The health benefits of being physically active in adolescence and young adulthood are well-documented. These benefits include enhanced cardiorespiratory and muscular fitness, superior bone and cardiometabolic health and a positive effect on body weight status (Guthold et al., 2020), as well as improved cerebrovascular and cognitive functioning abilities (Hwang et al., 2018). There is also evidence that these health benefits are carried forward into later adult life, with cardiorespiratory fitness (CRF) in young adulthood a modifiable, prognostic marker of long-term mortality and cardiovascular disease (CVD) risk in later life (Guthold et al., 2020). Specifically, higher fitness levels in early adulthood are associated with a lower future risk for all-cause mortality and adverse cardiovascular events (Valle et al., 2015; Shah et al., 2016), as well as being associated with a lower risk for developing cardiometabolic disorders, such as type 2 diabetes (T2DM), by middle-age (Chow et al., 2016).

Sedentary behaviour, defined as any waking activity with an energy expenditure <1.5 METs whilst in a sitting, reclining or lying posture (Tremblay et al., 2017), and physcial inactivity are becoming increasingly prevalent in modern societies based on changes in physical, social, and economic environments, especially for younger adults (Owen et al., 2010; Castro et al., 2018). Worryingly, it is well-established that sedentary behaviours are associated with both cardiovascular and all-cause mortality (Kodama et al., 2009; Ekelund et al., 2016; Young et al., 2016). Further, numerous physiological changes associated with chronic disease risk are seen with physical inactivity, including declines in CRF, insulin resistance, hypertension, and increased adiposity (Booth et al., 2017). Together, combined presentation of the latter three risk factors is often termed the Metabolic Syndrome (MetS) (Kassi et al., 2011), which confers an odds ratio for cardiovascular morbidity and mortality of 3 and 1.8, respectively (Isomaa et al., 2001). Independent of other factors, young people with a high body mass index (BMI) have an increased risk of developing MetS in later life and may already have established hallmarks of the cardiometabolic disease such as elevated blood glucose, increased carotid intima thickness and raised serum cholesterol levels (Magnussen et al., 2010). In addition, sedentary behaviour is associated with an increased risk of T2DM (Wilmot et al., 2012; Patterson et al., 2018), with 29% of T2DM incidence in the UK estimated to be associated with physical inactivity (Patterson et al., 2018).

Although physical inactivity is associated with declines in CRF (Booth et al., 2017), physical activity can mitigate these reductions (Sparling et al., 2015) as well as the aforementioned risks of mortality associated with sedentary behaviour (Ekelund et al., 2016). However, despite these clear benefits, up to 62% of young adults do not meet current physical activity guidelines (Valle et al., 2015), with “lack of time” a chief barrier in younger adults (Fisher et al., 2015). Other commonly reported barriers preventing exercise participation in young adults include a lack of access to equipment and negative body image when exercising in public (Korkiakangas et al., 2011; Scott et al., 2020).

High intensity interval training (HIIT) has emerged as a time-efficient strategy to rapidly improve CRF (Gillen et al., 2014; Weston M. et al., 2014) and indices of cardiometabolic health, such as weight loss and insulin sensitivity, in younger adults (Gibala and Little, 2010; Weston K. S. et al., 2014). However, these favourable adaptations, that have been shown to be achieved in a shorter time-frame than those achieved by traditional aerobic exercise training (Gillen et al., 2014) have, for the most part, been achieved by supervised training employing specialist equipment such as cycle ergometers (Karlsen et al., 2017; Scott et al., 2019). To date, very few studies have investigated the efficacy of home-based HIIT regimes for improving CRF, with those that have been conducted studying middle-aged individuals (Blackwell et al., 2017; Scott et al., 2019) or those with disease (Scott et al., 2019). In addition, of those studies that have compared laboratory and home-based HIIT (Blackwell et al., 2017; Scott et al., 2019), different modalities of exercise were employed in each of these settings, and as such the impact of supervision was not explored.

Reducing sedentary behaviour in younger adults should be considered a key strategy to promote health and well-being across the life-course (Carnethon et al., 2003; Vainshelboim et al., 2019). Given the purported barriers to exercise participation in this particular age-group, and the allied growing interest in optimising the magnitude of adaptation from exercise, whilst minimising time and effort (Fisher et al., 2015; Foster et al., 2015; Shepherd et al., 2015), a home-based HIIT regime that can improve CRF without the need for specialist equipment could be largely beneficial in this space (Fisher et al., 2015). Therefore, we investigated both the efficacy of, and the impact of supervision on, short-term, equipment-free HIIT for improving CRF in young, recreationally active adults. We hypothesised that this HIIT regime would improve the CRF of young adults, irrespective of supervision status.

This study has shown that just 4 weeks, time-efficient, equipment-free, bodyweight-based HIIT is able to elicit improvements in the CRF and BMI of young healthy adults, and also aspects of skeletal muscle architecture, all irrespective of exercise supervision. This HIIT protocol was not however able to elicit improvements in resting BP, or indices of glucose and insulin metabolism in this cohort.

Although HIIT has been well-documented to be a potent stimulus to improve CRF in a short period of time in younger adults (Gibala et al., 2012; Gillen and Gibala, 2014; MacInnis and Gibala, 2017), the role of supervision and the requirement for equipment to facilitate efficacious HIIT in this age group has been scarcely studied (Blackwell et al., 2017; Scott et al., 2019). This study addresses this evidence gap and demonstrates that: (1) HIIT is acceptable and effective for young individuals in a non-laboratory environment; (2) bodyweight-based HIIT can achieve comparable improvements in CRF to those achieved using equipment (e.g., cycle ergometers); and (3) these improvements can be achieved using an unsupervised protocol with no direct feedback. The improvements we report in both our L-HIIT and H-HIIT groups provides evidence that the exercises employed were sufficient to generate a high enough intensity to replicate the previously reported benefits of HIIT with equipment (Boereboom et al., 2016; MacInnis and Gibala, 2017); with exercise intensity shown to be key to achieving improvements in CRF (Phillips et al., 2017). Our HIIT programme was well-received by all participants and no adverse events or injuries were reported during completion of the programme, irrespective of supervision. This data supports the growing notion that HIIT can be performed outside a laboratory/supervised environment, and that expensive specialist equipment is not needed (Hardcastle et al., 2014; Metcalfe et al., 2020).

Higher fitness levels at an earlier age are associated with a lower future risk for all-cause mortality and adverse cardiovascular events (Valle et al., 2015; Shah et al., 2016). As such, the significant improvements CRF that we report after just 4 weeks of intervention provides further evidence that HIIT as an independent exercise modality potentially mitigates future disease risk. Compared to health-based physical activity guidelines, which until recently have primarily focussed on moderate intensity continuous training (MICT), the ~10% increase in CRF that we report for both of our HIIT groups is consistent with those previously observed with MICT [e.g., 11% after 5-weeks cycling (Kong et al., 2016)].

It must be recognised that HIIT is an umbrella term used for numerous different exercise profiles which intersperse intense efforts with periods of rest or active recovery (Gibala and Little, 2010). Some of these protocols are not time-efficient and as such do not address the commonly cited barrier to exercise of “lack of time.” Conversely, some HIIT protocols are incredibly time-efficient (Whyte et al., 2010) but require “supra-maximal” efforts [often known as sprint-interval training (SIT)], which are often considered not suitable, or indeed attainable, by those with no prior exercise training experience (Gillen and Gibala, 2014). Using a temporal HIIT protocol which is both time-efficient (<16 min per session) and achievable by all [including cancer patients (Blackwell et al., 2020) and elderly individuals (Boereboom et al., 2016)], translated into an equipment free regime, our results are superior to that observed in a 6-week, cycle ergometer-based HIIT study which employed a four-by-30 s protocol in overweight or obese young males. This study reported only a 2.8% increase in VO₂peak (Fisher et al., 2015) and concluded that 6 weeks of MICT using continuous cycling (+11.1%) was superior to HIIT for improving CRF in this group. Although between group differences (BMI and gender split) may explain the discrepant findings between the work of Fisher and colleagues and those presented herein, we postulate that it was likely the duration of high-intensity time (2 vs. 5 min) per session, as session frequency and target intensity were matched.

The findings from this study are comparable to another bodyweight based HIIT study in young, obese individuals, which at 4 weeks post intervention, reported an improvement in VO₂peak of 2.6 ml/kg/min (Scott et al., 2019). This non-randomised study utilised a four-by-60 s protocol performed 3 times per week for 12 weeks, with interim results at 4 weeks. That our results are similar to this study further validates the use of a 60 s effort for bodyweight HIIT protocols to elicit improvements in CRF.

The findings from the HIIT regime employed in our study are also superior to the findings of a meta-analysis of <13 sessions of low volume HIIT which reported a 6.2% and 3.6% improvement in the VO₂peak of young, active, non-athletic males and females, respectively (Weston M. et al., 2014). The majority of studies in this meta-analysis were fully supervised and had equipment requirements. As such, that the improvements we report in this study were based on a protocol with no equipment and a much-reduced time commitment when compared to both MICT and some other forms of HIIT, our HIIT regimen may also improve adherence given that both equipment and time are commonly cited barriers to exercise (Foster et al., 2015).

An increase of one MET or 3.5 ml/kg/min (Jetté et al., 1990) in VO₂peak is associated with a 15 and 19% lower risk of all-cause and CVD mortality, respectively (Lee et al., 2011). The increase in VO₂peak that we observed in both HIIT groups is close to this level and not different based on supervision, supporting the notion of this HIIT protocol being a feasible strategy for reducing the risk of all-cause mortality in later life. However, for this benefit to be achieved exercise adherence would be essential, and as such further large-scale trials are needed to not only assess exercise adherence but also retention of favourable adaptations beyond the 4-week intervention period reported in this study.

BMI is another parameter known to be closely associated with cardio-metabolic disease risk (Isomaa et al., 2001; Kassi et al., 2011) and we report small but significant reductions in BMI in both HIIT groups, representing another favourable physiological impact of our HIIT regime. These relatively small improvements in BMI in our non-obese cohort are similar to those reported in a 12-week rowing HIIT study in non-obese young females (Brown et al., 2018). Further, that an 8-week SIT programme comparing improvements in BMI and body composition in young obese and normal weight males reported significant improvements in the obese group only (Ouerghi et al., 2017), suggests that greater improvements in BMI may be observed using our HIIT regime in individuals with a less favourable body composition profile at baseline.

A second aspect of body composition associated with overall health is skeletal muscle mass (Franchi et al., 2018). In this study, although we were not able to assess whole-body muscle mass as reported in a number of HIIT studies (Boereboom et al., 2016; Brown et al., 2018; Marcos-Pardo et al., 2019; Scott et al., 2019), we saw no change in VL MT, reported to be representative of muscle mass (Franchi et al., 2018) or VL FL (Randhawa and Wakeling, 2015) in any of the groups. We did however observe an increase in PA in both HIIT groups which is thought to represent an increase in cross sectional area and associated force and contractile capability (Lee et al., 2015; Monti et al., 2020). Whilst improvements in muscle PA have been shown following training modalities commonly associated with hypertrophy and strength gains, e.g., resistance and plyometric exercise training (Monti et al., 2020), the impact of HIIT on PA has been less well-studied.

Despite many HIIT studies reporting improved blood glucose and insulin sensitivity in healthy young, adults after training (Ciolac et al., 2010; Fisher et al., 2015; Shepherd et al., 2015), and in those with impaired metabolic function (Whyte et al., 2010; Jelleyman et al., 2015; Scott et al., 2019), we did not demonstrate any significant improvements in indices of glucose or insulin metabolism, each key features of whole-body health and future disease risk. Of these previous studies in healthy young adults, the shortest training duration was 6-weeks and as such it may be that the 4-week protocol employed in this study was too short to elicit improvements in indices of glucose handling and insulin sensitivity. However, somewhat against the suggestion of training duration being the limiting factor for improvements in these parameters, Babraj et al. (2009) showed improvements in glucose and insulin metabolism in healthy young males following just 2 weeks of a supervised, cycle-based SIT protocol. However, as already mentioned, the “supra-maximal” style of SIT would be difficult to achieve outside of a laboratory setting, especially without equipment, and may not appeal to many individuals (Gillen and Gibala, 2014). As such it may be that a longer duration of training is needed for our well-tolerated, HIIT protocol to elicit favourable changes in glucose and insulin metabolism, especially in young healthy individuals without metabolic impairment.

A number of studies have shown HIIT to be an excellent way to improve BP in distinct patient cohorts, and young and older adults (Ciolac et al., 2010; Gillen et al., 2014; Bruseghini et al., 2015; Grace et al., 2018), including those using the same temporal HIIT profile (Herrod et al., 2020) and/or training duration reported herein (Boereboom et al., 2016). However, in this study, we failed to demonstrate any change in resting SBP or DBP in either HIIT group. We are not however the first to report a lack of improvement in BP with HIIT. For example, a 6 week cycle ergometer-based study in young, overweight males also failed to demonstrate improvements (Fisher et al., 2015). As with our suggestion for improvements in metabolic function, it may be that in normotensive, younger participants, a longer duration of HIIT is required to elicit changes in BP, especially via our well-tolerated regime. Furthermore, a separate study in young, overweight males performing 6-weeks of cycle ergometer-based HIIT reported that BP reductions were less pronounced in normotensive subjects (Clark et al., 2020), which may go some way to explain our findings.

Self-monitoring has been shown to be a promising technique to invoke positive behavioural changes in physical inactivity, thereby increasing rates of compliance and adherence to exercise interventions (Gardner et al., 2016). As such, we must consider that our request for self-report in the H-HIIT group may have impacted their behaviour. In addition, more modern eHealth technologies have been shown to reduce the burden and improve the accuracy of physical activity data collection, when compared to traditional paper-based methods (Compernolle et al., 2019). Further, these technologies have also been shown to improve compliance, adherence and in turn greater achievement of intervention goals (Turner-McGrievy et al., 2013). Although self-reported levels of adherence and compliance in our H-HIIT group were high, the use of eHealth technologies with the opportunity to integrate assessment and feedback, may further enhance motivation and adherence/compliance, especially over longer periods of training. For example, in a recent unsupervised, equipment-free HIIT study, Scott et al. (2019) reported improvements in cardiometabolic health and good levels of adherence in younger adults with diabetes completing a 12-week protocol. Scott and colleagues monitored HR remotely with feedback and instruction provided to participants via a mobile phone application (Scott et al., 2019). Whilst unsupervised HIIT has not been extensively investigated, the application of HIIT in this way may prove to be a crucial cost-effective public health tool to battle the rise of inactivity, cardiometabolic disorders and the associated later poor health outcomes in younger adults.

As with all studies, we recognise some limitations to this work. Firstly, as a volunteer study, self-selection to take part may have created bias within our final cohort. Although this would not impact between-group comparisons, or indeed conclusions on the efficacy of our intervention, it may be that compliance and adherence data may not represent that to be achieved by a larger scale public-health “roll-out.” More expansive measures of body composition would provide additional information on other aspects pertaining to whole body health that may be achieved by our HIIT regimen [i.e., adipose distribution (Scott et al., 2019) and or bone facets (Ravnholt et al., 2018)]. The use of a heart rate monitor, ideally with remote monitoring capability, would have improved the reliability of the data on heart rate achieved during each session/effort, and this is scope for future work. Whilst participants were asked to maintain their current level of physical activity for the duration of the study, this was not measured, and as such it cannot be concluded that alterations in this aspect of their lifestyle did not influence outcomes of this study. Finally, although the inclusion of males and females is a strength of this study given its potential application for public health, hormonal changes during the menstrual cycle are known to impact exercise adaptation and performance (Janse de Jonge, 2003). Despite this, numbers of females in each group was roughly equal, and the 4-week training duration would span a regular menstrual cycle in those not take contraceptive medication, and as such we do not feel that not controlling for menstrual cycle phase will have likely impacted our outcomes.

A 4-week, time-efficient, equipment-free HIIT programme is able to elicit improvements in the CRF and BMI of young, healthy adults. Furthermore, this programme is equally successful when performed in an unsupervised setting such as the home. Our home-based HIIT regime could offer an attractive exercise intervention for younger adults, counteracting some of the effects of the rise in sedentary behaviour, and potentially mitigating chronic disease risk across the adult life-course.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by University of Nottingham Faculty of Medicine and Health Sciences Research Ethics Committee (C16122016). The patients/participants provided their written informed consent to participate in this study.

TS, JL, and BP: study conception and design. TS, TI, and AG: acquisition of data. TS, BD, NG, PA, and BP: analysis and interpretation of data. TS, PA, and BP: wrote manuscript. All authors contributed to the final approval of manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.