monedero and Donne (2000) used 2 × 5-km self-paced maxi-
mal effort work intervals separated by either a 20-min PA 
recovery, a massage, ACT recovery at 50% of ̇VO2max , or a 
combined ACT recovery/massage. The combined recovery 
condition was found to be the most effective for maintenance 
of 5-km performance time. Both studies provide informative 
insights into the effect of recovery intensity on the perfor-
mance of high-intensity AIT. However, further research uti-
lising different HIIT protocol designs and recovery intensi-
ties is required in order to broaden the understanding of the 
role of recovery interval intensity on the acute responses to 
self-paced AIT. The current study therefore sought to inves-
tigate the role of recovery intensity on the physiological and 
perceptual responses during cycling-based AIT.
Methods
Participants
Fourteen trained cyclists participated in the study. All par-
ticipants had a minimum of 2 years competitive racing 
experience and were in training for the next competitive 
season. According to De Pauw et al. (2013), participants 
were classified as follows: nine were performance level 3 
(trained), four were performance level 4 (highly trained) 
and one was performance level 5 (professional). The study 
was completed with full ethical approval, according to the 
Declaration of Helsinki standards. All participants provided 
signed informed consent prior to testing,
Study design
Each participant completed seven visits to the laboratory. 
Visit 1 being incremental exercise tests to identify the lactate 
threshold (LT), ̇VO2max and to familiarise the participants 
with the laboratory environment and equipment. In visits 
2–7, participants performed six HIIT sessions in a ran-
domised order (using simple randomisation; Roberts and 
Torgerson 1998) using different recovery intensities: PA, 
ACT at 80% of power output (PO) at the LT (80A) and ACT 
at 110% of PO at the LT (110A). The 80A and 110A recov-
ery intensities were selected to straddle the LT and intended 
to provide differing levels of recovery. The 4-min and 8-min 
work durations were selected having previously been used 
in HIIT research to bring about training adaptation (Stepto 
et al. 1999; Seiler et al. 2011).
Visits were conducted on non-concurrent days and par-
ticipants were instructed to refrain from any exercise in the 
day prior to testing and intense exercise in 2 days prior. 
Participants were instructed to arrive euhydrated and in a 
post-prandial state, having eaten at least 4-h prior to test-
ing. Participants were told to not consume caffeine within 
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European Journal of Applied Physiology (2021) 121:425–434 
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4-h and alcohol within 24-h of testing. Each participant 
completed all their visits to the laboratory at the same time 
of day to avoid any circadian variance. An electric fan was 
placed 2 m in front of the participants to provide cooling 
during all tests.
Participants used their own bike at all visits, affixed to a 
Cyclus2 ergometer (PO ± 2% maximal error; Rodger et al. 
2016) calibrated to the manufacturer’s instructions (Leip-
zig, Germany). At all visits respiratory gas exchange data 
were assessed using breath by breath gas analysis (Meta-
lyzer 3B; CORTEX Biophysik GmbH, Leipzig, Germany). 
Prior to all testing, the analyser was calibrated according to 
the manufacturer recommendations. Heart rate (HR) was 
assessed at all visits using Garmin HR monitors (Garmin, 
Kansas, USA).
Preliminary testing
Participants were measured for anthropometric values: 
height and mass. Prior to starting the LT test resting blood 
lactate (B[La]) samples were taken. The participants then 
completed a 10-min warm-up at 50 W followed by an incre-
mental exercise test during which PO was initially set at 
80 W for 4 min, and then increased by 20 W every 4 min. 
The 4-min increments continued until B[La] > 4 mmol L−1. 
Participants completed a cool down for 10 min at 50 W, after 
which they completed seated rest for 10 min, before com-
mencing the ̇VO2max test protocol.
During the LT test B[La], samples were collected using 
fingertip capillary blood 30 s before the end of each stage. 
Blood samples were analysed using a Biosen C-Line (EKF 
Diagnostic, London, UK). PO and HR were continuously 
measured throughout the test, and rating of perceived exer-
tion (RPE) measurements were asked at the end of each 
stage using the Borg 6 to 20-point scale (Borg 1982). The 
first LT was assessed as the point at which B[La] breaks 
from linearity (Yoshida et al. 1987). The lactate turnpoint 
(LTP) was assessed as the second break point after which 
B[La] begins to rise above 4 mmol L−1 (Faude et al. 2009).
The ̇VO2max test protocol started with a 10-min warm-up 
at 100 W, after which the required cycling PO was increased 
by 20 W every 1 min until the participant reached volitional 
exhaustion (operationally defined as a cadence of < 60 revo-
lutions/min for > 5 s, despite strong verbal encouragement). 
PO and HR were measured continuously throughout the test, 
with RPE measurements taken in the last 10 s of each 1-min 
stage of the test (Borg 1982). The participant’s ̇VO2peak was 
assessed as the highest pulmonary oxygen consumption 
( ̇VO2 ) that was attained during a 1-min period in the test. 
Maximal minute power (MMP) and maximal minute heart 
rate  (HRmax) were assessed as the highest mean 1-min PO 
and HR achieved during the test.
HIIT sessions
Participants completed both the 6 × 4-min and 3 × 8-min 
HIIT sessions three times (6 HIIT sessions in total), once 
with each of the three recovery interval intensities: PA, 80A 
and 110A. The ACT recovery intensities were calculated as 
80% and 110% of the participants PO at the LT (Table 1). 
During the PA recovery intensity, HIIT session participants 
were instructed to remain seated with their right leg at the 
bottom of the pedal stroke.
All HIIT sessions had an equal work duration of 24 min. 
Work intervals were prescribed as self-paced on a ‘maximal 
session effort’ basis, with participants instructed to achieve 
the highest PO possible during each interval. Participants 
were only shown time elapsed during the HIIT sessions. 
Consistent verbal encouragement was given throughout 
every session. HIIT sessions commenced with a 10-min 
warm-up at 100 W and finished with a 10-min cool down at 
100 W. Recovery interval durations were a standardised 2:1 
work:recovery ratio (2 min and 4 min for the 6 × 4-min and 
3 × 8-min HIIT sessions, respectively).
PO, HR, near-infrared spectroscopy (NIRS) and respira-
tory gas data were measured continuously throughout the 
HIIT sessions. B[La] was measured via a fingertip capillary 
blood sample and analysed as outlined above. Samples were 
taken prior to the warm-up and during the last 30 s of each 
work interval. RPE measurements were taken during the last 
15 s of each work interval (Borg 1982). Session RPE (sRPE) 
Table 1  Participants characteristics and preliminary test results 
(mean ± SD)
PO power output, LT lactate threshold, LTP lactate turnpoint, VL 
vastus lateralis muscle, ̇VO2peak maximal oxygen consumption, MMP 
maximal minute power, HRmax maximal minute heart rate
Age (years)
33 ± 13
Height (cm)
176.6 ± 5.9
Mass (kg)
70.6 ± 8.1
VL skin fold (mm)
9.5 ± 2.7
̇VO2peak (L min−1)
4.3 ± 0.6
Relative ̇VO2peak (mL kg min−1)
62 ± 9
MMP (W)
370 ± 56
Relative MMP (W kg−1)
5.2 ± 0.8
HRmax (bpm)
187 ± 11
PO at LT (W)
205 ± 44
PO at LTP (W)
273 ± 48
RPE at LT (6–20)
11 ± 1
RPE at LTP (6–20)
15 ± 1
80A recovery intensity (W)
164 ± 35
110A recovery intensity (W)
225 ± 48
Years training
6.8 ± 6
Years competing
6.3 ± 5.4
Mean weekly training hours
9.1 ± 2.9
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1 3
measurements were taken using a 0 to 10-point scale at the 
end of the 10-min cool down (Foster et al. 2001).
NIRS data were acquisitioned at 10 Hz from the right 
vastus lateralis muscle (VL; 8 cm from the knee joint on 
the vertical axis) using a portable continuous-wave NIRS 
device (Portamon, Artinis Medical Systems, The Nether-
lands), which simultaneously uses the Beer-Lambert and 
spatially resolved spectroscopy method. Changes in tissue 
oxyhaemoglobin  (O2Hb) and deoxyhaemoglobin (HHb) 
were measured using the differences in absorption charac-
teristics at three wavelengths 770, 850 and 905 nm (corre-
sponding to the absorption wavelengths of  O2Hb and HHb). 
An ischemic calibration procedure was performed before 
each session to scale the NIRS  O2Hb and HHb signals to 
the maximal physiological range, as previously described by 
Ryan et al. (2013). Skinfold thickness at the site of applica-
tion of the NIRS optode was determined before each HIIT 
sessions using Harpenden skinfold callipers (British indica-
tors Ltd, Burgess Hill, UK).
Data analyses
Time above percentages of MMP,  HRmax and ̇VO2peak dur-
ing the work intervals was calculated by summing all raw 
PO, HR and ̇VO2 measures over the established cut off. 
Raw PO, HR and ̇VO2 data were averaged over each work 
and recovery interval. The Δ  O2Hb and Δ tissue saturation 
index (TSI%) were calculated as the change from the last 
30-s average of the work interval to the last 30-s average of 
the recovery interval.
Statistical analyses
Data were presented as individual values or mean ± SD 
(unless specified otherwise). Statistical analyses were con-
ducted using IBM SPSS Statistics 26 (IBM, Armonk, New 
York, USA). Visual inspection of Q–Q plots and Shap-
iro–Wilk statistics were used to check whether data were 
normally distributed. Three separate two-way repeated 
measures ANOVA, (1) two HIIT protocols (6 × 4 min vs 
3 × 8 min) × three recovery intensities (PA, 80A and 110A); 
(2) three recovery intensities (PA, 80A and 110A) × num-
ber of work intervals; (3) three recovery intensities (PA, 
80A and 110A) × number of recovery intervals were used 
to determine between and within condition effects for all 
dependent variables. Bonferroni post hoc comparisons were 
used when a main effect or interaction was significant. Par-
tial eta squared (ηp
2) was computed as effect size estimates 
and were defined as small (ηp
2 = 0.01), medium (ηp
2 = 0.06) 
and large (ηp
2 = 0.14; Lakens 2013). The significance level 
was set at P < 0.05 in all cases.
Results
Participants’ characteristics/anthropometrics are presented 
in Table 1.
The PA recovery protocol resulted in a longer time 
spent at > 80% MMP (P ≤ 0.001; ηp
2 = 0.54), > 90% MMP 
(P ≤ 0.001; ηp
2 = 0.62) and > 95% MMP (P ≤ 0.001; 
ηp
2 = 0.49) during the work intervals, when compared to 
the 80A and 110A recovery protocols of the 6 × 4-min and 
3 × 8-min HIIT sessions. Despite the differences in time 
spent at high percentages of MMP, there was no effect 
of recovery intensity on the time spent at > 80% ̇VO2peak 
(P = 0.10; ηp
2 = 0.15), > 90% ̇VO2peak (P = 0.11; ηp
2 = 0.16) 
and > 95% ̇VO2peak (P = 0.50; ηp
2 = 0.05) during the work 
intervals of the 6 × 4-min and 3 × 8-min HIIT sessions 
(Table 2).
There was no effect of recovery intensity on the time spent 
at > 90%  HRmax during the work intervals of the 6 × 4-min 
HIIT session (P = 0.07; ηp
2 = 0.42). The PA recovery proto-
col did increase the time spent at > 95%  HRmax (P ≤ 0.001; 
ηp
2 = 0.53) during the work intervals, when compared to the 
80A and 110A recovery protocols of the 6 × 4-min HIIT 
session. The PA recovery protocol increased the time spent 
at > 90%  HRmax (P = 0.012; ηp
2 = 0.52) during the work inter-
vals of the 3 × 8-min HIIT session, when compared to the 
Table 2  Time spent in seconds above percentages of ̇VO2peak ,  HRmax and MMP during the work intervals
̇VO2peak peak oxygen consumption, HRmax maximal minute heart rate, MMP maximal minute power, Ω significant difference between PA and 
110A, β significant difference between PA and 80A, α significant difference between 80 and 110A
Prescription
Time at % ̇VO2peak
Time at %HRmax
Time at %MMP
80
90
95
80
90
95
80
90
95
PA 6 × 4
1168 ± 141
806 ± 266
516 ± 263
1265 ± 63
954 ± 145
591 ± 221 Ωβ
940 ± 386 Ωβ
89 ± 76 Ωβ
52 ± 50 Ωβ
80A 6 × 4
1034 ± 358
669 ± 392
444 ± 328
1272 ± 96
734 ± 267
254 ± 251
625 ± 506
19 ± 28
15 ± 25
110A 6 × 4
1161 ± 372
749 ± 417
523 ± 384
1327 ± 99
902 ± 165
333 ± 236
465 ± 470
26 ± 32
15 ± 23
PA 3 × 8
1217 ± 131
841 ± 321
499 ± 301
1313 ± 59
962 ± 218 β
539 ± 268
654 ± 372 Ωβ
48 ± 39 Ωβ
27 ± 29 Ωβ
80A 3 × 8
1116 ± 334
686 ± 320
383 ± 274
1301 ± 84
817 ± 299
363 ± 288
362 ± 362
19 ± 28
14 ± 24
110A 3 × 8
1101 ± 323
640 ± 373
377 ± 332
1337 ± 54
887 ± 215
350 ± 220
209 ± 215
17 ± 25
10 ± 14
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1 3
80A recovery protocol (P = 0.12) but not the 110A recovery 
protocol (P > 0.05). There was no effect of recovery intensity 
on the time spent at > 95%  HRmax during the work intervals 
of the 3 × 8-min HIIT session (P = 0.10; ηp
2 = 0.32; Table 2).
Recovery intensity had an effect on perceptual responses 
with participants reporting a higher sRPE during the 110A 
recovery protocol, when compared to the PA and 80A recov-
ery protocols of the 6 × 4-min HIIT session (PA, 8.3 ± 0.7 vs 
80A, 8.7 ± 0.6 vs 110A, 9.1 ± 0.5 [95% CL: PA, 7.9–8.6 vs 
80A, 8.3–9.0 vs 110A, 8.8–9.4]; P ≤ 0.001; ηp
2 = 0.81) and 
the 3 × 8-min HIIT session (PA, 8.6 ± 0.7 vs 80A, 8.5 ± 0.6 
vs 110A, 9.1 ± 0.5 [95% CL: PA, 8.2–9.0 vs 80A, 8.1–8.8 vs 
110A, 8.8–9.4]; P ≤ 0.001; ηp
2 = 0.79).
Statistics and effect size estimations from the ANOVA 
for each work interval variable are shown in Table 3. There 
were interactions found between recovery intensity and work 
interval for PO (3 × 8; Fig. 1b), HR (Fig. 1c, d) and ̇VO2 
(Fig. 1e, f). No interactions between recovery intensity and 
work intervals were found for PO (6 × 4; Fig. 1a), B[La] 
(Fig. 1g, h) and RPE (Fig. 1i, j). There was a main effect of 
recovery intensity for PO and B[La] (6 × 4), but not for ̇VO2 , 
HR, B[La] (3 × 8) and RPE. There was a main effect of work 
interval number found for PO (6 × 4), HR, ̇VO2 , B[La] and 
RPE, but not for PO (3 × 8). A main effect of session type 
was only found for PO. Higher work interval PO was pro-
duced during the 6 × 4-min HIIT sessions, when compared 
to the 3 × 8-min HIIT sessions.
Recovery intensity had an effect on the physiologi-
cal response of the recovery intervals. Both ACT recov-
ery protocols produced significantly higher mean recov-
ery interval HR (6 × 4-min: PA, 145 ± 8 vs 80A, 157 ± 11 
vs 110A, 164 ± 9 bpm; 3 × 8-min: PA, 128 ± 10 vs 80A, 
148 ± 11 vs 110A, 161 ± 12 bpm; P ≤ 0.001; ηp
2 = 0.89) and 
̇VO2 (6 × 4-min: PA, 1.9 ± 0.3 vs 80A, 3.4 ± 0.9 vs 110A, 
3.8 ± 0.8 L min−1; 3 × 8-min: PA, 1.4 ± 0.2 vs 80A, 3.0 ± 0.6 
vs 110A, 3.5 ± 0.7 L min−1; P ≤ 0.001; ηp
2 = 0.91) when 
compared to the PA protocol, during both HIIT sessions.
Percentage HHb was significantly higher at the end of 
the recovery intervals during the 80A and 110A recovery 
protocols, when compared to the PA recovery protocols dur-
ing both HIIT sessions (P ≤ 0.001; ηp
2 = 0.95; Fig. 2a, b). 
There was a greater change in percentage  O2Hb during the 
PA recovery intervals, when compared to the 80A and 110A 
recovery intervals during both HIIT sessions (P ≤ 0.001; 
ηp
2 = 0.95; Fig. 2c, d). There was a greater change in TSI 
% during the PA recovery intervals, when compared to the 
80A and 110A recovery intervals during both HIIT sessions 
(P ≤ 0.001; ηp
2 = 0.91; Fig. 2e, f).
Discussion
The main finding of the study was the prescription of ACT 
recovery intervals significantly impairs work interval per-
formance. Specifically, mean work interval PO (Fig. 1a, b) 
and time spent > 80%, > 90% and 95% of MMP (Table 2) 
were significantly higher during the PA recovery protocols, 
when compared to both ACT recovery protocols. Work 
interval POs were significantly higher during the 6 × 4-min 
HIIT protocols compared to the 3 × 8-min HIIT protocols; 
however, the manipulation of recovery intensity resulted in 
similar physiological and perceptual responses during the 
work intervals of both HIIT protocol designs (Table 3).
Table 3  Statistics and effect-
size estimations from analysis 
of variance for each work 
interval variable analysed
PO power output, HR heart rate, ̇VO2 oxygen consumption, B[La] blood lactate concentration, RPE rating 
of perceived exertion
*Statistical significance. Effect sizes defined as small (ηp
2 = 0.01), medium (ηp
2 = 0.06), and large 
(ηp
2 = 0.14)
Variable
Prescription
Interaction (inten-
sity × interval)
Main effect of 
recovery intensity
Main effect of 
work interval 
number
Main effect of ses-
sion type (6 × 4 vs 
3 × 8)
P
ηp
2
P
ηp
2
P
ηp
2
P
ηp
2
PO
6 × 4
0.11
0.11
0.001*
0.44
0.001*
0.26
< 0.001*
0.68
3 × 8
0.04*
0.17
0.021*
0.26
0.69
0.03
HR
6 × 4
< 0.001*
0.43
0.09
0.19
< 0.001*
0.89
0.21
0.14
3 × 8
< 0.001*
0.50
0.10
0.17
< 0.001*
0.83
̇VO2
6 × 4
< 0.001*
0.32
0.06
0.20
< 0.001*
0.72
0.84
< 0.01
3 × 8
0.006*
0.24
0.52
0.05
< 0.001*
0.74
B[La]
6 × 4
0.08
0.12
< 0.001*
0.49
< 0.001*
0.59
0.26
0.10
3 × 8
0.10
0.15
0.06
0.22
< 0.001*
0.53
RPE
6 × 4
0.06
0.12
0.09
0.17
< 0.001*
0.87
0.24
0.11
3 × 8
0.19
0.11
0.02
0.26
< 0.001*
0.86
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1 3
The ACT recovery intervals increased the oxygen  (O2) 
demand at the exercising muscle, as shown by the greater 
deoxygenation of the VL (Fig. 2a, b).  O2Hb and TSI% were 
therefore unable to recover to the same extent by the end 
of the recovery interval, in comparison to the PA protocols 
(Fig. 2c–f). The increased deoxygenation of the VL mus-
cle (an important locomotor muscle during cycling perfor-
mance) would potentially impair key recovery processes, 
such as adenosine triphosphate and phosphocreatine resyn-
thesis, and muscle lactate clearance which require the avail-
ability of  O2 (Spencer et al. 2006). Moreover, insufficient  O2 
availability (i.e. local hypoxia) has been suggested to affect 
muscular performance and exaggerate the rate of develop-
ment of both central and peripheral fatigue (Amann and 
Calbet 2008). The more complete recovery provided by the 
PA protocols may explain the participant’s ability to sustain 
higher work interval POs, compared to the ACT recovery 
protocols. Buchheit et al. (2009), Kriel et al. (2016) and 
Ohya et al. (2013) support the findings of the current study 
by showing the increased deoxygenation of the VL muscle 
during ACT recovery lead to a reduction in work interval 
performance.
Time spent at high percentages of ̇VO2peak (≥ 90% and 
95%) is often used to quantify the effectiveness of a HIIT 
protocol (Thevenet et al. 2007; Buchheit and Laursen 
2013). When exercising close to ̇VO2peak , the  O2 delivery 
and utilisation systems are maximally stressed, which has 
been suggested to be an effective stimulus for improving 
̇VO2max and endurance performance (Buchheit and Laursen 
2013; Midgley et al. 2006). In the current study, recovery 
intensity had no effect on the duration participants spent 
at > 90% and > 95% of ̇VO2peak during both HIIT sessions 
(Table 2), despite the PA recovery protocols significantly 
reducing ̇VO2 at the start of subsequent work intervals. It 
has been suggested that commencing work intervals from 
a lower metabolic rate, as observed in the PA protocols, 
results in a higher ̇VO2 amplitude and reduces the time 
to reach a ̇VO2 plateau during subsequent work intervals 
(Schoenmakers and Reed 2018). In addition, the speed of 
̇VO2 response has been shown to be increased at higher 
work rates (Hill et al. 2002). Thus, the higher work inter-
val POs and increased time spent > 90% and > 95% of 
MMP during the PA recovery protocols would have likely 
provided a more potent driver for ̇VO2 , in comparison to 
the significantly lower work intensity of the ACT recovery 
protocols. The combination of the aforementioned factors 
provides a likely explanation for the similar times spent at 
high percentages of ̇VO2peak between PA and ACT recovery 
protocols.
Monitoring HR during training is commonplace for 
coaches and athletes, whilst HR is not directly related 
to muscular energy turnover or systemic  O2 demand 
(Buchheit et al. 2012; Wu et al. 2005), accumulated time 
at > 90%  HRmax and > 95%  HRmax has been used to quan-
tify adaptive effects (Seiler et al. 2011). In the present 
study, PA recovery lowered mean work and recovery 
interval HR, yet increased the time spent > 90%  HRmax by 
52–220 s and > 95%  HRmax by 176–337 s, when compared 
to both ACT recovery protocols (Table 2). Aligned to the 
̇VO2 data, it can be inferred that PA recovery results in 
a faster mean response time and a higher amplitude of 
̇VO2 and HR during subsequent work intervals, when com-
pared to ACT recovery (performed at ≥ 80% PO at LT). 
It is improbable that the increase in time > 90%  HRmax 
and > 95%  HRmax would elicit a greater adaptive stimu-
lus. Nevertheless, our findings indicate that maintaining 
an elevated ̇VO2 and HR during recovery is not necessary 
for reaching high fractions of ̇VO2peak and  HRmax during 
subsequent work intervals.
Low-intensity ACT recovery between work intervals 
has been shown to be more effective in the removal of 
B[La] than PA recovery (Bogdanis et al. 1996; Coso et al. 
2010; Siegler et al. 2006; Mandroukas et al. 2011). Current 
data show the ACT recovery protocols resulted in lower 
B[La] values when compared to the PA recovery proto-
cols, although only significant during the 6 × 4-min HIIT 
session (Fig. 1g, h). This is unlikely the result of the ACT 
recovery intervals facilitating a greater removal of B[La] 
when compared to the PA recovery intervals. As B[La] 
measurements were taken at the end of the work intervals, 
it is possible that the lower B[La] values were simply due 
to the lower work interval intensity of the ACT protocols 
(Fig. 1a, b). In accordance with evidence showing B[La] 
does not inhibit exercise performance (Hall et al. 2016), 
the higher B[La] values attained during the PA protocols 
did not affect subsequent work interval PO. These data 
support the prescription of PA recovery for increasing the 
metabolic stress during HIIT sessions, without affecting 
work interval performance. Whilst research should be used 
to guide HIIT design, coaches and athletes are advised to 
be cautious when extrapolating the findings beyond the 
scope of the HIIT protocols used.
There was a clear linear increase in work interval RPE 
throughout all HIIT sessions, with reported RPE values 
reaching ≥ 18 (very hard) at the last work interval (Fig. 1i, 
Fig. 1  a, b Mean PO, c, d mean HR, e, f mean ̇VO2 , g, h B[La], i, 
j RPE. Data are displayed per work interval as mean ± SD for the 
6 × 4-min and 3 × 8-min HIIT sessions with PA recovery intensity 
(closed triangles), 80A recovery intensity (open circles) and 110A 
recovery intensity (closed circles). φ significant difference from inter-
val 1, T significant difference from previous interval, Ω significant 
difference between PA and 110A, β significant difference between 
PA and 80A, α significant difference between 80A and 110A, χ main 
effect of recovery intensity (all P < 0.01), $ main effect of work inter-
val number (all P < 0.01). *P < 0.05; **P < 0.001
◂
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European Journal of Applied Physiology (2021) 121:425–434
1 3
j). The upward drift in physiological stress throughout the 
HIIT sessions provides an explanation for the increase in 
RPE, whilst it is also highly likely that biomechanical and 
psychological processes also effected the participant’s 
RPE (Marcora et al. 2009; Ulmer 1996). The higher RPE 
values reported during the PA protocols maybe linked to 
Fig. 2  a Percentage HHb during the last 30 s of the recovery intervals 
during the 6 × 4-min HIIT sessions, b percentage HHb during the last 
30 s of the recovery intervals during the 3 × 8-min HIIT sessions, c Δ 
 O2Hb during the recovery intervals of the 6 × 4-min HIIT sessions, 
d Δ  O2Hb during the recovery intervals of the 3 × 8-min HIIT ses-
sions, e Δ TSI% during the recovery intervals of the 6 × 4-min HIIT 
sessions, f Δ TSI% during the recovery intervals of the 3 × 8-min 
HIIT sessions. PA recovery intensity (closed triangles), 80A recov-
ery intensity (open circles) and 110A recovery intensity (closed cir-
cles). Values are mean ± SD. φ significant difference from interval 
1, T significant difference from previous interval, Ω significant dif-
ference between PA and 110A, β significant difference between PA 
and 80A, α significant difference between 80A and 110A. *P < 0.05; 
**P < 0.001
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European Journal of Applied Physiology (2021) 121:425–434 
1 3
the higher work interval POs (Fig. 1a, b) and percentages 
of MMP (Table 2) achieved during the PA protocols in 
comparison to the ACT protocols. Despite within session 
RPE being higher during the PA protocols, participants 
reported significantly higher sRPE values at the end of the 
110A recovery protocol when compared to the 80A and 
PA recovery protocols during both HIIT sessions. This 
finding is of particular interest from an applied perspective 
when programming HIIT. A HIIT protocol design which 
reduces an individual’s sRPE without negatively affect-
ing the physiological and metabolic load would likely be 
seen as a favourable session prescription by both athlete 
and coach.
Conclusion
ACT recovery at 80% and 110% of the LT significantly impairs 
performance PO but has a limited effect on the physiological 
stress of the work intervals during two closely matched HIIT 
designs, when compared to PA recovery. Based on current 
evidence, PA recovery between long ‘aerobic’ work intervals 
facilitates a higher external training load whilst maintaining a 
similar internal stress for a lower sRPE and therefore may be 
the efficacious recovery intensity prescription.
Acknowledgements Not applicable.
Author contributions CF and JH designed the research. CF conducted 
the experiments, data collection and data analysis. CF and JH wrote the 
manuscript. All authors read and approved the manuscript.
Funding Not applicable.
Data availability Data transparency.
Code availability Data analysis software application used (SPSS) 
openly available.
Compliance with ethical standards 
Conflict of interest Not applicable.
Ethical approval The study was completed with full ethical approval, 
according to the Declaration of Helsinki standards.
Consent to participate All participants provided signed informed con-
sent prior to testing,
Consent to publication All participants consented to having research 
findings published. All authors consented to publication of manuscript.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long 
as you give appropriate credit to the original author(s) and the source, 
provide a link to the Creative Commons licence, and indicate if changes 
were made. The images or other third party material in this article are 
included in the article’s Creative Commons licence, unless indicated 
otherwise in a credit line to the material. If material is not included in 
the article’s Creative Commons licence and your intended use is not 
permitted by statutory regulation or exceeds the permitted use, you will 
need to obtain permission directly from the copyright holder. To view a 
copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.
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The acute physiological and perceptual effects of recovery interval intensity during cycling-based high-intensity interval training.
Fennell, Christopher R J,Hopker, James G