Regular exercise training improves maximal oxygen uptake (VO 2max ), but the optimal intensity and volume necessary to obtain maximal benefit remains to be defined. A growing body of evidence suggests that exercise training with low-volume but high-intensity may be a time-efficient means to achieve health benefits. In the present study, we measured changes in VO 2max and traditional cardiovascular risk factors after a 10 wk. training protocol that involved three weekly high-intensity interval sessions. One group followed a protocol which consisted of 4×4 min at 90% of maximal heart rate (HR max ) interspersed with 3 min active recovery at 70% HR max (4-AIT), the other group performed a single bout protocol that consisted of 1×4 min at 90% HR max (1-AIT). Twenty-six inactive but otherwise healthy overweight men (BMI: 25–30, age: 35–45 y) were randomized to either 1-AIT (n = 11) or 4-AIT (n = 13). After training, VO 2max increased by 10% (∼5.0 mL⋅kg −1 ⋅min −1 ) and 13% (∼6.5 mL⋅kg −1 ⋅min −1 ) after 1-AIT and 4-AIT, respectively (group difference, p = 0.08). Oxygen cost during running at a sub-maximal workload was reduced by 14% and 13% after 1-AIT and 4-AIT, respectively. Systolic blood pressure decreased by 7.1 and 2.6 mmHg after 1-AIT and 4-AIT respectively, while diastolic pressure decreased by 7.7 and 6.1 mmHg (group difference, p = 0.84). Both groups had a similar ∼5% decrease in fasting glucose. Body fat, total cholesterol, LDL-cholesterol, and ox-LDL cholesterol only were significantly reduced after 4-AIT. Our data suggest that a single bout of AIT performed three times per week may be a time-efficient strategy to improve VO 2max and reduce blood pressure and fasting glucose in previously inactive but otherwise healthy middle-aged individuals. The 1-AIT type of exercise training may be readily implemented as part of activities of daily living and could easily be translated into programs designed to improve public health.

Funding: The present study was supported by grants from the K.G Jebsen Foundation, Norwegian Council of Cardiovascular Disease, the Norwegian Research Council (Funding for Outstanding Young Investigators; Dr Wisløff), and funds for Cardiovascular and Medical Research at St Olav's University Hospital, Trondheim, and the Eckbos Foundation, Oslo. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2013 Tjønna et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

The global epidemic of overweight [body mass index (BMI) between 25.0–29.9] and obesity (BMI≥30) has become a major health, social and economic burden. It is estimated that at least 400 million adults are obese, and approximately 1.6 billion adults are overweight [1]. Overweight and obesity are associated with increased mortality from cardiovascular and metabolic causes [2], [3], [4], whereas exercise training protects against premature cardiovascular mortality [5]. Current public health guidelines generally recommended that adults accumulate at least 150 min per week of moderate intensity exercise (50%–70% of HR max ) or a minimum of 20 min of vigorous exercise (70% to 80% of HR max ) at least three times per week [6]. These recommendations appear difficult to achieve for most people with one of the most common cited barriers being lack of time [7].

There is evidence to suggest that a lower volume of exercise may confer health benefits. For example, Lee et al. [8] showed that apparently healthy elderly men who exercised once or twice per week (so-called “weekend warriors”) had a lower risk of all-cause mortality compared with sedentary counterparts. Consistent with these findings, a 18-year follow-up study revealed that a single, vigorous weekly bout of physical activity was associated with prevention of cardiovascular death among men and woman without known cardiovascular disease at the beginning of follow-up [9]. These data suggest that it may be possible to reduce cardiovascular mortality with substantially less exercise than is generally recommended, provided it is performed in a vigorous manner [10].

Although both overweight and VO 2max are strong and independent prognostic markers of cardiovascular mortality, the link between VO 2max and mortality seems to be stronger [11]. Moreover, recent analyses have shown that while meeting physical activity recommendations marginally reduced all-cause mortality risk, being physically fit (as reflected by VO 2max ) was associated with a marked reduction in all-cause mortality risk even when physical activity was below recommendations [12], [13]. It has therefore been suggested that improving VO 2max is more important than losing weight or simply engaging in increasing amounts of lower to moderate intensity physical activity [12], [14]. These studies by different research groups call into question public health policies and programs largely revolving around accumulating a daily volume of lower to moderate physical activity.

There is evidence that exercise programs that involve relatively high intensity are more effective in improving VO 2max , cardiac and endothelial function than isocaloric exercise programs of moderate intensities, in healthy individuals [15], [16], [17], patients with post-infarction heart failure [18], metabolic syndrome [19], coronary artery disease [20], and overweight and obese individuals [21]. We have demonstrated that an interval training program consisting of a 10-min warm-up followed by four, 4-min intervals at ∼90% of HR max interspersed by 3 min of active recovery at ∼70% of HR max , performed 2–3 times per week for 8–16 weeks improved VO 2max by 16–46%. Endothelial function measured as flow mediated dilatation (FMD) also improved by 2–9% (absolute changes) in individuals with initially low FMD. These are relatively large adaptations over a short time period.

With regard to the practical translation of results from interval training studies into public health practice, a fundamental question remains: how abbreviated can the stimulus be, and still achieve a robust training effect measured as increased VO 2max and/or FMD and improvement in other cardiovascular disease (CVD) risk factors? One study found that after a several minute graded warm-up, simply training for several minutes at about 75% of VO 2max (∼85% of HR max ) twice per week for 12 weeks was sufficient to increase the VO 2max of previously sedentary men and women by about 5 ml·kg−1·min−1 [22]. However, the brief high-intensity training intervention was only one aspect of a more comprehensive multi-component health behaviour program. It is not clear how effective such brief training can be when performed alone.