Sports Medicine

, Volume 45, Issue 10, pp 1469–1481 | Cite as

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

Systematic Review

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 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’, and ‘VO2max’. Inclusion criteria were controlled trials, healthy adults aged 18–45 years, 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 ± 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 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 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 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 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 VO2max of healthy, young to middle-aged adults, with the gains in VO2max being greater following HIT 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 compared with other well established risk factors such as hypertension, diabetes mellitus, smoking and obesity, cardiovascular fitness is a more powerful predictor of mortality [2, 3]. Fitness training programmes 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-recover 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 improving fitness 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 fitness [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 (VO2max) increases were higher for untrained men and women who participated in an 8-week HIT programme (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 cardiovascular disease in both men and women (relative risk: 0.61 and 0.49, respectively).

It is therefore not surprising that recent meta-analyses [14, 15, 16, 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 criteria to the aforementioned reviews, Bacon et al. [18] meta-analysed the effect of HIT on VO2max 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 VO2max 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 VO2max. 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, 15, 16, 17], to the best of the authors’ knowledge there is no systematic review and meta-analysis 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 VO2max of endurance training and HIT in healthy, young to middle-aged adults, when compared 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.

2 Methods

2.1 Search Strategy

Electronic database searches were performed using MEDLINE, 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 VO2max 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, cardiorespiratory fitness, VO2max, young adults. The literature search, quality assessment and data extraction were conducted 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 inclusion 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 Systematic Reviews and Meta-Analyses (PRISMA) statement [22].

2.2 Inclusion Criteria

2.2.1 Type of Study

Our meta-analysis included randomised and non-randomised 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 in our meta-analysis were healthy, untrained, sedentary, recreational and non-athletic men and women aged between 18 and 45 years, who were not suffering from any kind of acute or chronic diseases. No exclusion criteria were applied to participant baseline fitness; however, studies with overweight and obese participants were excluded from our review due to confusion over the proper expression of VO2max 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 moderate intensity (60–85 % maximum heart rate [HRmax]), with HIT intensity classified as ‘all-out’, ‘supramaximal’, ‘maximal’ or ‘high (90–95 % HRmax)’. Studies involving nutritional interventions were only included if the intervention was used by all participants, and studies 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 (VO2max).

2.3 Final Study Selection

Following database examination, 804 potential manuscripts were identified with another 17 selected on the basis 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).
Fig. 1

Flow diagram of the study selection process

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

Δ VO2max (%)

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 ↑ VO2max, 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 % HRmax

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 % HRmax

   

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 % HRmax

   

1800

 

7.2

VO2max ↑ 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 % HRmax

60

60

2700

8

0.67

26.4

HIT and END ↑ VO2max 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 % HRmax

   

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 ↑ VO2max

NC in resting HR and BP in any group

24

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 %VO2max

14

14

672

60

0.5

15.7

HIT and CON were equally ↓ ambulatory blood pressure and ↓↓ insulin

48

END

60–70 % VO2max

   

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 % Pmax

6

10

96

30

0.25

9.7

HIT ↑ power at VO2max (+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 % pVO2max

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 VO2max 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 ↑ VO2max compared with control group without changes between training groups

NC in percentage of body fat and energy intake in all groups

30

END

65 % VO2peak

   

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 % vVO2max

5

5

120

  

9.8

HIT ↑ in vVO2max 10.38 %

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, VLT 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 % VLT

   

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 ↑ VO2max 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 % WRmax

8

12

60

60

1

4.3

HIT and END ↑ VO2max after training programme; HIT and END ↓ time constant for VO2 response by ~20 % after only 2 days of training and by ~40 % post-training, with no difference between groups

8

END

65 % VO2max

   

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 % VO2max

7

8

225

20

2

14.6

END did not increase anaerobic capacity but ↑↑ in VO2max

HIT ↑↑ VO2max by 7 mL·kg−1·min−1 and anaerobic capacity by 28 %

30

END

70 % VO2max

   

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 ↑ VO2peak 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 % VO2peak

   

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 % VO2max

5

5

60

60

0.33

9.6

HIT and END ↑ VO2max 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 % VO2max

   

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 VO2peak and the LT (7–10 %), with no significant differences between groups

NC in percentage of VO2peak 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 % vVO2max

5

8

130

200

1

9.2

HIT1 ↑ in VO2max, vVO2max (+6.4 %), Tmax (5 %) and VLT (+11.7 %); HIT2 ↑ in VO2max, vVO2max (+7.8 %), Tmax (32 %), and VLT (+11.7 %) but not VLT; NC in these variables were found in END

HIT1↑ in VO2max and Tmax compared with END

20

HIT

130 % vVO2max

7

12

190

30

0.11

6.2

40

END

75 % vVO2max

   

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 VO2max; 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 % VO2max

   

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 ↑↑ VO2peak, fat-free mass, and maximum workload; NC in relative fat mass

30

END

65 % VO2peak

   

2400–3600

 

15.6

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 % HRmax

47

47

1128

15

1

6.4

HIT1 and HIT 2 ↑↑ VO2max compared with END1 and END 2; percentage increases in VO2max 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 % HRmax

4

4

96

240

1.33

8.8

24

END1

70 % HRmax

   

2700

 

1.8

24

END2

85 % HRmax

   

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 % VO2max

8

12

384

120

1

22.2

HIT and END ↑↑ VO2max and peak stroke volume, blood volume compared to CON; no differences between HIT and END in any parameters

36

END

65 % VO2max

   

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 % VO2max

15

20

445

60

1

21.0

HIT and END ↑↑ VO2max and pulmonary VO2max kinetics, compared with CON

22

END

60 % VO2max

   

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 % VO2max

3

3

120

180

1.5

22.5

HIT and END ↑↑ VO2max, HIT ↑↑ VO2max compared with END; only HIT ↑↑ left ventricular mass, stroke volume and resting HR

40

END

60–65 % VO2max

   

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 % VO2max

     

15.7

HIT and END ↑↑ VO2max, HIT ↑ HDL and ↓ LDL, NC in END for HDL and LDL

72

END

60 % VO2max

     

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 % HRmax

5

10

189

30

0.16

5.3

HIT and END ↑ VO2max, HIT ↑ insulin sensitivity and cholesterol profile while NC for END

24

END

70–80 % HRmax

   

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, HRmax maximum heart rate, HRR heart rate reserve, LT lactate threshold, MAS maximal aerobic speed, max maximal, NC no changes p > 0.05, Pmax power at VO2max, pVO2max maximal aerobic power, rep repetitions, STR strength training, Tmax time to exhaustion at vVO2max, VLT velocity of the lactate threshold, VO2max maximal oxygen uptake, VO2peak peak rate of oxygen consumption, vVO2max running speed at VO2max, WRmax work rate at maximal O2 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

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 numerical data to enable analysis or graph digitizer software was used to extract the necessary data (DigitiZelt, Germany).

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 final 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 determine the pooled effect size of HIT and endurance training on VO2max, 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 VO2max 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 probabilities that the true value of the effect was trivial, beneficial 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 VO2max) was high enough to warrant use of the intervention but with an unacceptable risk of harm (reduced VO2max). An odds ratio of benefit to harm of <66 was used to identify such unclear effects. Inferences were then subsequently 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 VO2max. Magnitude thresholds were 0.8, 2.4 and 4.7 mL·kg−1·min−1 (endurance 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 examining 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 reasonably influence the overall effect of training on VO2max 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 evidence of funnel plot asymmetry (Fig. 2) and the results indicated publication bias for all analyses (p < 0.10).
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

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 VO2max (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.
Fig. 3

Effects of endurance training vs no-exercise controls on maximal oxygen uptake. CL confidence limits

Table 2

Effects of endurance training on VO2max

 

Effect on VO2max (mL·kg−1·min−1)

Inference

Mean

±95 % CL

Main effect

 Endurance training vs control

4.9

±1.4

Possibly large ↑

Modifying effectsa

 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 VO2max lower by 12.6 mL·kg−1·min−1

1.4

±2.0

Possibly small ↑

CL confidence limits, VO2max maximal oxygen uptake, ↑ indicates increase

aModifying 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)

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 VO2max (5.5 mL·kg−1·min−1; ±1.2 mL·kg−1·min−1) (Fig. 4; Table 3). Meta-regression analysis revealed a likely moderate greater beneficial improvement in VO2max for subjects 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 putative modifiers were unclear. Random variation in the effect from study to study was 1.3 mL·kg−1·min−1.
Fig. 4

Effects of HIT vs no-exercise controls on maximal oxygen uptake. CL confidence limits, HIT high-intensity interval training

Table 3

Effects of HIT on VO2max

 

Effect on VO2max (mL·kg−1·min−1)

Inference

Mean

±95 % CL

Main effect

 HIT vs control

5.5

±1.2

Likely large ↑

Modifying effectsa

 Baseline VO2max 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, VO2max maximal oxygen uptake, ↑ indicates increase, ↓ indicates decrease

aModifying 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)

3.3 HIT vs Endurance Training

When compared with endurance training, there was a possibly small beneficial effect of HIT on VO2max (1.2 mL·kg−1·min−1; ±0.9 mL·kg−1·min−1) (Fig. 5; Table 4). The modifying effects of typically longer HIT repetitions, older and less fit subjects, longer interventions and a greater work:rest ratio were possibly to likely small increased beneficial improvements in VO2max. Random variation in the effect from study to study was 0.8 mL·kg−1·min−1.
Fig. 5

Effects of HIT vs endurance training on maximal oxygen uptake. CL confidence limits, HIT high-intensity interval training

Table 4

Effects of HIT vs endurance training on VO2max

 

Effect on VO2max (mL·kg−1·min−1)

Inference

Mean

±95 % CL

Main effect

 HIT vs endurance training

1.2

±0.9

Possibly small ↑

Modifying effectsa

 HIT repetition duration longer by 164 s

2.2

±2.1

Likely small ↑

 Age higher by 12.9 years

1.8

±1.7

Likely small ↑

 Intervention duration longer by 10.3 weeks

1.7

±1.7

Likely small ↑

 Work:rest ratio higher by 1.4

1.6

±1.5

Likely small ↑

 Baseline VO2max lower by 14.5 mL·kg−1·min−1

0.8

±1.3

Possibly small ↑

CL confidence limits, HIT high-intensity interval training, VO2max maximal oxygen uptake, ↑ indicates increase

aModifying 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)

4 Discussion

This study presents a quantitative evaluation of HIT and endurance training models for VO2max improvements in healthy adults aged 18–45 years. Our results show that when compared with no-exercise controls, both types of training elicit large improvements in VO2max. 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, 28, 29, 30, 36, 37, 51] that continuous aerobic endurance training is an effective method for VO2max 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 endurance training on VO2max 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 frequency lower than that currently recommend [2].

When compared with no-exercise controls, HIT elicits a likely large substantial improvement in the VO2max of healthy adults. The size of this effect was greater than that 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 fitness—a finding consistent with low-volume HIT programmes [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 VO2max were following longer intervention durations (p = 0.004). Additionally, we found an unclear effect on VO2max 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 previous HIT meta-analyses [18, 19, 21], the demographic 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 substantial improvements in VO2max and also measures of functional fitness and quality of life.

When compared with endurance training controls, HIT had a possibly small beneficial effect on VO2max. Previous comparisons between HIT and endurance training yielded either an unclear effect [19, 21] or a significantly higher increase in VO2peak 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 difference in the training effect between HIT and endurance was enhanced for older and less fit subjects, suggesting HIT to have appeal for those involved in the fitness programming of older adults and patient populations, especially 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. VO2max), functional fitness (e.g. sit-to-stand 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 VO2max following either endurance training or HIT helps to explain changes in VO2max, a discussion of physiological adaptations is beyond the scope of our review. In this instance, 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 physiological 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 compared 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, between 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-analysis confirms that endurance training and HIT both elicit large improvements in the VO2max of healthy, young to middle-aged adults with the effects being greater for the less fit. Furthermore, when comparing the two modes of training, the gains in VO2max 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. repetition intensity, duration, work:rest ratio etc.) are therefore recommended to enhance our understanding of the beneficial effects of HIT.

Notes

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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Copyright information

© Springer International Publishing Switzerland 2015

Authors and Affiliations

  • Zoran Milanović
    • 1
  • Goran Sporiš
    • 2
  • Matthew Weston
    • 3
  1. 1.Faculty of Sport and Physical EducationUniversity of NisNisSerbia
  2. 2.Faculty of KinesiologyUniversity of ZagrebZagrebCroatia
  3. 3.Department of Sport and Exercise Sciences, School of Social Sciences, Business and LawTeesside UniversityMiddlesbroughUK

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