Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials

Abstract

Background Enhancing cardiovascular fitness can lead to substantial health benefits. High-intensity interval training (HIT) is an efficient way to develop cardiovascular fitness, yet comparisons between this type of training with traditional endurance training are equivocal.
Objective Our objective was to meta-analyse the effects of endurance training and HIT on the maximal oxygen consumption (VO2max) of healthy, young to middle-aged adults.
Methods Six electronic databases were searched (MEDLINE, PubMed, SPORTDiscus, Web of Science, CINAHL and Google Scholar) for original research articles. A search was conducted and search terms included ‘high intensity’, ‘HIT’, ‘sprint interval training’, ‘endurance training’, ‘peak oxygen uptake’, ‘VO2max’. Inclusion criteria were controlled trials, healthy adults aged 18-45 y, training duration ≥2 weeks, VO2max assessed pre- and post-training. Twenty-eight studies met the inclusion criteria and were included in the meta-analysis. This resulted in 723 participants with a mean ± SD age and initial fitness of 25.1 ± 5 y and 40.8 ± 7.9 mL•kg-1•min-1, respectively. We made probabilistic magnitude-based inferences for meta-analysed effects based on standardized thresholds for small, moderate and large changes (0.2, 0.6 and 1.2, respectively) derived from between-subject standard deviations (SDs) for baseline VO2max.
Results The meta-analysed effect of endurance training on VO2max was a possibly large beneficial effect (4.9 mL•kg-1•min-1; 95% confidence limits ±1.4 mL•kg-1•min-1), when compared with no exercise controls. A possibly moderate additional increase was observed for typically younger subjects (2.4 mL•kg-1•min-1; ±2.1 mL•kg-1•min-1) and interventions of longer duration (2.2 mL•kg-1•min-1; ±3.0 mL•kg-1•min-1), and a small additional improvement for subjects with lower baseline fitness (1.4 mL•kg-1•min-1; ±2.0 mL•kg-1•min-1). When compared to no exercise controls, there was likely large beneficial effect of HIT (5.5 mL•kg-1•min-1; ±1.2 mL•kg-1•min-1), with a likely moderate greater additional increase for subjects with lower baseline fitness (3.2 mL•kg-1•min-1; ±1.9 mL•kg-1•min-1) and interventions of longer duration (3.0 mL•kg-1•min-1; ±1.9 mL•kg-1•min-1), and a small lesser effect for typically longer HIT repetitions (-1.8 mL•kg-1•min-1; ±2.7 mL•kg-1•min-1). The modifying effects of age (0.8 mL•kg-1•min-1; ±2.1 mL•kg-1•min-1) and work:rest ratio (0.5 mL•kg-1•min-1; ±1.6 mL•kg-1•min-1) were unclear. When compared to endurance training, there was a possibly small beneficial effect for HIT (1.2 mL•kg-1•min-1; ±0.9 mL•kg-1•min-1) with small additional improvements for typically longer HIT repetitions (2.2 mL•kg-1•min-1; ±2.1 mL•kg-1•min-1), older subjects (1.8 mL•kg-1•min-1; ±1.7 mL•kg-1•min-1), interventions of longer duration (1.7 mL•kg-1•min-1; ±1.7 mL•kg-1•min-1), greater work:rest ratio (1.6 mL•kg-1•min-1; ±1.5 mL•kg-1•min-1) and lower baseline fitness (0.8 mL•kg-1•min-1; ±1.3 mL•kg-1•min-1).
Conclusion Endurance training and HIT both elicit large improvements in the VO2max of healthy, young to middle-aged adults with the gains in VO2max being greater following HIT, when compared to endurance training.

Full text

SYSTEMATIC REVIEW
Effectiveness of High-Intensity Interval Training (HIT)
and Continuous Endurance Training for VO
2max
Improvements:
A Systematic Review and Meta-Analysis of Controlled Trials
Zoran Milanovic
´
1
•
Goran Sporis
ˇ
2
•
Matthew Weston
3
Published online: 5 August 2015
Ó Springer International Publishing Switzerland 2015
Abstract
Background Enhancing cardiovascular fitness can lead to
substantial health benefits. High-intensity interval training
(HIT) is an efficient way to develop cardiovascular fitness,
yet comparisons between this type of training and tradi-
tional endurance training are equivocal.
Objective Our objective was to meta-analyse the effects of
endurance training and HIT on the maximal oxygen con-
sumption (VO
2max
) of healthy, young to middle-aged adults.
Methods Six electronic databases were searched (MED-
LINE, PubMed, SPORTDiscus, Web of Science, CINAHL
and Google Scholar) for original research articles. A search
was conducted and search terms included ‘high intensity’,
‘HIT’, ‘sprint interval training’, ‘endurance training’, ‘peak
oxygen uptake’, and ‘VO
2max
’. Inclusion criteria were
controlled trials, healthy adults aged 18–45 years, training
duration C2 weeks, VO
2max
assessed pre- and post-training.
Twenty-eight studies met the inclusion criteria and were
included in the meta-analysis. This resulted in 723 partici-
pants with a mean ± standard deviation (SD) age and initial
fitness of 25.1 ± 5 years and 40.8 ± 7.9 mLkg
-1
min
-1
,
respectively. We made probabilistic magnitude-based
inferences for meta-analysed effects based on standardised
thresholds for small, moderate and large changes (0.2, 0.6
and 1.2, respectively) derived from between-subject SDs for
baseline VO
2max
.
Results The meta-analysed effect of endurance training on
VO
2max
was a possibly large beneficial effect
(4.9 mLkg
-1
min
-1
; 95 % confidence limits
±1.4 mLkg
-1
min
-1
), when compared with no-exercise
controls. A possibly moderate additional increase was
observed for typically younger subjects (2.4 mLkg
-1-
min
-1
; ±2.1 mLkg
-1
min
-1
) and interventions of longer
duration (2.2 mLkg
-1
min
-1
; ±3.0 mLkg
-1
min
-1
), and
a small additional improvement for subjects with lower
baseline fitness (1.4 mLkg
-1
min
-1
; ±2.0 mLkg
-1-
min
-1
). When compared with no-exercise controls, there
was likely a large beneficial effect of HIT (5.5 mLkg
-1-
min
-1
; ±1.2 mLkg
-1
min
-1
), with a likely moderate
greater additional increase for subjects with lower baseline
fitness (3.2 mLkg
-1
min
-1
; ±1.9 mLkg
-1
min
-1
) and
interventions of longer duration (3.0 mLkg
-1
min
-1
;
±1.9 mLkg
-1
min
-1
), and a small lesser effect for typi-
cally longer HIT repetitions (-1.8 mLkg
-1
min
-1
;
±2.7 mLkg
-1
min
-1
). The modifying effects of age
(0.8 mLkg
-1
min
-1
; ±2.1 mLkg
-1
min
-1
) and work/rest
ratio (0.5 mLkg
-1
min
-1
; ±1.6 mLkg
-1
min
-1
)were
unclear. When compared with endurance training, there was
a possibly small beneficial effect for HIT (1.2 mLkg
-1-
min
-1
; ±0.9 mLkg
-1
min
-1
) with small additional
improvements for typically longer HIT repetitions
(2.2 mLkg
-1
min
-1
; ±2.1 mLkg
-1
min
-1
), older subjects
(1.8 mLkg
-1
min
-1
; ±1.7 mLkg
-1
min
-1
), interventions
of longer duration (1.7 mLkg
-1
min
-1
; ±1.7 mLkg
-1-
min
-1
), greater work/rest ratio (1.6 mLkg
-1
min
-1
;
±1.5 mLkg
-1
min
-1
) and lower baseline fitness
(0.8 mLkg
-1
min
-1
; ±1.3 mLkg
-1
min
-1
).
Conclusion Endurance training and HIT both elicit large
improvements in the VO
2max
of healthy, young to middle-
& Zoran Milanovic
´
zoooro_85@yahoo.com
1
Faculty of Sport and Physical Education, University of Nis,
C
ˇ
arnojevic
´
eva 10a, 18000 Nis, Serbia
2
Faculty of Kinesiology, University of Zagreb, Zagreb,
Croatia
3
Department of Sport and Exercise Sciences, School of Social
Sciences, Business and Law, Teesside University,
Middlesbrough, UK
123
Sports Med (2015) 45:1469–1481
DOI 10.1007/s40279-015-0365-0

aged adults, with the gains in VO
2max
being greater fol-
lowing HI T when compared with endurance training.
Key Points
When compared with no exercise, endurance training
and high-intensity interval training elicit large
improvements in maximal oxygen uptake.
Endurance training and high-intensity interval
training elicit additional benefit for individuals with
lower pre-training fitness.
In healthy, young to middle-aged adults, high-
intensity interval training improves maximal oxygen
uptake to a greater extent than traditional endurance
training.
1 Introduction
Improving or maintaining cardiovascular fitness can
reduce the risk of all-cause and cardiovascular diseases
[1]. Indeed, when com pared with o ther well established
risk factors such as hypertension, diabetes mellitus,
smoking and obe sity, cardiovascular fitness is a more
powerful predictor of mortality [2, 3]. Fitnes s training
programme s aimed at the improvement of cardiovascular
fitness therefore have broad appeal to the general
population.
The fitness industry has recently seen a surge of interest
in high-intensity interval training (HIT)—a burst-and-re-
cover cycle that is suggested to be a viable alternative to
the traditional approach to enhancing aerobic fitness,
namely continuous endurance training [4]. However,
specifying an optimal training regimen for im proving fit-
ness in the general community requires knowledge of how
these different types of training influence adaptations in
physiological parameters [5]. Consequently, there has been
a substantial amount of research examining which modality
of training, endurance or HIT, is superior for aerobic fitness
improvements.
Endurance training and HIT both increase aerobic fit-
ness [6] and thus relate to benefits in cardiovascular risk
factors, fitness and all-cause mortality [7]. Some studies,
however, have suggested that HIT leads to improvements
in both aerobic and anaerobic fitness [8] and improves
endurance performance to a greater extent than endurance
training alone [9]. For example, Daussin et al. [10] found
that maximal oxygen uptake (VO
2max
) increases were
higher for untrained men and women who participated in
an 8-week HIT program me (15 %) than they were for
untrained participants undertaking an endurance training
programme (9 %). High-intensity interval training has also
been reported to be more effective than continuous, steady-
state exercise training for inducing fat loss in men and
women, despite requiring considerably less total energy
expenditure during training [11, 12]. Recent studies have
demonstrated that the cardiovascular adaptations occurring
following HIT are similar, and in some cases superior, to
those following endurance training [5, 13], and further
beneficial effects of HIT were provided by the Nord-
Trøndelag Health Study [13], which indicated that just a
single weekly bout of HIT reduced the risk of cardiovas-
cular disease in both men and women (relative risk: 0.61
and 0.49, respectively).
It is therefore not surprising that recent meta-analyses
[14–17] have confirmed HIT to be an appropriate training
stimulus to improve cardiorespiratory fitness and reduce
metabolic risk factors in patient populations. Using similar
inclusion criteri a to the aforementioned reviews, Bacon
et al. [18] meta-analysed the effect of HIT on VO
2max
but
only calculated an overall effect size, irrespective of the
type of control group (no-exercise or endurance training).
Consequently, we cannot conclude that HIT is better than
endurance training because the effect of HIT is, naturally,
much higher in comparison with no-exercise control groups
than the effect when compared with endurance training
controls. A separate analysis (HIT vs endurance training;
HIT vs no exercise) is therefore necessary to determine
more precise effects of HIT. Gist et al. [19] reported a
moderate effect (0.69) of sprint interval training (SIT)—
classified as a form of HIT at the highest end of the
intensity spectrum [20]—on VO
2max
in comparison with
no-exercise control groups; yet a trivial effect (0.04) when
compared with endurance training controls. However, this
meta-analysis [19], as well as the recent meta-analyses
performed by Weston et al. [21] and Sloth et al. [20], only
addressed the effect of SIT on VO
2max
. In doing so, these
reviews excluded HIT research utilizing longer interval
durations and shorter recovery periods. While there have
been meta-analyses on longer duration HIT repetitions in
patient populations [14–17], to the best of the authors’
knowledge there is no systematic review and meta-anal ysis
examining the effect of longer duration HIT repetitions in
comparison with either endurance training or no-exercise
controls. Therefore, our aim was to meta-analyse the
effects on VO
2max
of endurance training and HIT in heal-
thy, young to middle-aged adults, when compare d with no-
exercise controls and also when the two types of training
were compared with one another. A further aim was to
examine the modifying effects of study and subject
characteristics.
1470 Z. Milanovic
´
et al.
123

2 Methods
2.1 Search Strategy
Electronic database searches were performed using MED-
LINE, PubMed, SPORTDiscus, Web of Science, CINAHL
and Google Scholar using all available records up to 28
February 2014. The search terms covered the areas of high-
intensity interval training, continuous endurance training
and VO
2max
using a combination of the following key
words: high-intensity interval training, high-intensity
intermittent training, sprint interval training, endurance
training, continuous endurance training, aerobic exercises,
maximal oxygen uptake, peak oxygen uptake, cardiores-
piratory fitness, VO
2max
, young adults. The literature
search, quality assessment and data extraction were con-
ducted independently by two authors (ZM and GS). Papers
that were clearly not relevant were removed from the
database list before assessing all other titles and abstracts
using our pre-determined inclusi on and exclusion criteria.
Inter-reviewer disagreements were resolved by consensus
opinion or arbitration by a third reviewer. Full papers,
including reviews, were then collected and when not
available the corresponding author was contacted by mail.
Reference lists of the selected manuscripts were also
examined for any other potentially eligible papers. This
systematic review and meta-analysis was undertaken in
accordance with the Preferred Reporting Items for Sys-
tematic Reviews and Meta-Analyses (PRISMA) statement
[22].
2.2 Inclusion Criteria
2.2.1 Type of Study
Our meta-analysis included randomised and non-ran-
domised controlled trials, written in English. Uncontrolled
and cross-sectional studies were excluded from analysis
and only studies published in the last 20 years (after 1995)
were included in our review.
2.2.2 Type of Participants
The type of participants included i n our meta-analysis
were healthy, untrained, sedentary, recreational and non-
athleticmenandwomenagedbetween18and45years,
who were not suffering from any kind of acute or chronic
diseases. No exclusion criteri a w ere applied to partici-
pant baseline fitness; however, studies with overweight
and o bese participants were excluded from our re view
due to confusion over the proper expression of VO
2max
data when comparing obese and normal weight
individuals.
2.2.3 Type of Interventions
To be included in our meta-analysis, training programmes
had to last at least a minimum of 2 weeks, with participants
allocated to endurance training, HIT or a no-exercise
control group. Endurance training intensity was classified
as moderat e intensity (60–85 % maximum heart rate
[HR
max
]), with HIT intensity classified as ‘all-out’,
‘supramaximal’, ‘maximal’ or ‘high (90–95 % HR
max
)’.
Studies involving nutritional interventions were only
included if the intervention was used by all participants,
and stud ies were excluded if training was combined with
strength training.
2.2.4 Type of Outcome Measure
The outcome measure for this meta-analysis was maximal
oxygen uptake (VO
2max
).
2.3 Final Study Selection
Following database examination, 804 pote ntial manuscripts
were identified with another 17 selected on the basi s of the
reference lists of the potential manuscripts (Fig. 1). After
removal of duplicates and elimination of papers based on
title and abstract screening, 84 studies remained. The full
texts of the remaining papers were examined in more
detail. According to our eligibility criteria, 56 did not meet
the inclusion criteria leaving 28 studies that met our
inclusion criteria and were therefore included in the meta-
analysis (Table 1).
2.4 Data Extraction
Cochrane Consumers and Communication Review Group’s
data extraction protocol was used to extract participant
information including age, health status and sex, sample
size, description of the intervention (including type of
exercises, intensity, duration and frequency), study design
and study outcomes. This was undertaken by one author
(ZM) while GS checked the extracted data for accuracy and
completeness. Disagreements were resolved by consensus
or by a third reviewer. Reviewers were not blinded to
authors, institutions or manuscript journals. In those studies
where the data were shown in figures or graphs, either the
corresponding author was contacted to get the nume rical
data to enable analysis or graph digitizer software was used
to extract the necessary data (DigitiZelt, Germany).
HIT vs Endurance Training 1471
123

2.5 Assessment of Bias
Risk of bias was evaluated according to the PRISMA
recommendation [23] and two independent reviewers
assessed the risk of bias. Agreement between the two
reviewers was assessed using k statistics for full-text
screening, and rating of relevance and risk of bias. When
there was disagreement about the risk of bias, a third
reviewer checked the data and took the fin al decision on it.
The k agreement rate between reviewers was k = 0.95.
2.6 Statistical Analysis
A random effects meta-analysis was conducted to deter-
mine the pooled effect size of HIT and endurance training
on VO
2max
, using Comprehensive Meta-Analysis software,
Version 2 for Windows (Biostat company, Englewood, NJ,
USA). We performed separate analyses to determine the
pooled effect of the change in VO
2max
for endurance
training vs no exercise, HIT vs no exercise, and HIT vs
endurance training. The precision of the pooled effect was
reported as 95 % confidence limits (CL) and also as prob-
abilities that the true value of the effect was trivial, bene-
ficial or harmful in relation to threshold values for benefit
and harm. These probabilities were then used to make a
qualitative probabilistic inference about the overall effect
[24]. Given that enhanced aerobic functioning has clear
clinical applications [21], our meta-analysed effects were
assessed via clinical inferences. Here, the effects were
considered unclear if the chance of benefit (improved
VO
2max
) was high enough to warrant use of the intervention
but with an unacceptable risk of harm (reduced VO
2max
).
An odds ratio of benefit to harm of\66 was used to identify
such unclear effects. Inferences were then subsequently
Records identified through database
searching
(n = 804)
ScreeningIncluded
Eligibility
Identification
Additional records identified
through references list
(n = 17)
Records after duplicates removed
(n = 548)
Records screened by title or
abstract
(n = 316)
Records excluded after abstract
analysis
(n = 232)
Full-text articles assessed
for eligibility
(n = 84)
Full-text articles excluded, with
reasons
(n = 56)
Not original investigation (n=8)
Not relevant outcomes (n=22)
Not young adult (n=10)
Other (n=16)
Studies included in
qualitative and quantitative
synthesis (meta-analysis)
(n = 28)
Fig. 1 Flow diagram of the study selection process
1472 Z. Milanovic
´
et al.
123

Table 1 Summary of characteristics of all studies meeting the inclusion criteria
Study Population, age (year), no. of subjects, groups (n) Duration
(weeks)
Total
sessions
Group Exercise
intensity
No. of reps Total
reps
Reps
duration
(s)
Work/
rest
ratio
D
VO
2max
(%)
Outcomes and results
Start End
Astorino et al.
[26]
Recreational active men (n = 16) and women
(n = 13), age 25.3 ± 4.5 years
HIT (n = 20), CON (n = 9)
3 6 HIT All-out 4 6 30 30 0.10 6.1 HIT : VO
2max
, oxygen pulse and power output
NC in resting BP, HR and force production
Nybo et al. [27] Untrained inactive men (n = 36), age 20–43 years
HIT (n = 8), END (n = 9), CON (n = 11), STR
(n = 8)
12 36 HIT 95–100 %
HR
max
5 5 180 120 2.0 14.0 HIT was less efficient than END for resting
HR, fat percentage and ratio between total
and HDL cholesterol. END ; body mass and
fat percentage
NC in total bone mass and lean body mass in
HIT and END groups
36 END 80 % HR
max
3600 7.4
Osei-Tutu and
Campagna
[28]
Healthy Caucasian sedentary men and women
(n = 40), age 20–40 years
END (n = 15), CON (n = 10)
8 40 END 60–79 % HR
max
1800 7.2 VO
2max
: in END. END ; fat percentage
(-6.7 %), tension and total mood
disturbance
Trapp et al. [11] Healthy nonsmoking, inactive women (n = 45),
age 18–30 years
HIT (n = 15), END (n = 15), CON (n = 15)
15 45 HIT 95–100 %
HR
max
60 60 2700 8 0.67 26.4 HIT and END : VO
2max
compared with CON
group; only HIT ; total body mass, fat mass,
trunk fat and insulin level
NC in adiponectin levels in HIT and END
groups
45 END 75 % HR
max
1200–2400 19.4
Gormley et al.
[29]
Healthy young men and women (n = 61), age
18–44 years
HIT (n = 13), END (n = 13), CON (n = 14)
6 18 HIT 100 % HRR 5 5 90 300 1 20.2 HIT and END : VO
2max
NC in resting HR and BP in any group24 END 75 % HRR 2400 9.6
Ciolac et al. [30] Healthy young college women (n = 44), age
20–30 years
HIT (n = 16), END (n = 16), CON (n = 12)
16 48 HIT 80–90 %VO
2max
14 14 672 60 0.5 15.7 HIT and CON were equally ; ambulatory
blood pressure and ;; insulin
48 END 60–70 %
VO
2max
2400 8.0
Bayati et al. [31] Young active males (n = 16), age 25.0 ± 0.8
years
HIT (n = 8), CON (n = 8)
4 12 HIT 125 % P
max
6 10 96 30 0.25 9.7 HIT : power at VO
2max
(?16.1 %) and peak
power output (?7.4 %); blood lactate
recovery : in HIT compared with CON
NC in mean power output
Metcalfe et al.
[32]
Healthy sedentary young men and women
(n = 29), age 22.5 ± 2.0 years
HIT (n = 15), CON (n = 14)
6 18 HIT All-out 1 2 35 10–20 13.4 HIT : insulin sensitivity by 28 % in men
Ziemann et al.
[33]
Recreationally active men (n = 21), age
21.3 ± 1.0 years
HIT (n = 10), CON (n = 11)
6 18 HIT 80 % pVO
2max
6 6 108 90 0.5 11.0 HIT : anaerobic threshold
(3.8 mLkg
-1
min
-1
), work output
(12.5 Jkg
-1
), glycolytic work (11.5 Jkg
-1
),
mean power (0.3 W kg
-1
), peak power
(0.4 Wkg
-1
), and max power (0.4 Wkg
-1
)
Ben
Abderrahman
et al. [34]
Male physical education students (n = 15), age
20.6 ± 0.7 years
HIT (n = 9), CON (n = 6)
7 21 HIT 105–110 %
MAS
8 10 66 30 1 5.9 NC in time spent above 95 % of VO
2max
in
absolute and relative values
Burgomaster
et al. [35]
Healthy young men (n = 10) and women
(n = 10), age 23.56 ± 1.0 years
HIT (n = 10), END (n = 10)
6 18 HIT All-out 4 6 30 30 0.11 7.3 HIT and END : in mitochondrial markers for
skeletal muscle and lipid oxidation; both
groups : VO
2max
compared with control
group without changes between training
groups
NC in percentage of body fat and energy
intake in all groups
30 END 65 % VO
2peak
2400–3600 9.8
Chtara et al.
[36]
Male physical education students (n = 48), age
21.4 ± 1.3 years
HIT (n = 10), CON (n = 9)
12 24 END 100 % vVO
2max
5 5 120 9.8 HIT : in vVO
2max
10.38 %
HIT vs Endurance Training 1473
123

Table 1 continued
Study Population, age (year), no. of subjects, groups (n) Duration
(weeks)
Total
sessions
Group Exercise
intensity
No. of reps Total
reps
Reps
duration
(s)
Work/
rest
ratio
D
VO
2max
(%)
Outcomes and results
Start End
Hottenrott et al.
[6]
Recreational endurance men (n = 15) and women
(n = 15), age 43.4 ± 6.9 years
HIT (n = 14), END (n = 16)
12 36 HIT All-out 4 10 936 30 0.33 18.5 HIT and END :: peak oxygen uptake, resting
HR, V
LT
and visceral fat, body mass; END :
total body fat and fat-free mass compared
with HIT
NC in maximal lactate for both groups
24 END 75–85 % V
LT
1800–7200 7.0
Lo et al. [37] Healthy nonathletic men (n = 34), age
20.4 ± 1.36 years
HIT (n = 10), STR (n = 10), CON (n = 14)
24 72 END 75–85 % HRR 1800 20.5 END and STR : VO
2max
and lower body
strength; STR : upper body strength, lean
mass and body size of arm and calf
compared with END and CON groups
McKay et al.
[38]
Young adult men (n = 12), age 25.0 ± 4.0 years
HIT (n = 6), END (n = 6)
3 8 HIT 120 % WR
max
8 12 60 60 1 4.3 HIT and END : VO
2max
after training
programme; HIT and END ; time constant
for VO
2
response by *20 % after only
2 days of training and by *40 % post-
training, with no difference between groups
8 END 65 % VO
2max
5400–7200 7
Tabata et al.
[39]
Young male students (n = 14), age 23.0 ± 1.0
years
HIT (n = 7), END (n = 7)
6 30 HIT 170 % VO
2max
7 8 225 20 2 14.6 END did not increase anaerobic capacity but
:: in VO
2max
HIT :: VO
2max
by 7 mLkg
-1
min
-1
and
anaerobic capacity by 28 %
30 END 70 % VO
2max
3600 9.4
Cocks et al. [40] Young sedentary men (n = 16), age 21.0 ± 0.7
years
HIT (n = 8), END (n = 8)
6 18 HIT All-out 4 5 85 30 0.11 7.6 HIT and END : VO
2peak
and maximal power
output (END 16 %, HIT 9 %); both groups ;
in HRR, mean and diastolic BP with no
difference between group; NC in systolic BP
in both groups
30 END 65 % VO
2peak
2400–3600 15.6
Dunham and
Harms [41]
Physically active, healthy, untrained subjects
(n = 15), age 21.3 ± 2.3 years
HIT (n = 8), END (n = 7)
4 12 HIT 90 % VO
2max
5 5 60 60 0.33 9.6 HIT and END : VO
2max
and time trials
following training with no differences
between groups; HIT : in maximum
inspiratory pressure compared with END
NC in expiratory flow rates in both groups
12 END 60–70 %
VO
2max
2700 5.5
Edge et al. [42] Recreationally female students (n = 16), age
20.0 ± 1.0 years
HIT (n = 8), END (n = 8)
5 15 HIT 120–140 % LT 2 10 100 120 2 14.0 HIT and END : in VO
2peak
and the LT
(7–10 %), with no significant differences
between groups
NC in percentage of VO
2peak
at which LT
occurred
15 END 80–95 % LT 14
Esfarjani and
Laursen [43]
Healthy recreational men (n = 17), age 20.0 ± 2.0
years
HIT1 (n = 6), HIT2 (n = 6), END (n = 5)
10 20 HIT 75 % vVO
2max
5 8 130 200 1 9.2 HIT1 : in VO
2max
, vVO
2max
(?6.4 %), T
max
(5 %) and V
LT
(?11.7 %); HIT2 : in
VO
2max
, vVO
2max
(?7.8 %), T
max
(32 %),
and V
LT
(?11.7 %) but not V
LT
;NCin
these variables were found in END
HIT1: in VO
2max
and T
max
compared with
END
20 HIT 130 % vVO
2max
7 12 190 30 0.11 6.2
40 END 75 % vVO
2max
3600 2.1
Macpherson
et al. [44]
Healthy young recreationally active men (n = 12)
and women (n = 8), age 24.0 ± 3.0 years
HIT (n = 6), END (n = 5)
6 18 HIT All-out 4 6 90 30 0.11 11.5 HIT and END : body composition, 2000-run
time trial performance and VO
2max
; ft mass
; by 12.4 % with HIT and 5.8 % with END;
lean mass : 1 % in both groups. None of
these improvements differed between
groups
18 END 65 % VO
2max
1800–3600 12.5
Shepherd et al.
[45]
Healthy sedentary men (n = 16), age 21.5 ± 1.0
years
HIT (n = 8), END (n = 8)
6 18 HIT All-out 4 6 90 30 0.11 7.6 HIT and END :: VO
2peak
, fat-free mass, and
maximum workload; NC in relative fat mass
30 END 65 % VO
2peak
2400–3600 15.6
1474 Z. Milanovic
´
et al.
123

Table 1 continued
Study Population, age (year), no. of subjects, groups (n) Duration
(weeks)
Total
sessions
Group Exercise
intensity
No. of reps Total
reps
Reps
duration
(s)
Work/
rest
ratio
D
VO
2max
(%)
Outcomes and results
Start End
Helgerud et al.
[5]
Healthy nonsmoking men (n = 24), age
24.6 ± 3.8 years
HIT1 (n = 6), HIT2 (n = 6), END1 (n = 6),
END2 (n = 6)
8 24 HIT1 90–95 % HR
max
47 47 1128 15 1 6.4 HIT1 and HIT 2 :: VO
2max
compared with
END1 and END 2; percentage increases in
VO
2max
for the HIT1 and HIT 2 groups were
5.5 and 7.2 %, respectively. Stroke volume
of the heart : in HIT1 and HIT2
NC in blood volume, high-density lipoprotein
and low-density lipoprotein in any groups
after training programme
24 HIT2 90–95 % HR
max
4 4 96 240 1.33 8.8
24 END1 70 % HR
max
2700 1.8
24 END2 85 % HR
max
1455 2.0
Warburton et al.
[46]
Healthy untrained men (n = 20), age 30 ± 4 years
HIT (n = 6), END (n = 6), CON (n = 8)
12 36 HIT 90 % VO
2max
8 12 384 120 1 22.2 HIT and END :: VO
2max
and peak stroke
volume, blood volume compared to CON;
no differences between HIT and END in any
parameters
36 END 65 % VO
2max
1800–2880 23
Berger et al.
[47]
Healthy sedentary men (n = 11) and women
(n = 12), age 24 ± 5 years
HIT (n = 8), END (n = 8), CON (n = 7)
6 22 HIT 90 % VO
2max
15 20 445 60 1 21.0 HIT and END :: VO
2max
and pulmonary
VO
2max
kinetics, compared with CON
22 END 60 % VO
2max
1800 20.0
Matsuo et al.
[48]
Sedentary men (n = 42), age 26.5 ± 6.2 years
HIT (n = 14), END (n = 14)
8 40 HIT 80–85 %
VO
2max
3 3 120 180 1.5 22.5 HIT and END :: VO
2max
, HIT :: VO
2max
compared with END; only HIT :: left
ventricular mass, stroke volume and resting
HR
40 END 60–65 %
VO
2max
2400 10.0
O’Donovan
et al. [49]
Sedentary men (n = 42), age 41 ± 4
HIT (n = 13), END (n = 14), CON (n = 15)
24 72 HIT 80 % VO
2max
15.7 HIT and END :: VO
2max
, HIT : HDL and ;
LDL, NC in END for HDL and LDL
72 END 60 % VO
2max
22.5
Sandvei et al.
[50]
Healthy young men (n = 8) and women (n = 15),
age 25.2 ± 0.7 years
HIT (n = 11), END (n = 12)
8 24 HIT 100 % HR
max
5 10 189 30 0.16 5.3 HIT and END : VO
2max
, HIT : insulin
sensitivity and cholesterol profile while NC
for END
24 END 70–80 % HR
max
1800–3600 3.8
BP blood pressure, CON control group, END continuous endurance training, HDL high-density lipoprotein, HIT high-intensity interval training, HR heart rate, HR
max
maximum heart rate, HRR heart rate reserve, LT lactate threshold,
MAS maximal aerobic speed, max maximal, NC no changes p [ 0.05, P
max
power at VO
2max
, pVO
2max
maximal aerobic power, rep repetitions, STR strength training, T
max
time to exhaustion at vVO
2max
, V
LT
velocity of the lactate
threshold, VO
2max
maximal oxygen uptake, VO
2peak
peak rate of oxygen consumption, vVO
2max
running speed at VO
2max
, WR
max
work rate at maximal O
2
uptake, : indicates significant increase p \ 0.05, :: indicates significant
increase p \ 0.01, ; indicates significant decreases p \ 0.05, ;; indicates significant decreases p \ 0.01
HIT vs Endurance Training 1475
123

based on standardised thresholds for small, moderate and
large changes of 0.2, 0.6 and 1.2 standard deviations (SDs),
respectively [24] and derived by averaging appropriate
between-subject variances for baseline VO
2max
. Magnitude
thresholds were 0.8, 2.4 and 4.7 mLkg
-1
min
-1
(en-
durance vs no exercise), 0.8, 2.3 and 4.7 mLkg
-1
min
-1
(HIT vs no exercise) and 0.9, 2.6 and 5.3 mLkg
-1
min
-1
(HIT vs endurance training). The chance of the true effect
being trivial, beneficial or harmful was then interpreted
using the following scale: 25–75 %, possibly; 75–95 %,
likely; 95–99.5 %, very likely; [99.5 %, most likely [24].
Random variation in the effect from study to study was
expressed as an SD, with the SD doubled to interpret its
magnitude [25]. Publication bias was assessed by examin-
ing asymmetry of funnel plots using Egger’s test, and a
significant publication bias was considered if the p \ 0.10.
2.7 Meta-Regression Analysis
Meta-regression analyses were conducted to explore the
effect of putative moderator variables on the pooled effect.
Here, we selected five moderator variables that could rea-
sonably influence the overall effect of training on VO
2max
and these were age, baseline fitness, intervention duration,
work:rest ratio and HIT repetition duration. The modifying
effects of these five variables were calculated as the effect
of two SDs (i.e. the difference between a typically low and
a typically high value) [24].
3 Results
The Egger’s test was performed to provide statistical evi-
dence of funnel plot asymmetry (Fig. 2) and the results
indicated publication bias for all analyses (p \ 0.10).
3.1 Endurance Training vs No-Exercise Controls
The meta-analysed effect of endurance training, when
compared with controls, was a possibly large beneficial
effect on VO
2max
(4.9 mLkg
-1
min
-1
;95%CL
±1.4 mLkg
-1
min
-1
) (Fig. 3; Table 2). Meta-regression
analysis revealed a greater beneficial effect (possibly
moderate) for typically younger vs older subjects and
interventions of longer duration, and a greater beneficial
improvement (possibly small) for subjects with typically
lower baseline fitness. The random variation in the overall
pooled effect from study to study, expressed as an SD, was
1.3 mLkg
-1
min
-1
.
3.2 High-Intensity Interval Training (HIT) vs No-
Exercise Controls
The meta-analysed effect of HIT, when compared with
controls, was a likely large beneficial effect on VO
2max
(5.5 mLkg
-1
min
-1
; ±1.2 mLkg
-1
min
-1
) (Fig. 4;
Table 3). Meta-regression analysis revealed a likely mod-
erate greater beneficial improvement in VO
2max
for sub-
jects with typically lower baseline fitness and interventions
of longer duration and a likely small lesser effect for longer
HIT repetitions. The effects of all other puta tive modifiers
were unclear. Random variation in the effect from study to
study was 1.3 mLkg
-1
min
-1
.
3.3 HIT vs Endurance Training
When compared wi th endurance training, there was a
possibly small beneficial effect of HIT on VO
2max
(1.2 mLkg
-1
min
-1
; ±0.9 mLkg
-1
min
-1
) (Fig. 5;
Table 4). The modify ing effects of typically longer HIT
repetitions, older and less fit subjects, longer interventions
Fig. 2 Funnel plot of standard
difference in means vs standard
error; the aggregated standard
difference in means is the
random effects mean effect size
weighted by degrees of freedom
1476 Z. Milanovic
´
et al.
123

and a greater work:rest ratio were possibly to likely small
increased beneficial improvements in VO
2max
. Random
variation in the effect from study to study was
0.8 mLkg
-1
min
-1
.
4 Discussion
This study presents a quantitative evaluation of HIT and
endurance training models for VO
2max
improvements in
healthy adul ts aged 18–45 years. Our results show that
when compared with no-exercise controls, both types of
training elicit large improvements in VO
2max
. In studies
where HIT and endurance were directly compared, there
was a small beneficial effect for HIT.
The results of our systematic review and meta-analysis
confirm the conclusions of previous studies [11, 27–30, 36,
37, 51] that continuous aerobic endur ance training is an
effective method for VO
2max
improvement in young adults.
The training effect was greater for less fit adults, which is
consistent with previous work demonstrating that aerobic
training has an adaptive effect that favours the less fit [21].
Further to this, the beneficial effect of continuous endur-
ance training on VO
2max
is greater for younger subjects and
with interventions of longer duration. Most of the studies in
this particular analysis undertook three moderate-intensity
sessions per week lasting 40–60 min, yet the American
College of Sports Medicine (ACSM) recommends to
undertake moderate-intensity continuous exercises for a
minimum of 30 min on 5 days each week or 20 min of
vigorous exercises 3 days each week, or a combination of
the two [52]. As such, it is clear from the findings of this
review that substantial gains in aerobic fitness can be
obtained with a moderate-intensity training session fre-
quency lower than that currently recommend [2].
When compared with no-exercise controls, HIT elicits a
likely large substantial improvement in the VO
2max
of
healthy adults. The size of this effect was greater than that
-20.0 -10.0 0.0 10.0 20.0
Bayati et al. [31]
Astorino et al. [26]
Esfarjani and Laursen [43]
Ben Abderrahman et al. [34]
Ciolac et al. [30]
Ziemann et al. [33]
Metcalfe et al. [32]
Nybo et al. [27]
Gormley et al. [29]
Berger et al. [47]
O' Donovan et al. [49]
Trapp et al. [11]
Warburton et al. [46]
Overall
Favours
control
Favours
HIT
Study name
Mean difference (mL·kg
-1
·min
-1
)
with 95% CL
Fig. 4 Effects of HIT vs no-exercise controls on maximal oxygen
uptake. CL confidence limits, HIT high-intensity interval training
Table 2 Effects of endurance training on VO
2max
Effect on VO
2max
(mLkg
-1
min
-1
) Inference
Mean ±95 % CL
Main effect
Endurance training vs control 4.9 ±1.4 Possibly large :
Modifying effects
a
Age lower by 13.7 years 2.4 ±2.1 Possibly moderate :
Intervention duration longer by 13 weeks 2.2 ±3.0 Possibly moderate :
Baseline VO
2max
lower by 12.6 mLkg
-1
min
-1
1.4 ±2.0 Possibly small :
CL confidence limits, VO
2max
maximal oxygen uptake, : indicates increase
a
Modifying effects are presented as the effect of two standard deviations of the numerical covariates (i.e. a typically high value minus a
typically low value)
-20.0 -10.0 0.0 10.0 20.0
Favours
control
Favours
endurance training
Ciolac et al. [30]
Gormley et al. [29]
Nybo et al. [27]
Osei-Tutu and Campagna [28]
Chtara et al. [36]
O' Donovan et al. [49]
Lo et al. [37]
Berger et al. [47]
Trapp et al. [11]
Warburton et al. [46]
Overall
Study name
Mean difference (mL·kg
-1
·min
-1
)
with 95% CL
Fig. 3 Effects of endurance training vs no-exercise controls on
maximal oxygen uptake. CL confidence limits
HIT vs Endurance Training 1477
123

reported by Gist et al. [19], who reported a moderate effect
(effect size 0.69) for low-volume HIT when compared with
no-exercise controls, with differences in the overall dose of
exercise possibly accounting for these inconsistent results.
Irrespective of dose, HIT has a clear beneficial effect on the
aerobic fitness of healthy young adults when compared
with no exercise. This effect is moderated by initial fitness
as the training benefits individuals with lower initial fit-
ness—a finding consistent with low-volume HIT pro-
grammes [21]. With regard to HIT programming, a
moderating beneficial effect for longer intervention
duration is consistent with the subgroup analysis performed
by Bacon et al. [18]. Here, the authors reported that the
largest increases in VO
2max
were following longer inter-
vention durations (p = 0.004). Additionally, we found an
unclear effect on VO
2max
with an increased work:rest ratio
(e.g. greater recovery between HIT repetitions), a finding
consistent with that reported by Weston et al. [21]. Future
studies are therefore needed to resolve this unclear effect,
although the prescription of an ‘optimal’ work:rest ratio is
challenging as variables such as age, sex, baseline fitness
and training experience may need to be considered when
designing HIT programmes. We also found an unclear
modifying effect of age on HIT and consistent with pre-
vious HIT meta-analyses [18, 19, 21], the dem ographic of
participants in the studies analysed was mainly young
adults. As such, we suggest that more HIT studies need to
be undertaken in older populations, especially given the
recent encouraging findings reported by Adamson et al.
[53] and Knowles et al. [54] whereby HIT elicited sub-
stantial improvements in VO
2max
and also measures of
functional fitness and quality of life.
When compared with endurance training controls, HIT
had a possibly small beneficial effect on VO
2max
. Previous
comparisons between HIT and endurance training yielded
either an unclear effect [19, 21] or a significantly higher
increase in VO
2peak
after HIT compared with endurance
training (3.03 mLkg
-1
min
-1
; ±2.0 to 4.1 mLkg
-1-
min
-1
)[21]. Discrepancies in the overall training dose (e.g.
low-volume HIT vs HIT) and study participants (e.g.
healthy participants vs patient populations) no doubt
account for the inconsistency in these findings. The dif-
ference in the training effect between HIT and endurance
was enhanced for older and less fit subjects, suggesting
Table 3 Effects of HIT on VO
2max
Effect on VO
2max
(mLkg
-1
min
-1
) Inference
Mean ±95 % CL
Main effect
HIT vs control 5.5 ±1.2 Likely large :
Modifying effects
a
Baseline VO
2max
lower by 18.5 mLkg
-1
min
-1
3.2 ±1.9 Likely moderate :
Intervention duration longer by 13 weeks 3.0 ±1.9 Likely moderate :
Age higher by 11.7 years 0.8 ±2.1 Unclear
Work:rest ratio higher by 1.1 0.5 ±1.6 Unclear
HIT repetition duration longer by 161 s -1.8 ±2.7 Likely small ;
CL confidence limits, HIT high-intensity interval training, VO
2max
maximal oxygen uptake, : indicates increase, ; indicates decrease
a
Modifying effects are presented as the effect of two standard deviations of the numerical covariates (i.e. a typically high value minus a
typically low value)
-20.0 -10.0 0.0 10.0 20.0
Cocks et al. [40]
Shepherd et al. [45]
McKay et al. [38]
Edge et al. [42]
Macpherson et al. [44]
Berger et al. [47]
Warburton et al. [46]
Sandvei et al. [50]
Burgomaster et al. [35]
Dunham and Harms [41]
Trapp et al. [11]
Tabata et al. [39]
Nybo et al. [27]
Ciolac et al. [30]
O' Donovan et al. [49]
Gormley et al. [29]
Helgerud et al. [5]
Hottenrott et al. [6]
Matsuo et al. [48]
Overall
Study name
Mean difference (mL·kg
-1
·min
-1
)
with 95% CL
Favours endurance
training
Favours
HIT
Fig. 5 Effects of HIT vs endurance training on maximal oxygen
uptake. CL confidence limits, HIT high-intensity interval training
1478 Z. Milanovic
´
et al.
123

HIT to have appeal for those involved in the fitness pro-
gramming of older adults and patient populations, espe-
cially given that the safety concerns associated with HIT
are unfounded [55, 56]. Our supposition is supported by
recent evidence whereby HIT induced substantial
improvements in cardiovascular (e.g. VO
2max
), functional
fitness (e.g. sit-to-sta nd test) and health- related quality of
life/physical functioning following short (3 weeks) [53]
and long duration (13 weeks) [54] interventions. Our
findings of enhanced beneficial effects for HIT with longer
repetitions, greater work:rest ratios and longer training
interventions provides valuable information to those
involved in the design and implementation of HIT
programmes.
While information on the physiological mechanisms
subtending the improvements in VO
2max
following either
endurance training or HIT helps to explain changes in
VO
2max
, a discussion of physiological adaptations is
beyond the scope of our review. In this instanc e, we direct
readers to the articulate and comprehensive reviews of
Jones and Carter [57], Gibala et al. [58] and Sloth et al.
[20] for a detailed discussion of the underlying physio-
logical adaptations to endurance training and HIT.
Finally, the observed magnitude of the between-study
variation in the mean effect was moderate for endurance
training vs control and HIT vs control, and small for HIT vs
endurance training. As such, the mean effect, when com-
pared with control, lies typically between 3.6 mLkg
-1-
min
-1
(very likely moderate) and 6.2 mLkg
-1
min
-1
(very likely large) for endurance training, betwee n
4.2 mLkg
-1
min
-1
(most likely moderate) and
6.8 mLkg
-1
min
-1
(very likely large) for HIT, and
between -0.4 mLkg
-1
min
-1
(most likely trivial) and
2.0 mLkg
-1
min
-1
(likely small) for HIT compared with
endurance training.
5 Conclusion
Our meta-analysi s confirms that endurance training and
HIT both elicit large improvements in the VO
2max
of
healthy, young to middle-a ged adults with the effects being
greater for the less fit. Furthermore, when comparing the
two modes of training, the gains in VO
2max
are greater
following HIT. Given the well established link between
aerobic fitness and mortality, further investigations into the
manipulations of the HIT dose (e.g. repe tition intensity,
duration, work:rest ratio etc.) are therefore recommended
to enhance our understanding of the beneficial effects of
HIT.
Compliance with Ethical Standards No sources of funding were
used to assist in the preparation of this review. The authors have
no conflicts of interest that are directly relevant to the content of
this review.
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