High-intensity interval training (HIT) has been proposed as a time-efficient alternative to traditional cardiorespiratory exercise training, but is very fatiguing. In this study we investigated the effects of a reduced-exertion HIT (REHIT) exercise intervention on insulin sensitivity and aerobic capacity.Twenty-nine healthy but sedentary young men and women were randomly assigned to the REHIT intervention (men: n=7, women n=8) or a control group (men n=6; women n=8). Subjects assigned to the control groups maintained their normal sedentary lifestyle, whilst subjects in the training groups completed 3 exercise sessions per week for 6 weeks. The 10-min exercise sessions consisted of low intensity cycling (60 Watts) and one (1 st session) or two (all other sessions) brief 'all-out' sprints (10 s in week 1, 15 s in weeks 2-3 and 20 s in the final 3 weeks). Aerobic capacity (V O2peak) and the glucose and insulin response to a 75-g glucose load (OGTT) were determined before and 3 days after the exercise program. Despite relatively low ratings of perceived exertion (RPE: 13±1), insulin sensitivity significantly increased by 28% in the male training group following the REHIT intervention (P<0.05). V O2peak increased in the male training (+15%) and female training (+12%) groups (P<0.01). In conclusion we show that a novel, feasible exercise intervention can improve metabolic health and aerobic capacity. REHIT may offer a genuinely time-efficient alternative to HIT and conventional cardiorespiratory exercise training for improving risk factors of T2D.

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Over the past decade, it has been convincingly shown that regularly performing repeated brief supramaximal cycle sprints (sprint interval training [SIT]) is associated with aerobic adaptations and health benefits similar to or greater than with moderate-intensity continuous training (MICT). SIT is often promoted as a time-efficient exercise strategy, but the most commonly studied SIT protocol (4-6 repeated 30-s Wingate sprints with 4 min recovery, here referred to as 'classic' SIT) takes up to approximately 30 min per session. Combined with high associated perceived exertion, this makes classic SIT unsuitable as an alternative/adjunct to current exercise recommendations involving MICT. However, there are no indications that the design of the classic SIT protocol has been based on considerations regarding the lowest number or shortest duration of sprints to optimise time efficiency while retaining the associated health benefits. In recent years, studies have shown that novel SIT protocols with both fewer and shorter sprints are efficacious at improving important risk factors of noncommunicable diseases in sedentary individuals, and provide health benefits that are no worse than those associated with classic SIT. These shorter/easier protocols have the potential to remove many of the common barriers to exercise in the general population. Thus, based on the evidence summarised in this current opinion paper, we propose that there is a need for a fundamental change in focus in SIT research in order to move away from further characterising the classic SIT protocol and towards establishing acceptable and effective protocols that involve minimal sprint durations and repetitions. Key PointsOver the past decade, aerobic fitness adaptations and health benefits following sprint interval training (SIT) have received much attention. However, the most commonly used SIT protocol, involving 4-6 repeated 'all-out' 30-s cycle sprints, is very demanding and not as time efficient as often suggested.Recent studies demonstrate that both the number of sprint repetitions and the sprint duration of SIT protocols can be reduced (to a point) without attenuating the associated health benefits.Based on the evidence that we present in this article, we contend that the focus of SIT research should be moved towards establishing acceptable and effective protocols that involve minimal sprint durations and repetitions.

21(V O2max) in both sedentary men and women, but improves insulin sensitivity in men only. 22The aim of the present study was to determine whether there is a true sex difference in 23 response to REHIT, or that these findings can be explained by the large interindividual 24 variability in response inherent to all exercise training.